Industrial Drives and Traction MCQs 2026

Centrifugal pump affinity laws show flow scales with

1. A speed while power scales approximately with cube of speed ✓
2. B inverse cube of speed ✓
3. C square of voltage only ✓
4. D independent of speed ✓

Regenerative braking with VFD returns energy when

1. A motor operates as generator and DC bus energy is handled by braking chopper or active front end ✓
2. B motor always motoring forward ✓
3. C DC bus is disconnected ✓
4. D only mechanical brake is applied ✓

VFD cable requirements often specify shielded cable to reduce

1. A only conductor ampacity ✓
2. B only tower sag ✓
3. C only solar fill factor ✓
4. D common-mode currents and EMI ✓

Derating a standard induction motor on VFD may be needed because

1. A VFD always improves insulation life ✓
2. B frequency change has no thermal effect ✓
3. C insulation stress from PWM high dv/dt and reduced cooling at low speed ✓
4. D PWM eliminates all losses ✓

Multi-quadrant operation of a VFD drive means

1. A both directions of torque and speed are controllable ✓
2. B only forward motoring quadrant ✓
3. C only single speed forward ✓
4. D operation without any DC bus ✓

Soft starter differs from VFD in that soft starter

1. A limits inrush current at fixed frequency without continuous speed control ✓
2. B varies frequency for full speed range ✓
3. C always regenerates to grid ✓
4. D replaces motor entirely ✓

Parameter acceleration and deceleration times in VFD prevent

1. A any use of the motor ✓
2. B excessive current and mechanical shock during speed changes ✓
3. C connection to the grid ✓
4. D setting of carrier frequency ✓

Dynamic braking of a DC motor connects the armature across a

1. A open circuit ✓
2. B direct short without resistance ✓
3. C braking resistor while field remains excited ✓
4. D transformer primary only ✓

Plugging (counter-current braking) reverses

1. A only field flux to zero instantly ✓
2. B only mechanical brake pads material ✓
3. C only gearbox ratio ✓
4. D armature voltage or phase sequence to produce opposing torque ✓

Regenerative braking feeds energy back to

1. A only the atmosphere as heat always ✓
2. B the supply or DC bus instead of dissipating in resistors ✓
3. C only lubricating oil ✓
4. D only the motor frame magnetically ✓

Mechanical braking is still used with electric braking because

1. A electric braking replaces parking brakes entirely always ✓
2. B it holds the load at standstill and provides emergency stopping ✓
3. C it increases motor slip to infinity ✓
4. D it eliminates wear on pads completely ✓

Braking torque of an induction motor in DC injection braking is produced by

1. A applying controlled DC to stator creating stationary field ✓
2. B increasing supply frequency above synchronous ✓
3. C removing all stator current ✓
4. D only changing power factor capacitor ✓

Overhauling loads such as downhill conveyors require

1. A continuous braking capability in the drive system ✓
2. B only acceleration ramps ✓
3. C no braking at all ✓
4. D open-loop without torque control ✓

Braking resistor sizing depends on

1. A only motor colour ✓
2. B only line voltage frequency ✓
3. C only solar irradiance ✓
4. D energy to dissipate, duty cycle and resistance value limiting current ✓

Coast-to-stop means the motor

1. A decelerates freely without applied electrical braking ✓
2. B is plugged instantaneously ✓
3. C is reversed at full voltage ✓
4. D is disconnected from earth ✓

Eddy-current or hysteresis brakes provide

1. A contactless braking torque proportional to control excitation ✓
2. B only hydraulic pressure to pads ✓
3. C only increased motor slip by overspeeding ✓
4. D only solar MPPT ✓

Anti-lock or controlled braking in traction coordinates

1. A only maximum mechanical brake always ✓
2. B only regenerative without limits ✓
3. C electric and pneumatic braking to optimize adhesion ✓
4. D disabling all speed feedback ✓

DC chopper in traction braking controls

1. A only AC line frequency ✓
2. B only transformer tap ✓
3. C current through braking resistors by switching ✓
4. D only pantograph uplift ✓

Braking time decreases when

1. A braking torque magnitude increases within system limits ✓
2. B braking torque is zero ✓
3. C load inertia is infinite ✓
4. D supply is removed with no braking ✓

Thermal limit during repeated start-brake cycles is governed by

1. A only cable colour code ✓
2. B only insulator creepage ✓
3. C only wind Betz limit ✓
4. D motor and resistor I²t ratings ✓

