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Three-Phase Motor (Three-Phase Electric Motor)

Definition

A three-phase motor is an electric motor supplied by three alternating voltages of equal magnitude and frequency, offset from one another by 120 electrical degrees, which together produce a rotating magnetic field in the stator without any starting device.

The three phases are what make the field rotate. A single-phase supply produces a field that pulses rather than turns, which is why single-phase machines need a capacitor or an auxiliary winding to start at all and a three-phase machine does not.

That is the whole reason industrial motors above a few kilowatts are three-phase: the starting mechanism is free, the machine is smaller for the same output, and the power is delivered over three conductors instead of two.

Also written: 3 phase motor · three phase electric motor · three phase motors · 3-phase motor · 3 phase motors

At a glance

What the term fixes, and what it leaves open

Supply	Three voltages, equal magnitude and frequency, 120 electrical degrees apart
Rotating field	Produced by the supply itself — no starting winding, capacitor or centrifugal switch
Machine types covered	Both induction and synchronous machines are three-phase; the term does not choose between them
Winding connection	Star (Y) or delta (Δ) — set by the winding design and the supply voltage, not by preference
Terminal markings	U1 V1 W1 / U2 V2 W2 in the IEC system; T1–T6 (or T1–T9, T1–T12) in the NEMA system
Reversing	Swap any two of the three supply leads
LEADGO standard supply	380 V, 50 Hz; other voltages and 60 Hz on request
What it does not fix	Efficiency class, enclosure, cooling, duty type, speed range — all separate specifications

Why three, and why 120 degrees

Three windings spaced around the stator, fed by three currents that peak one after another, produce a magnetic field whose direction sweeps round the bore. The rotor is dragged after it. Nothing has to switch, commutate or engage — the rotation is a property of the supply.

That is also why the direction reverses by swapping any two leads: swapping two phases reverses the order in which the windings peak, and the field sweeps the other way. It is the single most common site adjustment on a three-phase machine, and it needs no parts.

Star and delta are not grades

A winding designed for 380 V in delta and 660 V in star is the same winding either way — the connection decides what voltage each coil sees, not how good the machine is. Connecting a machine in the wrong one is not a performance choice, it is an overvoltage or an undervoltage.

This is why the terminal box markings matter more than they look, and why the two systems — IEC letters and NEMA numbers — have to be read in their own convention rather than translated by guesswork.

Common misreadings

What Three-Phase Motor does not tell you

Three-phase does not mean induction

Synchronous machines, including permanent magnet ones, are three-phase as well. The phrase describes the supply, not how the rotor works.

Three-phase does not fix the voltage

380 V, 400 V, 415 V, 460 V, 690 V and 6 kV machines are all three-phase. Voltage and frequency are separate entries on the nameplate and both have to be matched.

Three-phase does not mean it can run on a drive

A converter output is three-phase too, but it is not a sine wave. Whether a machine is suitable for converter supply depends on its insulation and cooling, not on the number of phases.

Three-phase says nothing about starting current

Direct-on-line starting current is a property of the rotor design. It is the reason NEMA design letters and locked-rotor ratios exist, and none of that is implied by the term.

Questions

Frequently asked

What is a three-phase motor?

An electric motor supplied by three alternating voltages of the same magnitude and frequency, offset from one another by 120 electrical degrees. Because the three phases peak one after another, the magnetic field in the stator rotates on its own, so the machine needs no capacitor, auxiliary winding or other starting device.

How do you reverse a three-phase motor?

Swap any two of the three supply leads. That reverses the order in which the stator windings reach their peak, so the rotating field sweeps the other way and the shaft follows it. No parts are involved, which is why it is the standard site correction when a pump or fan turns the wrong way on first start.

What is the difference between star and delta connection?

They are two ways of connecting the same three windings, and they decide what voltage each individual coil sees. A machine marked 380 V delta / 660 V star has one winding set that is connected one way on a 380 V supply and the other way on a 660 V supply. Choosing the wrong connection is an overvoltage or an undervoltage, not a preference.

Are all industrial motors three-phase?

Nearly all of them above a few kilowatts, because the starting mechanism comes free with the supply and the machine is smaller for the same output. Every motor series on this site is three-phase; LEADGO does not build single-phase machines.

Does three-phase tell me anything about efficiency?

No. Efficiency class is a separate specification with its own standard — GB 18613 or IEC 60034-30-1 for line-fed low-voltage machines. Two three-phase motors of the same frame and output can sit two efficiency classes apart.

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