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# 3 Phase Induction Motors

By Karen Brooks,2014-12-03 15:47
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3 Phase Induction Motors

Transformers

1. The effect of increasing the load on a transformer would be to cause the

secondary current to ………………….. and the secondary voltage

to …………………..

2. State the two basic forms of tap changers

3. Briefly explain why it would be necessary to parallel two transformers

4. List three conditions that must be identical for both single phase and three phase

transformers before they can be paralleled

5. An ideal double wound transformer has 350 primary turns and 80 secondary

turns. Calculate

i. The secondary voltage when 230V is applied to the primary winding

ii. The secondary current if primary current is 4A

6. A single phase ideal autotransformer has 230 V on the primary and 220 V on the

secondary. If the secondary load current is 18A calculate:

i. The primary current

ii. The current in the common section of the winding

7. A single-phase transformer is wound with 280 turns on the primary and 80 turns

on the secondary. The maximum value core flux is 0.03 Wb and the supply

frequency is 50 Hz. Calculate the value of the primary voltage.

8. Calculate the percentage impedance (%Z) of a transformer that has a rated

voltage of 240V/12V if on a short circuit test the applied primary voltage required

to circulate rated secondary current is 10V

.

9. Determine the prospective short-circuit current of a 40 kVA three-phase

transformer with 3% impedance, to be operated from a 400/230V, 50 Hz supply.

10. A delta-star connected three phase transformer has a phase relationship

between primary and secondary of ………………. degrees

11. A star-star connected three phase transformer has a phase relationship between

primary and secondary of ………………. Degrees

12. A star-delta connected three phase transformer has a phase relationship

between primary and secondary of ………………. degrees

Motors

1. How do you reverse the direction in which the rotor of a three phase induction

motor rotates?

2. Identify two outcomes that would happen to a three phase squirrel cage induction

motor when it is started if the load torque was greater than the locked rotor torque

of the motor.

3. Briefly describe the purpose of a centrifugal switch in a split phase motor

4. Briefly describe how to reverse the rotation of a shaded pole motor.

5. What type of protection is suitable for preventing damage by overheating to a

motor due to blocked are vents.

6. An insulation resistance test is carried out on the terminals of a three phase

induction motor with the connection links removed. The following results were

recorded:

Test Connections Measurement Test Connections Measurement

U1 to Frame >200 MΩ U1 to V1 1.95 MΩ

V1 to Frame >200 MΩ V1 to W1 0.75 MΩ

W1 to Frame >200 MΩ W1 to U1 2.0 MΩ

Discuss the condition of the motor and whether it is suitable to be connected to

the supply:

7. Complete the following statement: Where electrical equipment is remotely

controlled, devices shall be provided for stopping motors. oAS/NZS 3000:2007 Clause/Table N

8. The single phase motor that is suitable to drive a circular saw is a:

9. Name the type of overload protection that is most suitable for a motor operating

in extremely hot or cold ambient temperatures.

10. What happens when starting a single phase split phase motor, if the centrifugal

switch failed to close when the motor was last stopped?

11. Identify two advantages of three phase synchronous motors.

12. Identify one reason why a single phase Hysteresis motor would be suitable for an

audio tape drive.

13. A four-pole 50-Hertz three-phase squirrel cage induction motor operates with a

full load rotor speed of 1435 rpm. Calculate the frequency of rotor currents.

14. A three phase 400V 50 Hz four pole induction motor draws 60 A on full load. If

the full load efficiency is 88%, power factor is 0.75 and full load slip is 3.5%,

calculate

i. Power input

ii. Power output

iii. Torque developed

15. The rotor of a four-pole 50-Hertz three-phase slip ring induction motor has a

resistance of 6.25 Ω per phase and an inductance of 150 mH per phase.

Calculate the slip speed at which maximum torque occurs

16. A single phase alternator has a terminal voltage of 230 Volts when delivering full-

load current. When the load is removed the terminal voltage rises to 249 Volts.

Calculate the percentage voltage regulation for this alternator

17. A load of 450 kVA operates at a power factor of 0.65 lagging. An additional

synchronous motor is added having an input power of 90 kW and a maximum

power factor of 0.85 leading. Determine reactive power and the overall power

factor.

18. Determine the input power of a three phase, 25 kW, 400 V 40 A wound rotor

induction motor that is 80% efficient.

