Tuesday, January 13, 2015

Three-Phase Induction Motors: Comparison between squirrel-cage and woundrotor induction motors.

COMPARISON BETWEEN SQUIRREL-CAGE AND WOUND-ROTOR INDUCTION MOTORS

The wound-rotor induction motor has the following advantages:

• High starting torque and low starting current if the motor is started with the maximum resistance inserted in the rotor circuit

• Variable speed

• Smooth acceleration under heavy loads

There are several disadvantages to this motor, including the following:

• Greater initial cost

• Higher maintenance and repair costs

• Low efficiency and poor speed regulation (when the motor is operated with resistance inserted in the rotor circuit)

SUMMARY

• A three-phase, squirrel-cage induction motor

1. is simple in construction and is easy to maintain.

2. is smaller in physical size than other motors for a given horsepower rating.

3. has very good speed regulation under varying load conditions.

4. is used for many industrial applications because of its low cost, rugged construction, and operating characteristics.

Other characteristics are as follows:

1. The basic components of a squirrel-cage motor are a stator, a rotor, and two end shields housing the bearings supporting the rotor shaft.

2. The stator is a three-phase winding that is placed in the slots of a laminated steel core. Three single-phase windings are spaced 120 electrical degrees apart. These windings may be connected in wye or delta.

3. The rotor consists of a cylindrical core made from steel laminations. Aluminum bars are mounted near the surface and are connected at the ends to two aluminum end rings.

• The squirrel-cage induction motor is also known as a synchronous speed motor.

1. The synchronous speed is determined by the number of poles in the stator winding and the frequency of the ac source

imagewhere

S = synchronous speed, in r/min

f = frequency, in hertz

P = number of poles (not pairs of poles)

2. The following table lists the speeds of a motor at two different frequencies for different numbers of poles:

image

• Slip of an induction motor:

1. Slip is the difference between the synchronous speed of the motor and the actual speed of the rotor. The rotor always turns at a speed that is slightly less than synchronous speed.

2. The speed performance of an induction motor is usually measured in terms of percent slip. The percent slip for an induction motor is generally in the range of 2% to 5%:

image

3. The standard squirrel-cage induction motor has excellent speed regulation, from no load to full load. Because of its good speed regulation, an induction motor is considered to be a constant-speed motor:

image

4. The torque output of an induction motor varies as the square of the terminal volt- age for a given percent slip.

5. The power factor of the induction motor is low at no load and high at full load. At full-load conditions, the in-phase component of current is large and the magnetizing component is small; the opposite is true at no-load conditions.

6. The losses of an induction motor consist of the stray power losses and the copper losses. The stray power losses are nearly constant at all loads. The copper losses increase as the current of the motor increases.

7. The direction of rotation of a three-phase, squirrel-cage induction motor can be changed by reversing two of the three incoming supply leads.

• The torque of an induction motor

1. results from the interaction between the stator flux and the rotor flux.

2. is produced only when the rotor turns at a speed that is less than the synchronous speed of the stator flux pattern.

3. is small at no load because there is only a slight difference between the rotor speed and the synchronous speed of the stator flux.

4. will increase to a larger value as the load on the shaft of the rotor increases. The resulting increase in the rotor current causes a larger torque at a slightly lower rotor speed.

• Rotor frequency:

1. This frequency is an important factor in the operation of the motor. A change in the rotor frequency causes the inductive reactance and the impedance of the rotor to decrease. A change in the frequency will affect the starting and running characteristics of the motor.

2. Rotor frequency is expressed by the following formulas:

image

• At the instant of start-up

1. the rotor is not turning and there is 100% slip. The rotor frequency is equal to the stator frequency.

2. the inductive reactance of the rotor is very large compared to the effective resistance component. The rotor has a very low lagging power factor and the starting torque is low.

3. the stator field cuts the rotor bars at a faster rate; thus, the induced voltage in the rotor causes a large rotor current and the stator current will also be high at start-up.

In addition:

1. Because of the high starting current, induction motors must have starting protection. This protection may be rated as high as three times the full-load current rating of the motor.

2. Some very large induction motors are started with auxiliary starters. These devices reduce the motor voltage at start-up to limit the starting surge of current. As a result, there is less voltage disturbance on the feeder circuit supplying the motor load.

• The point of the maximum torque output is called the breakdown point.

1. An increase in the load beyond this point means that less torque is developed by the motor and the rotor stops.

2. The breakdown point occurs at 200% to 300% of the rated torque.

• The efficiency of an ac induction motor is given by the following equation:

image

• Brushes and slip rings are not required on a squirrel-cage induction motor. Thus, this type of motor can be used in locations where there is a possibility of explosion due to arcing, such as in chemical plants and flour or lumber mills.

