Vfds

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I was hoping someone might have some imput on this question. I have been performing maintnence on a building recently. We had 2 vfds fry the contactor coils after an outage. Also the contacts were severely scored due to arching. This has occured 3 times. Everytime there seems to be a power outage these 2 vfds fry there contactors. 1 contactor is a drive contactor and the other is a bypass contactor. The contol wiring for the contactors has a mechanical interlock so that they can never be on at the same time. My theory is that the VFDS are not getting damaged by the outage. They are some how causing the outage. The VFDs are for a supply and return fan. There are lots of other VFDs in the building and these are the only 2 that are affected. Please help. Any input would be greatly appreciated.
 

mcclary's electrical

Senior Member
Location
VA
I was hoping someone might have some imput on this question. I have been performing maintnence on a building recently. We had 2 vfds fry the contactor coils after an outage. Also the contacts were severely scored due to arching. This has occured 3 times. Everytime there seems to be a power outage these 2 vfds fry there contactors. 1 contactor is a drive contactor and the other is a bypass contactor. The contol wiring for the contactors has a mechanical interlock so that they can never be on at the same time. My theory is that the VFDS are not getting damaged by the outage. They are some how causing the outage. The VFDs are for a supply and return fan. There are lots of other VFDs in the building and these are the only 2 that are affected. Please help. Any input would be greatly appreciated.

Your suspicions sound correct, it's likely causing the outage. If it's frying BOTH contactors, to me, they BOTH have to be ON for this to happen. I'm questioning what you're calling a mechanical interlock? How foolproof is this.?? Has the machine ever been tested in the bypass mode? If so, does it operate correctly in bypass? The reason I'm wondering is, if the bypass is swapped phases from the drive contactor, and they both pulled in , you would odviously buck phases and go off like a shot gun, destroying both contactors. So, does the bypass function operate correctly? the interlock?
 
The controls to the different coils go through a set of normal closed set of auxillary contacts on the ooposite controller. I will definetly check to see if they are wired accordingly. Also this problem finds its way to the main service and activates the ATS. Can you help me understand how this would cause this. I beleive it has something to do with the line voltage being affected by the drive voltage which may or may not be in phase with the drive contactor
 
Also as I understand it there is no dynamic breaking for either motor. And it is typical for one fan to be running and the other not. When this occurs the fan that is not energized is being rotated by the energized fan. Can this cause problem on start up?
 

mcclary's electrical

Senior Member
Location
VA
The controls to the different coils go through a set of normal closed set of auxillary contacts on the ooposite controller. I will definetly check to see if they are wired accordingly. Also this problem finds its way to the main service and activates the ATS. Can you help me understand how this would cause this. I beleive it has something to do with the line voltage being affected by the drive voltage which may or may not be in phase with the drive contactor

You have an electrical interlock, not mechanical, and I now suspect that to be the problem. The NC contacts are not opening when drive contactor pulls in. Thus, the other contactor remains operable. So, what controls that? If it's operable,,,,what starts it in bypass? And about the main, it's not uncommon for a main to trip on ground fault, from a much smaller branch circuit. So when it trips, it actuates the ATS. (I'm only assuming)
 

mcclary's electrical

Senior Member
Location
VA
Also as I understand it there is no dynamic breaking for either motor. And it is typical for one fan to be running and the other not. When this occurs the fan that is not energized is being rotated by the energized fan. Can this cause problem on start up?

If being turned in the wrong direction, I would say yes. And depending on how much speed, ramp times, and several other things,maybe catastrophic.
 

Besoeker

Senior Member
Location
UK
Also as I understand it there is no dynamic breaking for either motor. And it is typical for one fan to be running and the other not. When this occurs the fan that is not energized is being rotated by the energized fan. Can this cause problem on start up?
I would have thought it unlikely. Modern VSDs have very good protection. At worst, the drive might trip on overcurrent.

A few comments on your original question.
  1. If aVFD has a failure it should take out its own protection (fuses, breaker)rather than cause a general power failure as you seem to be suggesting.
  2. I'm assuming the VFD contactor is on its output. If the sequencing is correct, it should not normally make or break current so you would not expect to see pitting. Pitting on this contactor suggests incorrect sequencing.
  3. As mcclary has noted, you have electrical interlocking between the two contactors. I'd have used a mechanical interlock as well but I appreciate that it might be difficult as a retrofit.
  4. The burnt out coils suggest that there might be a contactor race (the contactors pulling in and out rapidly) at some point in the sequence possibly at start up or on restoration of power. Again, this could point to incorrect sequencing.

A few of the things we'd normally incorporate in a similar VSD system:
  1. A power on timer. This has two functions. It allows time for power supplies etc to be established and it does an automatic reset of any latching faults - unless of course there is a fault. The drive is not permitted to run until this time has elapsed.
  2. A drive ready contact (from the VSD) in the VSD contactor start circuit. In your case, if you are using a relay in the drive to control both, you might want it in both start circuits. Maybe the relay you mention is the ready relay.
  3. An auxiliary of the VSD contactor back into the VSD enable to prevent it from producing an output unless its contactor is closed.
  4. An opposite sense contact from the bypass to prevent drive enable if the bypass contactor is in.

Going back to the original problem, the more I think about it, the more I think a contactor race is a likely problem. It's difficult to envisage another situation that would cause coil failure in both contactors. Particularly if you don't experience similar problems elsewhere in the building.
 
