Page

SAIT, JABALPUR

DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING

LABORATORY – ELECTRICAL DRIVES (B-312A)

SUBJECT - ELECTRICAL DRIVES

SUBJECT CODE: EX-702

BRANCH – EX

SEMESTER - VII (GRADING)

LIST OF EXPERIMENTS

1. To control the speed of 3-Φ Slip Ring Induction Motor using Static Kramer

System.

2. To study the operation of Parallel Inverter.

3. To study the operation of Series Inverter.

4. To perform the operation of speed variation of PMDC motor using single phase

bridge converter.

5. To perform the operation of full wave half controlled SCR bridge with motor load-

PMDC motor

6. To study and perform the operation of speed control of Universal Motor in open

loop and closed loop system (P&PI).

7. To study Speed Control of Universal Motor using AC Voltage Controller.

8. To perform Voltage Commutated Chopper.

9. To study of Synchronous Motor Drives.

Page

INDEX

S. NO. NAME OF EXPERIMENT PAGE NO. DATE SIGN. REMARKS 1. To control the speed of 3-Φ Slip Ring

Induction Motor using Static Kramer System.

3-7

2. To study the operation of Parallel Inverter

8-11

3. To study the operation of Series Inverter.

12-15

4. To perform the operation of speed variation of PMDC motor using single phase bridge converter.

16-19

5. To perform the operation of full wave half controlled SCR bridge with motor load- PMDC motor.

20-23

6. To study and perform the operation of speed control of Universal Motor in open loop and closed loop system (P&PI).

24-27

7. To study Speed Control of Universal Motor using Single Phase Half Controlled Rectifier.

28-30

8. To perform Voltage Commutated Chopper.

31-34

9.To study of Synchronous Motor Drives.

35-37

Page

__/__/20__ EXPERIMENT NO.-1

AIM: - To control the speed of the 3N slip ring Induction motor using the PEC16HV10B trainer module.

APPARATUS REQUIRED:-

S.N. NAME TYPE RANGE QUANTITY1 Training module.2 Induction Motor3 Transformer4 Patch chords

5 Inductor. 6 Tachometer.

THEORY:-In the static Kramer drive, the step power from the rotor of Slip Ring Induction Motor is converted

to DC voltage which is then converted to line frequency and pumped back to the ac source. As the

slip power can flow only in one direction, this system offers a sub-synchronous speed control. As the

power flow is from rotor circuits to supply, static Kramer drive offers a constant torque operation.

Speed Control of 3 Phase Slip Ring Induction Motor

Slip-Ring Induction motors are preferred in applications with the requirement of high starting torque,

high over load capacity, nearly constant speed and low starting current. Applications include driver

for Line shafts, Pumps, Lights, Generators, Winding Machines, and Mills etc. This manual discusses

about the static Kramer system of slip power Recovery for speed control of 3N slip Ring induction

motor module. In this system, the slip power is connected to DC by means of a solid state rectifier

and by using static slip power recovery scheme the excess power is again fed back to the supply. The

amount of power fed back depends on the firing angle of the inverter, 3N supply voltage and DC

voltage to the inverter.

PRINCIPLE OF THE STATIC KRAMER SYSTEM

A sub synchronous static Kramer drive scheme is shown. It consists of 3N slip ring induction motor,

a rectifier bridge, a filter, an inverter bridge and a transformer. In this scheme of speed control, slip

power is recovered from the rotor of 3N slip ring induction motor and is fed back to the supply line

through rectifier, inverter and transformer. The slip frequency alternating current is first rectified by

Diode Bridge and this unidirectional current is smoothed by a filter circuit after which it is fed to the

Page

3-inverter. This inverter is line commutated. The slip power returned to the supply depends upon the

inverter terminals and the 3-phase supply voltage. The function of the 3-phase transformer is to

match the inverter output voltage with the 3-phase supply voltage.

The speed control is achieved by varying the firing angle of the inverter. By varying firing angle, the

voltage taken from the supply is reduced and so speed of motor is reduced. The usual range of firing

angle is for 90° to 160°for a wide speed range. The power flow through the rectifier is

unidirectional; therefore, only sub-synchronous speeds below the normal value are obtainable. Since

the slip power is returned to the supply, the scheme illustrated in Fig. results in a constant torque

drive.

CIRCUIT DIAGRAM:-

PROCEDURE:-

1. Switch ON the 3N AC Power Supply to the module.

2. Switch ON the power ON/OFF switch of the module.

3. The pulse release switch should be in OFF condition and the pulse cable should not be

connected.

4. Now check the “Phase Sequence” of the 3N AC i/p at the 3N SCR Bridge firing circuit test

points r, y, and b.

Page

5. Now check the test point’s r, y, b and the secondary output of the step down transformer r, y, b

are at “In Phase” using CRO at dual mode. They should be in phase with each other. If they are

not at in phase, change the connection of the transformer and make them in Phase.

6. Now connect the pulse cable.

7. Keep the pot meter at minimum position.

8. Connect the lamp load across the 3N SCR converter output.

9. Switch ON the pulse release switch.

10. Slowly vary the pot meter and verify the converter output.

11. Now switch OFF the pulse release switch and remove the lamp load connection.

12. Now connect the 3N SCR bridge output positive to the negative of the 3N diode bridge output at

the right side of the module.

13. Connect the iron core inductor (0mH) tapping to the positive of the 3N diode bridge Output and

connect the 150mH output to the negative of the 3N SCR bridge output.

14. Connect the positive of 3N SCR bridge output to the negative of the 3N diode bridge Output.

15. Now switch ON the stator MCB and the three phase diode bridge MCB at the back side of the

module.

16. Switch ON the pulse release switch.

17. Vary the pot meter gradually from the minimum position till it reaches 1450rpm (rated speed).

Note: The motor should be in no-load condition,

18. Check the speed of the 3N slipping motor using tachometer.

OBSERVATION TABLE:-

S. NO. FIRING ANGLE " (DEG) SPEED N(RPM)

RESULT:- Thus the speed of the 3N slip ring Induction motor is using the PEC16HV10B trainer

module.

Page

PRECAUTION:-

1. Using isolation transformer.

2. All the cables and patch chord should be made through 15A wire.

3. Don’t load the motor above 7.5A of rotor current.

4. Don’t vary the pot meter once the motor reaches 1450 rpm.

VIVA VOCE

Page

1. Why Induction motors are widely used in Industries?Ans. In Industries 80% of the motor drives used are induction motors. Induction Motors are widely used in Industries because of some features it possess. They are:a) Low Cost b) Simple and Ruggedc) Construction; Absence of Commentators d) Good power factor

2. What is the relation between Induction motor and Transformer?Ans. Induction motors are considered as transformers with secondary winding rotating. This is because both transformers and induction motor operates in such a manner that power is transferred to secondary windings (rotor) from primary windings (stator) by mutual induction principle.

