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]

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]

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]

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]

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.


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

 

[HRSADI]

Hi Mr.Whippee;

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

 

[waross]

Welcome to Eng-Tips HRSADI.

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

 

[ASimmons23]

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]

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]

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]

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
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[burnt2x]

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]

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]

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]

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 - or from imbalanced supply voltages.


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

 

[CH5OH]

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]

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

 

[LIBERTYELEC]

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]

Welcome to Eng-Tips LIBERTYELEC.

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

 

[ScottyUK]

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.


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