Design of Electrical Machines Questions and Answers – Design of Rotor Bars and Slots

This set of Design of Electrical Machines Questions and Answers for Experienced people focuses on “Design of Rotor Bars and Slots”.

1. What is the formula for current in each of rotor bar?
a) current in rotor bar = 2 * slot pitch * window space factor * stator torque * stator current * power factor * rotor slots
b) current in rotor bar = 2 * slot pitch * window space factor / stator torque * stator current * power factor * rotor slots
c) current in rotor bar = 2 * slot pitch * window space factor * stator torque * stator current * power factor / rotor slots
d) current in rotor bar = 2 * slot pitch / window space factor * stator torque * stator current * power factor / rotor slots

Explanation: For the calculation of the current through each rotor bar, firstly we find out slot pitch, window space factor and rotor slots. Then the stator torque and stator current are obtained with respect to stator side.

2. What is the relation between rotor mmf and stator mmf?
a) rotor mmf = 0.85 * stator mmf
b) rotor mmf = 0.80 * stator mmf
c) rotor mmf = 0.75 * stator mmf
d) rotor mmf = 0.70 * stator mmf

Explanation: First the stator mmf is calculated. Then it is multiplied by 0.85 to obtain the rotor mmf.

3. What is the relation of the rotor resistance with respect to the starting torque?
a) rotor resistance is indirectly proportional to the starting torque
b) rotor resistance is directly proportional to the starting torque
c) rotor resistance is indirectly proportional to the square of the starting torque
d) rotor resistance is directly proportional to the square of the starting torque

Explanation: The rotor resistance is directly proportional to the starting torques. High resistance leads to high starting torque.

4. What is the relation of the rotor resistance to efficiency and losses?
a) as rotor resistance, losses increase, efficiency increases
b) as rotor resistance, losses increase, efficiency decreases
c) as rotor resistance, losses decrease, efficiency has no change
d) as rotor resistance, losses decrease, efficiency decreases

Explanation: As the rotor resistance increases, the I2R losses increases and cause heating effects. This increase in losses decreases efficiency.

5. What is the relationship between current density, conductor area and resistance?
a) higher the current density, higher the conductor area, higher the resistance
b) higher the current density, higher the conductor area, lower the resistance
c) higher the current density, lower the conductor area, higher the resistance
d) lower the current density, lower the conductor area, lower the resistance

Explanation: Higher the current density leads to lower conductor area, as current density is the ratio of current per area. As the conductor area decreases, resistance increases.
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6. What is the formula for the calculation of rotor resistance?
a) rotor resistance = resistance of the bars + resistance of end rings
b) rotor resistance = resistance of the bars – resistance of end rings
c) rotor resistance = resistance of the bars * resistance of end rings
d) rotor resistance = resistance of the bars / resistance of end rings

Explanation: First the resistance of the bars are obtained. Next, the resistance of the end rings are calculated and the sum gives the rotor resistance.

7. What is the range of current density in rotor bars?
a) 4-9 A per mm2
b) 4-6 A per mm2
c) 4-7 A per mm2
d) 5-6 A per mm2

Explanation: The minimum value of the current density in the rotor bars is 4 A per mm2. The maximum value of the current density in the rotor bars is 6 A per mm2.

8. What is the formula for the area of each bar?
a) area of each bar = current of the rotor bars + current density in rotor bars
b) area of each bar = current of the rotor bars / current density in rotor bars
c) area of each bar = current of the rotor bars * current density in rotor bars
d) area of each bar = current of the rotor bars – current density in rotor bars

Explanation: For calculating the area of each bar, current flowing across the rotor bars should be first calculated. Then the current density in rotor bars should be calculated next and the ratio gives the area of each bar.

9. Closed slots are preferred for small machines.
a) true
b) false

Explanation: Closed slots are preferred for small machines. It is because the reluctance of the air gap is not large owing to absence of slot openings.

10. What is the relation of closed slots with leakage reactance?
a) closed slots give no leakage reactance
b) closed slots give high leakage reactance
c) closed slots give low leakage reactance
d) closed slots give negative leakage reactance

Explanation: It is an advantage that closed slots give large leakage reactance. If the leakage reactance is large, the current at the starting can be limited.

11. What is the relation of closed slots with leakage reactance and overload capacity?
a) closed slots give high leakage reactance, and increases the overload capacity
b) closed slots give high leakage reactance, and decreases the overload capacity
c) closed slots give low leakage reactance, and decreases the overload capacity
d) closed slots give low leakage reactance, and increases the overload capacity

Explanation: The closed slots have the main advantage of giving high leakage reactance. A high leakage reactance gives the advantage that the current at the starting can be limited.

12. What is the relation between surface of rotor and the operation?
a) smooth surface leads to the quiet operation
b) rough surface leads to the quiet operation
c) smooth surface leads to the noisy operation
d) rough surface leads to the noisy operation

Explanation: For the design of the rotor bars of the three phase induction machine, smooth surface is preferred. Smooth surface helps in the silent operation.

13. Rectangular shaped bars and slots are preferred to circular bars and slots.
a) true
b) false

Explanation: Rectangular shaped bars and slots are preferred to circular bars and slots. This is because while using the rectangular shaped bars, rotor resistance increases and this leads to the improvement of starting torque.

14. What is the relation between clearances and slots?
a) high clearances are provided for salient slots
b) low clearances are provided for skewed slots
c) low clearances are provided for salient slots
d) high clearances are provided for skewed slots

Explanation: When the skewed slots are being used, higher clearances are provided. High clearances can lead to the smooth and efficient operation of the machine for skewed slots.

15. What is the range of clearance that can be left between rotor bars and the core?
a) 0.1-0.4 mm
b) 0.2-0.4 mm
c) 0.15-0.4 mm
d) 0.4-0.6 mm

Explanation: The range of clearance is chosen based on whether the slots are skewed or not. The range is usually chosen between 0.15-0.4 mm.

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