19. Calculate the speed of the rotor at full speed of a 6 pole, 400 V, 45 A, 3 phase,

50 Hz synchronous motor operating at unity power factor.

20. A 4 pole, 400 V, 65 A, 50 Hz, 3 phase, wound rotor induction motor has a rotor

resistance of 3Ω per phase and an inductance of 159.2 mH per phase. Determine

the speed at which maximum torque occurs.

21. The existing load on a factory is 1600 kVA and operates at 0.6 lag power factor.

The customer requires a new 500 kW motor to drive a rolling mill. The choice of

motors is an induction motor that operates at 0.8 lagging power factor, or a 500

kW synchronous motor that operates at 0.8 leading power factor. Both machines

operate at 90% efficiency. Identify the most appropriate machine for the

installation, and calculate the reactive power of that motor.

22. A 400 V three phase 6 pole 50 Hz induction motor rotates at 940 rpm and draws

10 A at full load. If the motor efficiency is 80% and the full load power factor is

0.85, determine:

i. Power input

ii. Power output

iii. Torque developed

iv. Frequency of rotor currents at this load

23. A 50 kW 400 V three phase 4 pole 50 Hz induction motor draws a current of 95 A

at a power factor of 0.85 when operating at full load. Calculate the full load

efficiency of the motor.

24. A 400 V three phase two pole 50 Hz slip ring motor has a rotor resistance of 0.1Ω

and an inductance of 12 mH per phase. Determine the resistance that must be

added to the rotor circuit per phase to produce maximum torque at start.

25. A 230 V 2 pole 50 Hz induction motor operates with a full load slip of 3%.

Calculate the full load rotor speed.

26. A 230 V 4 pole 50 Hz induction motor has a rotor frequency of 2.5 Hz at a certain

i. Synchronous speed

ii. slip speed

iii. rotor speed

27. Determine the full load current of a three phase 10 kVA 400 V alternator.

28. Describe the changes that occur to power factor and current when the d.c.

excitation is reduced below the normal value at which a three-phase synchronous

motor operates.

29. Identify two types of single phase synchronous motors and provide one

application for each.

Motor Application Marks

30. Connect the components together to make a working single phase motor, label

the components and identify the type of motor.

31. Identify two categories of protection which are required for three phase motors

installed in a factory workshop environment.

32. An alternator that has a cylindrical rotor would be driven by a ……………………….

turbine at …………………….. speed, and an alternator that has a salient pole

rotor would be driven by a ……………………………turbine

at ………………………speed.

33. Describe the characteristics that are present when maximum torque occurs in a

three phase induction motor.

34. What size motors (kW rating) shall be provided with protection against overload? oAS/NZS 3000:2007 Clause/Table N

35. The most appropriate single phase motor for a portable electric grinder would be

a ………………………….., and for a bedroom reversible ceiling fan would be

a …………………………..

36. Identify one reason why rotor bars are skewed in squirrel cage motors.

37. The resistance of the outer cage of a double cage type rotor is ………………..

than the resistance of the inner cage.

38. The contact of a thermal overload relay is a normally ……………….contact and is

connected in …………… with the control circuit of the motor contactor.

39. The following two questions related to the graph below

i. Which curve is the Power factor curve?

ii. At what rotor current must the motor operate to provide an operating power

iii. At what rotor current must the motor operate to provide a line current of 60%

maximum with a lagging power factor?

iv. What is the motor power factor when the rotor current is 16 A?

40. The following three questions relate to the graph below

i. At what value of field current must the motor operate to provide an operating

ii. What percentage of stator current is flowing at unity power factor? iii. What is the lagging power factor when stator current is 50%?

41. On the following terminal blocks, label the terminals, draw in the horizontal or

vertical links and connect the supply lines for the configuration required.

42. A three phase induction motor has only three terminals at the terminal block, and

the nameplate indicates that the windings are connected in delta. When using an

ohm meter on the 100Ω range setting with the supply leads disconnected, the

following results were recorded:

Terminal Connections Winding Resistance

U1 to V1 2.6

V1 to W1 2.6

W1 to U1 5.2

Identify the fault in the motor, and explain how you came to that conclusion: 43. Permanent split capacitor main winding has …………………….resistance

and …………………inductance when compared to it’s auxiliary winding

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