On Fri, 25 Jan 2008 21:57:18 +0000,it is alleged that John Rumm
spake thusly in uk.d-i-y:


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).

Hi John, not sure how accurately you're wanting to assess it here, but
I was always told when training as an electrician, to assume 3x the
maximum run current as the inrush with induction motors, and
approximately 2x with universal motors. That was for BS88 (or BS1363
for smaller 1ph loads) fuselinks, which approximate (IIRC) a C-Curve
MCB.

In these H&S rich days I suspect that 'training as electrician' and
'assume' would be a bad mixture for the college/workplace, although
the late 80s/early 90s wasn't THAT long ago, it seems like it now.

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