Magnetism and motors
What you’ll learn
- How magnets and electromagnets behave, and what makes an electromagnet stronger.
- The motor effect: why a current in a magnetic field gets pushed sideways.
- How three phases make a field that rotates, and why an induction motor’s rotor chases it.
- Synchronous speed, Ns = 120 × f ÷ P, and slip — worked from real nameplates.
Every motor on a job site works on one fact: a current-carrying conductor in a magnetic field gets pushed sideways. Arrange enough conductors, feed them the right way, and the push becomes a turning force. This lesson builds that up from magnets you can hold, to the rotating field inside a three-phase motor, to the two numbers on every nameplate you should be able to explain: speed and slip.
Magnets and fields
A magnet has a north and a south pole, and a field around it that you cannot see but can map with iron filings or a compass. Opposite poles attract; like poles repel. The field lines leave north and come back into south, and they are densest near the poles, where the pull is strongest.
Run current through a wire and it grows a field of its own, in circles around it. Wind that wire into a coil and the circles add up into one field through the middle — an electromagnet, with a north end and a south end, that you can switch off. Its strength goes up with the current, the number of turns (together called ampere-turns), and an iron core, which carries a field far more easily than air. Contactor coils, relays, solenoid valves and transformer cores are all this idea.
The motor effect
Put a current-carrying conductor across a magnetic field and the two fields interact: they bunch up on one side of the wire and thin out on the other, and the wire is pushed toward the thin side. The push is bigger with a stronger field, more current, and more length of wire in the field. Reverse either the current or the field and the push reverses. Bend the conductor into a loop on a shaft and the two sides are pushed opposite ways — a turning force. That is a motor.
The same thing works backwards: move a conductor through a field and a voltage is pushed into it (the generator effect). A running motor does both at once, which is why it draws less current running free than it does stalled.
The rotating field
A three-phase motor’s stator has its windings laid in slots around the frame. Feed them three phases that peak 120° apart and the strongest part of the field moves from one winding group to the next, round and round, at a steady speed. Nothing moves mechanically; the field rotates.
In the common squirrel-cage induction motor the rotor is a drum of bars shorted together at the ends. The rotating field sweeps past those bars, induces currents in them, and those currents in that field get pushed — the motor effect again — so the rotor turns the same way as the field. No brushes, no connections to the rotor: that is why these motors are everywhere.
Synchronous speed and slip
- Ns = 120 × f ÷ Psynchronous speed, rpm
- slip = (Ns − N) ÷ Ns× 100 for percent
The field’s speed depends only on the frequency and the number of poles. More poles, slower field: on 60 Hz a 2-pole is 3,600 rpm, a 4-pole 1,800, a 6-pole 1,200, an 8-pole 900.
The rotor has to turn a bit slower than the field. If it caught up, the field would stop sweeping past the bars, no current would be induced, and the torque would vanish. So it runs just behind — and the more you load it, the further behind it falls, and the more torque it makes. That lag is slip.
Worked examples
1. A nameplate reading 1,750 rpm on 60 Hz
- The nearest synchronous speed above 1,750 is 1,800 rpm, so it is a 4-pole motor: 120 × 60 ÷ 4 = 1,800 rpm.
- Slip = (1,800 − 1,750) ÷ 1,800 = 50 ÷ 1,800 = 0.0278.
- So 2.8% slip at full load.
2. A 2-pole pump motor at 3,450 rpm
- Ns = 120 × 60 ÷ 2 = 3,600 rpm.
- Slip = (3,600 − 3,450) ÷ 3,600 = 150 ÷ 3,600 = 4.2%.
3. The 4-pole motor from example 1, on 50 Hz
- A 4-pole motor on 50 Hz: Ns = 120 × 50 ÷ 4 = 1,500 rpm.
- It runs roughly 1,500 ÷ 1,800 = 83% of its 60 Hz speed. This is also how a variable-frequency drive changes speed: it changes f.
Try one: an 8-pole motor on 60 Hz running at 870 rpm
Ns = 120 × 60 ÷ 8 = 900 rpm. Slip = (900 − 870) ÷ 900 = 30 ÷ 900 = 3.3%.
