Poles and phases explained
Two numbers that define how a motor's magnetic field rotates.
- Explain poles and phases explained at engineering level
- Read the numbers that define the behaviour
- Apply the idea to a real drive unit
Overview
Pole count and phase count are the first two numbers any motor engineer quotes, because between them they define how the rotating field is built and how fast it turns.
In depth
Three-phase windings
A three-phase motor has three sets of windings spaced 120 electrical degrees apart. Feeding them with three sinusoidal currents, each phase-shifted by 120 degrees, produces a magnetic field that rotates smoothly rather than pulsing — this is the basis of virtually every AC traction motor.
Synchronous speed depends only on electrical frequency and pole count — nothing else.
RPM Calculator
Synchronous speed from supply frequency and pole pairs, plus road speed.
n (rpm) = 120 × f ÷ poles ; v = n ÷ ratio × wheel circumferenceAn AC machine's synchronous speed is set by the inverter's output frequency and the number of magnetic poles. Induction machines run slightly slower than synchronous speed — that difference is slip.
Example: At 200 Hz with 8 poles: 120 × 200 ÷ 8 = 3,000 rpm.
- More poles means more torque per ampere but lower maximum speed for a given inverter frequency.
- Slip is zero for permanent-magnet synchronous machines by definition.
- Inverter switching frequency and output frequency are different things.
Key takeaways
- Three-phase windings, spaced 120° apart, create a smoothly rotating field.
- Synchronous speed is set purely by frequency and pole count.
- More poles means more torque per amp but a lower top speed for the same drive frequency.
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