How can I tell the difference between a single phase and three phase electric motor?

Question

this is for an air compressor I might purchase for my home, kellog american, 80 gal. upright. the motor looks a bit big for single phase. I cant find a tag on it. thank you

Answer 1

Look at the the electric motor's power cord plug: if there are 4 pins (3 phase and 1 ground wires), it is a 3 phase; if there are 3 pins (1 phase, 1 return, and 1 ground), it is a single phase.

One more double-check clue, if the motor has one or a bank of large capacitors mounted on its body, it is a single phase (provided it has an RPM of 1800 or higher: an assumption very likely for a few-HP motor of this compressor size). This starter cap creates a phase-offset voltage which drives a startup stator coil. This coil gives an itermediary jump point between the 180-degree-apart stator field increments. Without this capacitor the starting rotation direction would be ambiguous and the starting torque very weak. Weaker single phase brushless motors that are wound to turn at less than 1800 RMP have the stator field jump in 90 degree or smaller angle increments and hence do not need the phase offset voltage of a startup capacitor.

A 3-phase motor has the field rotate in 120 degree increments around the stator circle so it needs no such startup capacitor. It has a much stronger starting torque and a definite starting rotation direction which can be reversed by swapping any two of its three power cord phase wires.

Answer 2

The single phase motor will have probably no more than 4 leads coming from the windings. A 3 phase motor will have 6 or more. If the wires are marked with an A B or C it's 3 phase. This is usually a crimped ring around the wire. It could be a dual voltage motor with different ways to hook it up for either 230 V 3 phase or 480 V 3 phase. If single phase, it will be a 240 Volt.

Answer 3

Hi,
Fairly straighforwards, c.r.
No need to dismantle much.
It should have a rating plate riveted on somewhere, but if that has gone missing, how many terminals does it have in the connection box ?
If two + earth - single phase
If four + earth - 3 phase.

You mention American manufacture ?
Achtung the voltage difference.
UK any single phase to neutral should be circa 240V
Or 415V across phases.
US stuff will be 110V - 115V from any phase to the neutral line.

Not much that you can effectively do with the motor, so a step-down transformer, as used on building sites, etc, with yellow cable eminating from the yellow box should really be deployed in the UK., for powering 110V tools & equipment.
God bless H&S regs. A nuisance sometimes.

China is the odd one out in this department, with 342V across phases. Typical !

If the item transpires to be 3-phase, and only single phase is available, you are looking at a motor-alternator unit.
Single phase motor, tri-phase alternator.

Probably more costly than the device itself.

All the best with it, though.
Keep me posted via this section, interesting to hear the outcome !

Bob.

Answer 4

Three phase will have 3 power terminals. In the case of a three phase motor the earth goes onto the case and there is no neutral wire as the voltage is between any two of the phases which are 120 degrees apart. When coupling up ensure that the phases are connected to the correct terminal by colour.

Answer 5

Horse power for horse power, the 3 phase motor will actually be the smaller of the 2. The large motor you refer to is most likely a single phase motor.
The 3 phase can be smaller because you have 3 windings, each one 120 degrees apart providing the torque needed to move the load attached to the motor. It takes less material to accomplish this. A single phase motor has to have more material, that is wire in the coils, to produce the torque needed to move the load. This is why they tend to be physically larger than a 3 phase motor.

Answer 6

Single phase: one voltage phasor rotating at the system frequency. Usually limited to small loads such as individual residential units. Three phase: three voltage phasors, displaced 120 electrical degrees, rotating at the system frequency. Typical for generation, bulk power transmission, and industrial loads including large motors.

Answer 7

A single phase motor will have a capacitor to induce phase shift as there is no other way to stare the motor other than rotating it manually then switching the power on as it spins

Answer 8

A single phase motor will have the connections active, neutral and earth (frame).
A three phase motor will have phases A, B, C, Neutral and earth.

Answer 9

If it has more than 3 connections it is a 3 phase

Answer 10

Single pahse motors will have starting and run capacitors on it, they look like plastic or metal cylinder with two connection on top

Look on the data plate it should tell its voltage (3phase 400/415v single pahse 220-240v)