33 Drives & Motor Control questions from the Electrical Design bank, written for Indian campus drives and tech interviews. Every question has a verified answer and an AI-tutor explanation on placd.
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A.ABB scheme selecting inverter voltage vectors from hysteresis comparators on torque and flux estimates without a PWM modulator, torque response under 5 ms
B.common-mode voltage from PWM drives electrical discharge through bearings, mitigated by insulated bearings, shaft grounding brushes and common-mode chokes
C.motor acts as generator returning energy to the DC bus; a diode front end needs a braking chopper and resistor while an active front end returns it to mains
D.rectifier, DC link and PWM inverter that vary motor speed by adjusting both frequency and voltage, giving cube-law energy savings on fans and pumps
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2. Which term means: "rectifier, DC link and PWM inverter that vary motor speed by adjusting both frequency and voltage, giving cube-law energy savings on fans and pumps"?
Junior
A.Soft starter
B.Field-oriented control
C.V/f control
D.Variable frequency drive
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3. Which statement is correct?
Junior
A.Variable frequency drive — thyristor phase control that ramps motor voltage to reduce starting current and torque, then is bypassed; it gives no speed control unlike a VFD
B.Variable frequency drive — rectifier, DC link and PWM inverter that vary motor speed by adjusting both frequency and voltage, giving cube-law energy savings on fans and pumps
C.Variable frequency drive — keeps the voltage-to-frequency ratio constant to hold air-gap flux below base speed, with voltage boost at low frequency to overcome stator drop
D.Variable frequency drive — IEC 61800-5-2 drive safety function that removes gate pulses to prevent torque without opening a contactor, capable up to SIL 3
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4. What is V/f control?
Junior
A.keeps the voltage-to-frequency ratio constant to hold air-gap flux below base speed, with voltage boost at low frequency to overcome stator drop
B.motor acts as generator returning energy to the DC bus; a diode front end needs a braking chopper and resistor while an active front end returns it to mains
C.IEC 61800-5-2 drive safety function that removes gate pulses to prevent torque without opening a contactor, capable up to SIL 3
D.ABB scheme selecting inverter voltage vectors from hysteresis comparators on torque and flux estimates without a PWM modulator, torque response under 5 ms
Answer + AI explanation with Pro
5. Which term means: "keeps the voltage-to-frequency ratio constant to hold air-gap flux below base speed, with voltage boost at low frequency to overcome stator drop"?
Junior
A.Common DC bus
B.V/f control
C.Bearing currents
D.Variable frequency drive
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6. Which statement is correct?
Junior
A.V/f control — motor acts as generator returning energy to the DC bus; a diode front end needs a braking chopper and resistor while an active front end returns it to mains
B.V/f control — decouples stator current into a flux-producing d-axis and torque-producing q-axis component for DC-motor-like dynamic response
C.V/f control — several inverters share one rectifier and DC link so braking energy from one motor feeds motoring of another in multi-drive lines
D.V/f control — keeps the voltage-to-frequency ratio constant to hold air-gap flux below base speed, with voltage boost at low frequency to overcome stator drop
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7. What is Soft starter?
Junior
A.thyristor phase control that ramps motor voltage to reduce starting current and torque, then is bypassed; it gives no speed control unlike a VFD
B.keeps the voltage-to-frequency ratio constant to hold air-gap flux below base speed, with voltage boost at low frequency to overcome stator drop
C.IEC 61800-5-2 drive safety function that removes gate pulses to prevent torque without opening a contactor, capable up to SIL 3
D.operation above base speed with rated voltage and rising frequency so flux and torque fall inversely with speed at constant power
Answer + AI explanation with Pro
8. Which term means: "thyristor phase control that ramps motor voltage to reduce starting current and torque, then is bypassed; it gives no speed control unlike a VFD"?
Junior
A.Regenerative braking
B.Safe Torque Off
C.Bearing currents
D.Soft starter
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9. Which statement is correct?
Junior
A.Soft starter — keeps the voltage-to-frequency ratio constant to hold air-gap flux below base speed, with voltage boost at low frequency to overcome stator drop
B.Soft starter — IEC 61800-5-2 drive safety function that removes gate pulses to prevent torque without opening a contactor, capable up to SIL 3
C.Soft starter — thyristor phase control that ramps motor voltage to reduce starting current and torque, then is bypassed; it gives no speed control unlike a VFD
D.Soft starter — fast IGBT edges with long motor cables cause reflected-wave voltage doubling at the terminals, requiring dv/dt filters or inverter-duty insulation
Answer + AI explanation with Pro
10. What is Common DC bus?
Junior
A.several inverters share one rectifier and DC link so braking energy from one motor feeds motoring of another in multi-drive lines
B.ABB scheme selecting inverter voltage vectors from hysteresis comparators on torque and flux estimates without a PWM modulator, torque response under 5 ms
C.IEC 61800-5-2 drive safety function that removes gate pulses to prevent torque without opening a contactor, capable up to SIL 3
D.operation above base speed with rated voltage and rising frequency so flux and torque fall inversely with speed at constant power
Answer + AI explanation with Pro
11. Which term means: "several inverters share one rectifier and DC link so braking energy from one motor feeds motoring of another in multi-drive lines"?
