Ch06 - AC Motors

7/31/2019 Ch06 - AC Motors

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1

Chapter 6

AC Motors

7/31/2019 Ch06 - AC Motors


7/31/2019 Ch06 - AC Motors


3

Stator/Rotor

Action

• The diagram to the

right demonstrates

one complete AC

cycle being appliedto the windings of an

AC motor

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Stator Construction• Two-phase operation is necessary to create the magnetic field

conditions needed to generate torque in an AC motor

• Phase 1 is applied to the vertical stator windings

• Phase 2 is applied to the horizontal stator windings

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Three-Phase• Three-phase power is commonly used in industrial factories

• Three-phase power is ideal for powering rotating stator fields in

motors

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Rotor

Construction

• AC motors useelectromagnets for theirrotors

• Two methods are used forenergizing the rotor

– Connect an electricalcurrent to the windings

– Use induction as the meansof producing magneticfields

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Types of Rotors

• Squirrel Cage - “cage” is made of aluminum or brass, connected to end rings

• Wound Rotors - rotor is wound with coils

of wire. The number of coils must be equalto the number of stator coils

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Principles of Operation• The stator winding, when

energized, produces a

rotating magnetic fieldthat causes the motor toturn at synchronous speed

• However, the rotor willnever catch up to the

stator field

• Speed - determined byfrequency of the appliedvoltage and number of

stator poles per phase

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Speed and Slip

• Speed of a motor is determined by the following

formula:

– N = RPM

– P = Number of pole pairs

– f = Applied frequency– 60 = Formula constant based upon seconds/minute

N =f x66

P

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AC Motor Synchronous Speeds

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Single-Phase Induction Motors

• Single-phase commercial power is typically

supplied to residential customers

• AC induction motors require two phases

• Single-phase AC motors are classified by themeans of obtaining the two phases

– Resistance-start, induction-run

– Capacitor-start, induction-run

– Shaded-pole motor

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Resistance-Start Induction-Run

Motor• Has two separate windings connected in parallel

– Main (run) winding

– Auxiliary (start) winding

• Start winding has high resistance

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Capacitor-Start Induction-Run

Motor• Two windings, a start and a run winding

• Capacitor is placed in series with the start winding to obtain higher

starting torque

• At 70-80% of full speed, capacitor is disconnected by a centrifugal switch

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Shaded-Pole Motor

• Uses a squirrel cagerotor and a mainwinding

• Uses shaded poles todevelop rotating field

• Shaded-pole motorsare useful in light loadapplications

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Troubleshooting Split-phase

Motors

• The most frequent cause of failure incapacitor-start motors is a defective

capacitor

• Resistance-start motors usually fail as aresult of a defective centrifugal switch or

open start winding

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Universal Motors• Usually classed as an AC

motor, but can operate on

DC voltages as well

• Similar to a DC series-wound motor

• Universal motors use a

laminated iron core versusthe solid iron cores of DCmotors

• Output torque is lesscompared to a DC motor

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Three-Phase Motors• Most industrial motors operate on three-phase

power• Also referred to as polyphase motors

• Simpler in construction and more powerful thansingle-phase motors

• Three types:– Induction motor

– Wound-rotor motor

– Synchronous motor

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Induction Motor

• Most industrial machines are powered bythree-phase squirrel cage induction motors

• Small size, efficiency, relatively low-cost

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Rotating Field

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Wound-Rotor Motor

• Rotor consists of a set of three coils in place of

conducting bars found in

squirrel cage motors

• Wound-rotor motors areused where speed and

torque need to be

adjustable

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Synchronous Motor

• Synchronous motors turn at the same

speed as the stator’s magnetic field

• Synchronous motors perform two

functions:

– Convert electrical energy into mechanicalenergy

– Perform power factor correction

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Synchronous Motor Construction

and Operation• The most common type of synchronous motor has two different

rotor circuits:

– Damper or amortisseur winding

– Squirrel cage bars

• Damper windings are locked on theouter periphery of the pole core on pole face

• The other circuit coils are wound onlaminated core bodies called salient poles

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7/31/2019 Ch06 - AC Motors


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Power Factor Correction

• Synchronous motors are often used for power

factor correction in industrial settings

• Power factor correction is necessary because:

– Current-carrying capabilities of power systems are

reduced– Power companies assess penalties to industrial users

whose power factor is below a specified level

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Ch06 - AC Motors