60 Hz 3 Phase Water Pump

[johnmccafferty60]

Was asked to test a 440V 60 Hz 3 phase Star Configured Water Pump. The supply cables to the pump were 35mm Red Yellow Blue cables coming from a contactor in a panel. The test to be conducted was an IR at 500V between each phase and earth. When conducting the test I was getting a very low resistance between each phase and earth (0.2 Ohms). When I disconnected the cable and tested it alone to earth each phase was 4 G Ohms. Is the star point of this pump connected to earth? If so can someone explain the purpose of this?

PS. Very old pump, no electrical diagrams available. Been told it was Star configured, however the supply cable is not terminated in the same place where the motor is configured so I can't physically see if the star point is earthed. 

 

[ProDave]

Was that 0.2 ohms, or 0.2 Megohms? i.e did you try it on a continuity test having found such a low reading on an insulation test?

Are you sure the star point was not still connected to neutral?

 

[johnmccafferty60]

Hi. Yeah carried out continuity also same reading, 0.2 Ohms. Tested the disconnected cable which produced a reading of 4 GOhms each phase to earth. Motor still producing 0.2 ohms each phase to earth. There is no neutral in the system.

 

[ProDave]

And phase to phase on the motor?

If the star point is connected to earth (intentionally or not) that seems a pretty low motor winding resistance, what size is the motor?

 

[Marvo]

Yep 0.2 ohms is a very low resistance earth fault, even if by some wierd chance the star point was earthed the impedance reading would be higher than that unless you're talking about a very large motor. When you take the cover off the termination box does it smell like it's burnt?

 

[apprentice87]

It is a 60Hz motor. If it has been running on 50Hz, i think that they do tend to burn themselves out....... [but i am no expert!!!]

john..

 

[ProDave]

Yep 0.2 ohms is a very low resistance earth fault, even if by some wierd chance the star point was earthed the impedance reading would be higher than that unless you're talking about a very large motor. When you take the cover off the termination box does it smell like it's burnt?

I still don't really trust if he's measuring that with a continuity test or insulation test.

The other thing nobody has asked, was is the motor actually working?

If it is working without getting hot and bothered and making nasty noises, then there can't be a 0.2 ohm insulation problem and that's more likely 0.2 megohms (200 thousand ohms) you are reading on the insulation test mode.

If it's not working and you have been asked to test it,  I think you have found the problem.

 

[roys]

Not normally, it just runs a slower in speed (i.e. 4 pole motor at 1500rpm instead of 1750rpm - no account taken for slip)

Mmmm not sure canoe boy, about 10 years ago I was sent to look at a fan motor that was burnt out, when I checked the name plate details it was a 60 Hz motor that had been put on a 50 Hz supply. Got the correct motor and all was good but I did a bit of reading to find out why. Now memory ain't great so I stand to be corrected but from what I remember a 60Hz motor on a 50Hz supply will go slower as you say but if the supply voltage is not reduced the motors iron core will oversaturate overheat and burn out, as it does not have the same output HP.

As I say it was a few years ago and I can't provide any evidence so feel free to correct or give a more educated reasoning.

 

[Lurch]

I think it depends on the tolerance of the windings and how much it is run. Some will last for years running at ± 10Hz, some won't. Either way, it isn't recommended but is often done.

 

[Sidewinder]

Lots of motors these days are multiple voltage/frequency.

In that they can run at 50 or 60 Hz.

One I had the other day.

 

[roys]

I did say not normally, it will depend on the voltages (i.e. the motor nameplate) but it will be a very rare motor that would burn out if run at 50Hz instead of 60Hz. With a fan its a cube law for the power so it wont overload if you run a 60Hz motor at 50Hz. The older motors would have had a much bigger mass of iron core (unlike the newer highly efficient ones) and again this shouldn't of overheated (normally) by running a 60Hz motor at 50Hz. Like i said though normally, there are exceptions to all rules.......

Just out of interest ( your fault canoeboy ) I have just spent the last hour going through my notebooks and I found that I did the job in March 2005 and it was an oil centrifuge motor and it was the 2nd one that had been fitted and only lasted 48 hours, that was when I was sent to it and found it was a 60 Hz motor on a 50 Hz duty. As you say every case is different as I don't know how long the 1st motor lasted.

Back to the OP strange he has a 60 Hz motor in UK and if he is testing straight on to the motor tails it sounds knacked!

I was working abroad in 2005 when I met my 60Hz motor

 

[roys]

Send me the motor and ill take a look  :Salute

Long gone matey, long gone

 

[roys]

Strange it only lasted 48 hours, was it an american motor ?

Yes, what you thinking?

 

[Sidewinder]

Hot legs and stupid stuff like the Yanks use!

I suspect.

 

[roys]

Hot legs and stupid stuff like the Yanks use!

I suspect.

Are we still talking about motors

 

[roys]

I do remember it wasn't the worst American motor that I had worked on(except for the fact it was burnt out) as it did have a proper terminal block on it. Some of them had those daft wire nuts on them.