AHU with VFD and multiple motor fan wall

[barry harkins]

We have 3 AHU's with 18 10HP motors on the fan wall. 9 motors per VFD. 480v
One unit has lost motors and drives at an alarming rate. ABB drives with all motor conductors in a single pipe to the fan wall. Problems seem to start and escalate as soon as one motor goes and starts VFD to fault. Drives have been changed motors have been replaced multiple style faults. Ground faults ,over current, under voltage and more. Any ideas on newer high eff drives with old and new motors, conductors in same emt? Health care facility AHU run 24/7.
10 year old facility
clean power and generator run every month with full transfer
Identical ahu next to it feeding same space runs like new
A lot of info I knw but anything will help

 

[zbang]

Are those AHUs completely identical? They may look identical, but... look for differences- could be back pressure, uneven loading on the fan wall, lengths of VFD-motor conduits & wire, vibration, even the type of wire used. Is the VFD programming the same between units, or are the differences rational?

Also, what kind of motor faults happen? Could be the replacement motors aren't really identical to the existing ones.

 

[synchro]

We have 3 AHU's with 18 10HP motors on the fan wall. 9 motors per VFD. 480v
One unit has lost motors and drives at an alarming rate. ABB drives with all motor conductors in a single pipe to the fan wall.

So does each AHU have its own single conduit from VFDs to its motors, or are you saying there's one conduit feeding all of the AHUs?

 

[Russs57]

I'd suggest adding individual over current protection between the drive and each motor.

When one motor goes out the drive can now over amp the remaining motors. That increases the chance of another motor going. Manual overload relays are fine.

I'd use VFD cable to each motor if wiring is in a common raceway.

Keep in mind a generator run can subject equipment to four disturbances doing transfer and re-transfer. Might want to look into a closed transition ATS. Or at least changes to get it to swap over in synch/phase.

I know these are costly suggestions. I work at a hospital as well and having an AHU with 180 HP of fans in it is bound to effect a lot of spaces. I'd want to add a bypass contactor so I had something when a VFD blows. I'm hoping you aren't needing to run past 60 hertz

 

[petersonra]

So does each AHU have its own single conduit from VFDs to its motors, or are you saying there's one conduit feeding all of the AHUs?

good question. if so, very bad idea.

 

[petersonra]

I'd suggest adding individual over current protection between the drive and each motor.

Would be code required as each motor has to be protected from over current.

When one motor goes out the drive can now over amp the remaining motors. That increases the chance of another motor going. Manual overload relays are fine.

What might happen is the VFD ramps up due to the missing fan and causes more stress on the other fans.

I'd use VFD cable to each motor if wiring is in a common raceway.

best bet is to get the wiring out of the common raceway between the VFD and motors.

Keep in mind a generator run can subject equipment to four disturbances doing transfer and re-transfer. Might want to look into a closed transition ATS. Or at least changes to get it to swap over in synch/phase.

Keep in mind the generator is feeding the input of the drive. The drive won't care like a straight connection to a motor would. Worst case is the DC bus gets out of whack for a cycle.

I know these are costly suggestions. I work at a hospital as well and having an AHU with 180 HP of fans in it is bound to effect a lot of spaces. I'd want to add a bypass contactor so I had something when a VFD blows. I'm hoping you aren't needing to run past 60 hertz

 

[barry harkins]

So does each AHU have its own single conduit from VFDs to its motors, or are you saying there's one conduit feeding all of the AHUs?

No. A 2 1/2 " pipe to the unit and then to trough on fan wall. All have individual grounds to motors. Drives mounted in enclosure on the side of AHU. Built in 2010 and the two identical units are the same to same duct and space. All new motors have been in their own conduits on other projects throughout the facility.

 

[barry harkins]

I'd suggest adding individual over current protection between the drive and each motor.

When one motor goes out the drive can now over amp the remaining motors. That increases the chance of another motor going. Manual overload relays are fine.

I'd use VFD cable to each motor if wiring is in a common raceway.

Keep in mind a generator run can subject equipment to four disturbances doing transfer and re-transfer. Might want to look into a closed transition ATS. Or at least changes to get it to swap over in synch/phase.

I know these are costly suggestions. I work at a hospital as well and having an AHU with 180 HP of fans in it is bound to effect a lot of spaces. I'd want to add a bypass contactor so I had something when a VFD blows. I'm hoping you aren't needing to run past 60 hertz

All have individual motor overloads and they will trip. Drive will fault and usually unit will have static trouble after that but will reset ok.
built in 2010 have thought about trying a sample with vfd sheilded cable on half the motors but may try Cool Blue inductive reducers. ABB rep says drive should do al that. Cool Blue rep coming in .
Gens are synched
we have bypass in the drives .
Drives becoming to sensitive for older motors and wiring maybe

 

[barry harkins]

Are those AHUs completely identical? They may look identical, but... look for differences- could be back pressure, uneven loading on the fan wall, lengths of VFD-motor conduits & wire, vibration, even the type of wire used. Is the VFD programming the same between units, or are the differences rational?

Also, what kind of motor faults happen? Could be the replacement motors aren't really identical to the existing ones.