Junior
A.V/f control
B.Common DC bus
C.Variable frequency drive
D.Field-oriented control
Answer + AI explanation with Pro
12. Which statement is correct?
Junior
A.Common DC bus — thyristor phase control that ramps motor voltage to reduce starting current and torque, then is bypassed; it gives no speed control unlike a VFD
B.Common DC bus — rectifier, DC link and PWM inverter that vary motor speed by adjusting both frequency and voltage, giving cube-law energy savings on fans and pumps
C.Common DC bus — several inverters share one rectifier and DC link so braking energy from one motor feeds motoring of another in multi-drive lines
D.Common DC bus — IEC 61800-5-2 drive safety function that removes gate pulses to prevent torque without opening a contactor, capable up to SIL 3
Answer + AI explanation with Pro
13. What is Field-oriented control?
Mid
A.keeps the voltage-to-frequency ratio constant to hold air-gap flux below base speed, with voltage boost at low frequency to overcome stator drop
B.decouples stator current into a flux-producing d-axis and torque-producing q-axis component for DC-motor-like dynamic response
C.operation above base speed with rated voltage and rising frequency so flux and torque fall inversely with speed at constant power
D.ABB scheme selecting inverter voltage vectors from hysteresis comparators on torque and flux estimates without a PWM modulator, torque response under 5 ms
Answer + AI explanation with Pro
14. Which term means: "decouples stator current into a flux-producing d-axis and torque-producing q-axis component for DC-motor-like dynamic response"?
Mid
A.Variable frequency drive
B.Field-oriented control
C.Common DC bus
D.Soft starter
Answer + AI explanation with Pro
15. Which statement is correct?
Mid
A.Field-oriented control — operation above base speed with rated voltage and rising frequency so flux and torque fall inversely with speed at constant power
B.Field-oriented control — keeps the voltage-to-frequency ratio constant to hold air-gap flux below base speed, with voltage boost at low frequency to overcome stator drop
C.Field-oriented control — decouples stator current into a flux-producing d-axis and torque-producing q-axis component for DC-motor-like dynamic response
D.Field-oriented control — several inverters share one rectifier and DC link so braking energy from one motor feeds motoring of another in multi-drive lines
Answer + AI explanation with Pro
16. What is Direct torque control?
Mid
A.motor acts as generator returning energy to the DC bus; a diode front end needs a braking chopper and resistor while an active front end returns it to mains
B.ABB scheme selecting inverter voltage vectors from hysteresis comparators on torque and flux estimates without a PWM modulator, torque response under 5 ms
C.rectifier, DC link and PWM inverter that vary motor speed by adjusting both frequency and voltage, giving cube-law energy savings on fans and pumps
D.fast IGBT edges with long motor cables cause reflected-wave voltage doubling at the terminals, requiring dv/dt filters or inverter-duty insulation
Answer + AI explanation with Pro
17. Which term means: "ABB scheme selecting inverter voltage vectors from hysteresis comparators on torque and flux estimates without a PWM modulator, torque response under 5 ms"?
Mid
A.Bearing currents
B.Variable frequency drive
C.Soft starter
D.Direct torque control
Answer + AI explanation with Pro
18. Which statement is correct?
Mid
A.Direct torque control — ABB scheme selecting inverter voltage vectors from hysteresis comparators on torque and flux estimates without a PWM modulator, torque response under 5 ms
B.Direct torque control — operation above base speed with rated voltage and rising frequency so flux and torque fall inversely with speed at constant power
C.Direct torque control — decouples stator current into a flux-producing d-axis and torque-producing q-axis component for DC-motor-like dynamic response
D.Direct torque control — several inverters share one rectifier and DC link so braking energy from one motor feeds motoring of another in multi-drive lines
Answer + AI explanation with Pro
19. What is Regenerative braking?
Mid
A.IEC 61800-5-2 drive safety function that removes gate pulses to prevent torque without opening a contactor, capable up to SIL 3
B.rectifier, DC link and PWM inverter that vary motor speed by adjusting both frequency and voltage, giving cube-law energy savings on fans and pumps
C.motor acts as generator returning energy to the DC bus; a diode front end needs a braking chopper and resistor while an active front end returns it to mains
D.common-mode voltage from PWM drives electrical discharge through bearings, mitigated by insulated bearings, shaft grounding brushes and common-mode chokes
Answer + AI explanation with Pro
20. Which term means: "motor acts as generator returning energy to the DC bus; a diode front end needs a braking chopper and resistor while an active front end returns it to mains"?
