Induction motor amperage ratings and use

[cgallery]

I was playing with the little digital ammeter that Loring (LCHIEN) likes. This time on my bandsaw.

The saw has a 6-amp induction motor. Running, but doing no cutting, it was drawing about 3.4-amps.

When feeding 5.5" thick white oak into it at a moderate rate, it used about 4.5 to 4.75 amps.

Pushing harder, I was able to get the amperage up to 6.5 and even 7-amps, at least momentarily.

I've pushed the saw that hard before, never knowing I was exceeding the motor's rating. I've never noticed the motor getting especially warm or anything.

How dangerous is it to push an induction motor over its amperage rating?

[master53yoda]

An induction motor( one without brushes) will deliver about 250% of rated HP. If your overload conditions are no more then a couple minutes and not more the about 15% you won't do any damage. If you run over load long enough to generate some extra heat let the motor run for about a minute with out any load so that the fan can cool the windings down.

Electric motors such as HF tools I would be careful about overloading because I doubt the winding insulation is as good as american made motors would have.

If the service factor is 115 or 125 the motor is designed to operate at 115% or 125% of rated load continuous.

The current draw on an unloaded motor can be as much as 60% out of phase from the amperage, this is known as power factor. The more you load the motor the closer the voltage peak and the amperage peak are together and the closer the power factor will get to about 90% at full load.


hope this helps answer the question

[cgallery]

Thanks Art! Excellent explanation, as always.

[BrazosJake]

An induction motor( one without brushes) will deliver about 250% of rated HP.

So is that locked rotor amperage, or how they come up with the ridiculous "Peak developed" HP ratings?

[master53yoda]

The max delivered HP would be at about 75 to 80 % of full RPM much slower then that and the slippage is so great that the delivered power starts falling off. It is how the get max or peak horsepower numbers.