lect 14 1-phase induction machines

8/2/2019 Lect 14 1-Phase Induction Machines

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Single Phase Induction

Permanent-split capacitor motorOne way to solve the singlephase problem is to build a2-phase motor, deriving 2-

phase power from singlephase. This requires a motorwith two windings spacedapart 90o electrical, fed with

two phases of currentdisplaced 90o in time. This iscalled a permanent-splitcapacitor motor in Figure


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Main and Auxiliary windings


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1-Phase Induction Motor

This type of motor suffers increasedcurrent magnitude and backward timeshift as the motor comes up to speed,

with torque pulsations at full speed. Thesolution is to keep the capacitor(impedance) small to minimize losses.

The losses are less than for a shadedpole motor.


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1-Phase Induction Motor

This motor configuration works well up to1/4 horsepower (200watt), though,usually applied to smaller motors. The

direction of the motor is easily reversedby switching the capacitor in series withthe other winding. This type of motor can

be adapted for use as a servo motor,described elsewhere is this chapter


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Capacitor-start induction motor

In Figure a larger capacitor may beused to start a single phaseinduction motor via the auxiliarywinding if it is switched out by acentrifugal switch once the motor is

up to speed. Moreover, the auxiliarywinding may be many more turns ofheavier wire than used in aresistance split-phase motor tomitigate excessive temperature rise.The result is that more startingtorque is available for heavy loadslike air conditioning compressors.This motor configuration works sowell that it is available in multi-horsepower (multi-kilowatt) sizes.


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Capacitor-run motor induction motor

A variation of the capacitor-start motor Figure is to start themotor with a relatively largecapacitor for high starting

torque, but leave a smallervalue capacitor in place afterstarting to improve runningcharacteristics while notdrawing excessive current. The

additional complexity of thecapacitor-run motor is justifiedfor larger size motors.


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1-phase Capacitor start


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Capacitor Start-Run Induction Motor

A motor starting capacitor may be a double-anode non-polar electrolytic capacitor whichcould be two + to + (or - to -) series connected

polarized electrolytic capacitors. Such ACrated electrolytic capacitors have such highlosses that they can only be used forintermittent duty (1 second on, 60 seconds off)

like motor starting. A capacitor for motorrunning must not be of electrolyticconstruction, but a lower loss polymer type.


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Resistance split-phase motorinduction motor

If an auxiliary winding of much fewer turns of smaller wire is placed at 90oelectrical to the main winding, it can start a single phase induction motor. Withlower inductance and higher resistance, the current will experience less phaseshift than the main winding. About 30o of phase difference may be obtained. Thiscoil produces a moderate starting torque, which is disconnected by a centrifugalswitch at 3/4 of synchronous speed. This simple (no capacitor) arrangement

serves well for motors up to 1/3 horsepower (250 watts) driving easily startedloads.


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Wound rotor induction motors

A wound rotorinduction motor has a stator like the squirrelcage induction motor, but a rotor with insulated windingsbrought out via slip rings and brushes. However, no power isapplied to the slip rings. Their sole purpose is to allow

resistance to be placed in series with the rotor windings whilestarting.

This resistance isshorted out once themotor is started to

make the rotor lookelectrically like thesquirrel cagecounterpart.


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Wound Rotor Induction M/C

Why put resistance in series with the rotor?Squirrel cage induction motors draw 500% toover 1000% of full load current (FLC) during

starting. While this is not a severe problem forsmall motors, it is for large (10's of kW)motors. Placing resistance in series with therotor windings not only decreases start

current, locked rotor current (LRC), but alsoincreases the starting torque, locked rotortorque (LRT).


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Figure shows that by increasing the rotorresistance from R0 to R1 to R2, the breakdowntorque peak is shifted left to zero speed. Note

that this torque peak is much higher than thestarting torque available with no rotorresistance (R0) Slip is proportional to rotorresistance, and pullout torque is proportional

to slip. Thus, high torque is produced whilestarting.


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The resistance decreases the torque availableat full running speed. But that resistance isshorted out by the time the rotor is started. A

shorted rotor operates like a squirrel cagerotor. Heat generated during starting is mostlydissipated external to the motor in the startingresistance. The complication and maintenance

associated with brushes and slip rings is adisadvantage of the wound rotor as comparedto the simple squirrel cage rotor.


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Wound Rotor Induction

This motor is suited for starting highinertial loads. A high starting resistancemakes the high pull out torque available

at zero speed. For comparison, a squirrelcage rotor only exhibits pull out (peak)torque at 80% of its' synchronous speed


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Speed Control

Motor speed may be varied by putting variableresistance back into the rotor circuit. This reducesrotor current and speed. The high starting torqueavailable at zero speed, the down shifted break down

torque, is not available at high speed. See R2 curve at90% Ns, Resistors R0R1R2R3 increase in value fromzero. A higher resistance at R3 reduces the speedfurther. Speed regulation is poor with respect tochanging torque loads. This speed control technique is

only useful over a range of 50% to 100% of full speed.Speed control works well with variable speed loadslike elevators and printing presses.


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Speed Control


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Shaded Pole Induction Motor

Main windingsand Shaded Polewinding at theStator, while

shaded pole isshort circuited.


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Repulsion Motor

The machine is often converted into aninduction motor during the period ofrunning by arranging that all thecommutator segments are short-circuited by a centrifugally-operated

device when the motor is up to speed.The brushes are also lifted in samecases to reduce wear.To avoid the complication of the short-circuiting device, the rotor may be

arranged with a squirrel-cage winding atthe bottom of the slots. This takes overat speed and gives induction-motorcharacteristics


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Reversal ofRotation isachieved byswitching on rotor

windings


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lect 14 1-phase induction machines

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