Wound Rotor Induction Motors

Goals:

• Identify the terminal markings of a wound rotor induction motor.
• Discuss the operating characteristics of wound rotor motors.
• Connect a wound rotor motor for operation.
• Discuss speed control of wound rotor motors.

The wound rotor induction motor is one of the three major types of three-phase motors. It is often called the slip ring motor because of the three slip rings on the rotor shaft. The stator winding of a wound rotor motor is identical to the squirrel cage motor. The difference between the two motors lies in the construction of the rotor. The rotor of a squirrel cage motor is constructed of bars connected together at each end by shorting rings. The rotor of a wound rotor induction motor is constructed by winding three separate windings in the rotor (Ill. 1).

Ill. 1 Rotor of a wound rotor induction motor.

Ill. 2 The rotor of a wound rotor induction motor is connected to external resistors.

The wound rotor motor was the first alternating current motor that permitted speed control. It has a higher starting torque per ampere of starting current than any other type of three-phase motor. It can be started in multiple steps to provide smooth acceleration from 0 RPM to maximum RPM. Wound rotor motors are typically employed to operate conveyors, cranes, mixers, pumps, variable speed fans, and a variety of other devices. They are often used to power gear driven machines because they can be started without supplying a large amount of torque that can damage and even strip the teeth off gears.

The three-phase rotor winding will contain the same number of poles as the stator winding. One end of each rotor winding is connected together inside the rotor to form a wye connection, and the other end of each winding is connected to one of the slip rings mounted on the rotor shaft. The slip rings permit external resistance to be connected to the rotor circuit (Ill. 2). Placing external resistance in the rotor circuit allows control of the amount of current that can flow through the rotor windings during both the starting and running of the motor. There are three factors that determine the amount of torque developed by a three-phase induction motor:

• Strength of the magnetic field of the stator.

• Strength of the magnetic field of the rotor.

• Phase angle difference between rotor and stator flux.

Since an induction motor is basically a transformer, controlling the amount of rotor current also controls the amount of stator current. It is this feature that permits the wound rotor motor to control the in-rush cur rent during the starting period. Limiting the in-rush current also limits the amount of starting torque produced by the motor.

Ill. 3 The force of attraction is proportional to the flux density of the two magnets and the angle between them.

Ill. 4 Schematic symbol for a wound rotor induction motor.

The third factor that determines the amount of torque developed is the phase angle difference between stator and rotor flux. Maximum torque is developed when the magnetic fields of the stator and rotor are in phase with each other. Imagine two bar magnets with their north and south poles connected together. If the magnets are placed so there is no angular difference between them ( Ill. 3A), the attracting force is at maximum. If the magnets are broke apart so there is an angular difference between them, there is still a force of attraction, but it's less than when they are connected together (Ill. 3B). The greater the angle of separation, the less the force of attraction becomes ( Ill. 3C).

Adding resistance to the rotor circuit causes the induced current in the rotor to be more in phase with the stator current. This produces a very small phase angle difference between the magnetic fields of the rotor and stator. This is the reason that the wound rotor induction motor produces the greatest amount of starting torque per ampere of starting current of any three-phase motor.

The stator windings of a wound rotor motor are marked in the same manner as any other three-phase motor: T1, T2, and T3 for single voltage motors. Dual voltage motors will have nine T leads, like squirrel cage motors. The rotor leads are labeled M1, M2, and M3. The M2 lead is located on the center slip ring and the M3 lead is connected to the slip ring closest to the rotor windings. The schematic symbol for a wound rotor induction motor is shown in Ill. 4.

Manual Control of a Wound Rotor Motor

The starting current and speed of a wound rotor induction motor is controlled by adding or subtracting the amount of resistance connected in the rotor circuit.

Small wound rotor motors are often controlled manually by a three-pole make-before-break rotary switch.

