A rotating magnetic field is a resultant field that rotates in space when several phase-shifted coils are energized.
The transformer was great, but static. Now we want motion.
The historical problem with motors was how to get electricity to the rotating part (the rotor) without the cables tangling up. The classic solution was brushes (carbon contacts that rub), but they are dirty and wear out.
The rotating magnetic field allows torque to be transferred to the rotor without feeding it through a mechanical commutator. It is the foundation of three-phase synchronous and induction motors.
A Wave That Rotates
Think of the wave in a stadium. The spectators (the coils) remain in their seats. However, if they coordinate to stand up and sit down with a slight delay from one another… The Wave moves around the stadium.
In a three-phase motor, we do exactly the same thing:
We place three coils in a circle (Stator), physically separated by 120º.
We energize them with three phases (L1, L2, L3) that are phase-shifted in time.
What happens is pure mathematics:
- At millisecond 1, Coil A pulls strongly (North up).
- At millisecond 2, Coil A weakens a bit and Coil B pulls strongly (North to the right).
- At millisecond 3, Coil C takes over.
The result is that, although the copper coils are stationary and bolted to the chassis, the resulting Magnetic Field (the North-South arrow) rotates frantically in the center of the motor.
This is simply a consequence of how we generate three-phase power, by rotating a magnet in a generator.
Three-phase current is the reflection of this rotation. By introducing it into three coils, we are “reconstructing” the rotation.
The Parts of the Motor
We divide the motor into two independent parts that do not touch:
-
The Stator (The Stationary Part): This is the external housing. It contains the copper coils connected to the three-phase mains. Its sole purpose is to create the rotating magnetic field. It is the “ghost” that spins.
-
The Rotor (The Moving Part): This is the central cylinder that rotates. It needs no cables or brushes. It simply “feels” the stator’s magnetic field and chases it.
Synchronous Speed (
At what speed does this invisible magnetic field rotate? It is not random. It depends strictly on the mains frequency and how the winding has been constructed.
The formula every engineer should know is:
Where:
: Synchronous speed (in Revolutions Per Minute, rpm). : Mains frequency (Europe: 50 Hz, America: 60 Hz). : Number of Pole Pairs in the motor.
What Are Poles?
They are the “pairs” of magnetic coils.
- 1 Pole Pair (2 poles): One North and one South. The field completes one full revolution per mains cycle.
- 2 Pole Pairs (4 poles): The field advances more slowly, completing half a revolution per cycle.
Standard speeds in Europe (50Hz):
| Pole Pairs ( | Total Poles | Field Speed ( | Typical Use |
|---|---|---|---|
| 1 | 2 | 3000 rpm | Fast pumps, small fans. |
| 2 | 4 | 1500 rpm | Most common (conveyors, machinery). |
| 3 | 6 | 1000 rpm | Heavy loads, elevators. |
This is the speed at which the magnetic component travels. It is not necessarily the speed at which the physical shaft rotates (although they often coincide).
How the Rotor Responds
Now that we have an invisible magnet rotating at 3000 rpm (for example), what does the rotor do?
- Synchronous Motor: The rotor creates its own field using permanent magnets, electrical excitation, or reluctance. In steady state, it rotates at synchronous speed.
- Asynchronous or Induction Motor: The field induces currents in the rotor. When operating as a motor, it rotates slightly slower than the field (for example, at 2950 rpm versus 3000 rpm).
