45kW Motor Calc

[gtownspark]

Hi Guys

I recently installed a supply to a 45kW motor for cooling tower.

I carried out the following calculation:

IL=P/VLx1.73
=45000/400x1.73
=45000/692
=65.02A

There was only a short run from the cabinet to the motor so a 16mm SWA seemed the correct cable

After the motor was running I checked the current on each phase and found

L1 - 61A
L2 - 81A
L3 - 67A

Does anyone have an explaination as to why there would be so much more current being drawn through L2?

 

[johnboy6083]

Have you checked the windings? Before commissioning a motor, I think it's a good idea to megger the windings, and measure their resistance. This can then be used as a benchmark to test against. What's your Uo on each phase?

 

[johnboy6083]

Also mate, how is the motor started? If its DOL and maybe subject to frequent starting, then you should multiply your Iz by 1.4 to compensate. If its VSD, then manufacturers guidance should be followed due to the wide range of frequencies.

 

[rob@etc]

I = Kw / (1.732 v V x power factor)

 

[gtownspark]

Thanks for the advice johnboy, I'm on site tomorrow so will check those points, I need to check the Uo and I haven't checked the resistance of the windings.

 

[benji]

can you have 45kw on d.o.l.

 

[Darkwood]

can you have 45kw on d.o.l.

You can have even higher dol if your supply set-up is adequate but in general controlled starting is usually on 4.5 kw and higher as its possible for these to take out a standard 100amp dno fuse

 

[johnboy6083]

A lot of sites I work on have large motors on DOL. we're usually ripping the out and replacing with new motors on VSD's

 

[Knobhead]

Check the voltage at the motor for imbalance, (motor running). If the voltage is out of balance it points to the supply / starter. Balanced, check the motor coils for equal CR.

Quick way to find which direction to look.

 

[UKMeterman]

Hi,
Is it star or delta connected? I would suggest using a 3 phase power analyzer such as a fluke 434/ 435 to measure the harmonics at startup and running. The hire is not cheap, but is less than even weeks electricity for that fan...

 

[Knobhead]

Why hire equipment when basic tests haven’t been reported back?

There will be harmonics at start up no matter what method is used, what are you going to do about them? They don’t affect normal running.

Without knowing the basics what good is a load of globally gook from a condition monitor going to prove?
PF will be haywire, 5[SUP]th[/SUP], 7[SUP]th[/SUP], 11[SUP]th[/SUP], 13[SUP]th[/SUP] and 17[SUP]th[/SUP] harmonics will be high, that’s a normal motor start up!

We need basic steady running readings first to make a reasoned decision.

 

[UKMeterman]

The point is that there is something odd going on here with a very expensive bit of kit. If the windings do have the correct resistance then there could be a supply issue and need to identify what is going on. As for gobbdly gook, the unit is a three phase scope that can measure voltage and current and so again will help to point the finger.

 

[Knobhead]

There’s nothing “odd” going on, there is a fundamental fault. I don’t ask someone to take readings without reason.

As for the condition monitor:
I know what the kit is, I’ve used it on far larger bits of kit.

Back to my original statement, take basic readings first, don’t muddy the waters with a load of rubbish that isn’t needed. It’s a common failing now, information overload and then neglect the fundamentals of a situation.