WorksheetsElectrical Drives Situational-Based MCQs - Advanced to Expert
Total questions: 40
Worksheet time: 20mins
You are designing a high-speed spindle motor for CNC. Which control method ensures precision and stability?
Scalar control (V/f)
Field-Oriented Control (FOC) with encoder feedback
Open-loop PWM
Phase-angle control
A high-speed drive exhibits oscillations during acceleration. What control strategy can help stabilize it?
Increase PWM frequency
Apply notch filter and feedforward control
Use soft starter
Use DTC
A traction motor requires torque boost during low-speed startup. Best solution?
High rotor inertia
Voltage boost at low frequency or flux compensation
Phase reversal
Pulse injection
Your regenerative elevator drive fails under peak load. Likely reason?
Motor too large
DC link overvoltage and inadequate braking resistor sizing
Excessive slip
Bad alignment
In sensorless vector control, what can cause instability at low speeds?
Over-modulation
Inaccurate rotor flux estimation due to low back EMF
High V/f ratio
Encoder noise
In a PMSM drive with FOC, what causes torque ripple at high speed?
Demagnetization
Improper current vector angle control
Low frequency noise
Low carrier modulation
A BLDC motor used in robotics stalls under sudden load. Recommended strategy?
Increase speed
Implement current limit and acceleration ramping
Use DTC
Bypass sensor
During motor coasting, you notice slow deceleration. What would improve braking performance?
Lower frequency
Short motor leads
Dynamic braking using resistor or regenerative inverter
Lower DC link voltage
For torque-sharing between motors on a conveyor, which approach is best?
Equal V/f drive
Master-slave vector control with load balancing
DOL start
Use star-delta
A servo drive overshoots position at every cycle. What tuning change helps?
Reduce integral gain
Increase derivative gain to damp response
Increase proportional gain only
Reduce encoder resolution
For a DC drive using an armature chopper, increasing switching frequency has what effect?
More torque
Less torque
Smoother current waveform and lower ripple
Higher inductance
A DTC (Direct Torque Control) scheme exhibits high torque ripple. What can reduce it?
Coarse lookup tables
Using space vector modulation (SVM)
PWM off
Increase motor size
Which technique allows a PMSM to operate above base speed without weakening magnets?
Increasing field current
Field weakening through vector control
DC link bypass
Pulse injection
What is the effect of improper commutation in BLDC motor drive?
Increase in power factor
Torque drop and high acoustic noise
Over-excitation
Excessive flux
A multi-motor drive system shows phase imbalance. Which diagnostic step is most useful?
Measure neutral shift
Perform FFT of line currents to detect asymmetry and harmonics
Use IR thermometer
Increase load
In a servo positioning system, what reduces error due to frictional dead zone?
Higher encoder resolution
Feedforward compensation and deadband tuning
Stiff shaft
Longer cables
A DC series motor shows armature saturation at high current. What issue results?
Speed increase
Reduced torque per ampere and heating
Increased resistance
Commutator misfire
What is the purpose of anti-resonance filters in drive systems?
Limit voltage
Dampen mechanical oscillations caused by elastic shafts
Improve cos(φ)
Reduce harmonics
In an electric vehicle, drive torque suddenly drops at full throttle. What is the likely cause?
Regeneration
Thermal protection triggering due to excessive inverter IGBT temperature
MOSFET reverse recovery
Encoder fault
For applications requiring highly dynamic torque control (e.g., robotics), which control is best?
Scalar
Open-loop vector
Closed-loop vector or DTC with torque feedforward
V/f with PID
In a high-performance servo system, backlash is causing limit cycle oscillations. What’s the best compensator?
Integrator
Feedforward term
Deadband compensator or backlash model in position controller
Increased derivative gain
In sensorless control of induction motors at zero speed, rotor position cannot be determined. What’s a viable solution?
Voltage estimation
Signal injection (e.g., high-frequency injection)
Load torque observer
Increase V/f
A PMSM drive shows torque pulsations due to cogging. What mitigation technique is best?
Use skewed rotor or optimized winding slots
Increase stator resistance
Apply scalar control
Increase DC link voltage
A multilevel inverter-fed motor shows overmodulation. What advanced PWM strategy can help?
Fixed frequency PWM
Third harmonic injection PWM or Space Vector PWM
Pulse skip modulation
Square wave PWM
A high-speed BLDC drone motor emits EMI spikes on switching. Mitigation strategy?
Increase switching frequency
Use shielded cables and RC snubbers at FETs
Increase inductance
Use unipolar switching
In a matrix converter-fed drive, what is the key advantage?
Voltage boost
Bidirectional power flow and no DC link
Reduced speed
Lower slip
A load with strong torsional resonance interacts with the drive controller. What tuning strategy is used?
PID tuning only
Notch filters in speed loop or adaptive damping control
Increase inertia
Voltage boost
Which control technique allows sensorless field orientation without flux observer at medium speed?
Open-loop scalar
Back EMF estimation using real-time stator voltage equations
Resolver feedback
V/f linearization
In synchronous reluctance motors, what improves torque density?
Saliency ratio (Ld/Lq) optimization
Decrease rotor weight
Increase friction
Use Hall sensors
A drive uses real-time load torque observer to enhance current loop. What is the benefit?
Reduce voltage ripple
Compensate for sudden load changes with better dynamic response
Increase switching loss
Lower DC bus capacitance
For very low-speed, high-torque drives (e.g., robotics), which control method prevents cogging torque issues?
SPWM
Sensorless DTC
Current hysteresis control with cogging model and PI gain adaptation
V/f fixed ratio
A high-end servo fails position accuracy due to encoder drift. Which modern solution corrects this?
Encoder offset calibration
Observer-based correction using Kalman Filter
Add another encoder
Lower PWM
A wind turbine's drive trains have large inertia mismatch. What's the best method to decouple them?
Scalar open-loop
Model-based torque feedforward control with speed decoupling
V/f ramp
Short circuit DC bus
What can be inferred if motor speed oscillates even at steady load in a drive system?
Commutation is optimal
Controller gain too high or not tuned
Stable loop
Power factor is high
To suppress common-mode noise in high-frequency drives, what technique is used?
Delta connection
Common-mode chokes and split grounding
Lowering slip
Increasing rotor time constant
In dual-motor drives with equal loading, mismatch in speed occurs. What’s best remedy?
Dual PID with master-slave synchronization and cross-coupling loop
Fixed gain tuning
Larger motor
No feedback
A regenerative drive shares energy with another motor. What’s this system called?
Parallel braking
DC-bus energy sharing or common DC link system
Split-phase braking
Line-commutated rectification
In FOC, when the d-axis and q-axis currents are not properly decoupled, what results?
Higher torque
Loss of independent torque-flux control and performance degradation
Zero slip
Increased mechanical noise only
In EV applications, sudden torque reversal is needed. Which drive feature enables this?
Star-delta switching
Four-quadrant operation with current vector inversion
Phase delay
V/f boost
What’s a modern method to predict drive failure before it occurs?
Overcurrent trip
Predictive maintenance using data-driven AI models and vibration signatures
Stop motor at intervals
Change encoder
