Motors & Starters — Most Asked Questions
24 questions asked about motors & starters across the General and Fn 5 papers, heaviest first. 1 of them are must-master.
Learn the system first
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1 Explain star delta starter?
Answer
A star-delta starter spins a delta-wound motor up in star at one-third voltage and current, then a timer snaps it to delta for full power.
- Used on 6-terminal motors built for delta running as the cheapest reduced-voltage start.
- Start: main plus star contactors close, windings in star see V/√3 — current and torque about 30–33% of direct-on-line delta.
- Changeover: an on-delay timer drops the star contactor and picks up the delta contactor; overload relay 51 rides the main line throughout.
- Limit: poor for high-inertia loads that need starting torque — those need a high-torque motor at extra cost.
Surveyor follow-up: the motor hums in star but trips the instant it hits delta — electrical or mechanical, and why?
Full theory: Motor Starters — Taming 8–10× Current -
2 Working principle of induction motor?
Also asked: “How induction motor work?”
Answer
The stator makes a rotating magnetic field; the rotor chases it, cuts flux and makes its own torque — always slipping slightly behind.
- Three-phase stator windings fed in sequence produce a field rotating at synchronous speed.
- Rotor bars cut that field, inducing current in the squirrel cage — current plus field equals torque.
- The rotor must lag the field to keep cutting flux; that lag is slip, growing with load.
- No brushes, no separate excitation — speed is set by frequency and poles, trimmed by slip under load.
Surveyor follow-up: motor runs but slower than rated with high current — overload or supply dip, and which reading decides?
Full theory: Induction Motors — Principle, Faults & Testing -
3 difference between synchronous motor and induction motor?
Answer
Synchronous runs locked to supply frequency with its own excitation; induction always slips behind, making torque from induced rotor current.
- Induction: stator field induces rotor current — rotor must lag the field, so speed is always below synchronous by the slip.
- Synchronous: rotor carries DC excitation and locks to the rotating field — speed fixed by frequency and poles, slip zero.
- Starting differs: induction is self-starting DOL or reduced-voltage; synchronous needs damper windings, pony motor or drive to pull into step.
- Shipboard use: induction everywhere for pumps and fans; synchronous for big alternators, shaft generators and power-factor correction.
Surveyor follow-up: synchronous motor drops out of step on overload — what does it become, and what trips first?
Full theory: Motor Starters — Taming 8–10× Current -
4 How to check Insulation resistance of induction motor
Also asked: “How will check the insulation of the motor”, “How to check the insulation resistance of dc motor”
Answer
Megger the motor cold and hot: cables off, 500-volt megger, phase-to-phase and phase-to-earth logged against the maker floor.
- Isolate, discharge and disconnect cables, then megger stator windings to earth and between phases.
- Minimum roughly rated volts times three over rated output plus one thousand — never below 1 megaohm or dry her out.
- Dry below 90 degrees on windings until readings hold steady; heat, dust and damp drive readings down.
- Log cold checks routinely under planned maintenance — a falling trend fails before the motor does.
Surveyor follow-up: 0.5 megaohm after a humid port stay — run, dry or renew, and how do you dry her?
Full theory: Induction Motors — Principle, Faults & Testing -
5 Induction motor types
Also asked: “Different induction motors”
Answer
Induction motors split squirrel-cage for rugged duty and slip-ring for heavy starts, with double-cage bridging the two.
- Squirrel-cage: bars shorted by end rings in a laminated core — near 90% of motors, simple and almost indestructible.
- Slip-ring or wound-rotor: rotor winding out through rings to a starting rheostat for high-torque low-current starts.
- Double-cage: high-resistance outer cage starts, low-resistance inner cage runs — torque without the rings.
- Starters follow the rotor: reduced-voltage for cages, rotor resistance for slip-ring sets.
Surveyor follow-up: high-inertia fan stalls on star-delta — cage, slip-ring or double-cage replacement, and why?
Full theory: Induction Motors — Principle, Faults & Testing -
6 Explain soft starter?
Also asked: “what is soft starter He want full working of starter”
Answer
A soft starter ramps voltage with thyristors: gentle torque climb, capped current, full volts only at speed.
- Back-to-back thyristors chop each half-cycle, raising output voltage over the set ramp time.
- Current and torque follow voltage squared down — the motor pulls away without the DOL slam.
- Ramp, current limit and kick-start timers tune the start to pumps, fans and conveyors.
- All-thyristor bridges stay clean; thyristor-diode mixes are cheaper but feed harmonics into the windings.
Surveyor follow-up: motor stalls at the end of ramp with current capped — ramp longer, limit higher or load lighter?
