# Electrical Machines Questions and Answers – Measurement of Xd and Xq

This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Measurement of Xd and Xq”.

1. The d-axis reactance is determined by ______________
a) OCC & SCC
b) OCC
c) Slip test
d) SCC

Explanation: Both the OCC as well as short circuit test are used to calculate the d-axis reactance.

2. The slip test is used to determine ____________
a) Xq
b) Xd
c) Xd and Xq
d) None of the mentioned

Explanation: Slip test is used to determine the q-axis reactance.

3. During slip test it would be observed that ____________

```i) Swing of ammeter is wide
ii) Swing of voltmeter is  wide
```

a) i is true, ii is false
b) i is false, ii is true
c) both true
d) both false

Explanation: The impedance during slip test is low so the voltage drop will be less and the current will be more.

4. Slip test must be conducted at low armature terminal voltage. This is due to ____________
a) avoiding error due to large slip
b) avoiding error due to small slip
c) avoiding heating
d) all of the mentioned

Explanation: Slip test is conducted at low terminal voltage to avoid large slip in the synchronous machines.
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5. A 3 Phase 400, 100 MVA alternator is connected to infinite bus bar. If the mechanical power input is more than the maximum reluctance power, the reluctance generator will lose synchronizm if ____________
a) field is open circuited
b) field is short-circuited
d) damper is removed

Explanation: If the excitation itself is made zero, then machine will stop.

6. If a cylindrical rotor synchronous generator, is connected to the IBB, if the field is made zero, then it behaves as ____________
a) induction generator for a short time
b) induction motor
c) synchronous motor
d) synchronous generator

Explanation: It will act as induction motor as there will be difference in the speed.

7. When a 3–phase alternator is suddenly short–circuited at its terminals, the initial value of the short–circuit current is limited by which one of the following?
a) x”d
b) x’d
c) xs
d) Sum of x”d, x’d and xs

Explanation: When suddenly the terminals of the synchronous machine will be short circuited, at that immediate instant it will be subtransient reactance.

8. An alternator with higher value of SCR has ____________
a) better voltage regulation and higher stability limit
b) poor voltage regulation and lower stability limit
c) poor voltage regulation and higher stability limit
d) better voltage regulation and lower stability limit

Explanation: SCR is directly proportional to voltage regulation. Even stability limit is directly proportional to SCR.
SCR = 1/X.

9. The phasor diagram of a synchronous machine connected to an infinite bus is shown in the figure.

The machine is acting as
a) generator and operating at leading pf
b) generator and operating at lagging pf
c) motor and operating at leading pf
d) motor and operating at lagging pf

Explanation: The induced voltage leads the bus voltage, which means that it is at leading pf and then excitation magnitude is also more than terminal bus voltage.

10. If the excitation of a 3-phase alternator operating on infinite bus bars is changed, which one of the following will get affected?
a) Active real power of machine
b) Reactive power of machine
c) Terminal voltage of machine
d) Frequency of machine

Explanation: Excitation is coupled with the reactive power of the machine majorly.

11. In which one of the following is reluctance power developed?
a) Salient pole alternator
b) Non-salient pole alternator
c) Squirrel cage rotor type induction motor
d) All of the mentioned

Explanation: Because there is variable air gap due to saliency of the poles.

12. While conducting measurement for Xd and Xq, the information obtained is below.

```Id max = 10 A; Id min = 5 A
Vd max = 30 V; Vd min = 25 V
```

Which one of the following is correct?
a) Xd = 3, Xq = 2.5
b) Xd = 6, Xq = 3.86
c) Xd = 3, Xq = 3.86
d) Xd = 6, Xq = 2.5

Explanation: Xd=V(max)/I(min) = 30/5 = 6 ohms;
Xq=V(min)/I(max) = 25/10 = 2.5 ohms.

13. A synchronous motor is operated at a bus voltage of 1.5 pu and drawing 1.0 pu at zpf leading current. Its synchronous reactance is 0.4 pu. What is the excitation emf of the motor?
a) 2.4 pu
b) 1.5 pu
c) 1.0 pu
d) 1.9 pu

Explanation: E = V-I*Xs (Synchronous motor)
= 1.5 ∠0° – 1.0 ∠90° * 0.4∠90°
= 1.5 ∠0° – 0.4 ∠180°
= 1.5 ∠0° + 0.4 ∠0°
= 1.9 ∠0°

14. Consider a 3-phase alternator with negligible armature resistance and high rotational inertia delivering power to an isolated load when the armature terminals got short circuited. After sometime, the mechanical power input to the shaft of the machine is terminated at time t0. The short-circuit current circulating in the armature will change till the machine comes to halt at ‘ts’. Which one of the curves depicts this phenomena?
a)
b)
c)
d)

Explanation: The short circuit current will be a constant value till it flows.

15. A stationary alternator is connected to the infinite bus. It should not be connected because ____________
a) it will get short circuited
b) it will operate as induction motor
c) it will reduce bus bar voltage
d) all of the mentioned

Explanation: When a stationary synchronous machine is connected to the infinite bus, due to lack of synchronism there will be short circuit in the machine as the impedance reached very negligible.

16. Which of the options given will have no effect on the working of the synchronous motor if ignored?
a) It has comparable reactance
b) It requires dc excitation
c) It can not be started under no load
d) It has tendency to hunt

Explanation: The medium reactance is not any disadvantage to the synchronous machine.

17. When synchronous motor is running at synchronous speed, the damper winding produces?
a) no torque
b) eddy current torque
c) damping torque
d) torque aiding the developed torque

Explanation: As the relative speed will be zero in the damper winding and the torque too.

18. Slip test is performed to obtain _____________
a) direct axis reactance and quadrature axis reactance
b) slip
c) positive and negative sequence reactance
d) sub transient reactance

Explanation: Slip test is used to determine the direct axis and quadrature axis reactance.

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