The switch will contain as many contacts as there are steps of resistance (Ill. 5). A micro limit switch senses when the controller is set for maximum resistance. Most controllers won't start unless all resistance is in the rotor circuit, forcing the motor to start in its lowest speed. Once the motor has been started, the resistance can then be adjusted out to increase the motor speed. When all the resistance has been removed from the circuit and the M leads are shorted together, the motor will operate at full speed. The operating characteristics of a wound rotor motor with the rotor leads shorted together are very similar to those of a squirrel cage motor. A circuit for use with a manual controller is shown in Ill. 6.

Ill. 6 Control circuit for a manually controlled wound rotor motor.

Ill. 5 Manual controller for a wound rotor induction motor.

Timed Controlled Starting

Another method of starting a wound rotor motor is with the use of time delay relays. Any number of steps can be employed, depending on the needs of the driven machine. A circuit with four steps of starting is shown in Ill. 7. In the circuit shown, when the START button is pressed, motor starter M energizes and closes all M contacts. The load contacts connect the stator winding to the power line. At this point in time, all resistance is connected in the rotor circuit, and the motor starts in its lowest speed. When the M auxiliary contacts close, timer TR1 begins its time sequence. At the end of the time period, timed contact TR1 closes and energizes the coil of contactor S1. This causes the S1 load contacts to close and short out the first bank of resistors in the rotor circuit. The motor now accelerates to the second speed. The S1 auxiliary contact starts the operation of timer TR2. At the end of the time period, timed contact TR2 closes and energizes contactor S2.

This causes the S2 load contacts to close and shunt out the second bank of resistors. The motor accelerates to third speed. The process continues until all the resistors have been shorted out of the circuit and the motor operates at the full speed.

Ill. 7 Timed starting for a wound rotor induction motor.

The circuit shown in Ill. 7 is a starter circuit in that the speed of the motor cannot be controlled by permitting resistance to remain in the circuit. Each time the START button is pressed, the motor accelerates through each step of speed until it reaches full speed.

Starting circuits generally employ resistors of a lower wattage value than circuits that are intended for speed control, because the resistors are used for only a short period of time when the motor is started. Controllers must employ resistors that have a high enough wattage rating to remain in the circuit at all times.

Wound Rotor Speed Control

A time operated controller circuit's shown in Ill. 8. In this circuit, four steps of speed control are possible. Four separate push buttons permit selection of the operating speed of the motor. If any speed other than the lowest speed or first speed is selected, the motor will accelerate through each step with a 3 second time delay between each step. If the motor is operating at a low speed and a higher speed is selected, the motor will immediately increase to the next speed if it has been operating in its present speed for more than 3 seconds. Assume for example, that the motor has been operating in the second speed for more than 3 seconds. If the fourth speed is selected, the motor will immediately increase to the third speed and 3 seconds later increase to the fourth speed. If the motor is operating and a lower speed is selected, it will immediately decrease to the lower speed without time delay.

Ill. 8 Time operated speed control for a wound rotor induction motor

Ill. 9 Frequency control for a wound rotor induction motor

Frequency Control

Frequency control operates on the principle that the frequency of the induced voltage in the motor secondary (rotor) will decrease as the speed of the rotor in creases. The rotor windings contain the same number of poles as the stator. When the motor is stopped and power is first applied to the stator windings, the voltage induced into the rotor will have the same frequency as the power line. This will be 60 hertz throughout the United States and Canada. When the rotor begins to turn, there is less cutting action between the rotating magnetic field of the stator and the windings in the rotor. This causes a decrease in both induced voltage and frequency. The greater the rotor speed becomes, the lower the frequency and amount of the induced voltage.

The difference between rotor speed and synchronous speed (speed of the rotating magnetic field) is called slip and is measured as a percentage. Assume that the stator winding of a motor has four poles per phase. This would result in a synchronous speed of 1800 rpm when connected to 60 hertz. Now assume that the rotor is turning as a speed of 1710 rpm. This is a difference of 90 rpm. This results in a 5% slip for the motor.

A 5% slip would result in a rotor frequency of 3 hertz.

