1 VFD, 1 Magnetic Contactor, 2 Motors - Question?

[thebigjg]

Adding BAS to HVACR system.

I have two 10HP motors only 1 ever runs at a time the other is for redundancy and for cycling runtimes.

So I want to have one fused disconnect going to 1 vfd going to one mechanically and electrically interlocked contractor and each contractor goes to one motor.

Thoughts?

Also what are your thoughts on the attached image?

***Correction*** The 2nd set of contacts are tied together like the first set.***

 

[Bit_Bucket_07]

So, Motor 1 will always be variable speed and Motor 2 will always run across the line at full speed?

 

[rpoet]

By the time you buy the pair of interlocked contactors and necessary controlling equipment, as well as the necessarily larger panel, you could probably buy a second 10HP VFD. If the above design is for redundancy, what happens if the (only) VFD fails?


-rpoet

 

[thebigjg]

If you see the correction***

Basically this setup provides redundancy for both motor and or vfd failing. Motor fails then motor 2 runs on the vfd if vfd fails then motor runs on normal motor starter. Refering to the image*

 

[Bit_Bucket_07]

If you see the correction***

Basically this setup provides redundancy for both motor and or vfd failing. Motor fails then motor 2 runs on the vfd if vfd fails then motor runs on normal motor starter. Refering to the image*

I'm not seeing that in your drawing.

 

[thebigjg]

If you look at the first set of contacts they are jumped together and as i stated in my correction*** the 2nd set of contacts would be jumped the same way.

 

[thebigjg]

rpoet - the controlling equipment plc is already there it is only a matter of purchasing interlocked contacts. As there will be atleast 1 vfd installed anyway and the magnetic-motor starters are already there just need to add a 2nd contact on the one and remove the overload on the other.

If the only VFD fails it will run on the motor starter. (As the 2nd set of contacts would be jumped like the first set I just forgot 2 show it in my drawing that is why I put ***correction***)

 

[rdrast]

Still not clear, so you only really have one motor?
As I make it out, you have two motors, and want to have either motor run on either the VFD or the simple starter. That is not what you have diagrammed.

In any event, I wouldn't do something like that as you cannot isolate each independent power path. So a motor fails, and you switch to the backup. I wouldn't be having the failed motor changed if I could not completely isolate it's power, and a contactor isn't the way to do that (can't be locked out).

 

[thebigjg]

I dont know what is not clear about the below circuit. (Attached File)

Its a single line representation of exactly what I'm referring to without the control portion.

ABB Makes a VFD just like this without the 2nd set of interlocked contacts. I want the 2nd set of interlocked-contactors so I can run a backup motor if the motor fails and not the VFD.

http://patentimages.storage.googleapis.com/US20130087319A1/US20130087319A1-20130411-D00002.png

I know it works I just wanted some opinions and the one comment about not being able to isolate each motor independently is correct but this is not much of an issue in HVAC shut the source down for a min and isolate the issue then re-apply power. The motors are 3 feet apart.

 

[rdrast]

Then as diagrammed just above, no, that is not a good idea. You would be applying full line power to the output of the VFD when you are on your 'Redundant Contactor'.

 

[thebigjg]

Then as diagrammed just above, no, that is not a good idea. You would be applying full line power to the output of the VFD when you are on your 'Redundant Contactor'.

Yes you are correct line power would be going 2 load side of the vfd - drawing is not correct - thx

 

[DickDV]

Normally, the input side power splits with the VFD side going thru a service switch or contactor and the bypass side going to the starter contacts. There is no need to interlock these.

The output (load) side normally has a drive output contactor which is interlocked with the bypass starter so bypass power cannot feed backward into the drive IGBT's. The overload can then be in the single feed to the motor or in the bypass side in series with the starter contactor.

Since you have two identical (I hope) motors and you want redundancy, add a double-throw contactor in the single motor feed line to select which motor is on line. That should do it.

But............why not just install two drives and two motors and do the interlocking in the control circuit. I doubt that there would be any significant difference in cost

 

[Mark Snodgrass]

DO NOT energize the output of the VFD from the bypass contactor!!!!!!!!!!!! You will destroy your VFD. Been there, done that (not by design, but by operator manually pushing in contactor). The way you should design this is to have a separate breaker/disconnect for the VFD and the Bypass. Then you need two sets of interlocked contactors, one to select the power source (VFD or Bypass) then the second one to select what pump you want to use.

 

[jraef]

DO NOT energize the output of the VFD from the bypass contactor!!!!!!!!!!!! You will destroy your VFD. Been there, done that (not by design, but by operator manually pushing in contactor). The way you should design this is to have a separate breaker/disconnect for the VFD and the Bypass. Then you need two sets of interlocked contactors, one to select the power source (VFD or Bypass) then the second one to select what pump you want to use.

Absolutely. This will not work as shown, so now you are looking at 4 sets of contactors to provide proper isolation, it just gets too weird at some point. You are apparently trying to avoid buying that second VFD and throwing more money and complexity at it to the point where it stops making sense. I would just buy two VFDs, each with an across-the-line bypass. It will be simpler and much much more reliable in the long run.

 

[DickDV]

There is no need for an input side interlocked contactor set. The old two contactor bypasses had NO input contactors at all, just one fused bypass. A disconnect switch or contactor in the feed to the VFD is not necessary but is nice for drive isolation so it can be serviced safely while the bypass is active.

As long as the output side is properly interlocked, both the bypass and the drive can be energized at the same time if cost is the only consideration