Re: [CAD_CAM_EDM_DRO] Universal motor question

Tom Murray wrote:

> Is there a way to estimate the maximum current that a series wound
> universal motor will draw by looking at the voltage, no load current
> and rpm, and rated hp?
>
> The motor in question runs on 32 VAC/VDC, at 9800 RPM, with a draw of
> 5 amps (no load) and is rated at 1/2 hp.

You can calculate the instantaneous surge when starting from a dead stop,
but the running current is a factor of the motor's losses at the idling speed.
If it were perfectly efficient, the current would be zero.

Ahh, now rated HP helps a lot. Obviously, you can figure out what the
current draw would be for a perfectly efficient motor. DC makes it easier,
as there is no power factor to deal with. So, 1 HP = 746 W, so this
motor should draw 373 W. P = I * E, so I = P / E, or 373 / 32 = 11.66
So, the motor will draw at least 11.7 A at rated load. I would suspect the
rated load draw would be less than the ideal full load plus the idling current,
so maybe 14 A or so. Assuming 80 % efficiency, it should come out to
11.66 / 0.8 = 14.6. This would be a fairly safe value for acceptable
current. You could also run it at this load for a while, and see if it appears
to be overheating.

Jon