On Fri, 25 Jan 2008 21:57:18 +0000, John Rumm wrote:

Does anyone know if there is a way to assess the typical inrush current
for an induction motor?

I helped a mate install a new motor in a small saw table the other day.
The motor he had to hand was a monster 3kW jobbie that had been bought
for some other purpose but never used. I suggested this might be
overkill for the application, since we are only talking about a 7" blade
here!.

Anyway hooked it up via a DOL starter and set the max current trip on it
to something sustainable on a 13A plug. (the motors full load draw being
17A IIRC). Not unsurprisingly we could only spin it up on a circuit
protected by a 32A type B MCB - any circuits off 16A MCBs tripped
immediately. Load when running (but not cutting) was 7A (could not try
any cuts since we did not have a suitable belt and pulley).


Single phase DOL motors should be fused at about 6x full-load current,
using type gM or HRC fuses.

Alternatively you can use type C (5-10x FLC) or type D mcbs (10-20x FLC),
in which case you really need to check the tripping curves but you can
often get away with just picking the nearest size to your FLC.

Note that the size of protection used depends on the motor rating, how
long it takes to get up to speed and the load. A type C may be ok for a
pump, but put the same motor on a fan and you may need a type D.

Soft-starter units will allow motors to be started on much smaller fuses
or mcbs as they limit the available motor current during startup.

Hope this helps.

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info http://mixpix.batcave.net