3. Why the efficiency of the Induction motor is less compared to transformer?Ans. Induction motor and Transformer operates on the same principle that power is transferred to secondary by mutual induction. But in transformer, flux will be traveled from primary to secondary winding through the core of the transformer. In induction motor flux travels from stator to rotor through the air gap. Therefore power factor will be less for induction motor compared to transformer which also results in less efficiency for induction motor compared to transformer.

4. What is the condition for maximum torque in induction motor?Ans. Starting torque will be maximum when the rotor resistance / phase is equal to standstill rotor reactance / phase.

5. What are the advantages of static Kramer system, over static scherbius system? Ans. Since a static Kramer system possesses no line commutated inverter, it causes less reactive power and smaller harmonic contents of current than a static scherbius.

6. What is electrical power supply system? Ans. The generation, transmission and distribution system of electrical power is called electrical power supply system.

7. What type of starter is used for slip ring induction motor? Ans. Rotor resistance starter

8. Name the type of induction motor which produces high starting torque. Ans. Slip ring induction motor

9. State one major advantage and disadvantage of slip power recovery scheme based wound rotor induction motor drive. Ans. Advantage: The slip power can be recovered and fed back to the supply due to this overall efficiency is increased. Disadvantage: Power factor of the system is low.

10. Explain the rotor resistance starter allows fast start with heating of induction motor. Ans. In slip ring induction motor is normally started with rotor resistance starter. Here we can apply full voltage to the stator winding. Due to this, motor start with fast as well as heating of the motor.

Page

__/__/20__ EXPERIMENT NO. - 2

AIM: - To study the operation of Parallel Invertors

APPARATUS REQUIRED:-

S.N. Name Type Range Quantity1 Parallel inverter kit

2 Power supply

3 CRO

4 LOAD

5 Patch Chords

THEORY:-

The basic single phase Parallel Inverter utilizes capacitor ‘C’ for SCR commutation. The function of

‘L’ is to make the source current constant. The circuit configuration is as shown in fig. The operation

of this Inverter can be explained in defined modes as under.

Mode I:

TH1 is conducting and current flows through upper half of primary winding of transformer. This

current flux that links both the windings of primary. This produces voltage 2Vsacross whole of

primary winding and charges ‘C’ to 2Vs with upper plate positive. Now TH4 is forward biased

through TH1 by capacitor voltage 2Vs.

Mode II:

At time t = 0, TH4 is turned on by trigger pulse applied to its gate. As soon as TH4 is ON, 2Vs is

applied in reverse direction across TH1 and it turns OFF. TH4 being ON the constant current

maintained through TH1 is now transferred to TH4 and passes in reverse direction through lower

half of transformer winding. The capacitor also sends current through transformer primary and the

voltage across it changes from +2Vs to -2Vs. This mmf due to current through primary also

establishes secondary load voltage which varies from Vs to -Vs.

Mode III:

When capacitor is charged to -2Vs, with upper plate negative, TH1 may be fired again to reverse bias

TH4 and repeat the cycle.

By equating primary and secondary ampere turns and solving the respective differential equations.

If

Page

N 2

N1=n

tc= circuit turn off time; then

C=n2 . t c

4 R . ln 2

This means tc > toff, where toff is the SCR turn off time. It is also seen from above that if load resistor

is small, size of capacitor required is large.

CIRCUIT DIAGRAM:-

PROCEDURE:- 1. Make the connection as per wiring schedule given above to form a parallel inverter using SCRs

from “CON/INV” panel.

2. Use appropriate patch chords provided.

3. The commutating capacitor may be formed out of capacitors available from same panel where as

commutating Choke and Inverter Transformer may be used from “Main Unit”.

4. A 15W bulb should be connected as load which is also provided on main unit.

5. All the connections should be done using set of patch chords given.

Wiring Sequence: Charger (+)-choke1(upper), 4-choke 1(lower), 5-INV transformer 12V AC(upper), INV transformer 12V AC(lower)-12, 10-2, INV transformer 10.8V AC(upper)-29, INV transformer 10.8V AC(lower)-33, 32-36, 35-choke1(upper), 13-cap. Bank C1(lower), 6-Cap. Bank C1(upper), Charger(gnd) -INV transformer 0V(centre), INV transformer 230V(L)-choke 2(upper), Choke 2(lower)-lampload(L), lampload(N)-INV transformer 230V(N), Powerscope(red)-lampload(L), Powerscope(black)-lampload(N)

Page

6. The DC input to the Inverter is to be given by connecting battery wires to the battery Input

terminal provided on rear of the main unit.

7. The firing pulse selector on LSPT card is to be kept on Position 1st for getting required firing

pulses for TH3 and TH2.

8. Put ON AC input to main unit for establishing SCR pulses and then put ON DC supply.

9. Now observe various waveforms using 2 channel power Oscilloscope to study the operation as

described above.

10. Output AC voltage may also be measured by connecting meter provided on Main Unit. Now fill up the table.

OBSERVATION TABLE:-

Sr. No.

FrequencyHz

Vac on Panel Meter(Analog)

CONCLUSION:- The SCR Parallel Inverter uses reverse voltage across capacitor for commutating

the conducting SCR and the choke ensures continuous current while commutating. This reverse

voltage is generated using mutually coupled center tapped winding of transformer used. The circuit

works as a push pull inverter and produces output voltage as per transformer turns ratio.

Page

VIVA VOCE1. What is inverter? What are the types of Inverter?

Ans. A device which converts dc power into ac power at desired output voltage and frequency is called as Inverter. Inverters are broadly classified into:Voltage Source Inverter (VSI)Current Source Inverter (CSI)

2. What is meant by pulse triggered devices?Ans. As the name indicates, to trigger these devices just a pulse is sufficient. Continuous gate voltage of entire on time is not required. The advantage of pulse triggering is, it will avoid the hard triggering. Ex. Thyristor, GTO

3. What is meant by GTO?Ans. It is a three terminal, four layers PNPN Power Semiconductor device that can be turned on by a positive gate current and can be turned off by a reverse gate current.

4. Define Gate Charge in MOSFET?Ans. The gate charge is defined as the amount of charge required for the device during turn-on and turn-off time. It is the most important parameter we have to consider while selecting the MOSFET. The switching speed of the MOSFET depends on the speed at which the gate driver can charge or discharge the input gate charge.

5. What is load commutation?Ans. In load commutation, the load current flowing through the thyristor either becomes zero or is transferred to another device from the conducting SCR.

6. What is inverting operation of the converter?Ans. Converters are basically AC to DC controlled rectifiers. The power flows from AC supply to DC load. In inverting operation, the power flows from DC load to AC supply. Under such situation, the DC load is normally a battery. Thus the power of DC battery load is given to AC supply through controlled rectifier. For this operation, firing angle is greater than 90˚.

