(1) VFD for (2) Motors

[Rson]

Here is the situation:

I want a VFD to control a Fan motor - but I have a redundant fan that should not be used unless I am servicing the main Fan.

Is there a transfer switch of some type I can put between the VFD and the motor that would just toggle between the two motors - or would I be required to have a disconnect for each motor?

They should never both operate at the same time and I'm trying to think of the cleanest way to do this without having to provide a VFD for each motor.

Thanks for your opinions in advance!

 

[geniusintraining]

I have used these before on a similar application, just make sure you power down the drive before switching over or at least disable the drive that way it will ramp up and not switch instant

https://www.automationdirect.com/ad...uses_-z-_disconnects/merz_rotary_cam_switches

 

[Rson]

Thanks for the cam switch - I at least have an idea of what to look for.

This would be for something large, however. Probably 100-250 HP range.

 

[Ken Roach]

I think the search term you want is a "manual transfer switch".

UL 98 is for "double throw safety switches" and UL 1008 is for heavy duty transfer switches, so you might use those as search terms too.

 

[Rson]

Thanks, the double throw safety switch seems to be what I was looking for. It looks like most of them are Line-Load-Line rather than Load-Line-Load, but I suppose they could work either way.

 

[geniusintraining]

Thanks for the cam switch - I at least have an idea of what to look for.

This would be for something large, however. Probably 100-250 HP range.

Yah mine were a LOT smaller so the cam switch worked good, but a 250 would need some large contacts and a funny looking switch, not to mention the re-enforced door it would need to be mounted on

 

[alan_505]

I saw one at a site where it used key interlock switches so that the main isolator had to be off and the key then moved to the changeover switch to allow it to be switched.
This way nothing could be switched whilst it was running.
It was slightly different in that they had 2 VSD's for one motor but the principle is the same.

 

[roxusa]

I was recently at a new site that had 1 VFD controlling 2 motors similar to what you
are suggesting and they had 2 sets of contactors after the VFD to determine which motor was connected to the VFD. I did not look ate the schematic but I would think it was all interlocked & programmed through the PLC control so that only 1 could be on line at a time and that the VFD was in a stopped condition before the switch of Motors. It was a new machine install. I will be there next week and look closer.

 

[geniusintraining]

I would think it was all interlocked & programmed through the PLC control so that only 1 could be on line at a time

I think thats what I would do, just like a reversing motor starter

You can build one yourself with two contactors if they have aux contacts on them and just interlock through them its easy enough but probably faster just to buy a set then you can control some sort of time delay in the PLC

You would not swap the two phases, the pic is a reversing

 

[jstolaruk]

Here is the situation:

I want a VFD to control a Fan motor - but I have a redundant fan that should not be used unless I am servicing the main Fan.

Is there a transfer switch of some type I can put between the VFD and the motor that would just toggle between the two motors - or would I be required to have a disconnect for each motor?

They should never both operate at the same time and I'm trying to think of the cleanest way to do this without having to provide a VFD for each motor.

Thanks for your opinions in advance!

For servicing motors that large with that much current in play, you're going to want to have the individual disconnect switches along with the selecting contactors. Possibly interlocking those with a monitoring contact at least back to the PLC.

 

[TWS]

I am speculating here but if you have an application that needs a second fan for redundancy why would you not have a second drive? Or if your budget does not allow for the cost of a drive, then install a soft start on the second fan. then you can simply switch to the soft start in the event of a drive failure.
I think the fan is probably more reliable than the drive.

ABB has a wide array of manual and automatic transfer switches.

 

[geniusintraining]

I am speculating here but if you have an application that needs a second fan for redundancy why would you not have a second drive?....

What kind of fun would that be? then he would not even needed to post the question.

UNLESS, what if Rson had two drives and two motors but only one momentary push button and he needed to switch from one to the other, oh wait, thats a flip flop and I dont think we want to roll down that hill

 

[Rson]

Yes - in my view it would make the most sense to just have both drives. However, these drives are huge, take up panel space, and are expensive.

I was asked to look into a way to wire both motors to (1) VFD as a cost-savings measure. However, I think in my research the transfer switch is almost as much as a VFD (like 14k vs 16k or something close). I think what happened is that we quoted the customer and they had sticker shock, and now we are trying to find ways to cheapen our product.

 

[arpus4KM]

you just need to use 2 motor starters after the VFD, with Mechanical and Electrical lockouts on the starters. then control the two starters with a selector switch that will power either the first, or the second, (or neither if you opt for a third both off position).

I've even seen a single VFD used for multiple different sized motors. The PLC control for switching between setup/adjustment motors allowed you to switch between whichever contact was going to each motor, and selecting it also changed the parameters of the VFD to the correct motor voltage/FLA/etc that was necessary for each different motor that single vfd controlled

in position 1, motor starter 1 will be energized, and allowing power to flow from the top side to bottom side. and the NC contactor with the A1 coil power for contactor 2 will now be open and unable to electrically turn on. in addition, the contactor will have the mechanical lockout which would keep someone from mechanically pressing both in at the same time.

switch the position to 2, and starter 1 open, starter two closes and upon closing the NC contact on the starter 2 aux contact will open, preventing power from flowing to starter 1, and again mechanically lock out starter 1 from running while 2 is on.

 

[cidchase]

Install two disconnects. Interlock them (hard-wire or inputs) via auxiliary contacts so your single MS cannot pull in unless either one is on and the other off. Hard wire is safer. We do this whenever we have backup drop-lifter motors.