Overload tripping 2

[whippee2]

Hello all,

Got called to a lathe yesterday and found that the main motor contactor contacts where welding closed, thus not allowing the operators to stop the motor. Anyhow I changed the contactor with new one and installed a new overload (AB E1 plus 18-90 amp range) Started the motor and the overload tripped right away.
I then tong tested the motor phases out of the contactor and they are only pulling between 12-18 amps. BTW the motor is rated for 30 amps. I set the overload trip current up to 50 amps, it ran for approx. 5 minutes then tripped. AT restart it tripped right away. Anyhow the day ended and i then went home.
I went back there again this morning and the operator stated that the motor ran for approx 30 minutes then tripped out. He set the overload up to 80 amps and it tripeed right away. I arrived a little later and set the overload back to 50 amps and the motor ran for about 30 minutes and then tripped. I cranked the OL up to 80 and it tripped right away. I tong tested the phase cables again and none of the windings pulled more than 18 amps fulled loaded, but the OL was tripping even when set to 80 amps.
I suspect that the overload is faulty, even though it is brand new. I have ordered another for the customer, so I am hoping that is the problem.
Does anyone else have any input?

Bottom line: If i tong test the motor and its only pulling 18 amps max, there should be no reason why the OL would trip. The motor cables are the only thing coming out of the bottom of the contactor. BTW the contactor is a 85 amp contactor, due to alot of jogging action. but the ol was tripping during normal running operation.

thanks again
whippee

 

[waross]

You didn't say specifically and it is important. Are the motor currents equal? A good quality O/L may trip on a differential current less than FLC. Intermittent phase loss will cause a differential trip.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[whippee2]

hello thanks for reply.
I did check the motor currents on all three phases. And they were very close to the same, say within an amp or 2.

What do you think?

thanks

 

[CH5OH]

have a look under the hood of the lathe
some lathes have a hydraulic pump which drives the lathe. if the flow of pump is blocked (faulty oil circuit), it surely trips the motor no matter what the setting of OL is

 

[jraef]

Want to know my opinion? Your troubleshooting skills make me cringe! You can't make a 12A OL hold in so you turn it up to 50A!!!??? Are you nuts??? Then your other guy raises it to 80A!!!!!!??????

You guys are playing with fire here. You need a qualified engineer to look into this for you. I don't think anyone at Eng-Tips should further your dangerous endeavors by offering any other advice. Sorry to be so blunt but that is just plain dangerous. This forum is for serious professionals.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[whippee2]

thats for your 2 cents jraef. If the motor is only pulling 15-20 amps whats wrong with bumping up the overload, my test equipment assures me that its not pulling any larger amps

 

[wayne440]

Were you actually metering the motor current when these trips happened? If so, is your meter able to capture the peak current immediately before the trip?

If the answer to either question is no, you cannot be sure that the motor does not have an intermittent fault. The fact that it draws "12-18 amps" when it is running acceptably tells us nothing about what it does on a trip. Setting the trip point to 80amps may eventually show you something (look for the smoke), but it is far from the best way to troubleshoot. Parts and people often get damaged that way.

If you did have a suitable meter on all three phases when the trip(s) happened, and read no excessive current - disregard the above.

 

[waross]

If the O/L is tripping with a 12 amp load when set to 50 or 80 amps, it may be defective.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

I am with jraef. There is no need to help someone be more dangerous.

Rafiq Bulsara

http://www.srengineersct.com

 

[jraef]

If the motor is only pulling 15-20 amps whats wrong with bumping up the overload,...

I rest my case...

Good engineering practice 101:

1) Don't ASS-U-Me that protective devices are defective until you have effectively eliminated ALL OTHER possibilities, no matter how remote.

2) "Smoke testing" is the most dangerous form of trouble shooting. Sure we've all done it by accident, but you are purposefully doing it.

