Tuesday, January 13, 2015

Single-Phase Motors : Conductive and inductive compensation , Shaded-pole induction motor and Variablespeed motors .

CONDUCTIVE AND INDUCTIVE COMPENSATION

AC motors larger than 1⁄2 horsepower (hp) are used to drive loads when a high starting torque is required. For these motors, there is excessive armature reaction under load. One method of overcoming the armature reaction is known as conductive compensation. In this method, an additional compensating winding is placed in slots cut in the pole faces. The strength of this field increases with an increase in the load current. Thus, there is a reduc- tion in the distortion of the main field flux by the armature flux.

The compensating winding is connected in series with the series field winding and the armature (Figure 18–29). A motor with conductive compensation has a high starting torque and poor speed regulation. However, resistor-type speed controllers can be used to obtain a wide range of speed control.

A second method of overcoming armature reaction in ac series motors uses an inductively coupled winding (Figure 18–30). This winding acts like a short-circuited secondary winding in a transformer. This winding links the cross magnetizing flux of the armature. (The armature can be compared to the primary winding of a transformer.) Because the magnetomotive force of the secondary is equal in magnitude to the primary magnetomotive

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force but is nearly opposite in phase, the compensating winding flux almost neutralizes the armature cross-flux. An ac series motor of this type cannot be used on dc current. Its operating characteristics are similar to those of the universal motor with conductive compensation.

SHADED-POLE INDUCTION MOTOR

If a single-phase induction motor has a small fractional horsepower rating, it may be started by shading coils mounted on one side of each of the stator poles. A standard squirrel-cage rotor is used in this motor, which is known as a shaded-pole induction motor. The motor does not require a starter mechanism such as a centrifugal switch. Thus, there is no possibility of motor failure due to a faulty centrifugal switch mechanism.

Figure 18–31 shows a typical shaded-pole induction motor with four stator poles. The four poles are wound in alternate directions. Note that a shading coil is wound on one section of each of the stator field poles. The shading coil is a low-resistance copper loop.

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Action of Flux

As the current in the stator circuit increases, the stator flux also increases. A voltage is induced in each shading coil. The induced current in the shading loop opposes any increase in the main field flux through the loop. As a result, the flux increases in the other section of each pole face (Figure 18–32A).

When the stator current and flux both reach maximum values, there is an instant when no other change occurs in the current or the flux. At this instant, there is no voltage or current in the shading coil. Thus, the shading coil does not set up a magneto- motive force to oppose the stator flux. The resulting stator field is uniform across the pole face (Figure 18–32B). When the stator current and flux decrease, the induced volt- age and current in the shading coil set up a magnetomotive force. This force aids the stator field. As a result, the flux decreases less rapidly in the section of the pole face where the shading coil is mounted than it does in the other part of the pole face (Figure 18–32C).

Figure 18–32 shows that the shading coil causes the field flux to shift across the pole face. This shifting flux can be likened to a rotating magnetic field. The torque produced is small. Thus, the shaded-pole type of starting is used only on small motors rated at no more than 1⁄10 hp. Such motors are used in applications where a strong starting torque is not essential. Typical applications include driving small devices such as fans and blowers.

VARIABLE-SPEED MOTORS

The use of small variable-speed motors has increased greatly because of an increase in demand for products that employ their use. These motors are commonly used to operate light loads such as ceiling fans and blower motors. Two types of motors are used for these applications: the shaded-pole and the capacitor-start, capacitor-run motors. These motors

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are used because they operate without having to disconnect a set of start windings with a centrifugal switch. Motors intended to be used in this manner are wound with high-impedance stator windings. The high impedance of the stator limits the current flow through the motor when the speed of the rotor is decreased. Speed control for these motors is accomplished by controlling the amount of voltage applied to the motor or by inserting impedance in series with the stator winding.

Variable Voltage Control

The amount of voltage applied to the motor can be controlled by several methods. One method is to use an autotransformer with several taps (Figure 18–33). This type of controller has several steps of speed control. Notice that the applied voltage, 120 V in this illustration, is connected across the entire transformer winding. When the rotary switch is moved to the first tap, 30 V is applied to the motor. This produces the lowest motor speed for this controller. When the rotary switch is moved to the second tap, 60 V is applied to the motor. This provides an increase in motor speed. When the switch has been moved to the last position, the full 120 V is applied to the motor and it operates at its highest speed.

Another type of variable voltage control uses a triac to control the amount of volt- age applied to the motor (Figure 18–34). This type of speed control provides a more lin- ear control because the voltage can be adjusted from zero to the full applied voltage. At first appearance, many people assume this controller to be a variable resistor connected in series with the motor. A variable resistor large enough to control even a small motor would produce several hundred watts of heat and could never be mounted in a switch box. The

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variable resistor in this circuit is used to control the amount of phase shift for the triac. The triac controls the amount of voltage applied to the motor by turning on at different times during the ac cycle.

A triac speed control is very similar to a triac light dimmer used in many homes. A light dimmer, however, should never be used as a motor speed controller. Triac light dimmers are intended to be used with resistive loads such as incandescent lamps. Light- dimmer circuits will sometimes permit one-half of the triac to start conducting before the other half. The waveform shown in Figure 18–35 illustrates this condition. Notice that only the positive half of the waveform is being conducted to the load. Because only positive voltage is being applied to the load, it is dc. Operating a resistive load such as an incandescent lamp with dc will do no damage. Operating an inductive load such as the winding of a motor can do a great deal of damage, however. When direct current is applied to a motor winding, there is no inductive reactance to limit the current. The actual wire resistance of

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the stator is the only current-limiting factor. The motor winding or the controller can easily be destroyed if direct current is applied to the motor. For this reason, only triac controllers designed for use with inductive loads should be used for motor control. A photograph of a triac speed controller is shown in Figure 18–36.

Series Impedance Control

Another common method of controlling the speed of small ac motors is to connect impedance in series with the stator winding. This is the same basic method of control used with multispeed fan motors. The circuit in Figure 18–37 shows a tapped inductor connected in series with the motor. When the motor is first started, it is connected directly to the full voltage of the circuit. As the rotary switch is moved from one position

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to another, steps of inductance are connected in series with the motor. As more inductance is connected in series with the stator, the amount of current flow decreases. This produces a weaker magnetic field in the stator. Rotor slip increases because of the weaker magnetic field, and the motor speed decreases. A photograph of this type of controller is shown in Figure 18–38.