Why can an induction motor never reach synchronous speed on its own?

B. At synchronous speed the rotor bars would not cut any field lines, so no current would be induced and there would be no torque

A. Friction in the bearings always slows it

C. The supply frequency drops under load

The load on a running induction motor increases. What happens?

B. Speed falls slightly, slip increases, and current rises

A. Speed rises and current falls

C. Speed stays exactly the same; only voltage changes

D. The motor jumps to the next synchronous speed

How do you reverse a three-phase induction motor?

B. Swap any two of the three lines

Why does a motor draw a very large current for the first moment of an across-the-line start?

B. At standstill the slip is 100%: the field cuts the rotor at full speed and induces a large rotor current, which the stator must supply

C. The overload relay is bypassed

D. The windings are cold and have no resistance

Tercer año Lesson 6 of 13 · about 15 min

How an AC induction motor works

What you’ll learn

How three phases 120° apart make a magnetic field that rotates, and why the rotor chases it.
Synchronous speed from frequency and poles, and how to tell a motor’s pole count from its nameplate rpm.
What slip is, why the motor needs it, and how it changes with load.
The speed-torque curve: locked rotor, breakdown and full load — and why the starting current is so high.

Before this: Wye and delta · Reading a motor nameplate

An induction motor has no electrical connection to its rotor. Three-phase current in the stator makes a magnetic field that spins; the spinning field induces current in the rotor bars; that current makes the rotor a magnet, and it is dragged round after the field. It is a transformer whose secondary is free to turn.

The rotating field

The stator has three sets of windings spaced around it, one per phase. Each phase peaks a third of a cycle after the one before, so the strongest point of the magnetic field moves from one winding set to the next, round and round, without anything moving. On a 60 Hz supply with the simplest winding (two poles — one north, one south) the field goes round once per cycle: 60 revolutions a second, or 3,600 rpm.

Wind the stator with more poles and the field takes longer to get round. The speed of the field is the synchronous speed:

Ns = 120 × f ÷ Pf = supply hertz, P = number of poles
Slip = (Ns − N) ÷ NsN = actual shaft rpm; × 100 for percent
f rotor = slip × ffrequency of the rotor current

Polos	Sync speed at 60 Hz	Sync speed at 50 Hz	Typical nameplate (60 Hz)
2	3,600 rpm	3,000 rpm	about 3,450–3,550
4	1,800 rpm	1,500 rpm	about 1,725–1,775
6	1,200 rpm	1,000 rpm	about 1,140–1,180
8	900 rpm	750 rpm	about 850–880

The synchronous speeds are pure arithmetic. The nameplate ranges are typical of ordinary motors, not a rule — the plate in front of you is the authority. Swap any two of the three supply lines and the field turns the other way; that is all the forward/reverse lab is doing.

Slip: why it never catches up

The rotor only gets current because the field is sweeping past its bars. If the rotor ever turned as fast as the field, the bars would see a steady field, nothing would be induced, and there would be no torque. So the rotor always runs a little slow. That difference is slip.

Slip is self-adjusting. Put more load on the shaft and the rotor slows a touch; more slip means the field cuts the bars faster, which induces more current and more torque, until the torque matches the load again. The stator supplies that extra rotor current, which is why a clamp-on shows the line current rising as a motor is loaded.

Torque against speed. At standstill the motor makes its locked-rotor torque and draws its largest current. It accelerates past the breakdown peak and settles on the steep part of the curve, where a small change in speed makes a large change in torque. Load it past breakdown and it stalls.

Worked: reading speed off a nameplate

Nameplate: 60 Hz, 1,750 rpm. The synchronous speed just above 1,750 is 1,800 rpm, so it is a 4-pole motor (120 × 60 ÷ 4 = 1,800).
Slip = 1,800 − 1,750 = 50 rpm; 50 ÷ 1,800 = 2.8%.
Rotor frequency = 0.028 × 60 = 1.67 Hz. The rotor current alternates less than twice a second.
A 50 Hz motor plated 1,460 rpm: sync 1,500 (4-pole), slip 40 rpm = 2.7%.

Why the starting current is so big

At standstill the slip is 100%: the field sweeps past the bars at full synchronous speed and the rotor behaves like the shorted secondary of a transformer. The rotor current is huge, so the stator current is huge — commonly several times the full-load current, for the second or two it takes to come up to speed. The nameplate’s code letter describes how large that locked-rotor draw is. The next lesson is about the ways to tame it.