Spring-applied electrically released brake is fail-safe in that

1. A power loss applies the brake mechanically ✓
2. B power loss releases braking always ✓
3. C it never provides holding torque ✓
4. D it requires continuous high power to brake ✓

Combined electric and mechanical braking improves

1. A only motor starting performance ✓
2. B control of deceleration profile and energy recovery where possible ✓
3. C only corona on lines ✓
4. D only Y-bus sparsity ✓

Overhead catenary supplies traction vehicles with

1. A only diesel fuel ✓
2. B AC or DC contact via pantograph ✓
3. C only battery swap at every km ✓
4. D only optical fibre data ✓

Escalator and elevator drives require

1. A only uncontrolled direct-on-line start always ✓
2. B only fixed DC injection only ✓
3. C controlled torque profiles for passenger comfort and safety ✓
4. D no braking capability ✓

Fan and pump VFD energy savings at reduced speed arise because

1. A power is constant at all speeds ✓
2. B power demand drops sharply with cube of speed for centrifugal loads ✓
3. C efficiency is zero at low speed ✓
4. D only voltage is reduced without frequency change ✓

Synchronizing two conveyors with VFDs may use

1. A only mechanical clutch without feedback ✓
2. B only fixed 50 Hz without communication ✓
3. C master-follower or electronic line shafting ✓
4. D only DC plugging ✓

Motor thermal overload relay protects against

1. A instantaneous short-circuit only (fuse role) ✓
2. B only voltage transients from lightning ✓
3. C sustained overcurrent exceeding safe heating time ✓
4. D only bearing lubricant level ✓

STO (Safe Torque Off) function in modern drives

1. A increases torque to maximum ✓
2. B bypasses all interlocks ✓
3. C removes torque-producing capability for personnel safety ✓
4. D replaces mechanical guards entirely alone ✓

Common bus configuration of multiple VFDs shares

1. A only separate rectifiers without any DC tie ✓
2. B only mechanical line shafts ✓
3. C only pneumatic logic ✓
4. D single DC link allowing energy transfer between drives ✓

Fluid coupling in some conveyors provides

1. A soft torque transmission and limited starting shock ✓
2. B electronic vector control ✓
3. C regenerative grid tie ✓
4. D synchronous speed operation ✓

Selection of motor duty type (S1-S10) per IEC considers

1. A continuous vs intermittent load cycles and thermal limits ✓
2. B only paint colour ✓
3. C only cable length ✓
4. D only wind speed ✓

Full-load slip of standard induction motor is typically

1. A 50 percent always ✓
2. B 100 percent ✓
3. C zero by definition at load ✓
4. D small (few percent) ✓

Synchronized speed of induction motor is

1. A independent of frequency ✓
2. B 120f/P rpm for supply frequency f and pole pairs P ✓
3. C equal to slip times rotor speed only ✓
4. D fixed at 3000 rpm always ✓

Ward-Leonard system historically provided

1. A only PWM IGBT control ✓
2. B smooth DC motor speed control via generator-motor set ✓
3. C only solar MPPT ✓
4. D only SF6 breaker operation ✓

Servo drive system provides

1. A only fixed-speed across-the-line starting ✓
2. B only open-loop V/f without encoder always ✓
3. C only thermal protection of building ✓
4. D precise position, speed and torque control with feedback ✓

Individual drive assigns

1. A one motor for entire factory always ✓
2. B no speed control ever ✓
3. C dedicated motor and controller per machine for flexibility ✓
4. D only steam turbines ✓

Group drive in industry connects

1. A each machine to its own VFD always today ✓
2. B only hydraulic pumps without motors ✓
3. C one motor to multiple machines via line shafts or belts historically ✓
4. D only solar inverters in parallel ✓

Load compensation in train weight distribution affects

1. A only catenary height only ✓
2. B only ticket pricing ✓
3. C only solar panel tilt ✓
4. D adhesion per axle and braking balance ✓

Braking blending on EMUs prioritizes

1. A only mechanical brake until stop ✓
2. B no speed feedback ✓
3. C plugging at full line voltage always ✓
4. D electric regeneration when track is receptive, supplementing with friction brake ✓

Traction power substation converts utility supply to

1. A only signalling frequency ✓
2. B traction voltage and feeds one or more track sections ✓
3. C only passenger Wi-Fi ✓
4. D only diesel storage ✓

