On Fri, 27 Aug 2004 03:44:47 GMT, "Jerry Martes" wrote:
It might be concluded that 3 phase motors that arent loaded heavily for
long periods can be run from single phase without an idler.

Right, though they can't start on 1 ph. You need either a rotary converter,
a static converter, or a pony motor to start the 3 ph motor. It also won't run
smoothly on 1 ph power because torque will go through zero twice a cycle.

But, a 3 phase
motor will produce its full name plate HP even when fed single phase with no
idler.

It may, for a short period, but it will be drawing current in excess of its ratings
on the driven phase. A 3 ph motor driven by 3 ph power has its current divided
3 ways. In other words, its windings are sized to each handle 1/3rd of its rated
power. If you try to draw full nameplate HP from it while it is driven by 1 ph
power, the driven windings will be overloaded.

That's why there's a rule of thumb that a rotary converter needs to be sized
1.5 times the HP of the load motor, so the driven windings of the rotary converter
aren't overloaded when delivering full 3 ph power to the load motor.

Example, 1 HP load motor should be driven by a 1.5 HP rotary converter,
0.5 + 0.5 + 0.5 = 1.5, so that the current drawn by the load motor, 1/3 +
1/3 + 1/3 = 1 is the same as the maximum permissable 0.5 + 0.5 = 1 of
the two driven windings of the rotary.

Of course the rotary does the *transformation* from 0.5 + 0.5 to 1/3 +
1/3 + 1/3 by its rotary transformer action. But the key concept is that
power in must equal power out no matter how it is transformed, and
the currents must not overload any windings in the process.

Gary