En la obra

Tips

Nameplate rpm tells you the pole count in one glance: just under 3,600, 1,800, 1,200 or 900 at 60 Hz.
Bump new motors for rotation before coupling the load. Swap any two leads to reverse.
Current climbing while speed drops is the motor telling you the load has gone up — or a bearing has.
Unbalanced supply voltage causes much larger unbalance in the motor current and heats the windings. Measure all three.

Common mistakes

Expecting nameplate rpm to equal synchronous speed.
Counting pole PAIRS instead of poles in 120 × f ÷ P.
Reversing a motor by swapping all three leads — that keeps the same rotation.
Blaming the motor for a trip on start when the starting current was simply never allowed for.
Running a motor on the wrong frequency or voltage because it “turned over fine”.

Check yourself

0 right of 0 answered 8 questions · pick one answer each

1.What is the synchronous speed of a 4-pole motor on a 60 Hz supply?
2.A 60 Hz motor nameplate says 1,165 rpm. How many poles does it have?
3.A 4-pole, 60 Hz motor turns at 1,750 rpm at full load. What is its slip?
4.Why can an induction motor never reach synchronous speed on its own?
5.The load on a running induction motor increases. What happens?
6.How do you reverse a three-phase induction motor?
7.A 60 Hz, 4-pole motor runs at 1,750 rpm. What frequency is the current in its rotor bars?
8.Why does a motor draw a very large current for the first moment of an across-the-line start?

Answers

B. 1,800 rpm — Ns = 120 × f ÷ P = 120 × 60 ÷ 4 = 1,800 rpm. 1,750 is what a typical 4-pole motor actually turns at full load — a little below synchronous.
C. 6 — An induction motor runs just below synchronous speed. The synchronous speeds at 60 Hz are 3,600, 1,800, 1,200, 900… The one just above 1,165 is 1,200 rpm, which is 6 poles.
A. 50 rpm, 2.8% — Slip = (Ns − N) ÷ Ns = (1,800 − 1,750) ÷ 1,800 = 50 ÷ 1,800 = 2.8%.
B. At synchronous speed the rotor bars would not cut any field lines, so no current would be induced and there would be no torque — The rotor current is INDUCED by the field sweeping past the bars. No difference in speed, no sweeping, no current, no torque. Slip is what makes the motor work.
B. Speed falls slightly, slip increases, and current rises — More load slows the rotor a little. More slip means the field cuts the bars faster, inducing more rotor current and more torque — and the stator draws more current from the line to supply it.
B. Swap any two of the three lines — Swapping any two lines reverses the phase sequence, so the magnetic field rotates the other way and the rotor follows. Swapping all three in a rotation (L1→L2→L3→L1) keeps the same sequence.
B. 1.67 Hz — Rotor frequency = slip × supply frequency = (50 ÷ 1,800) × 60 = 1.67 Hz. The field only sweeps past the rotor at the slip speed, so the rotor current alternates slowly.
B. At standstill the slip is 100%: the field cuts the rotor at full speed and induces a large rotor current, which the stator must supply — Locked rotor is the worst case — the rotor behaves like a shorted transformer secondary. As it speeds up the slip falls and so does the current. The nameplate code letter describes how big that locked-rotor draw is.

Preguntas frecuentes

What is slip in an induction motor?

The difference between the speed of the rotating magnetic field (synchronous speed) and the actual speed of the shaft, usually given as a percentage of synchronous speed. A 1,800 rpm field and a 1,750 rpm shaft is 50 rpm, or 2.8% slip. It is not a fault — the motor needs slip to make torque.

Why does my 4-pole motor say 1,750 rpm and not 1,800?

1,800 rpm is the speed of the field on a 60 Hz supply. The rotor always runs a little slower so the field can keep cutting its bars and inducing current. 1,750 is the speed at full rated load; lightly loaded it runs closer to 1,800, overloaded it slows further.

Will a 60 Hz motor run on 50 Hz?

It will turn at five-sixths of its speed, and at the same voltage the core will be over-fluxed and run hot, because a lower frequency at the same voltage pushes more magnetising current. Some motors are dual-rated; the nameplate says so. Otherwise it is a question for the manufacturer, not a guess.

What is the difference between squirrel cage and wound rotor?

A squirrel-cage rotor has bars shorted by end rings — nothing to connect, nothing to wear. A wound rotor has real windings brought out through slip rings, so resistance can be added to the rotor circuit to control starting current and speed. Squirrel cage is what you will meet nearly everywhere.

Ver también

Reading a motor nameplate — rpm, code letter, service factor and the rest
Motor synchronous speed & slip — the arithmetic on this page
Motor starting methods — the next lesson
Forward / reverse lab — swapping two lines, with interlocks
How a VFD works — changing the frequency changes the speed
Motor troubleshooting flowcharts

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