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.
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.