It definetly sounds like I need to go through the starting circuit and verify the sequencing is right at start up.How would you incorporate a drive ready contact in the system? Is it based on a CT signal? Please explain
 

johnj

Member
VFDs

VFDs

Are these VFD's on a building management system or do they get started manually by an operator. (maintenance man)
 
They are controlled by the energy management system. The odd thing is the energy managment system should have had these in the stopped status. Everytime this outage occurs is on a saturday when these drives should be in the stopped status.
 

johnj

Member
Vfds

You have 2 VFD's. One is for the supply fan and the other is for the return fan if I'm reading this right. These VFD's also have a bypass on them. I've never seen both contactors try to pull in at the same time. (VFD and Bypass) These units have built in protection for that.
To me it seems it's in the programming of the building management system. They could have a bad line of code in their program which is pulsing their control point in their BMS cabinet. This would cause the control relay to be pulling in and out. The contactor is then following suit until it burns out. Because of the high starting current, the contacts would be arcing and thus pitting. I've seen what a messed up program can do. Just a thought.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
You have 2 VFD's. One is for the supply fan and the other is for the return fan if I'm reading this right. These VFD's also have a bypass on them. I've never seen both contactors try to pull in at the same time. (VFD and Bypass) These units have built in protection for that.
To me it seems it's in the programming of the building management system. They could have a bad line of code in their program which is pulsing their control point in their BMS cabinet. This would cause the control relay to be pulling in and out. The contactor is then following suit until it burns out. Because of the high starting current, the contacts would be arcing and thus pitting. I've seen what a messed up program can do. Just a thought.
I think this is the most likely scenario, or something like it. In a typical VFD-Bypass arrangement, the bypass is sometimes controlled by a manual switch for maintenance purposes only. However, another application is to have it controlled automatically so that if the VFD goes into fault, the control automatically switches over to Bypass mode. That doesn't / shouldn't mean the Bypass contactor COMES ON when the VFD faults, only that the Bypass is ENABLED should the VFD fault. The control circuit should still be determining when and where the FAN comes on and off.

I have seen a couple of related issues with BMS systems along these lines. One is that someone misinterpreted the concept of automatic bypass and it comes on every time the VFD faults, even if the fan was not needed. That's bad, because if the BMS is monitoring static duct pressure, the pressure goes up and it must do something about it, which could make it start to cycle fans on and off to try to maintain balance.

The other scenario is that someone did not think about the consequence of only one VFD faulting and going on Bypass, but the other is still on a VFD. So for instance if the Supply fan VFD faults and goes into Bypass, that means it will go full speed. But if the Return fan is still on a VFD and the accel rate is slow, the duct pressure builds up too quickly and the supply fan is shut down. But the return fan, being still on a VFD, then decels too slowly and the duct pressure drops quickly, so the supply fan is turned on again and the cycle repeats itself. This causes rapid cycling of the supply fan and on Bypass, can burn out the contactor. The solution is to make sure that if EITHER fan VFD faults and goes into Bypass, so does the other.

You need to get a full understanding of what the BMS is doing with these fans and why before you spin your wheels much more trying to find phantoms. VFDs don't usually affect an entire power system enough to cause something like this.
 

Besoeker

Senior Member
Location
UK
It definetly sounds like I need to go through the starting circuit and verify the sequencing is right at start up.How would you incorporate a drive ready contact in the system? Is it based on a CT signal? Please explain
Not CTs.
Most, maybe all, VFDs I have dealt with have an internal relay output for drive ready or drive healthy.
Nothing should be enabled for running until that is available.
 
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NE (9.06 miles @5.9 Degrees from Winged Horses)
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EC - retired
It definetly sounds like I need to go through the starting circuit and verify the sequencing is right at start up.How would you incorporate a drive ready contact in the system? Is it based on a CT signal? Please explain

Drive ready is a function assigned to a set of relay contacts on the drive. Some drives have one set, others two and maybe more. These relays can be programmed to close/open at various times. Ready - when the drive is waiting for a start. Full torque, low torque, @ rpm, etc. Read through the list in the programming manual and let your imagination go wild.
 

chaterpilar

Senior Member
Location
Saudi Arabia
One may suspect the two commands ( run/bypass) coming at the same time from BMS and but if properly wired then the two coils will not have the supply at the same time.

But,if they have been improperly wired then one of the coils will always will be energised and the mechanical interlock will not allow the closure and hence the coil will get fried due to overcurrent.

But this event should not trip the whole system. The breaker supplying power to the VSD should trip rather than the Main breaker of the facility.

I always avoid bypasses to VSD as they so easily can go wrong..

Please keep posted on your findings.

Cheers.
 

iwire

Moderator
Staff member
Location
Massachusetts
One may suspect the two commands ( run/bypass) coming at the same time from BMS and but if properly wired then the two coils will not have the supply at the same time.

But,if they have been improperly wired then one of the coils will always will be energized and the mechanical interlock will not allow the closure and hence the coil will get fried due to overcurrent.

I had to figure that exact situation out myself, 5 contactors fried when both coils of mechanically interlocked VFD/Bypass contactors where being energized at the same time.

A BMS contrator had improperly wired in an override signal from a smoke evacuation control board.
 

benaround

Senior Member
Location
Arizona
I would think that at least some of the problem is with the motor that is 'free wheeling'

and then the VFD is called on to ramp it up at a lower rate than it is spinning.

Last week there was a thread on the 'evils' of just this situation.
 
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