Tuesday, January 13, 2015

Single-Phase Motors : Capacitor-start, induction-run motor , Capacitor-start, capacitor-run motor and Multispeed motor .

CAPACITOR-START, INDUCTION-RUN MOTOR

Construction

Physically, the capacitor-start, induction-run motor is similar to the resistance-start, induction-run motor. However, the capacitor-start motor has a capacitor connected in series with the starting windings. Generally, the capacitor is mounted in any convenient external location on the motor frame. In some cases, the capacitor is mounted inside the motor housing. With regard to the starting torque, the capacitor-start motor has a higher torque than the resistance-start motor. The capacitor also limits the starting surge of current to a lower value as compared to the resistance-start motor.

The capacitor-start, induction-run motor is used in refrigeration units, compressors, oil burners, small-machine equipment, and in any application where split-phase induction motors are used. Because of its improved starting torque characteristics, the capacitor-start motor is replacing the split-phase motor in many applications.

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Principle of Operation

A typical capacitor-start, induction-run motor is shown in Figure 18–10. At start-up, both the running and starting windings are connected in parallel across the line voltage while the centrifugal switch is closed. The starting winding is also connected in series with the capacitor. When the motor reaches 75% of the rated speed, the centrifugal switch opens. The starting winding and the capacitor are disconnected from the line. The motor then operates on the running winding as a single-phase induction motor.

Starting Torque. As stated previously, the starting torque is due to to a revolving magnetic field that is set up by the stator windings. By adding a capacitor of the cor- rect value in series with the starting winding, the current in this winding will lead the running winding current by 90 electrical degrees.

The angle between the starting winding current and the running winding current is almost 90° (Figure 18–11). The magnetic field set up by the stator windings is almost iden- tical to that of a two-phase induction motor. Therefore, the starting torque for the capacitor- start motor is much better than the torque for a resistance-start, induction-run motor.

Capacitor Starting. This motor starts with the capacitor in the starting winding circuit. For normal running, only the running winding is energized and the motor operates as a single-phase induction motor. The capacitor is used to improve the starting torque. Because it is energized for just two or three seconds at start-up, the capacitor cannot make any improvement in the power factor.

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Defective capacitors often cause problems in the motors using them. For example, the capacitor may short-circuit and blow the fuse on the branch motor circuit. If the fuse rating is high so that the fuse does not interrupt the power supply to the motor, the starting winding may burn out. Starting capacitors may short-circuit if the motor is turned on and off many times in a short period of time. To prevent failures, many manufacturers recommend that this type of motor be started no more than twenty times per hour.

Paper or oil filled capacitors are used in these motors. If the motor is started too often in a short period of time, the current surge at start-up gradually damages the dielectric of the capacitor. Eventually, the dielectric breaks down and shorts (short- circuits) the capacitor plates. The capacitor-start motor is recommended for use only in those applications where there are few starts in a short period of time.

Speed Regulation. The speed regulation for a capacitor-start, induction-run motor is very good. The percent slip at full load is in the range from 4% to 6%. This means that the speed performance is the same as that of a resistance-start, induction-run motor.

Reversing the Motor. To reverse the direction of rotation of this motor, the leads of the starting winding circuit are reversed. The magnetic field developed by the start windings in the stator then rotates around the stator core in the opposite direction. Thus, the rotation of the rotor is reversed. The direction of rotation can also be reversed by interchanging the leads of the running windings.

The circuit connections for the capacitor-start motor are shown in Figure 18–12. In Figure 18–13, the starting winding leads are interchanged to reverse the direction of rotation of the motor.

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Dual-Voltage Ratings. Capacitor-start, induction-run motors can also have dual-voltage ratings of 120 V and 240 V. The connections in this case are the same as those for split- phase induction motors.

CAPACITOR-START, CAPACITOR-RUN MOTOR

In the capacitor-start, capacitor-run (CSCR) motor, the starting winding and the capacitor are connected into the circuit at all times. This motor has a very good starting torque. Because the capacitor is used at all times, the power factor at the rated load is 100%, or unity.

Operation

Several different versions of the CSCR motor are available. In one type of motor, two stator windings are placed 90 electrical degrees apart. The running winding is connected directly across the rated line voltage. A capacitor is connected in series with the starting winding. This winding is also connected across the rated line voltage. Because the starting winding is energized as long as the motor is operating, a centrifugal switch is not required. Figure 18–14 gives the internal connections for a CSCR motor.

Using One Capacitor. The motor shown in Figure 18–14 is quiet in operation. It is used on oil burners, fans, and small woodworking and metalworking machines. The direction of rotation of this motor is reversed by interchanging the lead connections of either winding.

