Re: [CAD_CAM_EDM_DRO] a little OT, Determing AC motor ratings

dave_ace_me wrote:

> Hi all,
>
> I was lucky enought to get a deal on some equipment and one of the
> motors is a single phase, 56 frame 208/230 vac motor. 1/2 inch shaft,
> made by Marathon.
>
> The nameplate is missing so I have no idea if it is a 1/2 to 1 hp
> unit.
>
> Is there some (simple) way to determine the motor hp or other
> ratings that would lead me to know that ?
>
> Sorry if this is too off topic, but I am at the end of me rope at to
> searching the net on this one.

Very roughly, there is. What you do is hook up the motor with no
load. You measure the current draw. The current for a 1 Hp 230
Volt motor at 230 V will be approximately 5.5 A, although it varies
more than a little from type to type, ie. a capacitor run motor will
be different from a plain induction motor.

What happens as you add load is that the current doesn't really
increase much, but the power factor increases dramatically, so
that the real power consumed goes from very small up to the
rated load. This change in power factor cannot be seen on
an ordinary ammeter, it needs a power factor meter, phase
angle meter or wattmeter to see the difference. Somewhere
near full load, the current draw starts to increase.

A 1/2 Hp motor should draw something like 3 Amps.

Jon