At 60 Hz, a 6-pole motor has what synchronous speed?
A 600 rpm
B 900 rpm
C 1200 rpm
D 1800 rpm
Answer: C. 1200 rpm. Synchronous speed equals 120f divided by poles, giving 120 times 60 divided by 6 or 1200 rpm.
A motor has ns 1200 rpm and nr 1180 rpm. Find slip.
A 0.83 percent
B 1.67 percent
C 2.5 percent
D 3.33 percent
Answer: B. 1.67 percent. Slip is 20 divided by 1200, or about 0.0167 and 1.67 percent.
A 15 kW motor runs at 1500 rpm. Estimate shaft torque.
A 95.5 Nm
B 47.7 Nm
C 75 Nm
D 150 Nm
Answer: A. 95.5 Nm. At 1500 rpm angular speed is about 157.1 rad/s; 15000 W divided by 157.1 rad/s is about 95.5 Nm.
A drive has J 0.5 kg m² and net torque 5 Nm. Find acceleration.
A 2.5 rad/s²
B 5 rad/s²
C 10 rad/s²
D 25 rad/s²
Answer: C. 10 rad/s². Angular acceleration equals net torque divided by inertia: 5 divided by 0.5 is 10 rad/s².
A 50 Hz induction motor has slip 0.03. Find rotor frequency.
A 0.03 Hz
B 1.5 Hz
C 3 Hz
D 50 Hz
Answer: B. 1.5 Hz. Rotor frequency is slip times stator frequency, so 0.03 times 50 Hz is 1.5 Hz.
Neglecting stator impedance, what sets maximum-torque slip approximately?
A Rotor resistance over reactance
B Stator voltage over poles
C Load torque over inertia
D Frequency over voltage
Answer: A. Rotor resistance over reactance. In the simplified induction-motor equivalent circuit, maximum torque occurs near slip R2 divided by standstill rotor reactance X2.
With stator effects neglected, what does higher rotor resistance do?
A Raises synchronous speed
B Eliminates peak torque
C Raises peak-torque slip
D Lowers pole count
Answer: C. Raises peak-torque slip. Increasing rotor resistance shifts the maximum-torque point to higher slip while the simplified model predicts nearly unchanged maximum torque.
Above base speed, what is a common induction-drive region?
A Constant flux without limit
B Constant torque without limit
C Zero-current operation
D Approximately constant power
Answer: D. Approximately constant power. Field weakening above base speed reduces flux and torque capability while many drives operate approximately at constant power.
Which signs identify reverse motoring?
A Negative speed, positive torque
B Negative speed, negative torque
C Positive speed, negative torque
D Positive speed, positive torque
Answer: B. Negative speed, negative torque. Reverse motoring occurs when torque and speed are both negative, the third quadrant.
Which signs identify reverse regenerative braking?
A Negative speed, positive torque
B Negative speed, negative torque
C Positive speed, positive torque
D Positive speed, negative torque
Answer: A. Negative speed, positive torque. In reverse braking, positive torque opposes negative rotation, corresponding to the fourth quadrant.
What happens to energy in resistor-based dynamic braking?
A It always charges battery
B It raises line frequency
C It becomes heat
D It becomes shaft torque
Answer: C. It becomes heat. The motor generates during braking and the resistor dissipates the electrical energy as heat.
Where does regenerative energy go in a receptive drive system?
A Supply or DC-link storage
B Motor bearings
C Gate resistor only
D Open circuit
Answer: A. Supply or DC-link storage. Regenerated energy must flow to a receptive supply, storage element or braking circuit; otherwise the DC-link voltage can rise.
What must a thyristor converter inverter preserve for line commutation?
A Zero DC current
B Infinite firing delay
C Open AC supply
D Adequate extinction angle
Answer: D. Adequate extinction angle. The outgoing thyristor needs sufficient reverse-bias recovery time before the AC voltage reverses again.
Why use an I²t or thermal overload model?
A Protect against heating accumulation
B Set synchronous speed
C Increase motor torque
D Change phase sequence
Answer: A. Protect against heating accumulation. Thermal protection reflects how current magnitude and duration accumulate heating in motor windings and devices.
What is the local stability condition at a drive operating point?
A Net torque slope is positive
B Net torque slope is negative
C Motor torque is zero always
D Load torque is negative always
Answer: B. Net torque slope is negative. A small speed increase should create a restoring negative net torque change, so the slope of motor torque minus load torque is negative.
At half speed, how does ideal fan power compare?
A One quarter rated power
B One eighth rated power
C Half rated power
D Twice rated power
Answer: B. One eighth rated power. Fan power scales approximately with the cube of speed, so half speed uses one eighth the power in the ideal affinity relation.
A constant-power load takes 12 kW at 300 rad/s. Find torque.
A 20 Nm
B 60 Nm
C 90 Nm
D 40 Nm
Answer: D. 40 Nm. Torque equals power divided by angular speed: 12000 W divided by 300 rad/s is 40 Nm.
A DC motor has V 220 V, Ia 20 A, Ra 0.5 Ω. Find back EMF.
A 200 V
B 210 V
C 219 V
D 230 V
Answer: B. 210 V. Ignoring brush drop, back EMF equals terminal voltage minus armature resistive drop: 220 minus 20 times 0.5 is 210 V.
At fixed back EMF, flux falls to 80 percent. What happens to speed?
A Falls by 20 percent
B Rises by 20 percent
C Stays unchanged
D Rises by 25 percent
Answer: D. Rises by 25 percent. Since back EMF is proportional to flux times speed, speed varies inversely with flux; dividing by 0.8 gives a 25 percent rise.
A 6-pole, 50 Hz motor runs at 970 rpm. Find slip.
A 1 percent
B 2 percent
C 6 percent
D 3 percent
Answer: D. 3 percent. Its synchronous speed is 1000 rpm; slip is (1000 minus 970) divided by 1000, or 3 percent.
For the motor above, what is rotor frequency?
A 0.5 Hz
B 3 Hz
C 1.5 Hz
D 50 Hz
Answer: C. 1.5 Hz. Rotor frequency equals slip times supply frequency: 0.03 times 50 Hz is 1.5 Hz.
In standard field-oriented control, what mainly sets flux?
A d-axis current
B q-axis current
C Zero-sequence voltage
D DC-link ripple
Answer: A. d-axis current. With the reference frame aligned to rotor flux, the d-axis current primarily controls flux.
In standard field-oriented control, what mainly sets torque?
A d-axis current
B Carrier frequency
C q-axis current
D Rotor resistance
Answer: C. q-axis current. The q-axis current is approximately torque-producing when the d-axis is aligned to the flux vector.
Why is low-frequency voltage boost used in scalar V/f control?
A Increase synchronous speed
B Reduce pole number
C Eliminate rotor slip
D Offset stator resistance drop
Answer: D. Offset stator resistance drop. At low frequency, stator resistance drop can reduce air-gap flux, so voltage boost compensates for it.
What is needed for a VFD to return braking energy to the grid?
A Regenerative front end
B A diode bridge only
C An open DC link
D A fixed resistor only
Answer: A. Regenerative front end. A diode-front-end drive cannot normally send power back to the grid; regeneration needs a receptive DC bus, storage or active front end.