3 phase motor current rating

[Jempe1012]

Hi

can Someone please show me how I can work out the current rating of a 3phase motor from its kw rating?

Singel phase is just kw/watts but I know 3phase is defferent.

Cheers

 

[thekingiam]

p=vI
I=P/V
?????????

 

[telectrix]

I = P/ (sq.rt.3. x V x PF)

so e.g. a 5kw motor

I = 5000/ (1.732 x 400 x 0.8) assuming a 3 phase voltage of 400 and a pf of 0.8

I = 5000/(554.25)

I = 9.02A

 

[thekingiam]

I = P/ (sq.rt.3. x V x PF)

OHH YEAH!! and what telectrix said.

 

[Knobhead]

I cheat and drop the figures in to a spreadsheet.

But he’s the formula for you:
A=((KW/3)/(V/√3))*1000

If power factor is known:
A=(((KW/3)/(V/√3))*1000)/cosΦ

If efficiency is known:
A=((((KW/3)/(V/√3))*1000)/cosΦ)/Eff%*100

Once you’ve get a few more posts under your belt I’ll e-mail the spreadsheet to you if you ask. Sorry but I’ve had quite a few newcomers asking for copies in their first post, I wrote it for the benefit of members.

 

[snowhead]

I normally just read the data plate on the motor :biggrin5:

 

[Knobhead]

Cheat!

 

[Marvo]

I normally just read the data plate on the motor :biggrin5:

Me too, or google the data-sheet and as a last resort drag out a clamp meter.

 

[Jempe1012]

I = P/ (sq.rt.3. x V x PF)

so e.g. a 5kw motor

I = 5000/ (1.732 x 400 x 0.8) assuming a 3 phase voltage of 400 and a pf of 0.8

I = 5000/(554.25)

I = 9.02A

Very helpful thankyou, what do I do if I don't know the power factor I think they just give the motor kw rating?
Thanks

 

[Marvo]

You can kinda guess the PF as 0.8. This may vary depending on how well loaded the motor is running (pf gets better as load increases) and whether or not it's a high efficiency motor.