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whippee2 (Industrial) (OP)
23 Jun 10 4:01
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 (Electrical)
23 Jun 10 8:49
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 (Industrial) (OP)
23 Jun 10 9:41
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 (Petroleum)
23 Jun 10 15:19
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
Helpful Member!(2)  jraef (Electrical)
23 Jun 10 20:29
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: Eng-Tips.com Forum Policies  

whippee2 (Industrial) (OP)
24 Jun 10 3:53
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 (Electrical)
24 Jun 10 10:21
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 (Electrical)
24 Jun 10 10:22
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 (Electrical)
24 Jun 10 13:36
I am with jraef. There is no need to help someone be more dangerous.

Rafiq Bulsara
http://www.srengineersct.com

jraef (Electrical)
25 Jun 10 1:48

Quote:

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: Eng-Tips.com Forum Policies  

whippee2 (Industrial) (OP)
25 Jun 10 3:42
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 (Electrical)
25 Jun 10 8:06
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 (Electrical)
25 Jun 10 8:48
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 (Petroleum)
25 Jun 10 13:22
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 (Electrical)
25 Jun 10 16:57
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: Eng-Tips.com Forum Policies  

whippee2 (Industrial) (OP)
25 Jun 10 17:51
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 (Electrical)
25 Jun 10 18:35
Have you looked at the contacts that are presently installed?  

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

jraef (Electrical)
25 Jun 10 20:28
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: Eng-Tips.com Forum Policies  

burnt2x (Electrical)
25 Jun 10 20:32
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 (Industrial) (OP)
26 Jun 10 17:57
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.
waross (Electrical)
27 Jun 10 5:20

Quote:

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.
These contacts.  

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

Shozza (Electrical)
28 Jun 10 2:53
Just throwing something out there from left field which others may be able to confirm.

The E1 Plus overload is a self powered device using the current flowing through the device to charge an internal capacitor which then powers the microprocessor. Given that this device is rated for 18-90 Amps but is only operating at 12-18 Amps is it possible that there is insufficienct power to keep the capacitor charged and after a period of time the capacitor is depleted to the point where the device fails to maintain normal operation. Perhaps using a more suitably sized overload i.e. the 9-45 Amp E1 Plus may help.

Anyway hopefully someone in the know may be able to comment on whether this is a valid argument.

waross (Electrical)
28 Jun 10 6:37
I don't think it will be very long before whatever is causing the problem lets the magic smoke out.

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

ScottyUK (Electrical)
29 Jun 10 1:00
Shozza,

I'm not sure about that particular relay but the CT-powered tripping units used on circuit breakers typically don't do anything at all when they have too little current passing for them to wake up. It's an interesting question though - hopefully someone can give a definitive answer.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

HRSADI (Electrical)
29 Jun 10 5:10
Hi Mr.Whippee;

I think the load is varing acording the process .And the amps variation more than Overload Relay setting

  
waross (Electrical)
29 Jun 10 12:13
Welcome to Eng-Tips HRSADI.

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

ASimmons23 (Electrical)
1 Jul 10 16:18
Shozza,
The relay isn't operating at 12-18 amps, the motor is drawing that much current.  The capacitor is charged off the control voltage at the relay.

 
Shozza (Electrical)
1 Jul 10 20:11
ASimmons23,

I'd have to disagree with you there as these overloads do not have a separate control voltage supply. They are self powered meaning they acquire their power supply directly from the line supply i.e. the 3-phase supply. Usually, as ScottyUK has suggested, these types of devices do not power up at all if the current through the device is below the minimum recommended current setting. My thoughts were that on startup, assuming DOL, a 30 Amp motor would draw upwards of 180 Amps momentarily regardless of the load attached. This may be enough to charge the capacitor and enable the Overload to operate temporarily. Once the motor current dropped below 18Amps the capacitor would start depleting to a point where the overload would shut down.

Whippee2,
I just read an article which states that if the current draw is below 30% of the FLA dial setting the E1 Plus may see this as 0Amps. This may be why setting it to 50Amps and even 80Amps didn't work. Another document also mentioned that this could be seen as a phase loss condition which may be the reason for these trips. Try setting the overload to around 18-20Amps and see if it continues to operate or whether it trips again. If anything it'll just over-protect the motor.  
 
waross (Electrical)
1 Jul 10 20:51
Can anyone say definitely how this device is powered? It seems a little obtuse to try to extract power from the current flowing through when there should be three phases of at least 208 Volts present whenever current is flowing. Self powered does not always mean loop current powered.
My feelin g is if they are trying to tap operating power from the passing current it may be time to switch brands. By the way, I have solved more than one problem by switching brands. Even with similar or equal specs, not all devices are created equal. And a particular device may work dependably in almost all applications but have issues with a certain type of application.

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

jraef (Electrical)
1 Jul 10 21:41
They are self powered by the current flow. It's a very tried and true method, invented by Furnas (now Siemens) with their ESP100 OL relays about 20 years ago. The basic design uses a very low power ASIC and a very good super capacitor for control power storage. If powered from anything else, it runs the risk of ceasing to function if the control power is lost.

I don't know for sure about the A-B, but on the ESP100, the minimum current was 25% of the lowest setting. So if the range of adjustment was 12-48A, the minimum current flow required to power the SSOL was 3A. I would imagine from what Shozza says he read that the A-B is similar (30%). So it appears the range is 18-90A, that means the minimum current to power it is 5.4A. I seriously doubt that's an issue here.