Full theory: Motor Starters — Taming 8–10× Current -
7 These are the rudders that have 30-40% of its area forward to the rudder stock. Here the torque required to turn / move is quite low as it lies on the centre of gravity
Answer
That is the balanced rudder: a third of its area ahead of the stock cancels most steering torque.
- With 30–40% of area forward, stock and pressure centres nearly meet at midships — torque collapses.
- Away from midships the centres part again, leaving only a reduced torque to fight.
- Smaller stock diameter and lighter gear follow; rudder angle stays capped near 35 degrees a side.
- Spade, open-sternframe and balanced-skeg forms all use the same trick with different supports.
Surveyor follow-up: balanced rudder heavy one way and light the other — bent stock, seized bearing or uneven flow?
Full theory: Rudder: Construction, Types & Support -
8 synchronous and asynchronous motor
Answer
Synchronous locks to supply frequency with its own excitation; asynchronous always slips behind on induced rotor current.
- Synchronous rotor carries DC excitation and runs at synchronous speed with zero slip.
- Asynchronous rotor must lag the field so flux cuts bars and induces torque current — slip grows with load.
- Starting differs: induction self-starts DOL or reduced-voltage; synchronous needs damper, pony or drive pull-in.
- Shipboard split: induction for pumps and fans, synchronous for alternators, shaft machines and power-factor duty.
Surveyor follow-up: synchronous motor drops out of step on overload — what does it become, and what trips first?
Full theory: Induction Motors — Principle, Faults & Testing -
9 Application of synchronous motor
Answer
Synchronous motors correct power factor, hold constant speed and steady voltage: over-excited leaders for lagging boards.
- Over-excited running leads the current, cancelling lagging induction loads across the board.
- Constant-speed drive above 600 rpm suits pumps, belts and compressors that never wander.
- Voltage regulation duty steadies weak networks where induction dips would sag the bars.
- More current at low factor means more losses and drop — synchronous correction pays in fuel.
Surveyor follow-up: board 0.7 lagging with a synchronous spare idle — run it, add capacitors or accept, and why?
Full theory: Induction Motors — Principle, Faults & Testing -
10 Squirrel cage motor vs wound rotor motor
Answer
Cage is fit-and-forget, wound-rotor is start-and-control: bars shorted for rugged duty, windings out to rings for torque.
- Cage bars brazed to end rings need no brushes — near-indestructible with direct starters.
- Wound rotors bring windings to slip rings with external resistance for high-torque low-current starts.
- Slip-ring brushes need spring pressure near 0.14–0.21 kg per square centimetre, bedded to curvature, renewed past 2 cm wear.
- Cage for pumps and fans; wound for winches and cranes that start heavy and crawl slow.
Surveyor follow-up: winch stalls on a cage motor — rewind, resistor-start wound or double-cage, and why?
Full theory: Motor Starters — Taming 8–10× Current -
11 Starting of synchronous motor
Also asked: “How is synchronous motor started?”
Answer
Start synchronous as induction, then pull into step: short the field, apply reduced volts, excite weak at speed, then full volts.
- Field shorted with auto-transformer reduced volts on the stator to spin up on damper windings.
- Near steady speed remove the short and apply weak DC excitation to pull into synchronism.
- Cut out the auto-transformer for full volts, then trim excitation for the wanted power factor.
- Pony-motor, VFD or shaft-drive alternatives spin to near synchronous before excitation locks the step.
Surveyor follow-up: fails to pull in with excitation correct — voltage, load or field, and what do you ease first?
Full theory: Motor Starters — Taming 8–10× Current -
12 Commutator and slip ring
Answer
Slip rings pass AC unchanged; split-ring commutators rectify it into DC — same coil, different tapping.
- AC machines take each coil end to its own slip ring, so the load sees the coil emf exactly as generated.
- DC machines join both ends to halves of one split ring that swaps connections each half-turn.
- Transformer action needs the AC tapping across pole pitches for maximum flux cutting at the brushes.
- Name the machine by the ring: continuous rings mean AC supply, split ring means DC output.
Surveyor follow-up: DC generator output pulsing AC at the terminals — brushes, split ring or winding, and what proves it?
Full theory: Motor Starters — Taming 8–10× Current -
13 Squirrel cage motor
Answer
The squirrel cage is the workhorse: stator field spins, rotor bars chase it by Lenz law, always a slip behind.
- Rotor bars shorted by end rings form the cage that gives the name — no brushes, no slip rings.
- Stator current builds the rotating field that induces rotor currents and drags the cage round.
- Rotor never catches the field: slip makes the changing flux that keeps torque alive.
- Cheap, rugged and sealed for pumps, fans and winches — speed set by poles and frequency with starters taming the inrush.