7. What is the inversion voltage of an inverter? Find out the inversion voltage of a CMOS inverter.Ans. The inversion voltage: The inversion voltage Vinv is defined as the voltage at which is defined as the voltage at which the output voltage Vo is equal to the input voltage Vin. For a CMOS inverter it can be expressed in terms of the threshold voltages of the MOS transistors and other parameters.

8. When we have to connect the SCRs in a parallel manner?Ans. To meet the high current demand we will connect the SCRs in a parallel manner. Similarly to meet the high voltage demand we have to connect the SCRs in series.

9. Define braking. Ans. In braking, the motor works as a generator developing a negative torque which opposes the motion.

10. What are the types of braking? Ans.

a) Regenerative`braking

Page

b) Dynamic brakingc) Plugging

11. What are the disadvantages of armature resistance control?Ans. In armature resistance control, speed is varied by wasting power in external resistors that are connected in series with armature.Since it is an inefficient method of speed control, it was used in intermittent load applications where the duration of low-speed operation forms only a small proportion of total running time.

12. What is brushless DC motor?Ans. An inverter fed trapezoidal permanent magnet AC motor drive operating in self control mode is called as brushless dc motor.

Page

__/__/20__ EXPERIMENT NO. - 3

AIM: - To study the operation of series Invertors.

APPARATUS REQUIRED:-

S.N. NAME TYPE RANGE QUANTITY1 Series inverter kit

2 Power supply

3 CRO

4 Load

5 Chords

THEORY: - The series inverter uses class A- type commutating circuit. The commutating components ‘L’ and ‘C’ are connected in series with load in each cycle using respective SCRs. This arrangement forms an under damped resonating circuit. In such circuit respective SCRs turn off themselves during first current zero or reversal. Single ended circuits in this category suffer from following two major drawbacks:

1. Maximum operating frequency is lower than the circuit resonating frequency. If this restriction is not allowed, DC fault will occur damaging the SCR.

2. Current drawn from source is intermittent as during alternate half load current is supplied by capacitor.

These limitations can be avoided by using a sort of push-pull circuit as shown in fig. The centre tapped choke with polarities as shown is used as commutating choke. Each half is labeled as L1 and L2. Two capacitors are used namely C1 and C2 for series resonance in each half.Here

Vs(+ve)-TH2-L1-INV transformer-C2-Vs(-ve) from resonating circuit during one half of output waveform.

Vs(+ve)-C1-INV transformer-L2-TH1-Vs(-ve) from resonating circuit during other half of output waveform.

Assuming TH1 to be conducting previously; while TH2 is turned ON following happens-a) Current through L1 induces voltage across L2 with such a polarity as to turn off TH2.b) Current rises through Inv transformer primary in one direction and induces voltage in Inv

transformer secondary so as to match primary and secondary ampere turns.c) The current as above is sinusoidal and passes through zero due to resonance. At this zero

crossing time TH2 turns off and TH1 is turned ON by gate trigger pulse.d) Now the direction of current through Inv transformer primary reverses as compared to ‘b’

above. As such generated secondary voltage also changes polarity. This current is also sinusoidal and passes through zero due to resonance, turning off TH1.

Page

e) Now the cycle as above repeats producing sine wave output in each half of output waveform. Gate pulses of TH2 and Th1 are placed more apart than their resonating time to ensure safe operation of the circuit.

Here the predominantly following points are observed while designing the inverters. Each SCR has to block forward voltage of magnitude Vc +Vs. Where Vc is capacitor voltage

and is function of resonating conditions and Vs is DC supply voltage. SCR forward voltage bloking rating should be chosen suitably.

The ringing frequency must be so chosen as to approximate the value of the desired output frequency so that OFF time of the SCR is greater than their Turn-off time.

If the load is variable the commutating components must be so chosen that the circuit always remains underdamped.

Following above guidelines and solving the differential equations for under damped conditions we can calculate suitable values of L and C as follows-

L1=L2=L in henries.

L=2×RL×t q=2×V s

I L×t q

C1=C2=C in Farads.

C=0.9×t qRL

=0.9×I LV s×tq

CIRCUIT DIAGRAM:-

Page

PROCEDURE:-1. Make the connections as per schedule given above.

2. Keep the firing mode selector switch on position 1.

3. Keep DC input switch in OFF position.

4. Now switch ON AC input to PE trainer and then connect DC input using either of the methods

explained above.

5. Use 15W lamp as a load.

6. Now observe various waveforms using 2 channel power Oscilloscope and fill up the table.

OBSERVATION TABLE:-

Sr. No.

FrequencyHz

Vac on Panel Meter(Analog)

CONCLUSION:- The series inverter as above is useful for generating direct “Sine Wave” output

with further tuning. The output frequency range will have obvious limitations. This inverter operates

in safer manner as mutually coupled chokes ensure proper shut down of outgoing SCR.

PRECAUTIONS: - 1) All the connection should be tight...

2) The electrical current should not flow the circuit for long time, otherwise its temperature will

increase and the result will be affected.

3) It should be care that the values of the components of the circuit is does not exceed to their ratings

(maximum value).

4) Before the circuit connection it should be check out working condition of the entire Component.

Wiring Sequence: Charger (+)-16, Charger(gnd)-5, 2-choke 1(lower), choke 1(centre)-INV transformer 12V AC(upper), INV transformer 0V AC(centre)-cap. bank C1(lower), cap. bank C1(lower)-Cap. Bank C2(lower), cap. bank C1(upper)-17, cap. bank C2(upper)-6, 20-choke1(upper), INV transformer(L)-lampload(L), INV transformer(N)lower-loadlamp(N), lampload(L)-powerscope(Red), Lampload(N)-powerscope(Black), INV transformer(L)-AC VM(L), INV transformer(N)-AC VM(N).

Page

VIVA VOCE

1. Compare the values of speed and torque in case of motors when in parallel and in series. Ans. The speed is one fourth the speed of the motor when in parallel.

The torque is four times that produced by the motor when in parallel.

2. Mention the speed control method employed in electric traction. Ans. Series-parallel speed control.

3. What is the effect of inserting resistance in the field circuit of a dc shunt motor on its speed and torque? Ans.For a constant supply voltage, flux will decrease, speed will increase and torque will increase.

4. What is Voltage-source inverter (VSI) drive?Ans. In a VSI drive, the DC output of the diode-bridge converter stores energy in the capacitor bus to supply stiff voltage input to the inverter. The vast majority of drives are VSI type with PWM voltage output.

5. What is Current-source inverter (CSI) drive?Ans. In a CSI drive, the DC output of the SCR-bridge converter stores energy in series-reactor connection to supply stiff current input to the inverter. CSI drives can be operated with either PWM or six-step waveform output.

6. What is Load commutated inverter (LCI) drive?Ans. In an LCI drive (a special CSI case), the DC output of the SCR-bridge converter stores energy via DC link inductor circuit to supply stiff quasi-sinusoidal six-step current output of a second SCR-bridge's inverter and an over-excited synchronous machine.

