# Design of Electrical Machines Questions and Answers – Synchronous Machines Output Equation

This set of Design of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Synchronous Machines Output Equation”.

1. What is the formula for output equations in synchronous machines?
a) kVA output = output coefficient * diameter2 * length * synchronous speed
b) kVA output = output coefficient / diameter2 * length * synchronous speed
c) kVA output = output coefficient * diameter2 / length * synchronous speed
d) kVA output = output coefficient * diameter2 * length / synchronous speed

Explanation: The output equation is found out by first calculating the output coefficient. Next, the diameter and length are obtained, and the synchronous speed is calculated using the tacho-generator to obtain the kVA output.

2. What is the formula of the output coefficient?

Explanation: The output coefficient is one of the terms required in the calculation of the output of the machine. The specific magnetic and electrical loading terms are first calculated along with the winding space factor.

3. What is the formula for the output equation with respect to the peripheral speed?
a) output = 1.11* specific magnetic loading * specific electrical loading * winding space factor * 10-3 * peripheral speed2 *Length * synchronous speed
b) output = 1.11* specific magnetic loading * specific electrical loading * winding space factor * 10-3 * peripheral speed2 *Length / synchronous speed
c) output = 1.11* specific magnetic loading * specific electrical loading * winding space factor / 10-3 * peripheral speed2 *Length * synchronous speed
d) output = 1.110 / specific magnetic loading * specific electrical loading * winding space factor * 10-3 * peripheral speed2 *Length * synchronous speed

Explanation: The output equation with respect to the peripheral speed depends on the square of the peripheral speed of the machine. It doesn’t consist of the diameter term in the output equation.

4. How many factors does the choice of specific magnetic loading depend upon?
a) 4
b) 2
c) 5
d) 8

Explanation: The choice of specific magnetic loading depends upon 5 factors basically. They are a) Iron Loss, b) Voltage, c) Transient short circuit current, d) Stability, e) Parallel Operation.

5. How is the iron loss related with the choice of specific magnetic loading?
a) choice of magnetic loading is directly proportional to the iron loss
b) choice of magnetic loading is indirectly proportional to the iron loss
c) choice of magnetic loading is directly proportional to the square of the iron loss
d) choice of magnetic loading is indirectly proportional to the square of the iron loss

Explanation: The choice of specific magnetic loading is directly proportional to the iron loss. The iron loss increases with the increase in the air gap density.
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6. How is the voltage related with the air gap density?
a) air gap density is directly proportional to the voltage
b) air gap density is indirectly proportional to the voltage
c) air gap density is directly proportional to the square of the voltage
d) air gap density is indirectly proportional to the square of the voltage

Explanation: The air gap density is indirectly proportional to the voltage. High voltage machine should have low air gap density, to avoid excessive values of flux density in the teeth and core.

7. How is the transient short circuit current related with the air gap density?
a) air gap density is directly proportional to the short circuit current
b) air gap density is indirectly proportional to the short circuit current
c) air gap density is directly proportional to the square of the short circuit current
d) air gap density is directly proportional to the square of the short circuit current

Explanation: The air gap density is directly proportional to the short circuit current. The air gap density should be kept low in order to reduce the initial electromagnetic forces under short circuit condition.

8. How is the steady state stability related with the air gap density?
a) air gap density is directly proportional to the steady state stability
b) air gap density is indirectly proportional to the steady state stability
c) air gap density is directly proportional to the square of the steady state stability
d) air gap density is directly proportional to the square of the steady state stability

Explanation: The air gap density is directly proportional to the steady state stability. The steady state stability is improved if the air gap density is high.

9. The machines having high air gap density operates poorly when connected in synchronism.
a) true
b) false

Explanation: The machines having high air gap density allows high amount of synchronizing power. Thus the machines having high air gap density provides high synchronism.

10. What is the range of the air gap density for salient pole machines?
a) 0.52-0.65 Wb per m2
b) 0.5-0.6 Wb per m2
c) 0.54-0.65 Wb per m2
d) 0.44-0.65 Wb per m2

Explanation: The range of air gap density for salient pole machines is 0.52-0.65 Wb per m2. The range of air gap density for turbo-alternators is 0.54-0.65 Wb per m2.

a) 2
b) 3
c) 4
d) 5

Explanation: There are 4 factors that influence the choice of specific electric loading. They are a) Copper loss and temperature rise, b) voltage, c) synchronous reactance, d) stray load loss.

12. How is the specific electric loading related to copper losses and temperature rise?
a) high specific electric loading gives high copper losses and high temperature rise
b) high specific electric loading gives low copper losses and high temperature rise
c) high specific electric loading gives high copper losses and low temperature rise
d) high specific electric loading gives low copper losses and low temperature rise

Explanation: The specific electric loading is directly proportional to the copper losses and the temperature rise. The high specific electric loading gives high copper losses and high temperature rise.

13. High value of the specific electric loading can be used for low voltage machines.
a) true
b) false

Explanation: High value of specific electric loading can be used for low voltage machines. This is because the space required for insulation is small.

14. How is the specific electric loading related to the synchronous reactance of the machines?
a) specific electric loading is high, leakage reactance is high, giving low synchronous reactance
b) specific electric loading is high, leakage reactance is low, giving low synchronous reactance
c) specific electric loading is high, leakage reactance is high, giving high synchronous reactance
d) specific electric loading is low, leakage reactance is high, giving high synchronous reactance

Explanation: The specific electric loading is directly proportional to the synchronous reactance. If the specific electric loading is high, the synchronous reactance becomes high.

15. What is the value of specific electric loading for the salient pole alternators?
a) 20,000-40,000 A per m
b) 50,000-75,000 A per m
c) 25,000-40,000 A per m
d) 20,000-45,000 A per m

Explanation: The value of specific electric loading for the salient pole alternators is 20,000-40,000 A per m. The value of specific electric loading for the turbo alternators is 50,000-75,000 A per m.

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