Surveyor follow-up: cage motor crawling with full volts and hot bars — single-phasing, overload or broken bars first?
Full theory: Motor Starters — Taming 8–10× Current -
14 working of synchronous motor
Answer
Synchronous motors lock poles to the field: excited rotor glued magnetically to the spinning stator at exact sync speed.
- DC through brushes and slip rings magnetises the rotor into strong north and south poles.
- Three-phase AC spins the stator field at synchronous speed fixed by frequency and poles.
- Unlike poles attract so hard the rotor locks step-for-step with zero slip — load angle stretches, speed never slips.
- Use where exact speed pays: clocks, compressors and power-factor correction with over-excitation.
Surveyor follow-up: synchronous motor hunting with excitation proved steady — load, supply frequency or damper winding first?
Full theory: Motor Starters — Taming 8–10× Current -
15 Advantage of soft starter
Answer
Soft starters start motors kindly: capped current, rising torque with speed, no demand peaks and gentle stops.
- Voltage ramps up as motor impedance rises so current stays capped while torque grows with the load.
- Kills peak-demand charges and burnouts on big horsepower starts with fewer nuisance trips of generators and electronics.
- Cuts downtime from snapped couplings and hammered pipework with torque control through start and stop.
- Installs simpler and cheaper than electromechanical reduced-voltage gear — ideal for pumps and fans started often.
Surveyor follow-up: soft-started pump stalling at the end of ramp with current capped — raise limit, extend ramp or check the load?
Full theory: Motor Starters — Taming 8–10× Current -
16 Describe the cffect of nunning an induction motor on reduced voltage. (6
Also asked: “9 (a) Describe the effect of running an induction motor on reduced voltage, (6”
Answer
Reduced voltage starves the torque: current falls, slip rises, speed sags slightly — and the motor heats at the load it still carries.
- Torque falls roughly with voltage squared, so a motor starting or running at 80% voltage delivers far less pull.
- A load that needs full torque instead slips more, drawing more rotor and stator current for the shaft demand.
- Speed falls only slightly, between 1% and 5% from no load to full load, because induction motors are nearly constant speed.
- Run below rated speed on a VFD and the motor must be rated at full-speed torque, with extra cooling if the torque is held.
Surveyor follow-up: motor stalling at reduced voltage with the current low — is the load, the voltage or the supply impedance?
Full theory: Motor Starters — Taming 8–10× Current -
17 Explain induction motor
Answer
An induction motor runs on induction alone: the stator field induces current in a rotor it never wires, and that current makes the torque.
- Three-phase supply creates a rotating magnetic field in the stator, rotating at synchronous speed fixed by poles and frequency.
- That field induces rotor current; the rotor current creates its own field, and the two fields chase each other.
- The motor can never reach synchronous speed — at that point no emf is induced, so no rotor current and no torque exist.
- The rotor therefore always slips slightly below synchronous speed, even on no load, to carry the losses.
Surveyor follow-up: motor drawing full load current with the nameplate slip not achieved — which assumption is wrong first?
Full theory: Motor Starters — Taming 8–10× Current -
18 Explain the term single phasing as applied to poly phase induction motors. (6) (b) State the likely causes of single phasing and the consequences if motors are not adequately protected. (5) (c) Describe with the aid of s
Also asked: “Explain the term single phasing as applied to poly phase induction motors. (6) (b) State the likely causes of single phasing and the consequences if motors are not adequately protected. (5) (c) Describe with the aid of sketches THREE methods for motor protection should single phasing occur. (5”
Answer
Single phasing is one phase lost: the motor runs on two phases, the healthy windings take more current, and it overheats without a reliable alarm.
- The remaining phases carry above rated current, so the motor overheats and eventually fails where a balanced motor would not.
- Causes are a lost supply phase, a blown fuse, a defective contact, or a loose connection in a three-phase breaker.
- The motor may hum louder than normal, vibrate, and draw excess current — visible signs that are often the only warning.
- Protection is a phase-failure or negative phase sequence relay, plus breakers that interrupt all three phases together.
Surveyor follow-up: motor single-phased on a breaker with one weak contact — is the relay, the contact or the supply phase the finding?
Full theory: Protection Chain — Contactors, Breakers, Overload vs Short Circuit -
19 Give a clear explanation of the following effects in a three phase induction motor, (1) the production of rotating field, (ii) the presence of an induced rotor current, (iii) the development of torque. A 4— pole, 250V mo
Also asked: “Give a clear explanation of the following effects in a three phase induction motor, (1) the production of rotating field, (ii) the presence of an induced rotor current, (iii) the development of torque. A 4— pole, 250V motor has its armature removed in order to test the continuity of the field windings which are connected in series and consist of 2000 turns each. What is the average e.m.f. induced”
Answer
Stator field rotates, rotor current is induced in it, and the two fields chase each other — that chase is the torque.
