Ac Series Motor
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INTRODUCTIONA 3-phase induction motor cannot be operated at variable speed.1-phase induction motor is not self starting.Thus, A.C. commutator motors have been developed to provide improved starting torque, good power factor and better speed control.A.C. Commutator Motor
1-Phase Motor 3-Phase Motor A.C. Series Motor 3- Series Comm. Motor Universal Motor 3- Shunt Comm. Motor Repulsion Motor Schrage Motor
AC SERIES MOTORIf A.C. supply is given to a d.c. series motor it can start and run because the torque produced is proportional to the product of the flux and the armature current.
During positive half cycle, the flux and the armature current are in positive direction thus torque is in positive direction.
During negative half cycle, both the flux and the armature current reverse but its product is positive. So, torque remains positive.
This kind of motor is known as A.C. Series motor.
PROBLEMS IN A.C SERIES MOTORThe alternating flux would cause excessive eddy current loss in the yoke and field cores which will become extremely heated.
Vicious sparking will occur at brushes because of the huge voltage and current induced in the short-circuited armature coils during their commutation period.
Power factor is low because of high inductance of field and armature circuits.
Torque developed is not of constant magnitude, but pulsates between zero and maximum value each half-cycles.
MODIFICATION IN DESIGN OF AC SERIES MOTORThe eddy current loss has been reduced by laminating the entire iron structure of the field cores and yoke by silicon steel.In order to improve power factor ,reactance of the field winding and the armature winding should be reduced.Field reactance is reduced by reducing the number of turns on the field windings. For a given current, it will reduce the field m.m.f. which will result in reduced air- gap flux.This will increase the speed but reduce motor torque. For same torque, the number of armature turns should be increase proportionately.
Increasing armature turns will increase inductive reactance of the armature, so the overall reactance of the motor will not be decreased.Increased armature m.m.f can be neutralized by compensating winding .On the basis of the compensating winding there are two types of connection :Conductively compensated motorInductively compensated motor
Conductively compensated motor:The compensated winding is connected in series with the armature.Inductively compensated motor:The compensated winding is short- circuited and has no interconnectionwith the motor circuit.The compensating winding act as a short-circuited secondary ,for which the armature winding acts as a primary. The current in the compensating winding will be proportional to the armature current and 180 out of phase with it.
In A.C Series motor,the field winding associated with the armature coil undergoing commutation acts as primary and the armature coil during its commutating period acts as a short-circuited secondary. This transformer action produces heavy current in the armature coil as it passes through its commutating period and results in vicious sparking.Unless the transformer voltage is neutralized
Speed- Torque Characteristics
UNIVERSAL MOTORThere are small capacity series motors which can be operated on d.c. supply or single phase alternating supply of same voltage with same characteristics, called universal motors.
They are of two types.i) Projected pole - Non compensated typeii) Distributed field-Compensated type
Non-compensated Type: in this the pole and yoke are not separate but there is single lamination for the two.Series field winding is wound over the poles.Armature winding is placed in the armature slots.Ends of armature coils are connected to the risers of the commutator bars.
Non Compensated Universal MotorCross-section of non compensated universal motor
Compensated Type: There are no projected pole in this type.Distributed type field winding similar to single phase induction motor is placed in the stator.Armature winding is placed in the armature slots.Ends of armature coils are connected to the risers of the commutator bars.
Compensated type Universal Motor
Working of Universal MotorSupply is given
Flux is produced due to field winding carrying current.
Torque is produced on the armature
When the current flows through armature winding
Armature rotatesDirection of torque remains same during the negative half cycle of a.c supply as the direction of both the field & armature current are reversed.
Speed Torque Characteristic
Speed Control of Universal MotorResistance Method Tap Field MethodCentrifugal MechanismThe variable resistance connected in series with the motor used in sewing machineThe field coil number of turns are adjusted by tapping method using switch S.Switch S is controlled by external lever. S open, R is introduced to reduce speed.S close, R is shorted increasing speed.The C reduces the sparking.Used in domestic machines such as mixers.
Reversal of Direction of RotationBy reversing the flow of current through the armature or field windings, the direction of rotation can be reversed for salient pole non compensated type universal motor. This is possible by interchanging the terminals on brush holders .In case of compensated type, the armature or field loads are interchanged and brushes are shifted against the direction of rotation of motor, to achieve the reversal of direction.
Application of Universal MotorThe universal motors are used for domestic applications likevacuum cleanersfood processorMixershair drierscoffee grinderselectric shavers etc.
Their other applications are blowers, portable tools likedrilling machines and small drivers.