### EE2302 Electrical Machines II Question bank fifth semester

1. Calculate the distribution factor for a 36- slot, 4 -pole, single layer three phase winding.
2.      Define voltage regulation of an alternator.
3.      What are the two components of field current required for the predetermination of regulation by MMF method ?
4.      What are the conditions to be fulfilled for  connecting  two alternators in parallel ?
5.      Define short circuit ratio of an alternator.
1. Why is the field system of an alternator made as a rotor?
2. Sketch salient pole and non salient pole rotors.
3. What is synchronous reactance?
4. Define Pitch factor.
5. Calculate the distribution factor for an 18 slots, 4-pole three-phase winding.
6. Draw the phasor diagram of synchronous motor working with lagging power factor.

1. Sketch the V and inverted V- curve of a synchronous motor, clearly indicating the X and Y axes for each curve.

1. State the differences in construction between squirrel cage and slip ring rotors of an induction motor.

1. A 3 phase, 4 pole, 50 Hz induction motor runs at a speed of 1425 rpm. Calculate the slip and frequency of the rotor EMF.

1. Sketch the circle diagram of a 3 phase induction motor.

1. Calculate the pitch factor for a 36 slot, 4 pole winding having a coil pitch of 6 slots.

1. A 5 MVA, 10 kV, 1500 rpm, 3 phase, 50 Hz,   4 pole, star connected alternator is operating on infinite bus bars. Determine the synchronizing power per mechanical degree of displacement under no-load condition. Xs = 20 %.

1. When is a synchronous motor said to be under excited? What will be the nature of
power factor under that condition?

1. What is hunting in a synchronous motor?

1. Under what condition, the slip in an induction machine is i) negative ii) greater than one?
2. How do you reverse the direction of rotation of a 3 phase induction motor?

1. Show the construction of a double cage rotor.

1. How much is the line current and starting torque reduced in a 3-phase induction motor, when a star-delta starter is used, compared to direct on line starting?

1. What are the four possible speeds that could be obtained in the cascaded operation of two induction motors with pole numbers, P1 and P2

1. Draw the phasor diagram of a single- phase capacitor start induction motor,
showing the stator applied voltage and the main and auxiliary winding currents.

Summary questions

1.The open circuit and short circuit test readings obtained on a 3 phase, star
connected, 1000 kVA, 2000 V , 50 Hz alternator  are :

Field Current, A       10           20           25          30          40          50

OC terminal
voltage , V               800        1500       1760       2000        2350     2600

SC armature
current, A                      -        200         250         300           -             -

The effective armature resistance is 0.2 Ω per phase.  Draw the characteristic
curves and estimate the full-load regulation at a power factor of 0.8 lagging by
synchronous impedance method.

2.Derive the EMF equation of an alternator.

3  What do you understand by direct and quadrature axis reactance in a salient
pole alternator?  Draw the phasor diagram of  this alternator for a lagging
4. Derive the expression for power developed in a 3 phase synchronous motor in
terms of E, V, Zs, θ and δ .

5. Sketch an excitation circle and a power circle of a synchronous motor and state
what each one represents

6.  Using relevant phasor diagrams, describe the effect of changing the excitation,
on the armature current and power factor of a synchronous motor and thereby
obtain ‘V’ and inverted ‘V’ curves.
7.  Deduce the Equivalent circuit of 3 phase induction motors

8. The power input to a 3 phase, 500 V, 50 Hz, 6 Pole, induction motor is 40 kW.
The motor runs at a speed of 975 rpm. The stator losses amount to 1 kW and
the friction and windage losses amount to 2 kW.  Calculate the i) slip ii) rotor
copper loss iii) hp output and iv) efficiency.

9. Draw the Circle diagram for a 5.6 kW, 400 V, 3 phase, star connected 4 pole
50 Hz   slip ring induction motor which gave the following test data :

No-load:    400 V,   6 A,  360 W     :  Blocked rotor  :  100 V,  12 A,  720 W

The ratio of stator to rotor turns is 2.6. The stator resistance per phase = 0.67 Ω .
The rotor resistance per phase = 0.185 Ω. Calculate the full load current and
maximum output.