- The three-phase stator winding produces a rotating field at synchronous speed, 120f divided by P — 1500 rpm for a 4-pole 50 Hz motor.
- The rotor bars cut that field, inducing rotor currents, which produce their own field at the same space speed.
- Rotor and stator fields are stationary relative to each other, so their interaction develops the torque.
- Because the induction needs relative motion, the rotor must always slip — which is why the motor cannot reach synchronous speed.
Surveyor follow-up: 4-pole motor on 50 Hz measuring 1470 rpm — what is the slip, and what would zero slip mean?
Full theory: Motor Starters — Taming 8–10× Current -
20 Induction motor in detail, synchronous speed and slip
Answer
Synchronous speed is 120f over p; slip is the difference between it and actual rotor speed, and slip is what makes the motor work.
- Synchronous speed n_s = 120f / P, where f is supply frequency and P the number of poles — a 4-pole motor on 50 Hz is 1500 rpm.
- The rotor runs below n_s because it must be moving relative to the field for any emf to be induced at all.
- Slip is the difference between synchronous speed and actual rotor speed, usually expressed as a percentage of n_s.
- At 5% slip on a 1200 rpm synchronous speed the rotor is 60 rpm behind, running at 1140 rpm — the working point, not a fault.
Surveyor follow-up: 4-pole motor on 60 Hz measuring 1760 rpm — what is the slip, and is the supply what the nameplate expects?
Full theory: Motor Starters — Taming 8–10× Current -
21 Motor Protection Device
Answer
Motor protection is a set, not one device: overload, short circuit, single phasing, and earth fault each catch a different failure.
- Thermal overload relays follow winding temperature and trip the contactor on sustained overload.
- Magnetic protection clears instantaneous high currents — short circuits and heavy inrush faults.
- Single-phasing and negative phase sequence protection catch a lost phase that overload relays miss.
- Earth fault protection covers the leakage case, and the devices are coordinated so the right one acts for the right fault.
Surveyor follow-up: motor tripping on overload with the load proved normal — which element, and what settings does the panel hold?
Full theory: Protection Chain — Contactors, Breakers, Overload vs Short Circuit -
22 What is meant by the term ‘back e.m.f.” as applied to an electric motor? (6
Answer
Back e.m.f. is the voltage the motor generates against the supply as it rotates — the counter-voltage that limits its own current.
- By Faraday’s law, a conductor moving in a magnetic field generates an emf; in a motor it is a by-product of producing torque.
- It is called back emf because it opposes the supply emf, and the difference between the two is what drives the current.
- It rises with speed, so the starting current is highest when the motor is stationary and back emf is zero.
- The term is applied to every emf not produced directly by the prime generating source — generators, batteries, emergency sets are excluded.
Surveyor follow-up: motor drawing full rated current on load and hardly any at rest — what does the back emf explain?
Full theory: Motor Starters — Taming 8–10× Current -
23 Why we use y not dol
Answer
Star, not direct-on-line, where the current would be unbearable: star puts a third of the voltage on each phase and cuts the torque to a third.
- In the star position each phase winding sees about 58% of line voltage, so starting current drops to a third.
- Starting torque also falls to roughly one-third of direct-on-line, which suits centrifugal pumps and fans where starting load is light.
- Direct-on-line would draw eight to ten times full-load current, stressing the supply, the cables and the contactor at every start.
- Star suits machines that accelerate easily to speed before the changeover to delta, which then gives full running torque.
Surveyor follow-up: star-delta starter on a conveyor that will not pull away in star — is it DOL, autotransformer or a lighter load?
Full theory: Motor Starters — Taming 8–10× Current -
24 how would you get to know an induction motor is fit?
Also asked: “• how would you get to know an induction motor is fit?”
Answer
A motor is fit when it draws rated current, holds its temperature and makes no noise — and insulation resistance proves the windings are sound.
- Take a megometer reading between windings and earth, comparing it with the last reading rather than any single figure.
- Run it on load and check current against the nameplate, temperature rise against the frame, and absence of noise and vibration.
- Check bearings, coupling alignment, cooling air and the mechanical load, because a healthy motor is often an unhealthy machine.
- A falling insulation trend is the finding that matters — it condemns the set long before the current gauge does.
Surveyor follow-up: motor drawing high with insulation healthy and bearings cool — is it load, alignment or supply voltage?
Full theory: Motor Starters — Taming 8–10× Current
Source: 123 written papers and surveyor oral banks, 24,613 entries, merged into 17,210 topics. Ticks are stored on this device only.