Remember the original problem here is that the contacts are weldng. Low current is not a usual suspect in that case. The only reason it reared up was the possibility that the SSOL was not functioning and the contactor was actually seeing high enough currents to damage the contactors. From Whipeee2's further descriptions, that appears not to be the case.

I come back to the rapid cycling possibility. A bad control connection somewhere causing a chattering of the contact under specific (and difficult to replicate) conditions is the most likely suspect. Or Joe-Bubba the lathe operator is doing indirect speed control by banging the switch on and off rapidly and then denying it later when the damage is done. It's not like that has never happened...


"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: Eng-Tips.com Forum Policies  

burnt2x (Electrical)
1 Jul 10 22:36
Well, things appear differently from other vantage points! Others looked at the OL protection, while Jraef saw the contact welding as the prime concern! Machines have no brains, it's sometimes the operator to blame - rapid cycling, I guess. Either you change brands; insert some "on" delay; or replace Joe Bubba! My two cents.
CH5OH (Petroleum)
3 Jul 10 2:49
may be not a very scietific explanation...
but may be its a result of a generation conflict.
if you have a lathe of the ´40 conflicting with a thermal overload from today...
if you look at a motor of those days and compare it to a motor of todays, with the same specification plate you looking at two different machines.the modern motor probably uses less material to be constructed,has a better efficiency and is less forgiving for abuse.
why not put an old type overload protection with bimetallic contacts, instead of an electronic device?
if I was the customer, I would be fuming by now, my problem still not being fixed.  
waross (Electrical)
3 Jul 10 3:23
Good catch, CH5OH.
Those U frame motors were rugged beasts, I remember one in a sawmill that the operators would stick an air hose in the MCC bucket to keep the O/L heaters cool when clearing jams, the motor would be so badly overloaded. Most of the motors had O/L heaters at least one and often two sizes too large. These were the old U frame motors. Had they been T frame motors, 1/4 of them would have been destroyed during the first day.
The NEMA frame motors that preceeded the U frames were even tougher.
Forty years ago there were still a lot of U frame motors being installed.
Those U frames were sometimes protected by dual element fuses. Any O/L that will protect a T frame motor will surely protect a U frame motor.
But, memories aside, the welded contacts indicate a problem elsewhere. And, if the relay has been false tripping, it may also have been causing the contactor to chatter.  

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

ScottyUK (Electrical)
3 Jul 10 4:02
If the line currents are as low as indicated, are they balanced? The relay possibly has some form of current imbalance protection: imbalanced currents cause severe heating of the rotor. Such a condition could result from a slightly high resistance contact in one phase - not impossible on a 70 y.o. machine smile - or from imbalanced supply voltages.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

CH5OH (Petroleum)
3 Jul 10 4:02
statement:99% of difficult to solve equipment failures occur when the failure is alowed to pass the boundries of expertise.in this case:
electrical expertise:motor,supply, safety devices
mechanical expertise:motor bearings,gearing,....
whippee, when the lathe is stopped, does the spindle stop imediately (proof of a braking device being present) or does it keep on turning driven by inertia.
modern lathes all have this device.
idea: the brake engages a few milliseconds to early, while the motor contactor is pulled open, extreme high torque is produced by the brake device, causing contact to weld together.
jraef (Electrical)
3 Jul 10 13:12
Generally, those old brakes (if one were installed at all) were mechanical; spring loaded and held open by a DC solenoid powered from the a rectifier connected to leads feeding the motor by tapping off right in the peckerhead. They only engaged when power was already cut to the motor. If their solenoid or rectifier failed, it failed all the time, there would be no way for it to "release early".

DC injection brakes didn't come along until the 70's. However if someone retrofitted a DC injection brake and did it without an isolation contactor, that might be something to consider. Generally the SCRs in the DCIB would blow immediately if the timing was incorrect, but certainly it's possible the contacts could get damaged instead.


"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: Eng-Tips.com Forum Policies  

LIBERTYELEC (Electrical)
3 Jul 10 15:50
I believe you may need to select a smaller overload relay/contactor configuration to bring the parameters of the protection design more into the middle of the operating current range of the motor.  Also, you may have replaced the overload setup with an AB overload which has dip switches on it.  If it is the EE type of E1-Plus, this may be the case.  Consider which type of trip class you may need to set the dip switches to meet your desired trip class if indeed it is a type EE overload.  Short of this and what already has been mentioned here, I believe you could be correct in your assumption that you have purchased a faulty overload.  Its been known to happen.  Along with this, were you always present when the overload tripped?  In my experience, operators are not always forth coming with ALL needed information to provide you with all the necessary information to troubleshoot machinery.  In other words...they lie to cover their butt or they're not always as knowledgeable about how a machine is supposed to be operated to keep the machine within operational design parameters.  So if you have been present during a tripping or two then that may not be the case.  Oh, and make sure all of your connections and terminations are TIGHT!  Look for the simple stuff!  Look for skinned wires too, even though you megged it out.
waross (Electrical)
3 Jul 10 21:51
Welcome to Eng-Tips LIBERTYELEC.

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

ScottyUK (Electrical)
4 Jul 10 4:14
Are we all assuming that the welding contactor and the overload tripping are caused by the same problem? I am beginning to wonder if there are two separate problems which are present, and we have made the assumption that they are related.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

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