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AC Motors

Instructional Plan

Universal Motors

• Universal motors use armatures, brushes, commutators, and field coils

• These motors can use ac or dc voltages.• High torque and varying motor speed are two

properties of the universal motor– Inductive reactance affects current flow in the coils– The resistance of the coil to ac current flow requires greater ac

voltages to develop motor torque and speed

• Uses of universal motors– Household appliances.– Workshop tools such as electric drills

Synchronous Motors

• Do not have commutators and brushes– Stators and rotors are used in synchronous motors.

• Stators are fields coils• Rotors rotate within the stator.

– AC applied to the stator produces a constant reversal of the magnetic lines of force

Synchronous Motors

Operation of the synchronous motor At the positive ac peak voltage, the magnetic field strength is the greatest for the half-cycle.

As the positive peak voltage drops to zero, the magnetic field strength weakens.

Current reverses at the zero peak voltage

The magnetic field strength will start building in the opposite direction.

At the peak of the negative voltage, the field strength is again at the peak for that half cycle.

The rate of reversals of the lines of force is determined by the frequency of the ac voltage.

The motor speed is synchronized to the ac frequency.– Rotors of made of soft iron disks are often used in

small synchronous motors.– Rotors made of electromagnets require dc voltages

applied to the rotor to create north and south poles.

Synchronous Motors

Synchronous Motors

Synchronous motors are usually not self-starting. Some rotors use damper windings to cerate enough torque to turn the rotor and start the motor.

Speed depends on the frequency of the ac voltage

Rotor can be operated clockwise or counter clockwise.