7. What is Cycloconverter or matrix converter (MC)?Ans. Cycloconverters and MCs are AC-AC converters that have no intermediate DC link for energy storage. A cycloconverter operates as a three-phase current source via three anti-parallel-connected SCR-bridges in six-pulse configuration, each cycloconverter phase acting selectively to convert fixed line frequency AC voltage to an alternating voltage at a variable load frequency. MC drives are IGBT-based.

8. What is a closed loop control system?Ans. A closed loop system is mainly used to maintain constant speed operation. It is a system in which the output has control over the input.

9. What are the advantages of closed loop system?Ans. a. System protection.b. Greater accuracyc. Improved dynamic responsed. Reduced effects of disturbances such as loading.

10. What are the basic blocks of a closed loop system of a dc motor?Ans. The system consists of a dc motor, power converter, feedback path, comparator and speed controller.

Page

__/__/20__ EXPERIMENT NO. - 4

AIM: - To perform the operation of speed variation of PMDC motor using single phase bridge

convertor.

APPARATUS REQUIRED:-

S.N. NAME RANGE TYPE QUANTITY1 Power thyristors2 Rheostat

3 CRO

4 Transformer (1-phase) 230V/24V

5 Connection wires

THEORY:- A fully controlled converter or full converter uses thyristors only and there is a wider

control over the level of dc output voltage. With pure resistive load, it is single quadrant converter.

Here, both the output voltage and output current are positive. With RL- load it becomes a two-

quadrant converter. Here, output voltage is either positive or negative but output current is always

positive. Figure shows the quadrant operation of fully controlled bridge rectifier with R-load. Fig

shows single phase fully controlled rectifier with resistive load. This type of full wave rectifier

circuit consists of four SCRs.

During the positive half cycle, SCRs T1 and T2 are forward biased. At ωt = α, SCRs T1 and T3 are

triggered, then the current flows through the L – T1- R load – T3 – N. At ωt = π, supply voltage falls

to zero and the current also goes to zero. Hence SCRs T1 and T3 turned off. During negative half

cycle (π to 2π). SCRs T3 and T4 forward biased. At ωt = π + α, SCRs T2 and T4 are triggered, then

current flows through the path N – T2 – R load- T4 – L. At ωt = 2π, supply voltage and current goes

to zero, SCRs T2 and T4 are turned off. The Fig-3, shows the current and voltage waveforms for this

circuit. For large power dc loads, 3-phase ac to dc converters are commonly used. The various types

of three-phase phase-controlled converters are 3 phase half-wave converter, 3-phase semi converter,

3-phase full controlled and 3-phase dual converter. Three-phase half-wave converter is rarely used in

industry because it introduces dc component in the supply current. Semi converters and full

converters are quite common in industrial applications. A dual is used only when reversible dc drives

with power ratings of several MW are required. The advantages of three phase converters over

single-phase converters are as under: In 3-phase converters, the ripple frequency of the converter

Page

output voltage is higher than in single-phase converter. Consequently, the filtering requirements for

smoothing out the load current are less. The load current is mostly continuous in 3-phase converters.

The load performance, when 3- phase converters are used, is therefore superior as compared to when

single-phase converters are used.

V o=1π∫απ

V m sinωtd (ωt )

V o=V mπ

¿

I o=V 0

R=V m

πR¿

CIRCUIT DIAGRAM:-

PROCEDURE:-

1. Make the connections as per wiring schedule given above.

2. Keep 2P-6W switch on LSPT card at 4th position.

3. Vary the converter conduction angle converter pot on LSPT card and observe variation in speed

of DC motor.

4. Observe firing signals and motor current waveforms on oscilloscope.

Wiring Sequence: 230V AC(L)- Inv transformer(L), 230V AC(N)- Inv transformer(N), Inv transformer 12V AC(upper)-4,Inv transformer 12V AC(lower)-18, 2-13, 16-27, 6-19, 11-25, 20-choke1(upper), Choke 1(lower)-1000µF/50V(left), 24-1000µF/50V(right), 1000µF/50V(left)-DC motor(+), 1000µF/50V(right)-DC motor(-).

Page

OBSERVATION TABLE:-

Sr. No.

Firing Angle Load voltages on panel meter (DC)

SpeedRPM

CONCLUSION:- SCR can be used as controlled half wave Rectifier. It can be fired at any point in

half conduction cycle. Thus controlled DC voltage can be made available to load.

Thus the operation of single phase fully controlled converter using PMDC motor load has been

studied and the output waveforms has been observed.

Page

VIVA VOCE

1. Can fuses with an AC voltage rating be used in a DC application?Ans. Fuses must be rated for the voltage AC or DC in which they will be used.Generally, fuses have a DC voltage rating that is half of the maximum AC voltage rating.

2. What is meant by PMDC? PMDC stands for Permanent Magnet DC MotorAns. A Permanent Magnet DC Motor is similar to an ordinary dc shunt motor except that its field is provided by permanent magnets instead of salient-pole wound field structure.There are three types of permanent magnets used for such motors namely;(i) Alnico Magnets(ii) Ceramic magnets(iii) Rare-earth magnetsThe major advantages are low noise, small size, high-efficiency, low manufacturing cost.

3. What is the necessity to use the special machines? Ans. General purpose motors (Induction motors, synchronous motors)  are neither precision speed nor precision position motors. For many automated systems require high precise speed and high precise positioning motors. In such cases special purpose motors like stepper motors, PMDC motors etc. are used.

4. What are the different means of controlling induction motor? Ans. Stator voltage control. Frequency control Pole changing control. Slip power recovery control.

5. What are the two ways of controlling the RMS value of stator voltage? Ans. Phase control Integral cycle control

6. Mention the two slip-power recovery schemes. Ans. Static scherbius scheme Static Kramer drive scheme.

7. Give the basic difference between the two slip-power recovery schemes. Ans. The slip is returned to the supply network in scherbius scheme and in Kramer scheme, it is used to drivean auxiliary motor which is mechanically coupled to the induction

8. What is inverter? What are the types of Inverter?Ans. A device which converts dc power into ac power at desired output voltage and frequency is called as Inverter.Inverters are broadly classified intoVoltage Source Inverter (VSI) Current Source Inverter (CSI)

9. What are the two types of feedback in dc drive?Ans. a. Current feedback b. Speed feedback

10. Why the load commutated inverter fed synchronous motor drive found suitable for high-speed and high power applications?

Page

Ans. The load commutated inverter fed synchronous motor drive used for high-speed and high power applications because it has high-efficiency and four quadradent operation with regenerative breaking.

