Single phase induction motor


Transcript of Single phase induction motor

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• AC current flows through the Stator two fields are generated

1.φf- forward rotating field2.φb-backward rotating field• Pulsating field- due to various magnitudes of

field at diff time

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Resultant flux φr = 2 x =

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Why 1 phase induction motor is not self starting

• Rotor = squirrel cage type rotor

• Alternating flux – not required to produce rmf

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Split phase induction motor

• Two winding• Main winding & auxiliary

winding• 90 degree phase angle• Main winding – low

resistance and high reactance

• Aux winding- high resistance and low reactance

• Switch = motor pickup its 75 % of its rated speed.

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• Is lags by V because low reactance and high resistance

• Im lags V by very large angle due to high reactance

• Starting torque proportional to sin α

• Disconnect switch after reach its 75 % of rated speed

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Capacitor induction motor

1. Capacitor start motor2. Permanent capacitor motor3. Capacitor start capacitor run motor

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Capacitor start motor

• Electrolytic capacitor • 90degree phase angle• Im and Is • Im lags V due to high

reactance of main winding

• Due to capacitor Is lead V• Im and Is angle 90 • High starting torque • Apps: Lathes, drilling

machiine,fan etc

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Capacitor start, capacitor run motor

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Synchronous motor

• Machine which converts electrical energy into mechanical energy that rotates at a constant speed equal to synchronous speed

• Alternator can runs as a synchronous motor if

AC -> armature winding DC-> field winding

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Synchronous motor

Parts of 3 phase synchronous motor1. Laminated stator core

with 3 phase armature winding

2. Rotating field structure complete with damper winding and slip rings

3. Two end shields to house the bearings that support the rotor shaft

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Principle of operation

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Starting of synchronous motor

1. Dc motor method2. Pony motor method3. Damper winding method4. Starting as a slip ring induction motor

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Dc motor method

• Attaching an external motor to it to bring syn. Machine up to full speed

• Then syn. Machine be paralleled with its power system as a generator

• Now starting motor can be detached from machine shaft, then its slow down

• machine change its mode to be motor• Once paralleling completed syn. Motor can be

loaded down in an ordinary fashion

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Pony motor method

• Since starting motor should overcome inertia of syn. machine without a load & starting motor can have much smaller rating

• since most syn. motors have brushless excitation systems mounted on their shaft, often these exciters can be used as starting motors

• For many medium-size to large syn. motors, an external starting motor or starting by using exciter may be the only possible solution , because the connected power system source may not be able to feed the required starting current for amortisseur winding

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Damper winding method

• most popular method is to employ amortisseur or damper winding

• armortisseur windings are special bars laid into notches carved in face of a syn. motor’s rotor & then shorted out on each end by a large shorting ring

• pole face shown in next slide• To understand what a set of amortisseur windings

does in a syn. motor, examine salient 2 pole rotor shown next

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Starting as a slip ring induction motor

• Damper winding method of starting doesn’t provide high starting torque

• To get high starting torque instead of shorting the damper winding – designed a three phase star winding , end brought to slip ring

• External rheostat connected in series with rotor circuit.

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Effect of change in excitation of synchronous motor

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V Curves• Plot armature current as a function of field current or

armature current as a function of excitation voltage.

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Applications • Synchronous motors are particularly attractive for low speeds (< 300r.p.m.) because the power factor can always be adjusted to unity and efficiency is high.• Overexcited synchronous motors can be used to improve the power

factorof a plant while carrying their rated loads.• They are used to improve the voltage regulation of transmission lines.• High-power electronic converters generating very low frequencies enableus to run synchronous motors at ultra-low speeds. Thus huge motors in the• 10 MW range drive crushers, rotary kilns and variable-speed ball mills.