motor tripping out 1

[ronbol]

a motor has been tripping out the breaker. (single phase 240v (UK voltage) )
If i disconnect the motor from the contactor the breaker stays latched the contactors also latch in indicating a fault on the motor.

How can I be sure that the motor is faulty?.
the motor has a capacitor on the top so i assume it's a capacitor start/run motor.

The only wires going from the contactors to the motor are LIVE, NEUTRAL, EARTH (3 core cable)

If i put a meter across the live and neutral I get 2000 ohms which sounds very high to me.
There dosen't seem to be any leakage from the live or neutral down to earth.

am i reading this right?

 

[Marke]

Hello ronbol

There are number of things that could be happening here, but in order to help, we need more information.

First, can we assume that this has been a working installation that has gone wrong, or is it a new installation?

When is the breaker tripping? is it immediately on start, or after a period of time, if so how long?

Does the motor begin to rotate before the trip?
Does the motor get to full speed before the trip?

Have you done a megger test between the LIVE input and earth of the motor?

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[ronbol]

yes, fully working installation

the breaker trips straight away.

no haven't got a megger to test, only meter.

any other ways round this

 

[raisinbran]

Ronbol,
Something that you could try - disconnect the neutral lead. If the breaker trips, it is due to ground fault(or a wiring problem), as no current should be flowing. This is kinda like a meggar.
Be careful,
Raisinbran

 

[Marke]

Hello ronbol

This sounds like an insulation breakdown within the motor, or a failed breaker.

If you do not have a megger, things get more difficult for breakdown fault finding.

You could measure the resistance between the LIVE input and earth and if you see any resistance at all it will indicate a problem. The reading should be better than a megohm. - watch for hand resistance.
Unfortunately, a resistance check will tell you if there is a hard fault, but not an insulation breakdown fault.

With the motor connected to the breaker, disconnect the NEUTRAL and see if the breaker still trips. If it does, then you have a problem to earth.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[machmech]

Hi ronbol

If all of the above is good. Does the motor spin easy? could be bad bearings, What about the load it's connected to, do you have a jam or too much resistance?

Chuck

 

[buzzp]

Sounds like it could be the cap as well. You really need a megger to make any definite conclusions unless you see an obvious fault to ground by measuring with an ohmmeter from hot to ground.
I have seen caps (not in motors but drives) fail under a load that otherwise test fine without voltage applied. If the breaker is correct for this installation, it sounds like there is a definite short somewhere. If the motor were jammed or had bad bearings or something (mechanical), I would think the overload protection would take the motor out rather than the breaker (breaker is for short circuit protection and should not trip if you have an overload on the motor - the overloads should trip). What is the breaker rating and what is the locked-rotor amps for your motor?

 

[bigbillnky]

ronbol

I am not very familiar with foreign power systems, so I have to ask if you are you sure you have a neutral connection with this system? This would normally have 2 line wires(both hot) ran with a grounding(earth) conductor to the motor here in the US on a 240 volt system. How many conductors pass thru the breaker? It will make a difference when trying to determine where the problem exists.

You do not mention if the breaker faults immediately when the motor attempts to start, or if it faults intermittently while running. A ground fault or short circuit will almost always trip the breaker as soon as the motor attempts to start. At least, that is the case until the fault point has been completely "blown clear" or melted open. Here is a few steps I will suggest:

1.Ground out the capacitor and disconnect the leads, being careful to take note of the correct connections BEFORE you disconnect them. This will ensure the proper connections are remade when the tests are complete. Test the capacitor.


2.Try to rotate the motor by hand. If the motor does not turn, disconnect the load to determine if the problem is the with the load or the motor.

3.Test the MOTOR leads to ground with a megger and check the motor winding resistance. Usually a high quality DMM can be used in place of a megger, but it may not accurately read the ground fault resistance. Resistance to ground should be at least 1.2 Megohms. The winding resistance should be greater than 0.

4.Check the resistance thru the contactor contacts for each pole with the wires disconnected. Again, make sure to note the correct connections before you disconnect them. The best method of completing this test is to energize the coil with the proper voltage, but it can also be completed by manually closing the contactor. Use caution when working with energized circuits! The contact resistance should be perfect thru each set of contacts, and MUST BE infinity BETWEEN each set.



Bigbillnky,C.E.F.....(Chief Electrical Flunky)

 

[machmech]

Hello Buzzp

OOPs I stand corrected....better not quit my day job!

Chuck

 

[buzzp]

machmech,
I am not sure, but they may have breakers with dual ratings now (one for ol and one for short circuit). I have been out of the loop for a while.

I do know they have fusing (from Cooper) that are dual element. I never really checked to see if they are listed as official overload devices but I believe they are.

Perhaps the breaker world has not started integrating the overload and short circuit protection yet, I don't know.