11. Give the advantages of using cycloconverter for synchronous motor speed controlAns.

a) Smooth low-speed operationsb) Four quadrant operation with regenerative brakingc) Good dynamic response

12. Define four quadrant operation.Ans. A motor operate in two modes namely motoring and braking.In motoring, it converts electrical energy into mechanical energy, which support its motion.In braking it works as a generator converting mathematical energy into electrical energy and thus, opposes the motion. Motor can be provided motoring and braking operations for both forward and reverse directions.

__/__/20__ EXPERIMENT NO. - 5

Page

AIM: - To perform the operation of full wave half controlled SCR Bridge with motor load PMDC

motor.

APPARATUS REQUIRED: -

Sr.No. NAME TYPE RANGE QUANTITY1 Trainer kit,

2 Patch cards,

3 Multi meter,

4 Connection wires

THEORY:- As shown in the diagram this configuration of two SCRs and two DIODEs in bridge.

Here for pure resistive load TH1, D1 conduct for positive half of input AC, while TH3, D2 conduct

for negative half. In both the halves actual instance of conduction is decided by firing angle of

respective SCR and the average output voltage can be varied as required, by deciding firing angle.

Respective output waveforms are as shown in fig.

In case of inductive load, load current is assumed to be continuous. The difference in operation is

that the load current continuous to flow through the outgoing SCR even if the voltage applied to it

reverses. This is the effect of inductance. Here if TH1, D1 are conducting up till now, then TH1

continuous to conduct but current through D1 is transferred to D2. Now whenever TH3 is triggered,

it reverses biases TH1 with input supply voltage and turns it off. Now TH3-D2 pair, supplies load

current.

For period π ≤ ωt ≤ π+α, TH1, D2 are conducting and the load voltage is clamped to forward drop

across them with –ve sign.

The configuration explained above is the most economical SCR converter configuration. Here

cathodes of both SCRs are joined together, as such ‘no isolated trigger pulses’ can also be given to

the SCRs. However there are two limitations to this configuration namely,

1. If second SCR is not fired positively at proper instance, then 1st SCR will continue conducting

even in next cycle and incoming SCR may not fire at all and the circuit will lose control over

firing angle.

2. For highly inductive loads with time constant greater than supply frequency, the above situation

may lead to circuit losing control over firing angle.

Page

This situation can be avoided by providing a freewheeling diode. This converter has a better power

factor at input and is very commonly used in applications wherein one quadrant operation is

acceptable. This option is cost effective also as diodes are chipper than SCRs.

CIRCUIT DIAGRAM:-

PROCEDURE:-

1. Make the connections as per wiring schedule given above.

2. Keep 2P-6W switch on LSPT card at 4th position.

3. Vary the converter conduction angle control pot on LSPT card & observe variation in speed

of PMDC motor.

4. Observe firing signals and motor current waveforms on oscilloscope.

OBSERVATION TABLE:-

Sr. No.

Firing Angle Load voltages on panel meter (DC)

SpeedRPM

Wiring Sequence: 230V AC(L)- Inv transformer(L), 230V AC(N)- Inv transformer(N), Inv transformer 12V AC(upper)-4,Inv transformer 12V AC(lower)-18, 3-31, 16-35, 6-19, 30-34, 5-choke1(upper), Choke 1(lower)-1000µF/50V(left), 33-1000µF/50V(right), 1000µF/50V(left)-PMDC motor(+), 1000µF/50V right) -PMDC motor(-).

Page

CONCLUSION:- The output current is always maintained positive in this half controlled converter

configuration. Line commutated Inverter operation is not possible in this configuration. The

configuration is very simple, reduces complications in gate control circuit and is very commonly

used in applications wherever single quadrant operation of SCR converter is sufficient to meet load

requirements.

Page

VIVA VOCE1. What is known as half-controlled rectifier and fully controlled rectifier?

Ans. The rectifiers that provide control of dc voltage in either direction and therefore, allow motor control in quadrants I and IV. They are known as fully-controlled rectifiers. The rectifiers that allow dc voltage control only in one direction and motor control in quadrant I only. They are known as half-controlled rectifiers.

2. What is called continuous and discontinuous conduction?Ans. A dc motor is fed from a phase controlled converter the current in the armature may flow in discrete pulses in called continuous conduction. A dc motor is fed from a phase controlled converter the current in the armature may flow continuously with an average value superimposed on by a ripple is called discontinuous conduction.

3. What is holding current in SCR?Ans. It is the minimum current required to hold the SCR in forward conduction state.When the forward current  becomes less than holding current, SCR turns from forward conduction state to forward blocking state.

4. What is latching current in SCR?Ans. It is the minimum current required to latch (turn on) the SCR from forward blocking state to forward conduction state.

5. What are the different turns on methods of SCR?Ans. a) Forward voltage triggering d) Temperature triggeringb) Gate Triggering e) Light triggeringc) dv/dt triggering

6. What are the difference between transistor and SCR? Ans. a) SCR is four layer PNPN device, whereas transistor is three layer (PNP or NPN) device.b) SCR turns on by regeneration but there is no regeneration in BJT.c) Gate has no control over the conduction once SCR turns on. BJT can be turned off by base

drive.d) BJT’s have higher switching frequencies than SCR’s.

7. What is snubber circuit?Ans. The snubber circuit is used for the dv/dt protection of the SCR. It is a series combination of a resistor and a capacitor in parallel with the SCR.

8. What is hard switching of the thyristor?Ans. When gate current is several times higher than the required gate current, the SCR is said to be hard fired. It reduces the turn ON time and enhances the di/dt capability.

9. What is firing angle?Ans. The angle between the zero crossing of the input voltage and the instant the SCR is fired is called as delay angle or firing angle.

10. Name some of the current controlled (current driven) devices...Ans. SCR,  GTO,  GTR 

Page

__/__/20__ EXPERIMENT NO. - 6

AIM: - To study speed control of Universal motor in open loop & close loops (P&PI).

APPARATUS REQUIRED:-

Sr. No. NAME RANGE TYPE QUANTITY1 Trainer kit,

2 Patch cards,

3 Multi meter,

4 Connection wires

THEORY:- The universal motor is so named because it is a type of electric motor that can operate

on both AC and DC power. It is a commutated series-wound motorwhere the stator's field coils are

connected in series with the rotor windings through a commutator. It is often referred to as an AC

series motor. The universal motor is very similar to a DC series motor in construction, but is

modified slightly to allow the motor to operate properly on AC power. This type of electric motor

can operate well on AC because the current in both the field coils and the armature (and the resultant

magnetic fields) will alternate (reverse polarity) synchronously with the supply. Hence the resulting

mechanical force will occur in a consistent direction of rotation, independent of the direction of

applied voltage, but determined by the commutator and polarity of the field coils. The Universal

Motor is a phase angle Triac controller having all the necessary functions for universal motor speed

control in washing machines. It operates in closed loop configuration and provides two ramp

possibilities. The TDA 1085C triggers a Triac accordingly to the speed regulation requirements.

Motor speed is digitally sensed by a tachogenerator and then converted into an analog voltage.

