Electronic breaker for large VFD tripping under low load

[rick hart]

There is a situation where a 480V 200 HP motor and Allen-Bradley drive are pumping chilled water. The system was designed for additions to a loop that has not expanded to its full potential. The drive runs constantly at 30- 35 Hz at what flow I am not sure but could find out if that is needed.
There has been a lot of nusance tripping of the 600A Micrologic circuit breaker. The circuit breaker has been checked and tested, both by primary injection and secondary injection methods and is fine. I suspect that the motor and drive are severly oversized for the actual load but lack the mechanical expertise to say for sure.
Is there a reason that a large motor and drive running at less than a third FLA would trip an electronic circuit breaker? The drive guys want a thermal magnetic breaker but I am afraid of the lack of ground fault protection. The next ground fault protection is the 3000A main circuit for the switchboard.

 

[jim dungar]

It is possible to add ground fault protection to thermal magnetic breakers equipped with shunt trip. But, the ground fault is only protecting the "input" section of your drive. Is it likely that you will experience a gound at that point of your system?

What is the input section of your drive? Ground faults are not uncommon on SCRs.

When does the breaker trip? At start-up, after several hours of constant speed, during a speed change, or on shut down?

Why does the breaker trip Ground Fault, or Short Time?

 

[boater bill]

With the drive running at basically half speed, what is the amp load? The only problem I have run into an oversized drive was expense. the drive will put out the necessary amps for the motor to operate the load.

Thought, what is the motor's cooling system? If it is TEFC then the fan isn't pushing enough air to keep it cool. An external cooling blower would be needed if there are thermal problems with the motor.

I am also wondering if there is an input line reactor, with a drive of this size, you would certainly want one to reduce the harmonics on the input line.

Let us know what else you find.

 

[coulter]

...There has been a lot of nusance tripping of the 600A Micrologic circuit breaker. ...I am afraid of the lack of ground fault protection. ....

SQD Micrologic is a medium sophisticated trip unit. Depending on which model you have, it has a lot of functions in addition to the normal overcurrent and ground fault (LSIG).

The list includes OV, UV, voltage imbalance, current imbalance, under freq, over freq. As Jim said, why is the breaker tripping?

Could be a coordination problem (protective relay settings) or something is broke and the protective relay is doing it's job.

So, time to troubleshoot - What is the trip function? Not much to help on with out knowing that

carl

 

[brian john]

Had 2 different projects with Micro Logic CBs tripping randomly, tested primary and secondary injection, both passed. Finally put a high speed recorder and captured the trip. Result both trip units were defective. Replaced and have not had any issues since then.

The CBs were tripping well within their rated OCP but when there was any current change they would trip.

We tested these and the end user on one job had the manufacture rep test as it was a warranty issue.

 

[khixxx]

How is the resistance readings when you meg the cables, from the drive to the motor (disconnect cables from drive so you don't damage the unit)?

It has been awhile sense I worked on this stuff, but I do recall units tripping out on under frequency. They also would get down to 30hz or 35hz and the pump would not be pumping any water. Motor was just turning.

What fault was displayed on the read out?

 

[rick hart]

Sorry to take so long to respond. Been out of the office for a bit:
Jim,
1. The breaker trips indicating short circuit short circuit
2. Not sure what you mean by input section but the drive is approx 8 years old.
3. the drive is controlled by the building automation system and is very hard to determine if the motor is ramping up or down. The breaker trips after runnning a while and has been observed tripping ramping down.
Boater,
1. The running load is btetween 75 and 100A typically
2. No input line reactor
3. Motor does not feel hot to the touch but, I will thermograph

 

[boater bill]

FWIW, the input section is whether the output frequency is derived from SCR's (old school >10 years), or IGBT's. You probably have IGBT's, but check the manual.

With those low of running amps, I am thinking a harmonic is causing the breaker to trip. I've seen it before, especially on bigger drives like you have. See if a 3% input reactor from MTE or TCI can be added to the drive input power. An input breaker tripping indicates an overhauling load on a process machine, but this a pump and shouldn't be having that problem.

See if you put the drive on local control. It may require a plant shutdown and not really possible, but try and simulate the operating conditions and range.

I am not totally familiar with the breaker in use, maybe someone else has some input.

I hope that helps.

 

[jim dungar]

Sorry to take so long to respond. Been out of the office for a bit:
Jim,
1. The breaker trips indicating short circuit short circuit
2. Not sure what you mean by input section but the drive is approx 8 years old.
3. the drive is controlled by the building automation system and is very hard to determine if the motor is ramping up or down. The breaker trips after runnning a while and has been observed tripping ramping down.
Boater,
1. The running load is btetween 75 and 100A typically
2. No input line reactor
3. Motor does not feel hot to the touch but, I will thermograph

Square D type LE and LX breakers are true RMS sensing. They have an adjustable short time pickup and delay. You may simply need to determine the correct settings for the breaker.

If your drive input section is an SCR design it is possible that it ocassionally misfires causing minor phase-phase short circuits or it may even have a re-generative feature which can have similar problems. These are one reason why old drives commonly required delta-wye input isolation transformers.

It is unlikely that any thing on the drive output section (i.e. the motor) is causing your problem.