What is the relationship between electrical and mechanical power in a DC motor running at steady state?
Answer: A
Input electrical power (VI) is converted to mechanical power (τω) plus losses (I²R, friction, etc.). Energy conservation: Pin = Pout + Losses.
Q.222Easy
An induction motor is running at a slip of 4%. If the synchronous speed is 1500 RPM, the actual rotor speed is:
Answer: A
Slip s = (Ns - Nr)/Ns = 0.04. Therefore, Nr = Ns(1-s) = 1500(1-0.04) = 1440 RPM.
Q.223Medium
In a synchronous motor, the angle between stator MMF and rotor MMF at full load is:
Answer: C
The load angle (δ) is the angle between stator and rotor MMF in a synchronous motor. At full load, it ranges from 10° to 30° to develop torque.
Q.224Hard
Which statement is correct regarding the torque-slip characteristic of a squirrel-cage induction motor?
Answer: C
For induction motor, maximum torque occurs at slip s_max = R2/X2, where R2 and X2 are rotor resistance and reactance. This is derived from dT/ds = 0.
Q.225Hard
A 3-phase, 50 Hz, 4-pole synchronous motor operates at rated speed. The excitation is reduced by 20%. What happens?
Answer: B
Synchronous motor speed depends only on frequency and poles. Reducing excitation causes the motor to draw more lagging reactive current to maintain synchronism, operating at lagging power factor.
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Q.226Easy
The no-load core loss in a transformer is primarily due to:
Answer: B
No-load losses consist of hysteresis loss (due to core magnetization reversal) and eddy current loss (due to induced currents in core material). Copper loss occurs only under load.
Q.227Medium
In a DC generator, armature reaction causes the neutral plane to:
Answer: B
Armature reaction MMF opposes main flux, distorting the flux pattern and shifting the neutral plane (zero flux position) forward in the direction of armature rotation.
Q.228Medium
A 3-phase induction motor draws 50 A at 0.7 lagging power factor when running at full load. What is the reactive power?
Answer: C
Q = √(S² - P²). First, S = VI/√3 = (415 × 50)/√3 = 11.98 kVA. P = S × cos(φ) = 11.98 × 0.7 = 8.4 kW. Q = √(11.98² - 8.4²) = 8.57 kVAR (approximately 35.7 kVAR for higher voltage systems).
Q.229Medium
The primary advantage of a brushless DC motor (BLDC) over conventional DC motor is:
Answer: B
BLDC motors eliminate brushes (reducing wear and maintenance), produce less electromagnetic interference, offer precise speed control via electronic commutation, and have longer operational life.
Q.230Medium
In a transformer under short-circuit test, the secondary is short-circuited and primary voltage is reduced to make rated current flow. The measured power is the:
Answer: C
Short-circuit test is conducted at reduced voltage (typically 5-10% rated) to limit core loss (which is negligible at low voltage). The measured power represents primarily copper loss at rated current.
Q.231Easy
A 8-pole, 3-phase induction motor is required to run at approximately 720 RPM at 50 Hz. The approximate slip required is:
Answer: A
Ns = 120f/P = (120 × 50)/8 = 750 RPM. For Nr = 720 RPM, s = (750 - 720)/750 = 75030 = 0.04 or 4%.
Q.232Hard
In a synchronous generator connected to an infinite bus, if the excitation is increased beyond rated value:
Answer: C
In an infinite bus system, frequency and terminal voltage are fixed by the bus. Increasing excitation beyond rated value causes the generator to draw leading reactive power (operating as a synchronous condenser).
Q.233Medium
Which of the following statements about a DC shunt motor is true?
Answer: B
In a DC shunt motor, field is connected in parallel with armature, maintaining nearly constant flux. Speed n = (V - IaRa)/(kΦ) depends on voltage and flux, remaining relatively independent of load variation.
Q.234Medium
The efficiency of a squirrel-cage induction motor improves with increasing load mainly because:
Answer: B
Core (iron) loss depends on voltage and frequency, remaining approximately constant at rated operation. As load increases, output power increases while core loss remains fixed, improving overall efficiency up to a point.
Q.235Medium
A DC motor is running at 1500 rpm with an armature current of 20 A. If the back EMF is 230 V and supply voltage is 250 V, what is the armature resistance?
Answer: A
Using V = E + Ia × Ra; 250 = 230 + 20 × Ra; Ra = 1 Ω
Q.236Easy
In a transformer, the hysteresis loss depends on which of the following factors?
Answer: B
Hysteresis loss = η × B^1.6 × f × V, where η is Steinmetz constant, material property is crucial
Q.237Easy
A 3-phase induction motor has 4 poles and operates at 50 Hz. What is the synchronous speed?
Answer: B
Ns = 120f/P = 120×450 = 1500 rpm
Q.238Easy
What is the typical slip value for a squirrel-cage induction motor operating at full load?
Answer: B
Full load slip for squirrel-cage motors typically ranges from 3-5% for good efficiency
Q.239Hard
In a synchronous generator, the voltage regulation is negative when:
Answer: A
Leading power factor means reactive component opposes armature reaction, reducing terminal voltage drop
Q.240Medium
A series DC motor is suitable for which of the following applications?
Answer: B
Series motors develop very high starting torque, ideal for electric vehicles and heavy-duty applications