Where: F _ Frequency in Hertz P _ Number of Poles per Phase S _ Speed in RPM 120 _ Constant

A diagram of a wound rotor motor starter using frequency relays is shown in Ill. 9. Note that the frequency relays are connected to the secondary winding of the motor and that the load contacts are connected normally closed instead of normally open. Also note that a capacitor is connected in series with one of the frequency relays. In an alternating current circuit, the current limiting effect of a capacitor is called capacitive reactance. Capacitive reactance is inversely proportional to the frequency. A decrease in frequency causes a corresponding increase in capacitive reactance.

When the START button is pressed, M contactor energizes and connects the stator winding to the line.

This causes a voltage to be induced into the rotor circuit at a frequency of 60 hertz. The 60 hertz frequency causes both S1 and S2 contactors to energize and open their load contacts. The rotor is now connected to maximum resistance and starts in the lowest speed. As the frequency decreases, capacitive reactance increases, causing contactor S1 to de-energize first and re-close the S1 contacts. The motor now increases in speed, causing a further reduction of both induced voltage and frequency. When contactor S2 de-energizes, the S2 load contacts re-close and short out the second bank of resistors. The motor is now operating at its highest speed.

The main disadvantage of frequency control is that some amount of resistance must remain in the circuit at all times. The load contacts of the frequency relays are closed when power is first applied to the motor. If a set of closed contacts were connected directly across the M leads, no voltage would be generated to operate the coils of the frequency relays and they would never be able to open their normally closed contacts.

Frequency control does have an advantage over other types of control in that it's very responsive to changes in motor load. If the motor is connected to a light load, the rotor will gain speed rapidly, causing the motor to accelerate rapidly. If the load is heavy, the rotor will gain speed at a slower rate, causing a more gradual increase in speed to help the motor overcome the inertia of the load.

QUIZ:

1. How many slip rings are on the rotor shaft of a wound rotor motor?

2. What is the purpose of the slip rings located on the rotor shaft of a wound rotor motor?

3. A wound rotor induction motor has a stator that contains six poles per phase. How many poles per phase are in the rotor circuit?

4. Name three factors that determine the amount of torque developed by a wound rotor induction motor.

5. Explain why the wound rotor motor produces the greatest amount of starting torque per ampere of starting current of any three-phase motor.

6. Explain why controlling the rotor current controls the stator current also.

7. What is the function of a micro limit switch when used with a manual controller for a wound rotor motor?

8. Why are the resistors used in the rotor circuit smaller for a starter than for a controller?

9. What is rotor slip?

10. A wound rotor has a synchronous speed of 1200 RPM. The rotor is rotating at a speed of 1075 RPM. What is the percent of rotor slip and what is the frequency of the induced rotor voltage?

11. Refer to the circuit shown in Ill. 6. Assume that the motor is running at full speed and the STOP button is pressed. The motor stops running. When the manual control knob is returned to the highest resistance setting, the motor immediately starts running in its lowest speed. Which of the following could cause this problem?

a. The STOP push-button is shorted.

b. The START push-button is shorted.

c. M auxiliary contact is shorted.

d. The micro limit switch contact did not re-close when the control was returned to the highest resistance setting.

12. Refer to the circuit shown in Ill. 7. Assume that the timers are set for a delay of 3 seconds each. When the START button is pressed, the motor starts in its lowest speed. After 3 seconds, the motor accelerates to second speed, but never reaches third speed. Which of the following cannot cause this problem?

a. TR1 timer coil is open.

b. S1 contactor coil is open.

c. TR2 timer coil is open.

d. S2 contactor coil is open.

13. Refer to the schematic diagram in Ill. 8.

Assume that the motor isn't running. When the 3RD SPEED push-button is pressed, the motor starts in its lowest speed. After a delay of 3 seconds, the motor accelerates to second speed and 3 seconds later to 3 speed. After a period of about 1 minute, the 4TH SPEED push-button is pressed, but the motor does not accelerate to fourth speed.

Which of the following could cause this problem?

a. Control relay CR2 coil is open.

b. S2 contactor coil is open.

c. CR3 coil is shorted.

d. S2 contactor coil is open.