One other comment for ronbol:
The 2000 ohms from hot to neutral does not sound that high to me for what appears to be a relatively small single phase motor.

 

[ronbol]

bigbillnky wroteTest the MOTOR leads to ground with a megger and check the motor winding resistance. Usually a high quality DMM can be used in place of a megger, but it may not accurately read the ground fault resistance. Resistance to ground should be at least 1.2 Megohms. The winding resistance should be greater than 0)

using the meter the motor leads to ground read O/L, winding resistance is still 2 Kilo ohms. As you can probably tell i am not electrical minded, but would like to know if this compressor for my garage is good or bad.

 

[buzzp]

Do you have a clamp on meter to read the current as soon as the motor starts? May trip to quick to get a good reading. You don't have many more options to check this. A megger would put a voltage on the leads and measure the resistance.
Everything appears ok based on the readings with your DMM. However, some problems only show up when a voltage is applied (insulation breakdown) so this is why a megger would be useful (applies a greater voltage than your DMM).
About the only thing left to do is assure all wires are tight, etc and then if that don't help then assume the breaker is good and take the motor to a repair shop and they can test it for you. If it tests good then the cable from the contactor to the motor is bad. Good luck.

 

[ronbol]

thanks everyone for being patient.
Looks like the only way to be sure is to megger.
I will try and borrow one tommorrow, but to clarify test the following

live to ground(earth) better than 1.2 megohms
neutral to ground(earth) better than 1.2 megohms

and live to neutral low resistance i.e greater than zero.

any other checks or is this it.

thanks once again

 

[machmech]

Hi ronbol

Since this is a compressor in your garage, could you post
motor information such as fla current, also what size wire
to outlet, and what amp circuit breaker? How far is the garage from supply panel?

Chuck

 

[ScottI2R]

Ronbol,
I would reccomend EBAY if you are buyin a meggar fro your own personal use. read:" NOt reimbursed by your company" That is how I got an old WW2 one for $50. Its crank style but it works! Just remember: If it's old, but it works. IT STILL WORKS!

Scott

In a hundred years, it isn't going to matter anyway.

 

[waross]

Check the centrifugal switch and the capacitor.
Check the centrifugal switch by physical inspection, (take the motor apart.)
First check of the capacitor,
Resistance;
The resistance should start low and then climb to the value of the discharge resistor or infinity if there is no resistor.
Then reverse the meter leads. The meter will probably "pin" for a few seconds as the capacitor discharges and then climb to the previous reading.
Second check of the capacitor,
Energise it. Not a bad idea to stand around the corner as a faulty capacitor can explode. If it trips the breaker, it's bad.
Measure the current if possible.
Alternate check of the capacitor. Buy a new one and replace it.
I suspect that you may be misreading your meter. I have never encountered 2000 Ohms winding resistance. Also, 2000 Ohms on 240 volts would draw 0.12 Amps. That's not going to trip anything. Did I misunderstand something?
One of the most common faults that I encounter in small motors is a faulty centrifugal switch. I repaired two in December.
You last post sounds beter. I would expect a resistance as you describe it, Low but not zero. Maybe 2 Ohms, or 6 Ohms, or 12 Ohms. At that low level of resistance I don't trust the accuracy of a small multi-meter to be within 4 or 5 Ohms.
Please give us some feed back on the final result of your efforts.
Yours

 

[waross]

Further to my post. When you have the back end bell off the motor to inspect the centrifugal switch, you can often visually inspect the windings. A frequent result of a faulty centrifugal switch is a failled starting winding, but not always. Depending on whether the switch fails open or closed, you may have a failled main winding, or all of the above.
My old domestic pump has had at least three centrifugal switch failures over the years and the thermal over load in the motor has saved me every time. The second time I considered myself unbelievably lucky. Saved the third time WOW, I'm considering buying a spare to have on hand. The next time will be the last I'm sure.

 

[ronbol]

I'm still trying to borrow a megger to be sure, but the resistance between the live and neutral (winding) is def 2kilo ohms), this always did sound high to me.
visibly the windings look very black, at least the part that i can see.

will also check the capacitor as you suggested.

will let you know how i get on

 

[Marke]

Hello robonol

If the resistance is 2k and the windings look very black, I would say that the smoke has come out and it is time for a rewind.

If you can not get your ands on a megger, try connecting the phase and neutral leads together, then connecting the frame to earth to neutral and a small lamp in series with the phase and neutral connection to the phase of the supply.
If the lamp glows at all, you have a problem to earth.
If the lamp does not glow, measure the voltage across the lamp. If you get any appreciable voltage, you have a problem within the motor. - a poor mans megger!!
Treat the frame of the motor as being alive at all times.

Best regards,

Mark Empson

http://www.lmphotonics.com