Single-phase motors
One phase on its own makes a field that pulses back and forth but does not rotate, so a single-phase motor needs help to start: a second “start” winding shifted in time by a capacitor or by its own design, which makes a lopsided rotating field for long enough to get the rotor moving. That is what the start capacitor on a compressor is for; many blowers instead keep a run capacitor and the second winding in circuit all the time (a PSC motor). A failed capacitor of either kind is the classic cause of “it hums but won’t start”.
En la obra
Tips from the job
- Nameplate rpm tells you the poles: 3,450-ish is a 2-pole, 1,750-ish a 4-pole, 1,150-ish a 6-pole. Useful when the plate is half painted over.
- Before you bump a new motor, know which way it should turn. Swapping two leads is the fix — at the disconnect side, with it locked out.
- A motor that hums and will not start on a three-phase supply has often lost one phase. Check all three line voltages before you blame the motor.
- Read the plate before anything else: volts, amps, rpm, frame, service factor. The motor nameplate page walks through one.
Common mistakes
- Using 120 × f ÷ P with pole pairs instead of poles. A 4-pole motor has 2 pairs; the formula wants 4.
- Expecting nameplate rpm to equal synchronous speed. The difference is slip, and it is normal.
- Swapping all three leads to reverse a motor — it still turns the same way.
- Treating a motor’s cold winding resistance as its running current limit. A running motor’s current is set mostly by its load and the field it builds, not by that small resistance.
Check yourself
8 questions. Tap an answer — it is marked straight away and the reason opens under it. Get 80% or better and the lesson is ticked off on the year page.
Not started yet.
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What is the synchronous speed of a 4-pole motor on 60 Hz?
Answer and why
C. Ns = 120 × f ÷ P = 120 × 60 ÷ 4 = 1,800 rpm.
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A 6-pole, 60 Hz induction motor’s nameplate says 1,140 rpm. What is its slip at full load?
Answer and why
A. Ns = 120 × 60 ÷ 6 = 1,200 rpm. Slip = (1,200 − 1,140) ÷ 1,200 = 60 ÷ 1,200 = 0.05 = 5%.
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Why can an induction motor’s rotor never quite reach synchronous speed?
Answer and why
B. The rotor current is induced by the field sweeping past the rotor bars. If the rotor turned as fast as the field, nothing would sweep past, nothing would be induced, and there would be no torque. It has to slip behind to work.
-
What makes an electromagnet stronger?
Answer and why
C. The field grows with ampere-turns (current × turns), and an iron core concentrates it far more than air or plastic.
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How do you reverse a three-phase induction motor?
Answer and why
A. Swapping any two lines reverses the order the phases peak in, so the field rotates the other way and the rotor follows. Swapping all three in a circle leaves the order the same.
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Like poles of two magnets…
Answer and why
B. North pushes north away, south pushes south away; opposite poles pull together.
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A conductor carrying current across a magnetic field…
Answer and why
A. That sideways push (the motor effect) is what turns every motor. Its size depends on the field strength, the current and the length of conductor in the field.
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The same 4-pole motor is run on a 50 Hz supply. Its synchronous speed is…
Answer and why
B. 120 × 50 ÷ 4 = 1,500 rpm. Frequency sets the field’s speed; a 60 Hz motor on 50 Hz runs about 17% slower.
Preguntas frecuentes
What is the synchronous speed formula?
Ns = 120 × f ÷ P, where f is the supply frequency in hertz and P is the number of poles. The 120 turns cycles per second into revolutions per minute (60 seconds) and allows for poles coming in pairs (÷ 2). On 60 Hz: 2 poles 3,600 rpm, 4 poles 1,800, 6 poles 1,200, 8 poles 900.
What is slip in a motor?
How far the rotor lags behind the rotating field, as a fraction of synchronous speed: slip = (Ns − N) ÷ Ns. A small general-purpose induction motor typically runs a few percent slip at full load. Slip grows with load: that is how the motor makes more torque.
Ver también
- Leer la placa de datos de un motor
- Motor control course — starting, stopping and reversing the motors on this page.
- Forward / reverse — the two-lead swap, done by contactors.
- Estrella y delta
- Intro to three-phase — the lesson before this one.
- All second year lessons
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