10. With neat diagrams , explain the working of any two types of starters used for
starting 3 phase squirrel cage induction motors.
11. Describe the following methods of speed control of slip ring induction motors:

i)        Using external rotor resistance
ii)      Slip power recovery scheme

12. Based on double field revolving theory and using the torque – speed
characteristics, explain the operation of single- phase induction motors.

ii) Draw the equivalent circuit of single-phase induction motor.

13. Describe the constructional features and the operation of the following:

i)        Stepper motor
ii)      Shaded pole motor
iii)    Hysteresis motor

14. A 3 phase star connected synchronous motor is rated for 6600 V. For a particular load, the motor takes 50 A line current from the supply. The effective armature resistance is 1.0 Ω per phase and synchronous reactance is 20 Ω per phase. Calculate (a) the power supplied to the motor and (b) induced EMF for a power factor of 0.8 lag.

15. Explain why a synchronous motor is not self starting. Describe the method of starting the synchronous motor using damper winding.

16. A 415 V, 29.84 kW, 50 Hz, 3 phase delta connected induction motor gave the following test data:

No load test:                           415 V              21 A                1250 W
Blocked rotor test:      100 V              45 A                2730 W

Calculate the parameters of the equivalent circuit, draw the circuit and then insert the values. Stator resistance per phase = 0.3 Ω.

17. Derive the equation for torque developed by a 3 phase induction motor. Draw a typical torque-slip curve and deduce the condition for maximum torque.
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18. Derive from fundamentals, the emf equation of an alternator

19. A 3 phase, 16 pole, star connected alternator has 144 slots with 10 conductors per slot. The flux per pole = 0.03 Wb. The speed = 375 rpm. The coil span = 7 slots. Calculate the phase and line induc3ed emfs.

20. A 3 phase, star connected alternator is rated at 1800 kVA, 11kV.The armature effective resistance and synchronous reactance are 1.4Ω and 28Ω respectively per phase. Calculate the percentage regulation for a load of 1250 kW at 0.8 p.f. lag.

21. A 3 phase, star connected alternator supplies a load of 1000kW at a p.f. of 0.8 lag with a terminal voltage of 11kV. Its armature resistance is 0.4 Ω/phase while synchronous reactance is 3Ω/phase. Calculate the line value of emf generated and the regulation at this load
22 What are the advantages of stationary armature and rotating field type of alternators compared to the other type? What are the two types of rotors used in alternators of rotating field type?

23 Derive from fundamentals, the EMF equation of an alternator.

24 Calculate the no-load phase and line voltage of a star connected 3-phase, 6-pole alternator which runs at 1200 rpm having a flux per pole of 0.1 Wb    sinusoidally        distributed. The stator of the alternator has 54 slots with 18 conductors per slot. The coil span is 7 slots.

25 Discuss the nature of armature MMF in an alternator, bringing out its effect on field MMF at various power factors.
1. A 25 kVA, 400 V, 3-phase, 50Hz star connected alternator gives rated current on short circuit with a field current of 2 A. The same field current gives 200 V on open circuit. The effective armature resistance per phase is 1.0 W. Calculate the full load voltage regulation at a Power Factors  of  i) 0.8 lag and  ii) 0.8 lead.
2. What are the advantages of stationary armature and rotating field type of alternators compared to the other type? What are the two types of rotors used in alternators of rotating field type?
3. Derive from fundamentals, the EMF equation of an alternator.
4. Calculate the no-load phase and line voltage of a star connected 3-phase, 6-pole alternator which runs at 1200 rpm having a flux per pole of 0.1 Wb    sinusoidally     distributed. The stator of the alternator has 54 slots with 18 conductors per slot. The coil span is 7 slots.
5. A 25 kVA, 400 V, 3-phase, 50Hz star connected alternator gives rated current on short circuit with a field current of 2 A. The same field current gives 200 V on open circuit. The effective armature resistance per phase is 1.0 W. Calculate the full load voltage regulation at a Power Factors  of  i) 0.8 lag and  ii) 0.8 lead.

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