A 4-pole induction motor is supplied at 50 Hz. Find synchronous speed.
A 1000 rpm
B 1500 rpm
C 750 rpm
D 3000 rpm
Answer: B. 1500 rpm. Synchronous speed is 120f divided by pole count, so 120 times 50 divided by 4 is 1500 rpm.
An induction motor has ns 1500 rpm and nr 1440 rpm. Find slip.
A 2 percent
B 6 percent
C 10 percent
D 4 percent
Answer: D. 4 percent. Slip is (ns minus nr) divided by ns; 60 divided by 1500 is 0.04, or 4 percent.
At 50 Hz and slip 0.04, what is rotor frequency?
A 2 Hz
B 0.5 Hz
C 4 Hz
D 50 Hz
Answer: A. 2 Hz. Rotor frequency equals slip times supply frequency: 0.04 times 50 Hz is 2 Hz.
A drive has J 4 kg mΒ² and net torque 12 Nm. Find acceleration.
A 3 rad/sΒ²
B 0.33 rad/sΒ²
C 16 rad/sΒ²
D 48 rad/sΒ²
Answer: A. 3 rad/sΒ². For constant net torque, angular acceleration equals net torque divided by inertia, so 12 divided by 4 is 3 rad/sΒ².
A motor delivers 7.5 kW at 1500 rpm. Estimate shaft torque.
A 23.9 Nm
B 47.7 Nm
C 75 Nm
D 95.5 Nm
Answer: B. 47.7 Nm. Angular speed is 1500 times 2Ο divided by 60, about 157.1 rad/s; torque is 7500 divided by this speed.
If a fan runs at half speed, what is its ideal torque ratio?
A One half
B One quarter
C One eighth
D Twice
Answer: B. One quarter. Fan torque is approximately proportional to speed squared, so half speed gives one quarter torque.
A fan uses 2 kW at rated speed. Ideal power at half speed is?
A 500 W
B 250 W
C 1 kW
D 4 kW
Answer: B. 250 W. Fan power is approximately proportional to speed cubed; 2 kW times one half cubed is 250 W.
At constant power, what happens to torque if speed halves?
A Torque doubles
B Torque halves
C Torque is unchanged
D Torque becomes zero
Answer: A. Torque doubles. For constant power, torque equals power divided by angular speed; halving speed doubles torque.
For a DC motor, what is back EMF proportional to?
A Current times resistance
B Torque times inertia
C Frequency divided by poles
D Flux times speed
Answer: D. Flux times speed. The generated back EMF of a DC motor is proportional to field flux and angular speed.
Below base speed, what commonly raises DC motor speed?
A Increase armature voltage
B Increase load inertia
C Reduce pole count
D Add rotor resistance
Answer: A. Increase armature voltage. With field flux held approximately constant, increasing armature voltage increases the motor speed below base speed.
At armature-current limit, what does field weakening reduce?
A Supply frequency
B Rotor inertia
C Pole count
D Available torque
Answer: D. Available torque. Motor torque is proportional to flux times armature current; weakening flux reduces torque at the same current limit.
What condition allows a DC motor drive to regenerate?
A Back EMF equals zero
B Field current is absent
C Back EMF exceeds terminal voltage
D Armature circuit is open
Answer: C. Back EMF exceeds terminal voltage. When back EMF exceeds the effective terminal voltage and the converter provides a return path, armature current can reverse for regeneration.
What signs describe forward regenerative braking?
A Positive speed, positive torque
B Negative speed, negative torque
C Positive speed, negative torque
D Zero speed, positive torque
Answer: C. Positive speed, negative torque. Forward braking has positive speed and torque opposite to rotation, placing it in the second quadrant.
What signs describe forward motoring?
A Positive speed, negative torque
B Negative speed, positive torque
C Positive speed, positive torque
D Negative speed, negative torque
Answer: C. Positive speed, positive torque. Forward motoring uses positive speed and positive torque, corresponding to the first quadrant.
What does added rotor resistance do in a wound-rotor motor?
A Changes pole count
B Raises slip at peak torque
C Raises synchronous speed
D Removes rotor current
Answer: B. Raises slip at peak torque. Increasing rotor resistance moves the maximum-torque point toward higher slip and can improve starting torque.
Why add voltage boost at low V/f frequency?
A Compensate stator resistance drop
B Increase pole count
C Eliminate rotor slip
D Reduce DC-link current
Answer: A. Compensate stator resistance drop. At low frequency, stator resistance drop becomes a larger share of applied voltage; a boost helps preserve air-gap flux.
What can happen under open-loop V/f control as load rises?
A Synchronous speed rises
B Speed drops with slip
C Rotor poles disappear
D Torque stays unlimited
Answer: B. Speed drops with slip. Open-loop V/f sets stator frequency but does not directly correct speed error, so increased load slip lowers rotor speed.
In common field-oriented control, which current component produces torque?
A d-axis voltage only
B Zero-sequence current
C DC-link ripple current
D q-axis current
Answer: D. q-axis current. With the d-axis aligned to flux, the q-axis current component mainly controls torque in the standard field-oriented model.
Which motor is commonly synchronized to stator electrical frequency?
A Synchronous motor
B Series DC motor
C Universal motor
D Shaded-pole motor
Answer: A. Synchronous motor. A synchronous motor rotor locks to the rotating stator field and runs at synchronous speed.
What can cause a stepper motor to lose commanded position?
A Missed steps under overload
B High insulation resistance
C Correct phase sequence
D Low rotor inertia
Answer: A. Missed steps under overload. If load torque exceeds available pull-out torque, the rotor may fail to follow pulses and lose position.
What produces torque in a switched-reluctance motor?
A Brush commutation
B Permanent-magnet repulsion
C Rotor seeks minimum reluctance
D Slip-ring current
Answer: C. Rotor seeks minimum reluctance. An energized stator phase pulls the salient rotor toward a position of lower magnetic reluctance.
What must a BLDC drive coordinate electronically?
A Bearing oil and load mass
B Pole count and line voltage
C Shaft seal and fan speed
D Phase current and rotor position
Answer: D. Phase current and rotor position. Electronic commutation switches phase currents according to rotor position to create continuous torque.
What should a traction drive often recover during deceleration?
A Battery heat only
B Gearbox lubricant
C Stator insulation
D Vehicle kinetic energy
Answer: D. Vehicle kinetic energy. Regenerative traction braking converts part of the vehicle's kinetic energy to electrical energy for storage or return.
What is the output of a speed-loop PI controller commonly limited by?
A Pole count only
B Bearing temperature only
C Current reference limit
D Line frequency only
Answer: C. Current reference limit. The speed controller often produces a torque-related current reference, which must respect motor and converter current limits.
Why is the current loop usually faster than the speed loop?
A Speed has no inertia
B Current is always constant
C Electrical dynamics are faster
D It removes motor feedback
Answer: C. Electrical dynamics are faster. Current responds on a faster electrical time scale, while speed changes through mechanical inertia, so the current loop is normally faster.