3) You ALREADY have a dangerous situation in the contactor welding and not allowing the motor to disconnect. There are dozens of tests I would do long before I would make matters worse by defeating protection devices.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[whippee2]

anyhow, this is why I am on this forum to ask a bit of advice from others. The whole issue is that I am working for a tight ass customer that doesnt want to pay for much at all. Therefore it is vital that we perform our work and get out as soon as we can. I know that I/we shouldnt be taking shortcuts, but working in the real world sometimes offers a different situation for us.
I tested the motor current with a calibrated analog tong tester. I would have expected to see a large spike in current immediately before the OL tripping.


Can other helpful advice would be greatly appreciated.

 

[rbulsara]

The customer is tight ass... Really? And that will make you what, based on the payment you are making for "helpful" advice?

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

Sounds like an itermitent ground fault. What are your megger readings?
If you are trying to trouble shoot without a megger. well...............

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[CH5OH]

again:
does the motor drives the lathe directly via the lathe gears or does it drive a hydraulic pump which then drives hydraulic motors to drive the lathe gears
when you operate the lathe, is there a start button (to start the pump) or does the direction lever to fwd and reverse the lathe immediatly switches the motor?

 

[jraef]

Speculation ad nauseum.

Test the circuit conductors and the motor windings (without the conductors) with a megger as waross said as a good start. If you don't find anything wrong (assuming you know what to look for), then start with the less likely issues. Check the connections for tightness, check the voltage balance, check the current balance, check the sizing of the conductors and components, do a thermal scan, use a recording meter to check for voltage and current anomalies, I could go on forever.



"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[whippee2]

thanks I have meggered the motor windings and have >200Mohms from each phase to earth and have the same readings across all three windings.

The motor directly drives the lathe via a set of v belts.

THe voltage balance is good within a volt right across the three phases, the current balance is good as well. I have checked for tight connections across the start/stop circuit, this is ok.

The other thing i was thinking about, is that they never had any problems with the old OL tripping only the new one. The only thing I was thinking as to why the main contactor stuck closed is because they do alot of jogging and these currents would eventually cause damage to the contacts.
thanks again

 

[waross]

Have you looked at the contacts that are presently installed?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Any chance that E1 Plus is connected to a DeviceNet network? Although the SSOL is self-powered when stand-alone, when connected to D-Net it is powered by the network and if the network goes down, it fails to act as an OL at all. At least it was like that last time I used them (which is the reason it was the last time). They may have fixed that by now, it was a very dangerous flaw in my opinion, but if you used an old one that's a possibility.

Rapid cycling creates a motor thermal condition that the E1 should pick up and remember. It's difficult to have enough cycling to damage the contacts without creating enough thermal build-up in the SSOL thermal register to cause a trip, but not impossible. The other aspect of this is a weak or intermittent control signal that is rapidly chattering the contactor, maybe only under certain conditions. The damage can happen faster than the thermal model for the motor would pick up in that kind of scenario, I have seen a contactor weld in seconds from chatter.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[burnt2x]

Ahh, belt-drive! Please check if your belts are not too tight. From my other life, most lathes of this kind tend to trip respective breakers when they do threading jobs (more forward-reverse, not braking at all)! We gave the belts a little slack. Any jogging decision on the operation does not transfer shock-load quickly to the motor due to some sort of slippage. Basically a trade-off on the belts and the contacts!
Or maybe you could introduce some "on-delay" on your forward-reverse switching so that operators will get used to braking before reversing; this will minimize contacts closing on an out-of-phase, remanence voltage, hence lower current spikes.
Hope above tips help.

 

[whippee2]

waross? what contacts are you referring to? I have changed the main motor contactor due to the old one having welded contacts?

jraef, there is no devicenet incorporated. It is a 1940s lathe and the wiring looks that old as well. no drawings, labelling, a real treat to work on.

burnt, there is no reversing, only forward. the motor drives some belts that run the whole machine. and continually run until the operators stop it via the stop button or open up an inspection door.