• There is no practical method of providing stepless speed control for the induction motor.

• The blocked rotor test is used to determine the equivalent resistance of the motor winding, per phase.

1. Two wattmeters are used in this test:

image

• Stray power losses:

1. Stray power losses include mechanical friction losses, windage losses, and iron losses.

2. To measure these losses, the motor is operated at no load with the rated voltage applied to the motor terminals:

image

3. The fixed losses at no load are obtained by subtracting the no-load copper losses from the power input at no load:

image

• Speed control of an induction motor:

1. The synchronous speed of the magnetic field of the stator is determined by the number of stator poles and the frequency of the ac source.

2. Generally, the speed of an induction motor cannot be varied by changing the frequency.

3. The speed of a squirrel-cage motor may be changed if the motor is provided with special stator windings. When these windings are reconnected using special switch controls, different numbers of stator poles are formed. Thus, different speeds can be obtained for a squirrel-cage induction motor connected to an ac source at a fixed frequency.

• Code letter identification for motors:

1. A system of code letters is used to identify certain induction motors. These motors are grouped according to their torque and starting current characteristics.

image

2. The code letter designates the ratio between the starting and full-load currents.

This letter appears on the nameplate of all squirrel-cage induction motors.

3. By referring to the National Electrical Code, this code letter can be used to deter- mine the current ratings of circuit breakers, fuses, and other overload protective devices.

• A motor nameplate contains the following information:

Full-load horsepower output (hp)

Full-load speed (r/min)

Full-load current amperage (FLA)

Locked-rotor current amperage (LRA)

Manufacturer name

Terminal voltage (volts)

Number of phases (phase)

Frequency (cycles) (Hz)

Temperature rise (rise °C) (or insulation system class and rated ambient temperature)

Time rating (5, 15, 30, 60 min, or continuous)

Code letter

[Note: Other general information may be placed on the motor nameplate.]

• A double-squirrel-cage motor has a low starting current, a strong starting torque, and excellent speed regulation. The starting torque for this type of motor can be as high as 250% of the rated torque.

• Single-phase operation of three-phase induction motors:

1. If a three-phase induction motor is running when it is subjected to single-phase conditions, it will continue to operate but at a greatly reduced capacity.

2. The motor will not have enough torque to start when it is energized from a single-phase source.

3. If the rated load is applied to the motor when it is operating as a single-phase motor, it will overheat and the insulation of the windings may be damaged.

• The wound-rotor induction motor:

1. This motor can be used for variable-speed applications. The squirrel-cage induction motor is a constant-speed motor.

2. The formula used to find the synchronous speed for a squirrel-cage induction motor can also be used for the wound-rotor motor:

image

3. A wye-connected speed controller provides control current through slip rings to the windings of the rotor.

a. At start-up, all of the resistance of the wye-connected speed controller is inserted in the rotor circuit. This additional resistance causes an excellent starting torque and a large percent slip.

b. As the motor accelerates, steps of resistance are cut out of the wye-connected speed controller.

c. When all of the resistance is cut out, the rotor slip rings are short-circuited.

The motor then operates at the rated speed like a squirrel-cage induction motor.

d. This motor can be operated at heavy loads by cutting in resistance to the rotor circuit to obtain a below-normal speed at a lower motor efficiency.

4. The direction of rotation can be changed by reversing any two leads of the three line leads feeding to the stator windings.

5. A wound-rotor induction motor is used when a strong starting torque and a range of speed control are required. Typical applications include cranes, large compressors, elevators, and pumps.

6. Compared to the squirrel-cage induction motor, the wound-rotor motor has the following advantages:

a. High starting torque and low starting current if it is started with the maximum resistance inserted in the motor circuit

b. Variable speed

c. Smooth acceleration under heavy loads

7. Compared to the squirrel-cage induction motor, the wound-rotor motor has the following disadvantages:

a. Greater initial cost

b. Higher maintenance and repair costs

c. Low efficiency and poor speed regulation (when it is operated with resistance inserted in the rotor circuit)

Achievement Review

1. Explain what is meant by the synchronous speed of a three-phase induction motor.

2. What two conditions determine the synchronous speed of a three-phase induction motor?

3. Explain what is meant by the following terms:

 

a.

b.

c.

Revolutions slip

Percent slip

Rotor frequency

4.

a.

b.

What is the rotor frequency of a three-phase, 60-Hz, squirrel-cage induction motor at the instant of start?

What is the approximate rotor frequency of a three-phase, 60-Hz, squirrel- cage induction motor when it is operating at the rated load?

