1. What is the formula for the mmf required for stator teeth?
a) stator teeth mmf = mmf per metre / length of flux path in rotor core
b) stator teeth mmf = mmf per metre + length of flux path in rotor core
c) stator teeth mmf = mmf per metre * length of flux path in rotor core
d) stator teeth mmf = mmf per metre – length of flux path in rotor core
Explanation: First the mmf per meter of stator slots is calculated by its equation. Then the length of the flux path in rotor core is obtained and the product of both gives the stator teeth mmf value.
2. What is the formula for the magnetizing current per phase?
a) magnetizing current per phase = (0.427 * no. of poles * total magnetizing mmf per pole) / stator winding factor * no of turns of stator slots
b) magnetizing current per phase = (0.427 / no. of poles * total magnetizing mmf per pole) / stator winding factor * no of turns of stator slots
c) magnetizing current per phase = (0.427 * no. of poles / total magnetizing mmf per pole) / stator winding factor * no of turns of stator slots
d) magnetizing current per phase = (0.427 * no. of poles * total magnetizing mmf per pole) * stator winding factor * no of turns of stator slots
Explanation: Firstly the total magnetizing mmf per pole is calculated. Then the no of poles and the stator winding factor is calculated. Next the no. of turns of stator slots is calculated and the magnetizing current per phase can be obtained.
3. What is the no load current percent of the full load current for the output of 0.75 KW?
a) 50%
b) 40%
c) 33%
d) 90%
Explanation: For output of 3 kW, the no load current is 40% of full load current. For output of 15 kW, the no load current is 33% of the full load current.
4. What is the no load current percent of the full load current for the output of 37 KW?
a) 50%
b) 30%
c) 27%
d) 67%
Explanation: For output of 0.75 kW, the no load current is 50% of full load current. For output of 75 kW and above, the no load current is 27% of the full load current.
5. The no load power factor is the ratio of full load current to no load current.
a) true
b) false
Explanation: For obtaining the no load power factor first the no load current value is obtained. Then the full load current value is obtained and the ratio gives the no load power factor.
6. How many factors influence the power factor of an induction motor?
a) 3
b) 2
c) 1
d) 4
Explanation: There are 2 factors which influence the power factor of an induction motor. They are magnetizing current and ideal short circuit current.
7. What is the relation between the magnetizing current and power factor?
a) magnetizing current is directly proportional to the power factor
b) magnetizing current is indirectly proportional to the power factor
c) magnetizing current is directly proportional to the square of the power factor
d) magnetizing current is indirectly proportional to the square of the power factor
Explanation: Magnetizing current is indirectly proportional to the power factor. As the magnetizing current is large, the power factor is poor.
8. What is the relation between the leakage current and power factor?
a) leakage current is directly proportional to the power factor
b) leakage current is indirectly proportional to the power factor
c) leakage current is directly proportional to the square of the power factor
d) leakage current is indirectly proportional to the square of the power factor
Explanation: Leakage current is indirectly proportional to the power factor. A small leakage current means a very good power factor.
9. What is the formula for dispersion coefficient?
a) dispersion coefficient = magnetizing current / ideal short circuit current
b) dispersion coefficient = magnetizing current * ideal short circuit current
c) dispersion coefficient = magnetizing current + ideal short circuit current
d) dispersion coefficient = magnetizing current – ideal short circuit current
Explanation: First the magnetizing current is calculated. Next the ideal short circuit current is calculated. The ratio of both gives the value of dispersion coefficient.
10. What is the formula for dispersion coefficient?
a) dispersion coefficient = 0.838 * 106 * 3.14 / air gap length * effective specific permeance / pole pitch * (window space factor)2 * number of slots per pole per phase
b) dispersion coefficient = 0.838 * 106 * 3.14 * air gap length / effective specific permeance / pole pitch * (window space factor)2 * number of slots per pole per phase
c) dispersion coefficient = 0.838 * 106 * 3.14 * air gap length * effective specific permeance * pole pitch * (window space factor)2 * number of slots per pole per phase
d) dispersion coefficient = 0.838 * 106 * 3.14 * air gap length * effective specific permeance / pole pitch * (window space factor)2 * number of slots per pole per phase
Explanation: For the calculation of dispersion coefficient, first the air gap length, effective specific permeance is calculated. Next the pole pitch, window space factor and the number of slots per pole per phase.