Dead section or neutral section in AC catenary prevents

1. A all train movement permanently ✓
2. B only regenerative braking ever ✓
3. C only DC third rail use ✓
4. D circulating currents between differently phased supply sections ✓

Pantograph uplift force must balance

1. A maximum possible downforce always ✓
2. B zero contact force ✓
3. C stable contact with minimal wear and lift loss at speed ✓
4. D only aerodynamic lift of roof ✓

Speed control of modern AC traction motors uses

1. A only rheostat in series with DC motor only ✓
2. B only fixed synchronous speed ✓
3. C VVVF drives with induction or permanent-magnet motors ✓
4. D only steam pressure ✓

Harmonics from mass transit rectifier loads can affect

1. A only solar fill factor ✓
2. B utility power quality requiring filtering and transformer connections ✓
3. C only biomass moisture ✓
4. D only cable tan delta ✓

Line voltage drop along long catenary sections is managed by

1. A only increasing train mass ✓
2. B feeding stations, booster transformers and conductor sizing ✓
3. C only reducing pantograph pressure to zero ✓
4. D only single-phase without neutral ✓

Multiple-unit (MU) operation allows

1. A only one motor car per train always ✓
2. B diesel-only operation ✓
3. C no electrical connection between cars ✓
4. D several railcars to be controlled from one cab with distributed traction ✓

Regenerative braking in electric trains returns energy to

1. A catenary or third rail when receptivity allows ✓
2. B only onboard resistor always without exception ✓
3. C only the atmosphere ✓
4. D only signalling cables ✓

Traction transformer on locomotive steps down

1. A catenary voltage to level suitable for rectifiers/converters ✓
2. B only mechanical wheel diameter ✓
3. C only air brake pressure ✓
4. D only track gauge ✓

Third-rail DC traction systems typically operate at

1. A 400 kV AC line voltage ✓
2. B only 12 V battery ✓
3. C only 25 kV without conversion ✓
4. D 600 to 750 V DC (and similar levels) ✓

25 kV 50 Hz single-phase AC traction is widely used because

1. A it eliminates all harmonics without conversion ✓
2. B it requires no insulation coordination ✓
3. C utility power can be transformed and rectified onboard efficiently ✓
4. D it uses only DC motors exclusively today ✓

Closed-loop vector control of induction motor requires

1. A only fixed 50 Hz supply ✓
2. B only wound-rotor external resistance ✓
3. C fast current and flux estimation or measurement for torque control ✓
4. D no feedback of any kind ✓

Open-loop V/f control without encoder is suitable when

1. A servo positioning with zero error is mandatory ✓
2. B speed accuracy and dynamic response requirements are moderate ✓
3. C synchronous motor field must be DC excited manually always ✓
4. D only DC motors are used ✓

Carrier frequency of PWM affects

1. A only utility tariff structure ✓
2. B only rotor bar material ✓
3. C only train track gauge ✓
4. D audible noise, heat loss and output waveform quality ✓

Input line reactor or DC choke on VFD reduces

1. A motor speed below zero only ✓
2. B harmonic current distortion and limits inrush ✓
3. C need for any earthing ✓
4. D insulation BIL of transformer ✓

Six-pulse diode rectifier front end of a VFD draws line current with

1. A perfect sinusoidal unity PF always ✓
2. B significant harmonic content ✓
3. C zero THD by design always ✓
4. D only DC component on AC mains ✓

DC link capacitor in a voltage-source inverter VFD

1. A replaces the motor entirely ✓
2. B eliminates need for any switching devices ✓
3. C converts AC to mechanical torque directly ✓
4. D smooths rectified DC and supplies ripple current ✓

PWM inverter in a VFD synthesizes AC by

1. A only 50 Hz transformer tap changing ✓
2. B only resistor divider ✓
3. C only mechanical commutator ✓
4. D switching DC bus voltage with pulse-width modulation ✓

Output frequency of a VFD below base frequency typically uses

1. A increasing V/f without limit ✓
2. B constant V/f ratio to maintain flux ✓
3. C zero voltage at all frequencies ✓
4. D fixed voltage with zero frequency ✓

A variable frequency drive (VFD) controls motor speed by varying

1. A only rotor resistance externally always ✓
2. B only number of poles mechanically on the fly ✓
3. C stator frequency and voltage in coordinated fashion ✓
4. D only air gap without frequency change ✓