Universal motors have high starting torque, can run at high speed, and are lightweight and compact.

They are commonly used in portable power tools and equipment, as well as many house hold

appliances. They're also relatively easy to control, electromechanically using tapped coils or

electronically. However, the commutator has brushes that wear, so they are much less often used for

equipment that is in continuous use. In addition, partly because of the commutators universal motors

are typically very noisy. The speed set is externally fixed and is applied to the internal linear

regulation input after having been submitted to programmable acceleration ramps. The overall result

consists in a full motor speed range with two acceleration ramps which allow efficient washing

Page

machine control (Distribute function). Additionally, the TDA 1085C protects the whole system

against AC line stop or variations, over current in the motor and tachogenerator failure.

CIRCUIT DIAGRAM:-

PROCEDURE:- Thyrister actuator panel EMT9 is used to control DC power & same is supplied to

universal motor.

1. Make the connections as per wiring sequence provided.

2. Do not connect motor terminal to respective sockets.

Wiring Sequence: (A) Open loop: 230(L)-7,230v(N)-8, +12V-EMT9(18),-12V-EMT9(20),GND-EMT9(15),AC sync.O/P-EMT9(21),EMT9(9)-DC AM(+)DC Motor(+),EMT9(13)-DC Motor(-)Switch setting: SW1, SW2-Right, SW4-Ext.position.(B) Close loop with P (Proportional) action: 230V (L)-7,230V (N)-8,+12V-EMT9(18),-12V-EMT9(20),GND-EMT9(15),AC Sycn.O/P-EMT9(22) EMT9 (21), EMT9 (9)-DC AM (+),DC AM(-) DC MOTOR (+),EMT9(14)-EMT9(28) Connect the audio connector cable from speed sensor assembly on motor to audio connector (l-h-s) on (EMT9) panel Switch setting: SW1-Right Sw2-Left SW2-Left, SW3-Down, SW4-ON p—action(C) Close loop with P+I( Proportional +Integral) action: 230V(L)-7, 230V(n)-8,+12 V-EMT9(22),17V unreg-EMT9(21),EMT9(9)-DC AM(+),DCAM(-)-DC MOTOR(+),EMT9(13)-DC MOTOR(-) EMT9(14)-EMT9(28),Connect the audio connector cable from speed sensor assembly on motor to audio connector(L-H-S) on EMT9 panel Switch setting : SW1-Right,SW2-Left,SW3- down,SW4- on P1- action

Page

3. Use 230V/100W lamp load, to check proper voltage variation of the bridge. Output voltage

variation achieved using SP pot on EMT9 should be around 0-180V DC for nominal input of

230V. If you are not getting proper control (smooth o/p voltage variation) interchange L & N.

4. Switch off the trainer and reconnect the motor terminals to respective sockets.

5. Before switch ON the trainer ensure some load is applied on motor.

6. Keep SP pot on EMT9 at minimum position.

7. Now switch ON the trainer and set SP pot on EMT9 to get input to motor e.g. 150V, 180V, etc.

8. Apply load on motor by tightening one wing nut while keeping other stationary and take

different readings as shown in observation table.

9. Typically speed can be measured at the socket provided at Tag no. 14 on DMM (0-2.5).2.5V =

5000RPM. Can use formula.

V(tag14) = [(F/8)*60]/2000,

Where F= Freq. in Hz at square wave TP provided

10. You can also measure speed with hand held lab tachometer directly in RPM.

OBSERVATION TABLE:-

Sr. No.

Input to(V)

motor

Current

(A)

Speed (open) Speed(P)

(closed)

Speed(PI)

(closed)

Open P PI Open P PI Volt RPM Volt RPM Volt RPM

CONCLUSION:-1. Do not run universal motor without some load as speed will shoot-up possibly damaging

bearings.

2. Speed drops in open loop considerably.

3. Speed drops somewhat in close loop (P).

4. Speed is well regulated under close loop (PI).

Page

VIVA VOCE1. What is the advantage of using PI type speed controller?

Ans. The addition of an integral feedback can be used to eliminate the steady-state error and to reduce the forward gain required.

2. Which type of converter can be selected if the input is ac?Ans. When the input is ac, the dc motor can be operated from rectifiers. If the motor ratings are low, we can use single phase controlled rectifiers and for high ratings, three phase controlled rectifiers are used.

3. What is the advantage of closed torque control scheme?Ans. It finds application in battery operated vehicles such as electric trains.

4. What is the use of current limiter in the closed loop control system?Ans. It saturates and sets current reference for inner current loop at a value corresponding to the maximum allowable current.

5. What are the advantages of using PI controller in closed loop controller of dc drive?Ans.

a. Stabilize the driveb. Adjust the damping ratio at the desired valuec. Makes the steady state speed error close to zero by integral action and filters out noise

again due to the integral action.

6. What happens if the control loop is without current loop?Ans. If inner current loop is not added in the control circuitry, transient over current is produced which is undesirable from the standpoint of converter rating and protection. This is particularly in case of starting or other large changes.

7. What is the advantage of using simulation package?Ans. Simulation packages are used for studying the nature of the system developed without being practically implementing it.

8. What are the main disadvantages of phase controlled converter fed dc motor drives?Ans. The phase controlled rectifiers always consume reactive power. Due to this, they are expensive to operate where the reactive power is to be paid for. It also generates harmonics

9. What is self-control mode of synchronous motors?Ans. In self-control mode, the stator supply frequency is changed in proportion to the rotor speed so that the rotating field produced by the stator always moves at the same speed as the rotor.

10. What is constant margin angle control of synchronous motor drive?Ans. In self-control synchronous motor drive, the load side converter is operated as an inverter and maximizes motor power factor. This control is called constant margin angle control.

11. Why a synchronous motor without damper winding is used, in the drive system employing a cycloconverter?Ans. A synchronous motor without damper winding is used because the damper winding reduces the inductance of the machine, and therefore, its ability to filter out harmonics in the output voltage of cycloconverter. Since the drive operates in self-control mode, the damper winding is not needed for its conversion purpose.

Page

__/__/20__ EXPERIMENT NO. - 7

AIM: - To study Speed Control of Universal Motor using AC Voltage Controller.

APPARATUS REQUIRED:-

S. NO. NAME TYPE RANGE QTY.

1 2 Channel Power Oscilloscope

2 PE trainer with INV/ Con panel

3 Multimeter4 Lamp5 Universal Motor6 Patch Cords

THEORY: A voltage controller, also called an AC voltage controller or AC regulator is an

electronic module based on either thyristors, TRIACs, SCRs or IGBTs, which converts a fixed

voltage, fixed frequency alternating current (AC) electrical input supply to obtain variable voltage in

output delivered to a resistive load. The given circuit is AC voltage controller. During the positive

half cycle of input AC supply TH1 conducts and voltage appears across load. During negative half

cycle TH2 conducts and negative voltage appears across the load. Changing the firing angle of both

the SCRs can control the load voltage. Thus we get controlled AC voltage across load. The direction

of load current reverses in every half cycle of input AC and the firing instance within these cycles

can be controlled as such this arrangement is used for controlling AC power to load. Typical

electrical heater based Ovens on Industrial furnaces employ this type of control. Here two distinct

are possible.

