Topic covered :

AC Series motors

Universal motor

Phasor Diagram of a.c. Series Motor

Speed control

A series ac motor is the same electrically as a dc series motor but construction differs slightly.

Special metals, laminations, and windings are used which reduce losses caused by eddy currents, hysteresis, and high reactance.

Dc power can be used to drive an ac series motor efficiently, but the opposite is not true.

The characteristics of a series ac motor are similar to those of a dc series motor. It is a varying-speed machine.

It has low speeds for large loads and high speeds for light loads.

Motor that can be used with a single-phase ac source as well as a dc source of supply voltages are called universal motor.

The stator and rotor windings of the motor are connected in series through the rotor commutator.

The universal motor is also known as an AC series motor or an AC commutatormotor.

MOTORS 5

Universal Motor

Series-connected◦ Rotor and Stator are

connected in series

Operates on either ac or dc

MOTORS 6

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Armature (Rotor)

Field Winding Field

(Stator)

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The Commutator Bar and Brushes are a switch that reverses the current in the armature coil as it rotates

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Force on conductor B is upwards, rotation is counter-clockwise (CCW)

Flux “Bunching”

+Positive Half-Cycle+

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Force on Conductor B is upwards, rotation is counter-clockwise (CCW)

Flux “Bunching”

Negative Half-Cycle

How do you reverse the direction of rotation?◦ Reverse the direction of the current in either the field or in

the armature, but not both!

How do you control the speed of the motor?

◦ Reduce the voltage applied to the motor Use an autotransformer or SCR/Triac This reduces the armature current, reducing the torque,

reducing the speed

Applications◦ Portable power tools ◦ Small appliances

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The compensating winding may beshort circuited on itself, inwhich case the motor is said to beinductively compensated

Series motor with inductively compensated winding

Phasor Diagram of a.c. Series MotorThe schematic diagram and phasor diagram for the conductivelycoupled single-phase ac series motor are shown in fig. below.

Schematic diagram of conductively coupled ac series motor

Speed control of ac series motor

The method adopted for the speed control of d.c series motor is also applicable to a.c series motor .

1.Armature resistance or reactance control2.Field flux control3.Armature voltage control

Method 1 is used for small motors, for large size motors method 3 is common.