5. What is the reason for the poor starting torque of a squirrel-cage induction motor?

6. A six-pole, three-phase, 60-Hz induction motor has a full-load speed of 1140 r/ min. Determine

a. the synchronous speed.

b. the revolutions slip.

c. the percent slip.

d. the rotor frequency.

7. a. Draw characteristic curves for a three-phase, squirrel-cage induction motor for the speed, percent slip, percent efficiency, power factor, and torque.

b. Discuss each of the five characteristic curves developed in part a of this question.

8. For a given value of slip, the torque output of an induction motor varies as the square of the impressed terminal voltage. Explain what is meant by this statement.

9. A 10-hp, 220-V, three-phase, 60-Hz, squirrel-cage induction motor is rated at 28 A per terminal. The full-load speed is 855 r/min, and the full-load power factor is 0.90 lag. The motor has eight poles. At the rated load, determine

a. the synchronous speed.

b. the slip in r/min.

c. the percent slip.

d. the rotor frequency at the rated speed.

10. A three-phase, 60-Hz, four-pole, 220-V, squirrel-cage induction motor takes 52 A per terminal at full load. The power factor is 0.85 lag, and the efficiency is 88%.

The slip is 3.0%. At the rated load, determine

a. the speed in r/min.

b. the horsepower output of the motor.

c. the total losses.

11. A three-phase, 60-Hz, six-pole, 220-V, squirrel-cage induction motor has a full- load output of 15 hp. The full-load efficiency is 87%, and the power factor is 0.88 lag. The windings of the motor are connected in delta. At the rated load, determine

a. the line current.

b. the phase winding voltage.

12. Assuming that the three-phase motor in question 11 is reconnected in wye with the same load and power factor, determine

a. the new rated line voltage.

b. the line current per motor terminal.

13. Explain how the direction of rotation is reversed for

a. a three-phase squirrel-cage induction motor.

b. a three-phase wound-rotor induction motor.

14. Explain why a three-phase squirrel-cage induction motor will not start when energized from a single-phase source.

15. Neither of the following alternating-current, three-phase induction motors is operating properly. Give a possible reason for the motor failure described, and state what should be done to correct each condition.

a. A 15-hp, 220 V, three-phase squirrel-cage induction motor overheats while operating at a normal load. The motor circuit is deenergized. When an attempt is made to restart the motor, it will not turn.

b. A 5-hp, 220-V, three-phase squirrel-cage induction motor stops as soon as the start push button for the across-the-line motor starter is released.

c. A newly installed 10-hp, 220-V, three-phase squirrel-cage induction motor has dual voltage ratings of 220 V and 440 V. The motor is supplied from a 220-V, three-phase source. At no load, the motor operates at a speed that is slightly below synchronous speed. When the rated load is applied, the motor stalls.

16. Show the connections for the nine terminal leads of a wye-connected, three-phase motor rated at 220/440 V for operation at

a. a line voltage of 440 V.

b. a line voltage of 220 V.

17. Compare a three-phase squirrel-cage induction motor and a three-phase wound-rotor induction motor, with regard to

a. construction.

b. starting torque.

c. speed control.

d. initial cost and maintenance.

e. efficiency.

18. a. Explain why the power factor of an induction motor is low at no load.

b. Explain what happens to the power factor as the load on an induction motor is increased.

19. Explain how both a good starting torque and small percent slip can be obtained using a squirrel-cage induction motor with a double-squirrel-cage rotor.

20. How is speed control provided for a squirrel-cage induction motor?

21. The following data are obtained in a prony brake test of a three-phase squirrel- cage induction motor:

image

Determine

a. the power factor.

b. the horsepower output.

c. the efficiency.

d. the torque output in pound • feet.

22. a. What are the losses in a squirrel-cage induction motor?

b. Which of these losses are constant and which of the losses vary with a change in load?

23. List the nameplate data of a typical squirrel-cage motor.

24. Code letters are included in the nameplate data for three-phase squirrel-cage induction motors. What is the purpose of these code letters?

25. What is the purpose of the protective starting device used on faceplate speed controllers and drum-type speed controllers?

26. The following test data are obtained on a 7.5-hp, three-phase, 220-V, wye- connected, four-pole squirrel-cage induction motor. The data are for a no-load condition and a full-load condition. The effective resistance of each single-phase winding of the wye-connected stator is 0.65 11. Determine the stray power losses of the motor.

image

27. Use the data given in question 26 and determine

a. the copper losses of the motor at full load.

b. the efficiency of the motor at rated load.

c. the horsepower output at full load.

d. the torque output in pound • feet at full load.

28. A three-phase, squirrel-cage induction motor must carry an additional load. List the sequence of steps, in chronological order, showing how the motor will adjust itself to carry this additional load.