Induction motor inrush current

[John Rumm]

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).

[Chip]

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

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.

[Pete C]

I'd measure the resistance with a multimeter and divide it into the supply voltage.

My 1.6kW pressure washer is 4 ohms and doesn't cause any problems with a 16A breaker, but the supply is a loooong length of 1.5mm (100m!).

Why not try a long (uncoiled) length of 13A cable? Might just do the trick.

Failing that try a google on 'inrush limiter', looks like RS and Farnell/CPC do them.

cheers, Pete.

[geoff]

In message , Chip writes

That's the rule of thumb figure I remember

[robgraham]

Been there, done that and have the ....., etc. I did a very similar exercise for a 12" saw table some years ago, using a similar size of motor off an old power washer, and found the anomaly that it would run as a power washer off a 13A supply, but not the saw - must have been something to do with the gearing of the belt drive I surmised while I reorganised the whole workshop wiring off a 32A MCB in the house CU !

I even dismantled the electronic starter, reverse engineered it and fiddled with the time constants, but it just continued to eat 13A fuses, which must have been the initial inrush current as you are suggesting.

Rob

[geoff]

In message , geoff writes

And as they get old and knackered, it goes up

[Andy Hall]

I have a couple of related situations in my workshop.

The cyclone dust extractor has a 2kW single phase motor which is fired up by a contactor, with that it turn operated by an impulse relay controlled by an RF remote - i.e. I can have a little box on the table saw and operate the extractor from there. Eventually I may do some current detection stuff on the saw's motor and switch it that way. At any rate, a 16A type B MCB was originally used for the dedicated circuit for this. It had a tendency to trip, but didn't always. A type C was fitted and resolved the problem.

My saw table has three 4kW three phase motors. These are individually switched in for saw, spindle and planer operations as required. The saw will take up to 315mm blades, so the power is a help.

There is also a VFD (variable frequency drive) essentially an electronics module, which is useful in several ways.

Firstly it ramps up rather than hard on (as it were) and thus reduces the inrush current.

Secondly, it provides single to three phase conversion.

Thirdly it does speed control.

Fourthly it does electronic braking.

I think that he should be concerned about the mechanical arrangements to drive the blade etc. This much power on a small blade could pose some safety problems with suitable belts, bearings, pullies etc.

[John Rumm]

Yup, that was the sort of assessment I was after...

The plate on the motor listed Imax as 17A so that would give 51A, although if fig 3.4 of BS7671 is to be believed the instantaneous trip region for a 16A type B ought to be 80A or more.

The 13A plug fuse also had no difficulty with repeated starts, so I guess that would set an upper limit round about 90 - 100A

Plenty good enough for this application! ;-)

[John Rumm]

I have a similar issue in mine if I plug a 3kVA transformer in too near to the CU - it can trip the 20A MCB. In the middle of the circuit or on a short extension however it is fine.

This soft start for motors can make a big difference to inrush. I think this is pretty much a requirement for motors much above 3kW IIUC.

I did suggest that not over egging it on the belt drive might be a good plan so as to maintain a intentional weak link. I also set the current limit on the starter to prevent the motor being able to ever develop full output power.

[mark]

Brook motors give inrush as 500-550% full load amps for single phase motors and 300-650% for three phase motors.

[John Rumm]

This was single phase. The 500% of max rating would be 85A which would tie in pretty closely with what I was seeing (i.e. 16A B MCB tripping, but not the 13A fuse).

[Dave Osborne]

Have a look here:

[John Rumm]

single phase ones - the MCB ratings suggested seem excessive, especially for type C devices.

[mick]

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.