1. Integral Cycle Control- Here the control circuit ensures that SCRs are kept ON for required

number of FULL AC cycles and then they are OFF. This cycle repeats so as to control average

power delivered to load. This arrangement minimizes generation of unwanted EMI and RFI

noise while switching power through SCRs, as the switching takes place almost near to load

current zero. This type is demonstrated in a separate unit on application of SCR namely 6.1

2. AC Phase Control- Here SCR phase control is achieved in every cycle as per power

requirement of load.

Page

CIRCUIT DIAGRAM:

PROCEDURE:1. Make the connections as per wiring diagram given to form AC voltage controller.

2. A hand drill motor/Fractional HP universal motor should be used for load. All connections

should be done using patch chords provided.

3. The firing pulse selector switch should be kept on 4th position for getting required gate pulse.

4. If you are using fractional HP universal motor, apply some load on motor using brake pully

loading arrangement provided. Else the motor will run at higher speed above its rated RPM.

While loading the motor, take care that the current should not exceed its rated value.

5. Now put on AC mains and measure the speed variation by using non-contact type tachometer.

6. Take the readings as per following table.

OBSERVATION TABLE:

Sr. No. Firing Angleα

AC voltage across Load

SpeedRPM

RESULT: In AC voltage controller two SCRs conducts alternately during positive and negative half

cycle and we get controlled AC output voltage across load.

Wiring Sequence: 230V AC-4, 12-2, 6-11, 10-Lampload(L), Lampload(N)-230V AC(N), Universal Motor(L)-Lampload(L), Universal Motor(N)-Lampload(N).

Page

VIVA VOCE1. What is Universal Motor?

Ans. It is defined as a motor which can be operated either on DC or single-phase AC supply at approximately the same speed and output. The universal motor is built exactly like a series DC motor. But a series DC motor cannot be run as a universal motor, even though both motors look the same internally and externally.

2. What are the advantages and disadvantages of AC voltage controller?Ans. Advantages: 1. They use line communication, hence no extra communication circuits are required.2. They have high efficiencies since device losses are reduced.Disadvantages:1. Large ripple and harmonics are present in the output.2. Output waveforms are not sinusoidal.

3. What is speed feedback?Ans. The motor speed can be sensed by any one speed sensor and this signal is compared with reference speed. This error signal is given to speed controller. The speed controller produce control signal to the power converter.

4. How is the speed of a motor sensed?Ans. The speed of a motor can be sensed by using a tachogenerator.

5. What are the two types of speed controller?Ans. Proportional controller & Proportional Integral controller

6. What is current feedback?Ans. The motor current can be sensed by current transducer. This signal is compared with reference signal. The error signal is fed to the current controller produces a control signal. This signal is fed to the power converter for controlling the output.

7. What is armature voltage control?Ans. The dc motor sped can be varied by varying armature voltage and field voltage is constant. This voltage can be varied by using power converter. This method only applicable for below base speed.

8. What is field weakening control?Ans. The dc motor speed can be varied by varying the field current and armature voltage is kept constant. The field current can be controlled by using power converter. By using this method the motor field flux decreases i.e., field weakening mode. This method is only applicable foe speeds above base speed because speed is inversely proportional to flux.

9. What is the purpose of current control in dc drives?Ans. The current control loop is used for the purpose of limiting the transient over current.

10. What are the advantages of constant margin angle control of synchronous motor drive fed by an inverter?Ans.

1. Eliminates the hunting and stability problems2. It does not require commutation circuits.

Page

__/__/20__ EXPERIMENT NO. - 8

AIM: - To perform voltage Commutated Chopper

APPARATUS REQUIRED:-

Sr. No. NAME RANGE TYPE QUANTITY1 Trainer kit

2 Patch cards

3 Multi meter

4 Connection wires

THEORY:-The junctions of any semiconductor exhibit some unavoidable capacitance. A changing voltage impressed on this junction capacitance results in a current, I = C dv/dt. If this current is sufficiently large a regenerative action may occur causing the SCR to switch to the one state. This regenerative action is similar to that which occurs when gate current is injected. The critical rate of rise of off-state voltage is defined as the maximum value of rate of rise of forward voltage which may cause switching from the off-state to the on-state. Since dv/dt turn-on is non-destructive, this phenomenon creates no problem in applications in which occasional false turn-on does not result in a harmful affect at the load. Heater application is one such case. However, at large currents where dv/dt turn-on is accompanied by partial turn-on of the device area a high di/dt occurs which then may be destructive. The majority of inverter applications, however, would result in circuit malfunction due to dv/dt turn-

on. One solution to this problem is to reduce the dv/dt imposed by the circuit to a value less than the critical dv/dt of the SCR being used. This is accomplished by the use of a circuit similar to those in Figure 3.8 to suppress excessive rate of rise of anode voltage. Z represents load impedance and circuit impedance. Variations of the basic circuit are also shown where the section of the network shown replaces the SCR and the R-C basic snubber. Since circuit impedances are not usually well defined for a particular application, the values of R and C are often determined by experimental optimization.A technique can be used to simplify snubber circuit design by the use of nomographs which enable the circuit designer to select an optimized R-C snubber for a particular set of circuit operating conditions. Another solution to the dv/dt turn-on problem is to use an SCR with higher dv/dt turn-on problem is to use an SCR with higher dv/dt capability. This can be done by selecting an SCR designed especially for high dv/dt applications, as indicated by the specification sheet. Emitter shorting is a manufacturing technique used to accomplish high dv/dt capability.

Page

CIRCUIT DIAGRAM:-

PROCEDURE:-

1) Make the inter connections in the power circuit as given is the circuit diagram. 2) Switch ON the firing circuit and observe the trigger outputs. 3) Make sure that the firing pulses are proper before connecting to the power circuit. 4) Then connect the trigger output from firing circuit to corresponding SCR’s / Triac. 5) If the output wave form is proper then you can connect the motor & increase the input voltage to rated value 0-230V gradually. Vary the firing angle and note down output voltage and speed of the motor.Note: 6) If you are not getting the output after all proper connections interchange AC output terminals, after switch OFF the MCB. This is just to synchronize the power circuit with firing circuit.

OBSERVATION TABLE:

RESULT:- Above experiment is done and results are verified.

