3PH Motor current requirments

[r10crw]

Apologies but it’s been a while since I did any of this and need some advice.

We’re looking at a new venture where by our workshop is used to build some large hydraulic pump units. One of things I need to look at is getting three phase into our workshop but am looking at getting a rough idea of the power requirements for the transformer etc.

Ive been told they will be using up to 37KW motors but they could only give the spec sheet for a 15KW.

I estimated previously that at 15Kw motor at 440V with a 0.8 power factor and 90% efficiency you would be looking at 27 amps. So allowing 4 times starting current, I need around a 100amp fuse for the 15Kw? Trouble is I just received the spec sheet for the motor and it quotes the starting current at 207 amps!!!!
Am I being stupid?

So, now I’m thinking that a 37KW motor is going to need 538 amps (around eight times nominal), surely this cannot be correct?

Sorry but know how smart some of you folks are!
Granted 4 times starting current is a number I picked out of my head but just seem to remember 3.5 from my uni days. Bearing in mind I run two houses and only one motor at any one time from this same source does anyone know what size of cable Im going to need to run to the house and what size of tranformer is likely to be needed?

Craig.

[s29ttc]

Use this

http://ecatalog.weg.net/TEC_CAT/tech_mo ... mpresa=110

Pull up a data sheet and check.

I know it's common for large machines that I have worked with to be 600% with DOL start. This is why some put in soft starts or even inverter starts to limit the starting current. I'm not sure how you size cabling though

[tut]

Was going to post the figures, but will wait a while to give Robin a chance, as he is probably walking into work.

tut

[r10crw]

Thanks Scott,

The equivelent motor on there has a locked rotor current of 430 amps so looks like the 530 I estimated was nt too far out. Questions is what do I really need as the starting current will only be for a short transient. Do I need to spec a transformer that can supply this full load?

Tut we could use your living room as you have three phase already! I hate to think how much your Halogens burn but you must be pretty close to the above figures!!

[robin]

If the motor is stalled then the starting current could easily be infinite, but it will also melt its windings very quickly in that case.

I assume there are time factors to the cable current limits and fuse blowing - if it only uses 100's of amps for 100ms, will the fuse still blow?

My guess is that your fuse needs to be of a type that has mutliple ratings - a sustained current rating and a transient rating to allow for inductive load startup.

Re: the transformer you'll find that it effectively limits current by dropping voltage. So if you try to pull more power through the transformer than it's built for the output voltage will droop, effectively providing a soft start for your motor (though you can reach steady state conditions other than full voltage running - this would be unlucky, though). Of course the transformer should not be run at its limits for sustained periods, but I would guess that like the fuse it will tolerate transients - in fact it will tolerate them better than the fuse.

Cheers,
Robin

[thinfourth]

Unless you are running them a lot then buy an old big 3phase genny as that is all you would need for test running them

And a transformer won't give you single phase to 3 phase you will need a phase converter

[kpb]

Have this issue a lot when putting ROV, s and Trenchers onto vessels.
The load current is only usually for the first 4 cycles when DOL starting but I have recorded as high as 8 times full load current and 6 times is the normal all dependant on motor design and lots of other black art things which want bore you with.
It is the trip curve on the circuit breaker protection device that is important and usually a "D" rated breaker or transformer rating is designed for the start-up current but the circuit does not have to be designed to take these loads on a 100% duty cycle.
Depending on the loading if any on the motor during start up a 63 amp supply would start this 37 KW motor with the correct trip curve protection rating but a 100 amp would definitely give some head room.
Have you got the option to allow for start delta operation on the motor?
A soft start would help but have a down side I had a lot of problems with harmonics on some vessels so cut their use down and prefer S/D or an inverter drive ramped up, if you go the soft start route make sure it is a 6 thyristor 6 pulse unit not a cheap 4 pulse and diode type.
Cable size if a multi core around 4 core 16mm or if a really long run you could use 4 core 25mm.
All IMHO of course.
Cheers
Kinnon