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Using Two Capacitors. A second type of motor uses two capacitors. At start-up, a large value of capacitance is connected in series with the starting winding, as shown in Figure 18–15.

At this moment, the two capacitors shown are in parallel. When the motor accelerates to nearly 75% of the rated speed, the centrifugal switch disconnects the larger capacitor. The motor then operates with the small-value capacitor connected in series with the start- ing winding.

This type of motor has a very good starting torque, good speed regulation, and a power factor of almost 100% at the rated load. It is used on furnace stokers, refrigerators, and compressors, and in other applications where its strong starting torque and good speed regulation are required.

Autotransformer and One Capacitor. Another version of the CSCR motor uses an autotransformer. One capacitor is used to obtain a high starting torque and a large power factor.

The internal connections for this motor are shown in Figure 18–16. As the motor starts, the centrifugal switch connects winding 2 to point A on the tapped autotransformer. The capacitor is connected across nearly 500 V. A large leading current is formed in wind- ing 2 and a strong starting torque results.

When the motor reaches nearly 75% of the rated speed, the centrifugal switch disconnects the starting winding from point A and reconnects it to point B on the autotransformer. In this way, less voltage is applied to the capacitor. The motor operates with both windings energized. The capacitor maintains the power factors at nearly unity at the rated load.

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The starting torque of this motor is very good, and its speed regulation is satisfactory. This motor is used in refrigerators, in compressors, and in other applications in which strong starting torque and good speed regulation are required.

MULTISPEED MOTOR

There are two basic types of multispeed motors. One type is known as the consequent pole motor. The other type is generally a capacitor-start, capacitor-run motor.

The Consequent Pole Motor

The speed of the rotating magnetic field of an ac induction motor can be changed in either of two ways:

1. Change the frequency of the ac voltage.

2. Change the number of stator poles.

The consequent pole motor changes motor speed by changing the number of its stator poles. The run winding in Figure 18–17 has been tapped in the center. If the ac line is connected to each end of the winding as shown, current flows through the winding in only one direction. Therefore, only one magnetic polarity is produced in the winding. If the winding is connected as shown in Figure 18–18, current flows in opposite directions in each half of the winding. Because current flows through each half of the winding in opposite directions, the polarity of the magnetic field is different in each half of the winding. The run winding now has two polarities instead of one. If the windings of a two-pole motor were to be tapped in this manner, the motor could become a four-pole motor. The synchronous speed of a two-pole motor is 3600 r/min, and the synchronous speed of a four-pole motor is 1800 r/min.

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The consequent pole motor has the disadvantage of having a wide variation in speed. When the speed is changed, it changes from a synchronous speed of 3600 r/min to 1800 r/min. The speed cannot be changed by a small amount. This wide variation in speed makes the consequent pole motor unsuitable for some loads such as fans and blowers.

The consequent pole motor, however, does have some advantages over the other type of multispeed motor. When the speed on the consequent pole motor is reduced, its torque increases. For this reason, the consequent pole motor can be used to operate heavy loads.

Multispeed Fan Motors

Multispeed fan motors have been used in industry for many years. These motors are generally wound for two to five steps of speed and are used to operate fans and squirrel-cage blowers. A schematic drawing of a three-speed motor is shown in Figure 18–19. Notice that the run winding has been tapped to produce low, medium, and high speed. The start

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winding is connected in parallel with the run winding section. The other end of the start lead is connected to an external oil-filled capacitor. This motor obtains a change in speed by inserting inductance in series with the run winding. The actual run winding for this motor is between the terminals marked “High” and “Common.” The windings shown between “High” and “Medium” are connected in series with the main run winding. When the rotary switch is connected to the medium-speed position, the inductive reactance of this coil limits the amount of current flow through the run winding. When the current of the run winding is reduced, the strength of the magnetic field of the run winding is reduced and the motor produces less torque. This causes the motor speed to decrease.

If the rotary switch is changed to the low position, more inductance is connected in series with the run winding. This causes less current to flow through the winding and another reduction in torque. When the torque is reduced, the motor speed decreases again.

Common speeds for a four-pole motor of this type are 1625, 1500, and 1350 r/min. Notice that this motor does not have the wide range between speeds that the consequent pole motor does. Most induction motors would overheat and damage the motor windings if the speed were to be reduced to this extent. This motor, however, has much higher imped- ance windings than do most motors. The run windings of most split-phase motors have a wire resistance of 1 to 4 f!. This motor will generally have a resistance of 10 to 15 f! in its run winding. It is the high impedance of the windings that permits the motor to be operated in this manner without damage.

Because this motor is designed to slow down when load is added, it is not used to operate high-torque loads. This type of motor is generally used to operate only low-torque loads such as fans and blowers.