SINGLE-PHASE INDUCTION MOTORS

SINGLE-PHASE

INDUCTION MOTORS

Another common single-phase motor is the single-phase version of the induction

motor. An induction motor with a squirrel-cage rotor and a single-phase stator is

shown in Figure 5.

Single-phase induction motors suffer from a severe handicap. Since there is

only one phase on the stator winding, the magnetic field in a single-phase induction

motor does not rotate. Instead, it pulses, getting first larger and then smaller,

but always remaining in the same direction. Because the re is no rotating stator

magnetic field, a single-phase induction motor has no starting torque.

This fact is easy to see from an examination of the motor when its rotor is

stationary. The stator flu x of the machine first increases and then decreases, but it

always points in the same direction. Since the stat or magnetic field does not rotate,

there is no relative motion between the stator field and the bars of the rotor.

Therefore, there is no induced voltage due to relative motion in the rotor, no rotor

current flow due to relative motion, and no induced torque

The fact that single-phase induction motors have no intrinsic starting torque

was a serious impediment to early development of the induction motor. When induction motors were first being developed in the late l880s and early 1890s, the

first available ac power systems were 133-Hz, single-phase. With the materials

and techniques then available, it was impossible to build a motor that worked

well. The induction motor did not become an off-the-shelf working product until

three-phase, 25-Hz power systems were developed in the mid- 189Os.

However, once the rotor begins to turn, an induced torque will be produced

in it. There are two basic theories which explain why a torque is produced in the

rotor once it is turning. One is called the double-revolving-field theory of singlephase induction motors, and the other is called the cross-field theory of single phase induction motors. Each of these approaches will be described below.

The Double-Revolving-Field Theory of Single-Phase Induction Motors The double-revolving-field theory of single-phase induction motors basically states that a stationary pulsating magnetic field can be resolved into two rotating magnetic fields, each of equal magnitude but rotating in opposite directions. The induction motor responds to each magnetic field separately, and the net torque in the machine will be the sum of the torques due to each of the two magnetic fields.

STARTING SINGLE-PHASE

INDUCTION MOTORS

As previously explained, a single-phase induction motor has no intrinsic starting

torque. There are three techniques commonly used to start these motors, and

single-phase induction motors are classified according to the methods used to produce their starting torque. These starting techniques differ in cost and in the

amount of starting torque produced, and an engineer normally uses the least expensive technique that meets the torque requirements in any given application.

The three major starting techniques are

1. Split-phase windings

2. Capacitor-type windings

3. Shaded stator poles

All three starting techniques are methods of making one of the two revolving

magnetic fields in the motor stronger than the other and so giving the motor

an initial nudge in one direction or the other.

Split-Phase Windings a split-phase motor is a single-phase induction motor with two stator windings, a main stator winding (M) and an auxiliary starting winding (A).

These two windings are set 90 electrical degrees apart along the stator of the

motor, and the auxiliary winding is designed to be switched out of the circuit at

some set speed by a centrifugal switch. The auxiliary winding is designed to have a higher resistance/reactance ratio than the main winding, so that the current in the

auxiliary winding leads the current in the main winding. This higher RIX ratio is

usually accomplished by using smaller wire for the auxiliary winding. Smaller

wire is permissible in the auxiliary winding because it is used only for starting and

therefore does not have to take full current continuously.

A cutaway diagram of a split-phase motor is shown in above Figure. It is

easy to see the main and auxiliary windings (the auxiliary windings are the

smaller-diameter wires) and the centrifugal switch that cuts the auxiliary windings

out of the circuit when the motor approaches operating speed.

Split -phase motors have a moderate starting torque with a fairly low starting

current. They are used for applications which do not require very high starting

torques, such as fans, blowers, and centrifugal pumps. they are available for sizes

in the fractional-horsepower range and are quite inexpensive.

Capacitor-Start Motors

For some applications, the starting torque supplied by a split-phase motor is insufficient to start the load on a motor's shaft. In those cases, capacitor-start motors

may be used (follow figure). In a capacitor-start motor, a capacitor is placed in series

with the auxiliary winding of the motor. By proper selection of capacitor size,

the magneto motive force of the starting current in the auxiliary winding can be adjusted to be equal to the magneto motive force of the current in the main winding,

Capacitor-start motors are more expensive than split-phase motors, and they

are used in applications where a high starting torque is absolutely required. Typical

applications for such motors are compressors, pumps, air conditioners, and

other pieces of equipment that must start under a load.

Permanent Split-Capacitor and Capacitor-Start,

Capacitor-Run Motors

The starting capacitor does such a good job of improving the torque-speed characteristic of an induction motor that an auxiliary winding with a smaller capacitor is sometimes left permanently in the motor circuit. If the capacitor's value is chosen correctly, such a motor will have a perfectly uniform rotating magnetic field at some specific load, and it will behave just like a three-phase induction motor at that point. Such a design is called a permanent split-capacitor or capacitor-start-and- run motor (next Figure). Permanent split-capacitor motors are simpler than

capacitor-start motors, since the starting switch is not needed. At normal loads,

they are more efficient and have a higher power factor and a smoother torque than

ordinary single-phase induction motors.

However, permanent split-capacitor motors have a lower starting torque than

capacitor-start motors, since the capacitor must be sized to balance the currents in

the main and auxiliary windings at normal-load conditions. Since the starting current

is much greater than the normal-load current, a capacitor that balances the

phases under normal loads leaves them very unbalanced under starting conditions.

If both the largest possible starting torque and the best running conditions

are needed, two capacitors can be used with the auxiliary winding. Motors with

two capacitors are called capacitor-start, capacitor-run, or two-value capacitor

motors (next Figure). The larger capacitor is present in the circuit only during

starting, when it ensures that the currents in the main and auxiliary windings are

roughly balanced, yielding very high starting torques. When the motor gets up 10

speed , the centrifugal switch opens, and the permanent capacitor is left by itself in

the auxiliary winding circuit. The permanent capacitor is just large enough to balance the currents at normal motor loads, so the motor again operates efficiently

with a high torque and power factor. The permanent capacitor in such a motor is

typically about 10 to 20 percent of the size of the starting capacitor.

The direction of rotation of any capacitor-type motor may be reversed by

switching the connections of its auxiliary windings.