BEE Electrical Machines 2 — Questions and Answers
Question 1: The power factor of a three-phase induction motor operating at no load is:
- Unity (1.0)
- Close to unity (0.9 lagging)
- Very low and lagging (0.1–0.2) (Correct answer)
- Very low and leading
Correct answer: Very low and lagging (0.1–0.2)
At no load the motor draws mainly reactive magnetizing current with very little active power, resulting in a very low lagging power factor.
Question 2: In a synchronous generator supplying a lagging power factor load, the effect of armature reaction on the air-gap flux is:
- Magnetizing (increases flux)
- Demagnetizing (reduces flux) (Correct answer)
- Cross-magnetizing (distorts flux)
- No effect on flux
Correct answer: Demagnetizing (reduces flux)
A lagging power factor load causes the armature MMF to oppose the main field MMF, producing a demagnetizing armature reaction.
Question 3: The rotor construction of a squirrel cage induction motor consists of:
- Copper windings connected to slip rings
- Short-circuited aluminum or copper bars embedded in slots (Correct answer)
- Wound rotor with external resistance connections
- Permanent magnets mounted on the rotor surface
Correct answer: Short-circuited aluminum or copper bars embedded in slots
A squirrel cage rotor has conducting bars (usually aluminum) embedded in rotor slots and short-circuited at both ends by end rings, requiring no external connections.
Question 4: Voltage regulation of a transformer is correctly expressed as:
- (VNL - VFL) / VNL × 100%
- (VFL - VNL) / VFL × 100%
- (VNL - VFL) / VFL × 100% (Correct answer)
- VNL / VFL × 100%
Correct answer: (VNL - VFL) / VFL × 100%
Voltage regulation = (VNL − VFL) / VFL × 100%, measuring the change in secondary voltage from full-load to no-load relative to rated (full-load) voltage.
Question 5: Speed control of a DC shunt motor can be achieved by:
- Varying armature resistance only
- Varying field current only
- Both armature resistance variation and field current variation (Correct answer)
- Varying the supply frequency only
Correct answer: Both armature resistance variation and field current variation
DC shunt motor speed N ∝ V/(φ), so speed can be controlled below base speed by adding armature resistance and above base speed by weakening the field (reducing field current).
Question 6: In a three-phase induction motor running at slip s, the frequency of the rotor currents is:
- f (supply frequency)
- s × f (Correct answer)
- f / s
- (1 - s) × f
Correct answer: s × f
The rotor cuts flux lines at slip speed (s × Ns), so the frequency of rotor currents is s × f, where f is the supply frequency.
Question 7: An over-excited synchronous motor (field current greater than normal) operates at:
- Unity power factor
- Lagging power factor
- Leading power factor (Correct answer)
- Zero power factor
Correct answer: Leading power factor
An over-excited synchronous motor generates more reactive power than it absorbs, acting like a capacitor and drawing leading current from the supply.
BEE Electrical Machines 2
The power factor of a three-phase induction motor operating at no load is: