Special machines

Special machines

there are a number of special-purpose motors which have not

been previously covered. These include reluctance motors, hysteresis motors,

stepper motors, and brushless dc motors.

Shaded -Pole Motors

A shaded-pole induction motor is an induction motor with only a main winding.

Instead of having an auxiliary winding, it has salient poles, and one portion of

each pole is surrounded by a short-circuited coil called a shading coil (see Figure

a). A time-varying flux is induced in the poles by the main winding. When

the pole flux varies, it induces a voltage and a current in the shading coil which

opposes the original change in flux.1llis opposition retards the flux changes under

the shaded portions of the coils and therefore produces a slight imbalance between

the two oppositely rotating stator magnetic field s. The net rotation is in the

direction from the unshaded to the shaded portion of the pole face. The

torque-speed characteristic of a shaded-pole molar is shown in Figure b.

Shaded poles produce less starting torque than any other type of induction

motor starting system. They are much less efficient and have a much higher slip

than other types of single-phase induction motors. Such poles are used only in

very small motors (Y..o hp and less) with very low starting torque requirements.

Where it is possible to use them, shaded-pole motors are the cheapest design

available.

Because shaded-pole motors rely on a shading coil for their starting torque,

there is no easy way to reverse the direction of rotation of such a motor. To

achieve reversal, it is necessary to install two shading coils on each pole face and

to selectively short one or the other of them. See Figures c and d.

Comparison of Single-Phase Induction Motors

Single-phase induction motors may be ranked from best to worst in terms of their

starting and running characteristics:

I. Capacitor-start, capacitor-run motor

2. Capacitor-start motor

3. Permanent split-capacitor motor

4. Split-phase motor

5. Shaded-pole motor

Naturally, the best motor is also the most expensive, and the worst motor is the

least expensive. Also, not all these starting techniques are available in all motor

size ranges. It is up to the design engineer to select the cheapest available motor

for any given application that will do the job.

SPEED CONTROL OF SINGLE-PHASE

INDUCTION MOTORS

In general, the speed of single-phase induction motors may be controlled in the

same manner as the speed of polyphase induction motors. For squirrel-cage rotor

motors, the following techniques are available:

1. Vary the stator frequency.

2.Change the number of poles.

.3Change the applied terminal voltage V_T.

In practical designs involving fairly high-slip motors, the usual approach to

speed control is to vary the terminal voltage of the motor. the voltage applied to

a motor may be varied in one of three ways:

1. An autotransformer may be used to continually adjust the line voltage. this

is the most expensive method of voltage speed control and is used only when

very smooth speed control is needed.

2. An SCR or TRIAC circuit may be used to reduce the rms voltage applied to

the motor by ac phase control. this approach chops up the ac wave form as

described in somewhat increases the motor's noise and vibration.

Solid-state control circuits are considerably cheaper than autotransformers

and so are becoming more and more common.

3. A resistor may be inserted in series with the motor's stator circuit. This is the

cheapest method of voltage control , but it has the disadvantage that considerable

power is lost in the resistor, reducing the overall power conversion efficiency.