Thermal Protection Relay - tripping

[Splitrings]

We recently installed a single phase, 10hp, vacuum pump. It has a Lavato thermal overload relay. It has a amperage range of 37-50 amps. The adjustment dial is set for the FLA rating of 50 amps. The relay keeps tripping. The first time you start the motor it will run for 4 or 5 mins and then trips. The second time it starts it will run for serval hours before tripping. The motor is only drawing anywhere from 28-33 amps. The motor was oversized to 10hp, the pump it runs was intended to have a 7.5 hp motor. Does it matter that we are under the thermal rating of the relay? It is rated for 37 amps and we are drawing only 33 or so. The relay is a Lavato 11RFN95350 or 11RF95350(couldn't read the number to be sure). At one point I did notice unbalanced current, about 23 amps on one leg and nearly 30 on the other. Any help would be appriciated. Thanks!

 

[Splitrings]

Culprit maybe insufficient oil flow to vacuum pump. Pump heats up, tolerances reduce and then motor overloads.

 

[MiketheEngineer]

Could be a bad relay - I have seen new ones "blow"

I am sure it must be under some kind of warranty. Call the mfg.

 

[Splitrings]

That is the problem. They bought it from a maple supply dealer who is also a sugarer themselves. They don't return calls. The sap is running and they are busy taking care of their own problems!

 

[iop995]

What mean unbalance current at a single phase motor?

 

[Splitrings]

Measured current on L1 was 23 amps and L2 was 33 amps.

 

[iop995]

It's USA 3 wire single phase electric system?. Difference of about 10A is a fault current, so check insulation motor / wires.

 

[Splitrings]

This is a capacitor start and capacitor run motor. One of the capacitor leads was laying against the opposite terminal on one of the capacitors. Is this the possible cause? The insulation on the wire appears fine. I can't tell if it is the run or start capacitor, both are round.

This is a 240v three wire (two 120 volt conductors and one grounded conductor) USA electrical system.

 

[ScottyUK]

Lovato are an Italian manufacturer. IEC motor overloads are sensitive to imbalance to protect against negative sequence heating of the rotor. If the overload sees the unbalanced current then it is likely to be the reason why it is tripping.

A circuit diagram would help - I can't quite imagine how this machine and its controlgear are connected. I would have guessed it was across the two hot lines with no connection to the neutral wire but from your description that is not the case.


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

 

[Splitrings]

This is the motor:

http://www.leeson.com/leeson/search...wProductDetails&motorNo=LM24684&productType=2

It is started with a Lavato magnetic starter with a Lavato Thermal Protection Relay.

The motor is directly coupled to the vacuum pump.

 

[Splitrings]

I'll post the wiring diagram tomorrow morning. Thanks for you comments

 

[ScottyUK]

Ok, I would expect the overload to have all three elements wired in series and to be connected in L1 (but could be equally in L2). The whole thing connects across the two hot lines with an appropriate contactor. You don't have the overload conncted in among the internal wiring, or a connection to the neutral wire do you?


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

 

[LionelHutz]

I don't understand how you could ge a current imbalance unless the motor has failed. You have 2 leads going into the motor so current in must equal current out unless it's going to ground in the motor.

 

[Compositepro]

If the run capacitor is bad or not connected, the motor will trip after a while. One sign is if the motor seems to struggle after the start capacitor switches out. Also, many vacuum pumps will trip if there is too much flow though them. They are often designed to take some overload while pulling the system pressure down, and then the load should decrease as the system pressure is reduced. They will trip if there is a large air leak that keeps the load high.

 

[waross]

You describe your system as two hot lines and a wire at ground potential. The wire at ground potential should be the equipment grounding conductor. This should not be connected to any of the motor leads. Line one should go through all three poles of the overload relay in series and then to one motor lead. Line two should go to the other motor lead.
Alternately, line one may go through two poles of the relay in series and the other motor lead may return to line two through the third relay pole.
If you have connected the grounded circuit conductor to any of the motor leads, the relay may trip on unbalanced current.
This may be the case, or you may have a faulty motor.

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

 

[jraef]

As the others have pointed out, IEC overload relays have a feature built-in that trips below the FLC setting if there is no current flowing through any one (or more) of the 3 sensing elements because on a 3 phase motor, current imbalance causes the motor to heat up disproportionately and damage can occur without ever exceeding the FLC setting of the OL relay. But that creates a problem on single phase motor applications for those relays, because with single phase you would never have more than two current carrying conductors. So on a single phase circuit, you simply loop the current carrying conductor(s) back through the OL re;lay poles. It doesn't matter that it's the same current going through 2 or all 3 of the poles, just so long as there is current.

However... You were not sure of the part number of the Lovato OL relay. If that part number is 11RFN95 3 50, then it is actually NOT phase loss sensitive, it is a version they sell specifically for 1 phase applications. If so, then this is not the problem and it is likely defective. If on the other had it is the 11RF95 3 50 (no N) then it is the type that would trip without current running through all 3 phases. So that would explain what is happening to you. I suspect it's the latter.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[Splitrings]

Compositepro,
I have seen the pump struggle shortly after starting, a minute or two after starting, and then quit. This may have been the time I measured unbalnaced current.

Waross,
The third conductor is the equipment ground. It is bonded to the motor frame only. I am beginning to wonder if the thermal relay is tripping due to unbalanced current, as I have never seen it approach the 50 amp upper limit.

jraef,
Each of the two hot leg pass through the contactor and thermal relay. As you mention the third connection is looped. I am not certain which relay it is. As much as I hate to admit it, I am now having trouble seeing small things close up! I'll take a picture of the system and confirm the part numbers this afternoon. It was too dark last night.

Thanks to everyone. This is a big help.

 

[jraef]

Also, without us knowing the ENTIRE control circuit, is there a chance that it is trippoing on something esle? In other words when it goes off line, is the remedy to hit the reset button on the overload relay? If not, it's likely something else.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[Splitrings]

I am beginning to wonder if we have two problems.

The first maybe a bad run capacitor or loose wire. This or something else has caused the relay to trip within the first minute or two of startup. Maybe this was when I measured unbalanced current??? I can't recall exactly when it was.

The second was the vacuum pump was not getting sufficient oil due to a flow restricting valve. This valve is now gone. This would cause the circuit breaker and or rely to trip. This would happen much later. At least 30 minutes later..I am guessing as no one was present when it happened.

Both times resetting the circuit breaker and or relay would allow the motor to run.

 

[ScottyUK]

I can't understand how you can have an imbalance with the overload relay connected as you describe, unless either Kirchoff was wrong (which is unlikely) or you're losing a lot of current to an earth fault. Can you do a megger test of the motor to earth?


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