VFD Tripping due to Overcurrent and Over Voltage

[Maysam]

Hi all,

Here is my first question:

Is it possible to have 2- 600v load reactors in VFD application each with 5% impedance put together in series to create a 10% load reactor? If this is possible could a person put 3 in series to create 15% impedance?

One of our clients have a 12 year old Toshiba VFD. Although, the motor distance from VFD is not more than 30? they had 9% load reactor installed. The incoming source is 600V and the motor is operating at 480V. Everything was working fine with 9% reactor till they had a ground-fault in load reactor. After that they changed the load reactor but they face the trip all the time. They tried 2% and 5% load reactor but nothing has changed yet.

It can be observed from the trends, there is a normal running one, one with no reactor, and one with 2- 3% reactors in series. Without the reactor they tripped on high current, 250 amps in 4 seconds, with 2 reactors we tripped on high voltage, 480 in 40 seconds. They also tried a single 3% reactor which did nothing. The 2 reactors they put in series were 600v / 250hp / 3% imp.

Do you think the problem is because of load reactors? They have performed megger test and it seems that the motor is just working fine. Do you think the problem is with the drive? Based on my knowledge it is recommended that IGBTs on the back end and Diodes on the front end be replaced after 9 years so maybe they are wearing out and that?s why they are facing overcurrent. Any thoughts on that?

Thanks.

 

[kwired]

Hi all,

Here is my first question:

Is it possible to have 2- 600v load reactors in VFD application each with 5% impedance put together in series to create a 10% load reactor? If this is possible could a person put 3 in series to create 15% impedance?

One of our clients have a 12 year old Toshiba VFD. Although, the motor distance from VFD is not more than 30? they had 9% load reactor installed. The incoming source is 600V and the motor is operating at 480V. Everything was working fine with 9% reactor till they had a ground-fault in load reactor. After that they changed the load reactor but they face the trip all the time. They tried 2% and 5% load reactor but nothing has changed yet.

It can be observed from the trends, there is a normal running one, one with no reactor, and one with 2- 3% reactors in series. Without the reactor they tripped on high current, 250 amps in 4 seconds, with 2 reactors we tripped on high voltage, 480 in 40 seconds. They also tried a single 3% reactor which did nothing. The 2 reactors they put in series were 600v / 250hp / 3% imp.

Do you think the problem is because of load reactors? They have performed megger test and it seems that the motor is just working fine. Do you think the problem is with the drive? Based on my knowledge it is recommended that IGBTs on the back end and Diodes on the front end be replaced after 9 years so maybe they are wearing out and that?s why they are facing overcurrent. Any thoughts on that?

Thanks.

Tell us more about the load being driven. Is the drive tripping during acceleration, deceleration, at a steady state?

Sounds like you could possibly have bad motor. If tripping in 4 seconds I kind of assume this means during acceleration. Shorts from turn to turn may pass a meg to ground test but will still draw too much current.

 

[petersonra]

I agree with the 2nd poster. This does not appear to be a reactor problem. It has that motor problem smell to it.

In any case, why would you ever need a 10% load reactor. That is just goofy.

 

[Jraef]

My guess on the 10% reactor was because with the line voltage at 600V, the DC bus voltage will be roughly 850VDC instead of 635VDC if it were a 480V service, so although you can program the drive to only put out 460VRMS, the pulses within the PWM output will be at a significantly higher potential, putting it at greater risk for winding insulation damage. Adding another 10% impedance may have been their attempt to mitigate that. Not how I would have done it, I'm just speculating here based on what I have seen at other Canadian lumber mills where they bought used equipment from the US but have 600V service..

Hopefully (he says in a hopeful manner) they were not so foolish as to think they would leave the output RMS voltage from the drive programmed at 600V and needed to drop the voltage to the motor by adding impedance to the circuit... Can you confirm that the VFD is programmed to reach 60Hz at 480V?

2nd issue: IGBTs either work or they fail, they don't "wear out" in a way that gives you nuisance OV or OC trips before they succumb to catastrophic failure. If a diode shorts, the DC bus goes to hell in a hand basket. In either case, the drive will trip on other protective functions and not allow you to reset it.

... Without the reactor they tripped on high current, 250 amps in 4 seconds, with 2 reactors we tripped on high voltage, 480 in 40 seconds. They also tried a single 3% reactor which did nothing. ...

This part makes no sense to me. First off, 250A means nothing because you do not indicate the size of the motor, other than by implication in that the reactors are 250HP. If you have a 250HP 460V motor, 250A is NOT high current! Same with the voltage trip, 480V is NOT high voltage. Are you by chance attempting to read these values on the output of the VFD with a common hand held meter? If so, these values are basically meaningless. If you are reading these from the VFD display, they are likely incidental in that they are not the cause of the trips, they are just what the average was at the moment of the trip, but the cause was something much more instantaneous and drastic. Typically the trip log will give you a deeper indication of the actual trip cause. For instance if it says the trip was DRIVE Over Current vs MOTOR over current, but the 250A was the MOTOR current at that instant, then that points to a failed IGBT, where it is likely shorting through the case to ground, or multiple IGBT failures where they are firing at the wrong times. Not sure where you are getting the OV trip from though, unless the damage in the drive has cascaded into capacitor failures and you have excess DC bus ripple.

 

[mike_kilroy]

Here is my first question:

Is it possible to have 2- 600v load reactors in VFD application each with 5% impedance put together in series to create a 10% load reactor? If this is possible could a person put 3 in series to create 15% impedance?

to answer this: YES & YES.

next, the system USED TO WORK for years, now it doesn't. the change is the inductor. Usually the last thing we do causes the problem..... hence, yes, it is fair to 'guess' that w/o 9+% Z in series is your problem now.

I am with Jraef and vote that an engineer decided to try to drop output voltage from 600v input to 480v by adding BIG choke on output.

Based on my knowledge it is recommended that IGBTs on the back end and Diodes on the front end be replaced after 9 years

Your comment about semiconductors 'wearing out' is not a common thought. General belief is they either work or don't and there is no in between. This is an acceptable industry belief and you can go with it and feel confident. In reality, they DO begin to wear out, and those of us who do in depth repairs actually often measure for it with resistance measurements. But the difference is often more 'preventive' maintenance while we have a drive to replace the weak SCRs or IGBTs rather than end them back to run another 3-8 years before they fail. So it is not a black and white question after all.

Why not stick all the inductors u own in series on the output and see if the old drive survives?