This set of Design of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “No Load Current”.
1. How many methods are present to obtain all the machine performance characteristics?
a) 3
b) 2
c) 1
d) 4
View Answer
Explanation: There are 2 methods in obtaining all the open circuit characteristics. They are no load characteristics and short circuit characteristics.
2. How many components does the no load current characteristics comprise of?
a) 2
b) 3
c) 4
d) 1
View Answer
Explanation: There are 2 main components under the no load current. They are Magnetizing current and Loss component of current.
3. How is the Magnetizing component with respect to the voltage?
a) the magnetizing component is in phase with the voltage
b) the magnetizing component is 90° leading the voltage
c) the magnetizing component is 90° lagging the voltage
d) the magnetizing component is 90° out of phase with the voltage
View Answer
Explanation: The magnetizing current component is 90° out of phase with the voltage. The loss component is in phase with the voltage.
4. How many parts does the flux produced by stator mmf passes through?
a) 3
b) 4
c) 5
d) 6
View Answer
Explanation: The flux produced by stator mmf passes through 5 parts. They are air gap, rotor teeth, rotor core, stator teeth, stator core.
5. The flux is distributed sinusoidally and the mmf varies sinusoidally in a DC Machine.
a) true
b) false
View Answer
Explanation: In a DC Machine, the flux is assumed to be uniform over any cross section and the same mmf for all the paths. But in an induction machine, the flux is distributed sinusoidally, and the mmf varies sinusoidally.
6. What factors does the value of magnetizing current depend on?
a) flux tube
b) output power
c) mean mmf
d) mean mmf and flux tube
View Answer
Explanation: If the permeability of iron were constant this would cause no difficulty. The value of magnetizing current would be accurately obtained by considering the mean mmf and the flux tube where this mean occurs.
7. When maximum values of the design factors are considered, what is the relation between flux and the magnetizing current?
a) flux is directly proportional to the magnetizing current
b) flux is indirectly proportional to the magnetizing current
c) flux is directly proportional to square of the magnetizing current
d) flux is indirectly proportional to square of the magnetizing current
View Answer
Explanation: The flux value is indirectly proportional to the magnetizing current. The flux is too small or rather the magnetizing current becomes high.
8. At what angle with respect to the interpolar axis does the flux tube gives a good approximation?
a) 30°
b) 45°
c) 60°
d) 90°
View Answer
Explanation: The flux tube crossing the air gap at 60° from the interpolar axis will always give a good approximation. The calculation of the magnetizing mmf should be based upon the value of the flux density at 60° from the interpolar axis.
9. What is the formula for mmf for air gap?
a) mmf for air gap = 800000 * air gap flux density * air gap factor * length of air gap
b) mmf for air gap = 800000 / air gap flux density * air gap factor * length of air gap
c) mmf for air gap = 800000 * air gap flux density / air gap factor * length of air gap
d) mmf for air gap = 800000 * air gap flux density * air gap factor / length of air gap
View Answer
Explanation: For calculating the mmf for air gap, the air gap flux density is first calculated. Next the air gap factor is calculated along with the length of air gap.
10. What is the formula for the mmf required for stator teeth?
a) mmf required for stator teeth = mmf per metre + depth of stator slots
b) mmf required for stator teeth = mmf per metre * depth of stator slots
c) mmf required for stator teeth = mmf per metre / depth of stator slots
d) mmf required for stator teeth = mmf per metre – depth of stator slots
View Answer
Explanation: First the mmf per meter is obtained separately from its design equation. Then the depth of the stator slots is obtained and the product of both gives mmf required for stator teeth.
11. 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
View Answer
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.
12. 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
View Answer
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.
13. 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%
View Answer
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.
14. 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%
View Answer
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.
15. The no load power factor is the ratio of full load current to no load current.
a) true
b) false
View Answer
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.
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