YEs. Yes. Yes. We get ground faults that we can't read with a meter. We also can change the ground faults to warnings instead of shutting down the units but why should we have to?

 

[barry harkins]

good question. if so, very bad idea.

We have a pretty good fluke power meter that we have put on the drive. Plan on putting on again for Thursday 3/24 for a generator transfer. Will compare readings. The THD-V and THD-A readings are interesting but not being an electrical engineer can be confusing when comparing to other loads we've tested.

 

[synchro]

I don't know whether it could be related to your problem, but the following app note from ABB mentions that a default "flying start" algorithm on their drives can lead to faults including overcurrent when starting fan arrays. That's because the drive will try to sense the speed and direction of the motor(s), but some motors might be free-spinning and others not (or not at the same speed). As an alternative, there is a "constant time" starting method which stops the motors first by applying a DC current before it starts them up.

https://library.e.abb.com/public/69c6cc111ba64361ba511d36f0445616/LVD-EOTKN024U-ENTechNote49.pdf

 

[tom baker]

Do the drives have line reactors?

 

[Jraef]

Do the drives have line reactors?

Similar to this point, it is HIGHLY recommended that there be a LOAD reactor on the output of the VFD when feeding multiple motors. Even though your motors may be close to the VFD, you have 9x the number of conductors, ergo 9X the total circuit length to contend with and thereby 9x the chance of conductor insulation problems. Also related to this issue is that the VFD should be a rated for a MINIMUM of 110% of the combined motor FLAs because the VFD can end up with issues in trying to handle the capacitive charging current of all of those parallel conductors and going into current limit, which can cause the motors to not run at full speed even though the system is demanding it.

 

[garbo]

We have 3 AHU's with 18 10HP motors on the fan wall. 9 motors per VFD. 480v
One unit has lost motors and drives at an alarming rate. ABB drives with all motor conductors in a single pipe to the fan wall. Problems seem to start and escalate as soon as one motor goes and starts VFD to fault. Drives have been changed motors have been replaced multiple style faults. Ground faults ,over current, under voltage and more. Any ideas on newer high eff drives with old and new motors, conductors in same emt? Health care facility AHU run 24/7.
10 year old facility
clean power and generator run every month with full transfer
Identical ahu next to it feeding same space runs like new
A lot of info I knw but anything will help

I retired from a large 550 bed hospital/research center/ ambulatory care campus. We had over 200 drives just in the main hospital and not one VFD feed more then 1 motor. Probably 75 % of the drives came with a bypass. I would run wire from the closer motor via cord to feed motor that burns out frequently and connect the other motor to wires that feed other motor. This would rule out a problem with drive & wiring from drive to motor. I had a 75 HP pump drive that mechanic swore the pump was bad but I had another mechanic check out pump and found no problems so I swapped out wires from the back up pump and found their was a problem with the drive which I later repaired. I always ordered TEFC motors for motors inside of AHU'S. Got called in for a tripped drive. 100 HP motor grounded out due to humidity control broke and was throwing out tons of water vapor into an open motor. With the cost of drives constantly coming down can not understand why anyone would place 6 motors on a single drive. Got called out to a remote site where they installed a roof top AHU to feed two very busy OR'S and office space. They had four 10 HP motors on a single drive. A day earlier they sent the least talented sparky to check it out. He reset the tripped motor overload & it tripped out again. He did not have the smarts to just remove the 3 wires from overload and have the remaining 3 motors run. Even though it was under warranty the office & OR'S were closed two days.

 

[paulengr]

No. A 2 1/2 " pipe to the unit and then to trough on fan wall. All have individual grounds to motors. Drives mounted in enclosure on the side of AHU. Built in 2010 and the two identical units are the same to same duct and space. All new motors have been in their own conduits on other projects throughout the facility.

So know how a transformer works? If you have multiple VFDs feeding multiple cables call jammed in a single conduit, imagine what the magnetic field of a pulse traveling through that conduit does to the other conductors at the same time. So with 3 VFDs your effective cable length before you have problems is 1/3rd the length. And each VFD is putting common mode curreht on each motor. Just imagine the potential ground potential rise situation.

Start by tariff down a motor. Look at how it failed. Look at the coils. Do you see burn marks on every coil in a symmetrical pattern (6 burn marks)? That’s reflected waves. Flush the bearings with brake cleaner and inspect. Are the balls polished or frosted? Are there fluting marks on the races? Is it uniformly burned all over. These give you clues why it’s failing. Everything else suggested is a shotgun approach…maybe it works maybe not. You can’t fix it until you know what the issue is.

 

[garbo]

Two other things. At the hospital that I retired from they had a very good contractor come in several times a month to check balance on air handlers. Are you using new 10 HP motors from a quality company like Balder ? Did anyone check that the fan is in good condition. We had a rewound motor that would trip out drive maybe every halve a hour on a guilitine that cut 30 rows of heavy Carmel 50 times a minute. Finally had our best machinists good over it with a fine tooth comb. He found that the shaft had a very small bend in it so we throw that motor away. I am not a fan of any foreign made 3 phase motor.

 

[petersonra]

I am not a fan of any foreign made 3 phase motor.

There are a lot of good motors made overseas. I don't have an issue with them. In any case, how would you know if it was made overseas or not?