WorksheetsElectrical Drives Quiz - Intermediate Level
Total questions: 20
Worksheet time: 10mins
In vector control of an induction motor, what is the role of rotor flux orientation?
To align torque with slip
To decouple torque and flux control
To reduce harmonics
To enable synchronous braking
You’re implementing closed-loop speed control in a DC motor. What component is essential for feedback?
Capacitor
Tachogenerator or encoder
Contactor
Resistor
A VFD drive connected to an induction motor is producing torque ripple. Most likely cause?
High modulation index
Improper carrier frequency
Zero slip
Low temperature
In regenerative braking using a four-quadrant chopper drive, which component manages power flow back to the supply?
Freewheeling diode
Step-down converter
Reversible switch matrix or controlled bridge
MOV
In a multi-motor hoisting system, speed mismatches are observed. What solution ensures equal torque sharing?
Separate VFDs with encoder feedback and master-slave synchronization
Same rated power
Increase slip
Use one large motor
Why is field weakening used in high-speed operation of drives?
To reduce voltage
To increase field flux
To allow operation beyond base speed at constant power
To improve starting torque
A synchronous motor lags at light load. How can you correct power factor?
Use damper windings
Increase stator current
Increase excitation (over-excited mode)
Increase rotor resistance
Which modulation strategy in inverters improves motor efficiency and reduces THD?
Square wave
Harmonic injection
Sinusoidal PWM (SPWM)
Cycloconverter
In field-oriented control, what quantity is transformed to dq reference frame?
Voltage only
Current and voltage
Phase currents and rotor flux
Speed only
Why is slip important in an induction motor under load?
It reduces starting torque
It allows rotor to generate torque
It increases power factor
It controls frequency
A drive fails to start the motor and shows “undervoltage” fault. What might be the root cause?
Low PWM frequency
Load torque mismatch
Weak input supply or bad rectifier
Excessive motor inertia
In regenerative braking for elevator drive systems, what ensures safety in case of power loss?
V/f curve
Anti-rollback relay
Battery backup or dynamic brake resistor
Auto-start
A DC motor controller shows erratic armature current. Which part should be checked?
PWM frequency
Armature reaction compensator
Commutator and brushes
Field current
What happens if the encoder connected to a drive fails during operation?
Motor stops instantly
Drive switches to open-loop or faults out
Motor runs faster
Drive switches direction
In servo drives, what limits overshoot in position control?
High inertia
Feedforward gain
Proportional-integral-derivative (PID) tuning
Phase converter
What determines maximum torque capability in a motor under V/f control?
Flux weakening
Motor core length
Constant voltage-to-frequency ratio
Load inertia
Your application requires a motor to maintain constant torque at low speeds. Best method?
Open loop V/f
Current control loop with torque feedback
Soft starter
Voltage boost circuit
A synchronous motor used with VFD becomes unstable at low frequency. Why?
Field winding is energized
Voltage/frequency mismatch causes insufficient torque
High inertia
Rotor misalignment
A chopper drive uses a high switching frequency. What is the likely advantage?
Larger filter size
Better efficiency and reduced ripple
Higher losses
Higher current
Which part of the drive system is most responsible for dynamic performance in closed-loop systems?
Motor frame
Sensor fusion
Controller tuning (PID, adaptive)
Armature resistance
