In general, what are the advantages of a three phase motor over a single phase motor.
I believe three phase motors run more efficient and have a longer life expectancy among other advantages.

Frank:

Check this out:
http://www.motorsanddrives.com/cowern/contents.html

The cowern papers are pretty handy for these kind of comparisons. Being more efficient and longer lasting are true, but also look at direction of rotation, size in HP, and versitility.

Carl

[This message has been edited by ctolbert (edited 02-06-2002).]

A few advantages for us is, Faster startup which means a lower startup cost. Lower initial cost of motor. They have less components, (capacitors, centrifigal switches)therefore less to go wrong.
If you have 480v service and use 3phase 460v motors, they do not have to go thru your own transformers thereby eliminating unnecessary load an transformers.
I know there are more. these are just a few of the benefits we see in our plant.

Polyphase motors are self starting and they may have a lower level of Reactive / shuttle power bouncing between the windings and the transformer.
Also, they require a lower FLA than their 1 phase counter parts.

All that stuff makes them have better overall efficiency for all areas concerned.

Other than this stuff, the HP [or wattage consumed] is equal on 1 phase motors and polyphase motors.

Oh ya, since they are self starting, they don't need the annoying aux/start circuit [or other starting means], and do not create double frequency pulsations on the shaft [or reflected back to the system in harmonic distortions].

Drawbacks:

• Require Polyphase AC Systems,
• Loss of one phase will create problems

Scott SET