# How a Synchronous Motor Works

How a synchronous motor locks its spinning rotor to the alternating current that drives it, and why it cannot start on its own\.

By Stephen Blackwell

Topic: Machines

[Open the interactive Jig](https://ask.jig.com/j/52PpwJ1aVBQo)

## Transcript

### 1. A motor that keeps perfect time

This is a synchronous motor\. It turns in perfect step with the electricity that feeds it, never faster and never slower\. Let's look inside and see how it manages that\.

Drag anywhere to turn the model and look around it\.

### 2. Coils that never move

The outer ring is the stator, the part that stands still\. It holds three pairs of copper coils\. Alternating current, electricity that keeps flipping direction, flows through each pair in turn\.

Watch the pole faces, the curved iron tips the coils sit on: red marks a north pole, blue a south pole\.

### 3. A magnetic field that spins

The three pairs peak one after another, so their magnetism adds up to one field that sweeps around the ring\. Nothing in the stator moves, yet the field spins\. The gold arrow shows where it points\.

Mains electricity in many countries flips back and forth 50 times a second, called 50 hertz \(Hz\)\. Then this field goes round 50 times a second; here it is slowed down about a hundred times\.

### 4. The rotor: a steady magnet

The rotor is the part that turns\. Its coils get direct current, electricity that flows one way only, through carbon brushes rubbing on two brass slip rings\. That makes the rotor a magnet with a fixed north and south pole\.

### 5. It cannot start by itself

Switch on with the rotor standing still and the field sweeps past far too fast\. The rotor is tugged one way, then the other, so it only trembles\. A synchronous motor is not self\-starting\.

### 6. Catching the field

So the motor gets help\. Copper bars in the rotor's pole faces let it spin up on its own, a little slower than the field\. Near full speed its north pole is pulled into step with the field and locks on\.

Synchronous means "at the same time": rotor and field now turn together\.

### 7. Add a load

Use the slider to press the brake onto the pulley\. The rotor leans back a little behind the field, an angle engineers call the load angle, but it keeps exactly the same speed\. Far too much load would pull it out of step and it would stop\.

### 8. Speed set by the supply

Its speed depends only on how often the current flips and on the number of poles\. Two poles on 50 Hz mains turn 3,000 times a minute; four poles turn 1,500\. That steady pace drives clocks, pumps and big compressors\.

Spun by a turbine instead, the same kind of machine becomes a power\-station generator\.