CONCLUSION:-1) Do not run universal motor without some load as speed.2) Speed drops in open loop considerably.3) Speed drops somewhat in close loop (p)4) Speed is well regulated under close loop (PI)

VIVA VOCE

S.R. Input VoltageVin

Firing Angle

Output Voltage Vo

Output Current Io

Speed RPM

Page

1. What is chopper? Ans. It converts the fixed dc voltage in to variable dc voltage.

2. What are the different types of chopper? Ans. a. First quadrant chopper b. Second quadrant chopperc. Third quadrant chopper d. Four quadrant chopper

3. State control strategies of choppers. Ans. a. Time ratio control b. Current limit control

4. Write the applications of DC chopper. Ans. Electric traction, Steel mills, Paper mills, Machine tools, Printing mills etc.

5. What is a type of Chopper?Ans. It is equivalent to a dc transformer in ac circuit. The Chopper is a static switch which is used to get the variable dc output voltage from a constant dc input voltage.Step down chopper: In this circuit, the average output voltage [VO = D. VS] is less than the input supply voltage. It is also known as Buck converter.Step up chopper: It is also known as Boost converter. Here the average output voltage [VO = VS / 1 - D] is more than the input supply voltage. 

6. What is meant by commutation?Ans. The process of changing the direction of current flow in a particular path of the circuit. It is used to turn off the SCR.

7. What are the types of commutation?Ans. Natural commutation Forced commutation

8. What is load commutation?Ans. In load commutation, the load current flowing through the thyristor either becomes zero or is transferred to another device from the conducting SCR.

9. What is natural commutation?Ans. The process of the current flowing through the thyristor goes through a natural zero and enable the thyristor to turn off is called as natural commutation.

10. What are the two types of SRM?Ans. Rotary SRM & Linear SRM

11. What are the types of rotor position sensors in SRM?Ans. Optical sensor & Hall sensor

__/__/20__ EXPERIMENT NO. - 9

Page

AIM: - To study of Synchronous Motor Drives.

THEORY:-

As the name suggests the synchronous motors rotates at synchronous speed. The main advantage of

synchronous motors are that they run on 3 ac supply and dc supply is given to the rotor when they

run on synchronous speed the loss is very minimal. We can say that if the synchronous motors are

designed to run only at rated synchronous speeds, then what is the use of introducing drives to them.

The answer is pretty simple, synchronous motor drives makes the starting, pull in and braking

process smooth and without any problems. We shall discuss about them one by one.

Starting Synchronous Motors

The problems with synchronous motors are that they are not self- starting. Before discussing the

starting method of this motor, we should know about the type of supply and the rotor and the stator

of the motor briefly. The stators of synchronous motors are similar to that of an induction motors but

the only difference lies in the rotor, the rotor of the synchronous motors are given dc supply.

Now before knowing, how the synchronous motors are started, we should know why they are not

self started? The answer can be given as, when 3 phase supply is given to the stator, there is a

rotating magnetic flux which rotates at synchronous speed, and if the rotor is also given dc supply,

rotor acts as a magnetic flux which rotates at synchronous speed, and if the rotor is also given dc

Page

supply, rotor acts as a magnet having two salient poles. As the rotor is at standstill position, it cannot

follow the magnetic field which is rotating at synchronous speed. The rotor stacks at its position

because the opposite poles move so rapidly that the rotor locks, this is the reason why synchronous

motors are not self starting. Now coming to the point how synchronous motors are started. At first

the synchronous motors are started as a normal induction motors, the rotor of the motor is not given

dc supply, when the motor reaches the rotor and pull in takes place, which is discussed later.

Another method of starting the synchronous motor drives is by external motor. In this method the

rotor of the synchronous motor is rotated by an external motor and when the speed of the rotor

reaches near synchronous speed, the dc-field is switched on and pull in takes place. In this method,

the starting torque is very low and it is not very popular method also.

Pull in of Synchronous Motors

When the rotor of the synchronous motors reaches near synchronous speed, then the dc field supply

is switched on and the pull in process begins. As during switching on the dc supply due to the phase

angle and torque angle there are various disturbances seen in the motor and there are several slip of

poles of air – gap flux is also seen. As the pull in process is completed the rotor acquires

synchronous speed. The complete pull in as fast as possible the dc supply should be switched on at

the most favorable angle. Like when the synchronous motor is running as induction motor, the dc

supply should be fed when the induction motor is at top speed, this will be the best moment because

the speed difference will be least at that point of time.

Braking of Synchronous Motors

As we know, there are three types of braking i.e, regenerative, dynamic and plugging type braking.

But for synchronous motor drives only dynamic braking can be applied though plugging can be

applied theoretically. Regenerative braking cannot be applied to them as they need higher speed than

synchronous speed. Dynamic braking is done by disconnecting the motor from supply and

connecting it across a three phase resistor. At that time the motor works as asynchronous generator

and energy is dissipated at the resistors. Plugging is not used for synchronous motors as high

plugging current can cause severe disturbance and damage in line.

VIVA VOCE1. What is meant by sub synchronous speed operation?

Page

Ans. The sub synchronous speed operation means, the slip ring induction motor speed can be controlled below synchronous speed. The slip power is fed back to the supply.

2. What is meant by supper synchronous speed operation? Ans. The supper synchronous speed operation means, the slip ring induction motor speed can be controlled above synchronous speed. The slip power is fed back to the rotor side.

3. What are the various function performed by an electric drive? Ans. a) Driving fans, ventilators, compressors & pumps etc.b) Lifting goods by hoists & cranes c) Imparting motion to conveyors in factories, mines & warehouses d) Running excavators & escalators, electric locomotives, trains, cars, trolley buses and lifts etc.

4. What is self-control mode of synchronous motors?Ans. In self-control mode, the stator supply frequency is changed in proportion to the rotor speed so that the rotating field produced by the stator always moves at the same speed as the rotor

5. What is constant margin angle control of synchronous motor drive?Ans. In self-control synchronous motor drive, the load side converter is operated as an inverter and maximizes motor power factor. This control is called constant margin angle control.

6. Why a synchronous motor without damper winding is used, in the drive system employing a cycloconverter?Ans. A synchronous motor without damper winding is used because the damper winding reduces the inductance of the machine, and therefore, its ability to filter out harmonics in the output voltage of cycloconverter. Since the drive operates in self-control mode, the damper winding is not needed for its conversion purpose.

7. What is brushless DC motor?Ans. An inverter fed trapezoidal permanent magnet AC motor drive operating in self control mode is called as brushless dc motor.

8. Why the load commutated inverter fed synchronous motor drive found suitable for high-speed and high power applications?Ans. The load commutated inverter fed synchronous motor drive used for high-speed and high power applications because it has high-efficiency and four quadradent operation with regenerative breaking.

9. Give the advantages of using cycloconverter for synchronous motor speed controlAns.a) Smooth low-speed operationsb) Four quadrant operation with regenerative brakingc) Good dynamic response

10. Define firing angle. Ans. The firing angle is defined as the angle between zero crossing of the input voltage and the instant the thyristor is fired. The firing angle is denoted as α.