Mid
A.Field weakening
B.Regenerative braking
C.Field-oriented control
D.Variable frequency drive
Answer + AI explanation with Pro
21. Which statement is correct?
Mid
A.Regenerative braking — thyristor phase control that ramps motor voltage to reduce starting current and torque, then is bypassed; it gives no speed control unlike a VFD
B.Regenerative braking — several inverters share one rectifier and DC link so braking energy from one motor feeds motoring of another in multi-drive lines
C.Regenerative braking — keeps the voltage-to-frequency ratio constant to hold air-gap flux below base speed, with voltage boost at low frequency to overcome stator drop
D.Regenerative braking — motor acts as generator returning energy to the DC bus; a diode front end needs a braking chopper and resistor while an active front end returns it to mains
Answer + AI explanation with Pro
22. What is dv/dt stress?
Mid
A.decouples stator current into a flux-producing d-axis and torque-producing q-axis component for DC-motor-like dynamic response
B.fast IGBT edges with long motor cables cause reflected-wave voltage doubling at the terminals, requiring dv/dt filters or inverter-duty insulation
C.motor acts as generator returning energy to the DC bus; a diode front end needs a braking chopper and resistor while an active front end returns it to mains
D.keeps the voltage-to-frequency ratio constant to hold air-gap flux below base speed, with voltage boost at low frequency to overcome stator drop
Answer + AI explanation with Pro
23. Which term means: "fast IGBT edges with long motor cables cause reflected-wave voltage doubling at the terminals, requiring dv/dt filters or inverter-duty insulation"?
Mid
A.Field weakening
B.dv/dt stress
C.Direct torque control
D.Field-oriented control
Answer + AI explanation with Pro
24. Which statement is correct?
Mid
A.dv/dt stress — fast IGBT edges with long motor cables cause reflected-wave voltage doubling at the terminals, requiring dv/dt filters or inverter-duty insulation
B.dv/dt stress — rectifier, DC link and PWM inverter that vary motor speed by adjusting both frequency and voltage, giving cube-law energy savings on fans and pumps
C.dv/dt stress — operation above base speed with rated voltage and rising frequency so flux and torque fall inversely with speed at constant power
D.dv/dt stress — decouples stator current into a flux-producing d-axis and torque-producing q-axis component for DC-motor-like dynamic response
Answer + AI explanation with Pro
25. What is Field weakening?
Mid
A.thyristor phase control that ramps motor voltage to reduce starting current and torque, then is bypassed; it gives no speed control unlike a VFD
B.decouples stator current into a flux-producing d-axis and torque-producing q-axis component for DC-motor-like dynamic response
C.motor acts as generator returning energy to the DC bus; a diode front end needs a braking chopper and resistor while an active front end returns it to mains
D.operation above base speed with rated voltage and rising frequency so flux and torque fall inversely with speed at constant power
Answer + AI explanation with Pro
26. Which term means: "operation above base speed with rated voltage and rising frequency so flux and torque fall inversely with speed at constant power"?
Mid
A.Common DC bus
B.Variable frequency drive
C.Direct torque control
D.Field weakening
Answer + AI explanation with Pro
27. Which statement is correct?
Mid
A.Field weakening — motor acts as generator returning energy to the DC bus; a diode front end needs a braking chopper and resistor while an active front end returns it to mains
B.Field weakening — operation above base speed with rated voltage and rising frequency so flux and torque fall inversely with speed at constant power
C.Field weakening — IEC 61800-5-2 drive safety function that removes gate pulses to prevent torque without opening a contactor, capable up to SIL 3
D.Field weakening — thyristor phase control that ramps motor voltage to reduce starting current and torque, then is bypassed; it gives no speed control unlike a VFD
Answer + AI explanation with Pro
28. What is Safe Torque Off?
Senior
A.IEC 61800-5-2 drive safety function that removes gate pulses to prevent torque without opening a contactor, capable up to SIL 3
B.rectifier, DC link and PWM inverter that vary motor speed by adjusting both frequency and voltage, giving cube-law energy savings on fans and pumps
C.keeps the voltage-to-frequency ratio constant to hold air-gap flux below base speed, with voltage boost at low frequency to overcome stator drop
D.operation above base speed with rated voltage and rising frequency so flux and torque fall inversely with speed at constant power
Answer + AI explanation with Pro
29. Which term means: "IEC 61800-5-2 drive safety function that removes gate pulses to prevent torque without opening a contactor, capable up to SIL 3"?
Senior
A.Common DC bus
B.Soft starter
C.Field weakening
D.Safe Torque Off
Answer + AI explanation with Pro
30. Which statement is correct?
Senior
A.Safe Torque Off — fast IGBT edges with long motor cables cause reflected-wave voltage doubling at the terminals, requiring dv/dt filters or inverter-duty insulation
B.Safe Torque Off — operation above base speed with rated voltage and rising frequency so flux and torque fall inversely with speed at constant power
C.Safe Torque Off — IEC 61800-5-2 drive safety function that removes gate pulses to prevent torque without opening a contactor, capable up to SIL 3
D.Safe Torque Off — motor acts as generator returning energy to the DC bus; a diode front end needs a braking chopper and resistor while an active front end returns it to mains
Answer + AI explanation with Pro
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