Three phase motor

[Richard]

Good evening,

I had to troubleshoot a three phase milling machine earlier. I'm not especially hot of plant like this, but i'm quite confident that the control system is good, but that the motor is faulty, In that when power is applied it pulls the breaker.

It's a 1HP 3 phase motor,drawing 1.4Amps according to the data plate. Is there a formula to work out what the winding to winding restance of the coils should be? as i'm guessing it isn't just as simple as ohms law.

I Megged to earth and they were all good, so I was wondering if there is a short between phases. When I tested, I was getting approx 11ohms between all three windings.

Thanks

 

[stef]

The fault could be with the contactor, i.e. starting on two phases. Does the breaker trip straight away or a few seconds after starting the motor?

 

[THE DUKE]

Simple 3Ph motor tests


I don't remove terminal box of motor until I have done the tests below, modern control panels are IP 22 (touch safe) do not open a motor terminal box lid if live normally you will have six studs on block with nothing stopping you from touching or shorting them out with the lid as you remove it


Motor running


Use clamp meter to measure current should be balanced +/-10% no more than nameplate details, in control panel, starter or local isolator (this is one reason you should leave a loop of cable at any switch or starter etc.)


Check for heat, motor windings (Class F insulation) can withstand 155 deg c bearings 125 deg c but would not want it to run any hotter than you can bear to touch


Motor noise and vibration this should be minimum but it depends on load and how its transmitted (pulley, coupling,sprocket etc.)


Motor Isolated


IR test do this at bottom of contactor O/Ls this will check your cables and any local isolator (make sure this is on) as well


Continuity again do this at bottom of contactor O/Ls same reasons as above small motors couple of hundred ohms large less than an ohm


To check for supply disconnect at bottom of contactor O/Ls test for all three phases, by doing it here you will check fuses, contactor and O/Ls all at once


Open terminal box lid and smell if it is burnt you will know


Check for loose connections


Check motor is connected correctly star or delta




Mechanical tests does motor turn (you might no turn large motors)
There should not be any lift in the motor shaft


Common mistakes testing motors


Testing voltage at motor terminals
One phase down will not show as you will get false reading though windings

 

[Gage]

Test O/L & contactor. Check the breaker is the correct size and starting current is not tripping it, and give all the above a go as well, especially checking if the mechanisms are free, just try to turn it by hand, if you can relatively freely it's ok

 

[Gage]

Also I hate motors on breakers

 

[Silly Sausage]

Check the local emergency stop button has the same number as the motor and make sure it's pressed in.

 

[Knobhead]

Maybe not in the right order. But I’m in grumpy bad tempered mode.

First question:
Do you know if the motor is Star or Delta connected?

Second question:
Have you separated the windings?

Third question:
What are the IR readings between the phases?

Fourth question:
Have you done a balanced CR test on the individual windings and what are the readings?

Fifth question:
Have you tested the voltage between phases at the terminal box?

The sixth question is an absolute killer:
Do you know how a 3Ph motor works?

I suspect the answer will be NO to most of the above.


Sorry I’ve spent all my working life around motors.
Unlike others, I don’t guess. I need sound figures to work on.

If I appear harsh it’s with reason. Motors have to be understood to fault find on them. Blundering around isn’t my style, concise answers are needed.

 

[Outspoken]

:iagree:

Tony nailed it...

 

[Engineer54]

Also I hate motors on breakers

Why??...

 

[telectrix]

because the salt in the water of the breakers is corrosive.

 

[Darkwood]

My question is ; is this a recent issue on an existing set-up or is it just tripping front end on a newly wired machine and you can't pinpoint the cause?

 

[Knobhead]

May as well give up with this one. Where's the OP?

 

[silva.foxx]

May as well give up with this one. Where's the OP?

Possibility A: fixed it and lives by the 'knowledge is power; I'm not sharing it' motto, or
Possibility B: backheeled it on to someone else who fixed it in minutes without asking us cybersparks, or
Possibility C: denied he knew there was a fault, hid all evidence of trying and is now on the sick whilst a spark is called in.

Or... a long shot... is too busy to post the solution as he's holidaying with the poster of the '50A SSR problem'.

 

[Knobhead]

And here’s me thinking I’m cynical!

 

[Spoon]

May as well give up with this one. Where's the OP?

Its happening more and more. The OP's must not like giving feedback.
Personally im going with Possibility C of silva's post