Answer: C. Motor torque minus load torque. Neglecting other effects, accelerating torque is motor torque minus opposing load torque.
What does load inertia resist?
A Change in line voltage
B Change in pole count
C Change in rotational speed
D Change in insulation
Answer: C. Change in rotational speed. Rotational inertia resists changes in angular speed, just as mass resists changes in linear velocity.
A fan load commonly has torque proportional to what?
A Speed only
B Inverse speed
C Constant voltage
D Speed squared
Answer: D. Speed squared. For many centrifugal fans, load torque rises approximately with the square of speed.
For a fan, how does ideal shaft power scale with speed?
A Approximately speed only
B Approximately speed cubed
C Inverse speed squared
D Constant power
Answer: B. Approximately speed cubed. If fan torque is proportional to speed squared, shaft power torque times speed is proportional to speed cubed.
What is a constant-torque load example?
A Conveyor belt
B Centrifugal fan
C Centrifugal pump
D Windage load
Answer: A. Conveyor belt. A conveyor often needs roughly constant torque over a useful speed range, though its exact profile depends on the process.
What does a constant-power load require as speed rises?
A Higher torque
B Lower torque
C Higher inertia
D Zero current
Answer: B. Lower torque. At constant power, torque equals power divided by speed, so torque decreases as speed rises.
How can a separately excited DC motor run above base speed?
A Reduce field flux
B Increase pole count
C Reverse armature polarity
D Raise armature resistance
Answer: A. Reduce field flux. Field weakening raises the speed range above base speed while reducing available torque.
What is a risk of running a DC series motor unloaded?
A Zero armature current
B Dangerously high speed
C Fixed synchronous speed
D Negative pole count
Answer: B. Dangerously high speed. A lightly loaded series motor can draw little current, weaken its field and accelerate to a damaging speed.
What does a DC shunt motor have in normal operation?
A No field winding
B Series-only field
C Alternating rotor field
D Nearly constant field flux
Answer: D. Nearly constant field flux. A shunt field is connected across the supply and normally produces approximately constant field flux.
What is the rotor speed of a synchronous motor in steady state?
A Synchronous speed
B Always below synchronous speed
C Twice synchronous speed
D Zero under load
Answer: A. Synchronous speed. A synchronous motor rotor locks to the rotating stator field and runs at synchronous speed in steady operation.
What provides commutation in a brushless DC motor?
A Mechanical commutator
B Slip rings only
C Electronic switching
D Centrifugal switch
Answer: C. Electronic switching. A BLDC drive electronically switches stator currents based on rotor position or an estimate of it.
What is a key use of vector control?
A Separate flux and torque control
B Remove the inverter
C Fix motor speed
D Replace all sensors
Answer: A. Separate flux and torque control. Field-oriented control transforms currents so flux-producing and torque-producing components can be controlled separately.
In induction-motor scalar control, what is commonly held roughly constant?
A Rotor resistance
B Shaft torque
C Voltage-to-frequency ratio
D Stator current angle
Answer: C. Voltage-to-frequency ratio. Scalar V/f control adjusts voltage with frequency to keep air-gap flux approximately constant below base speed.
What does increasing induction-motor slip generally increase?
A Rotor frequency
B Synchronous speed
C Pole count
D DC-link voltage
Answer: A. Rotor frequency. Rotor electrical frequency equals slip times stator supply frequency.
What changes when induction-motor phase sequence is reversed?
A Pole count
B Supply frequency magnitude
C Rotor resistance
D Rotating-field direction
Answer: D. Rotating-field direction. Reversing two stator phases reverses the rotating magnetic field and can reverse motor torque direction.
What does rotor-resistance control apply to a slip-ring motor?
A Stator capacitor only
B DC-link resistor
C Field weakening current
D External rotor resistance
Answer: D. External rotor resistance. External resistance can be inserted in a wound-rotor induction motor circuit to alter its torque-speed behavior.
What is a dual converter useful for in a DC drive?
A Reversible armature voltage
B Fixed speed only
C Removing field current
D Changing shaft inertia
Answer: A. Reversible armature voltage. A dual converter can provide reversible armature voltage and support motoring and regenerative operation in both directions.
What does a four-quadrant chopper allow?
A Unidirectional current only
B Fixed voltage only
C Bidirectional voltage and current
D AC frequency conversion
Answer: C. Bidirectional voltage and current. A four-quadrant chopper can control both voltage and current polarity, enabling motoring and braking in either rotation direction.
What does a flywheel do in load equalization?
A Smooths power demand
B Changes motor poles
C Blocks regeneration
D Raises line frequency
Answer: A. Smooths power demand. A flywheel stores energy during low-demand intervals and releases it during peaks, smoothing the source demand.
What is induction-motor regenerative braking condition?
A Rotor below standstill
B Rotor above synchronous speed
C Slip equals one
D Stator supply disconnected
Answer: B. Rotor above synchronous speed. When rotor speed exceeds synchronous speed, slip becomes negative and the induction machine can generate if the drive can return power.
What is the usual energy path in dynamic braking?
A Supply to rotor heat
B Motor to resistor heat
C Load to battery only
D Gate to stator
Answer: B. Motor to resistor heat. During dynamic braking, the motor generates and the braking resistor dissipates the returned energy as heat.
Why must plugging current be limited?
A It removes all flux
B Reversal can draw large current
C It guarantees zero torque
D It lowers supply frequency
Answer: B. Reversal can draw large current. Applying reverse torque while the motor is still moving can produce a large current, so current limiting is important.
What does an inner current controller often receive from a speed loop?
A Bearing temperature limit
B Pole-count command
C Load inertia estimate
D Torque-related current reference
Answer: D. Torque-related current reference. The outer speed controller commonly requests a torque-producing current, which the faster inner current loop regulates.
What does a traction drive commonly need at launch?
A Zero starting torque
B Fixed low current
C No braking capability
D High starting torque
Answer: D. High starting torque. Vehicle traction drives need substantial low-speed torque for acceleration and often also need regenerative braking.
What does motor nameplate base frequency mark?
A Zero torque point
B Rotor standstill point
C Rated-frequency operating point
D Thermal trip point
Answer: C. Rated-frequency operating point. Base frequency is typically the rated-frequency point up to which rated voltage and flux can be maintained.