VFD overload protection

Hi guys

i have some PF4 VFDs out of service at the same time, maybe it is the problem of incoming 480 VAC, there is a MOL before VFD, I know MOL will trip if the load side current/temperture is too high, my question here is if the infeed 480VAC is not clean enough, will the MOL trip if there is high curren or voltage from the 480 source?

Thanks

angi said:

Hi guys

i have some PF4 VFDs out of service at the same time, maybe it is the problem of incoming 480 VAC, there is a MOL before VFD, I know MOL will trip if the load side current/temperture is too high, my question here is if the infeed 480VAC is not clean enough, will the MOL trip if there is high curren or voltage from the 480 source?

Thanks

Click to expand...

1) If MOL = Motor Over Load, putting one AHEAD of a VFD is completely pointless. The VFD already IS the OL relay for the motor.

2) If what you are looking at is a Motor Protective Switch (see image), that's a different animal. That is the equivalent of a circuit breaker and many small VFDs are UL listed to have those ahead of them as the Branch Over Current Protective Device.


3) If you have multiple units (again, on the LINE side of the VFDs) that all turned off at the same time, all of your VFDs would have had to fail simultaneously. Since they are on the line side, and the VFDs would trip rather than allow the load to pull too much current, the only possible explanation would be that there was a severe voltage event that fried the diode bridges on all of those drives at the same time, meaning they are all toast and not repairable.

4) The only other possibility, IF those are the switches I showed above, is that they all have something like a Shunt Trip or Under Voltage Trip coil in them and something PURPOSELY opened them all at the same time. You would know because there would be 2 extra wires associated with each one,; if it's a shunt trip, they would be going somewhere else to a control circuit, if it's a UV trip, they would be tied to the line side wires.

To answer your question though, the harmonics on the input side COULD cause heating of the OL sensor elements, it's a known issue, but it's highly unlikely that multiple units would all trip at the same time, unless perhaps this is the first time it has been turned on and you have discovered a design flaw.

Thanks Jraef,

the things we talk about is item 2, motor protective switch, when I check the setting of the switch, they all set to Max, but should be to based on the size of VFD, so I mean if they are all set correctly, can the switch protect VFD from high voltage/current from source?

Thanks

High voltage, no.
You don't "get" high current FROM a source, you just get current. How MUCH current is dependent upon the load. With a VFD, if the MOTOR load is high, the VFD will trip itself. So like I said, if those units all tripped at the same time, it can only be the result of a massive simultaneous failure of the diode bridge rectifiers on ALL of the VFDs, which can happen from a massive voltage spike. Essentially all of the diode bridges became dead shorts, phase to phase.

Or, like I said, this is the first time you have energized this system and every one of those switches is set to a current level well below the motor current. If you post details I'll be glad to help, but I'm still flying blind here. Motor FLA, VFD rating, MPS switch rating / setting at minimum.

1. where could the massive voltage spike come from? lineside or load side. if it is the case, what we need to do?

2. I do not have all of the motor info, but I did see on site all of the protective switches for the damaged VFDs are all set to maximun current level.

Thanks

angi said:

1. where could the massive voltage spike come from? lineside or load side. if it is the case, what we need to do?

Click to expand...

Something happened somewhere upstream to put a massive voltage spike on it; lightning (off in the distance, if it was close by, lots of damage would be visible), car hits a power pole, neighbor has a major fault, all kinds of issues can lead to that.

2. I do not have all of the motor info, but I did see on site all of the protective switches for the damaged VFDs are all set to maximun current level.

Thanks

Click to expand...

Sure, but what does that mean? If the "maximum" is for example 4A, and the maximum VFD input current is 8A, then someone picked the wrong device. That's why I asked if it was the very first time or not. If it has EVER worked, this scenario is unlikely.

Check the VFD's for short circuits from L1-L2, L1-L3 & L2-L3...

You could have seen a high voltage spike which could have taken out the input side MOV's or Diodes.

As far as voltage protection, there's not much to prevent a catastrophic event like this, but line reactors are the normal solution when you see Over Voltage nuisance trips.

Gene

is the line reactor used for long distance between VFD and motor?

Thanks

An INPUT line reactor is used to help filter line noise both in and out of the drive (the drive absorbs noise into the DC bus, and can trip on overvoltage, and it also filters noise created by the drive, due to non-sinusoidal current charging the bus).

An OUTPUT line reactor is used to reduce the voltage spikes that are otherwise amplified due to long motor leads.

An input line reactor also makes the drive's input surge protectors work better by adding some impedance between the surge protectors and the source of the surge voltage. Often, without any input reactance, the incoming surge comes down the line with so much energy and so little impedance that the surge protectors can't manage the energy. The result is either that they die trying to do their job or they don't do their job and the surge manages to pass thru to the rectifiers and DC bus where further mischief can occur.