Multiple Motors From Same VFD

[Beadman]

I hava an application where I have 6 motors that will never be required to run at the same time. I would like to use just 1 VFD to save $$$. Can I use one motor starter and some downstream contactors to pull this off, and is there any code issues I should worry about. I am a ChE and have some knowledge of industrial control, but I am not an EE or electrician.

 

[buzzp]

You can do this as long as two motors are not going to run at the same time and the drive will be ramped down when switching motors. This can be accomplished as you suggested. Are they all the same motor? If not, the drive may have to be set-up different for the different motors. I know of no unusual code issues to worry about, just the basic stuff. I would incorporate some interlocks (electrically or mechanically) to prevent more than one motor running at the same time or starting another motor while one is online. This may or may not be a code requirement but would be a good idea to prevent accidental starting. Good luck.

 

[dpc]

You might want to search this forum - this has been discussed before. As buzzp says, you can do this provide you have adequate number of isolation contactors and the proper interlocking.

However, unless you are talking about large motors, you may find that it would have been cheaper to buy individual drives. Operations and maintenance is much simpler with individual drives as well.

 

[dydt]

hi...
how about the issue of all 6 motors cannot run because of the lone VFD failure?..unless of course you have a spare you can pull out quickly..

dydt

 

[Beadman]

Thanks everyone for you help. The issue is I have 19 idenitcal motors. Only 4 ever need to run at one time. I will build system with 5 so I have a spare, but 4 vs. 19 is a big cost savings.

 

[peebee]

Older VFD's were typically configured as current sources, which meant that running them without a connected load could possibly damage the VFD. I'm not sure about the newer models, but you might want to look into that and provide an interlock to prevent running the VFD with no load.

 

[akhaider]

All VFDs have watts and ampere ratings, specified by the manufacturer.
Damage can occur to the motor if the running watts exceed the name plate rating of the drive.
Similarly each drive has built in thermal protection. You must check the current you draw from the drive against its name plate rating.
As long as you remain in the limits, thru an electrical interlock, every thing will be OK.

 

[jbartos]

Suggestion: If the motors are small, selector switches downstream from VFD can complete the branch circuits. If the motors are larger, e.g. 5A or so, the contactors replacing the selector switches could be used.

 

[jraef]

dpc is right. NEC requirements for short circuit and overload protection of each individual motor may push the cost effectiveness out the window on small HP motors. Keep in mind that not only do you need separate circuits for each motor, the output terminals of the VFD must be isolated at all times from line power feeding back from another of the circuits (you will blow the transistors if not), so the cost of all of the necessary interlocking contactors becomes prohibitive unless the VFD is very expensive. Small (under 10HP) VFDs are now so cheap that it may make sense to use separate units. That which does not kill me, makes me stronger... and pissed off!

 

[busbar]

Additionally, it seems like horsepower-rated contactor switching may be equally or more expensive compared to dedicated VF controllers. Typically each motor would use two 3-pole, electrically and mechanically interlocked contactors. It is possible unit pricing on a quantity of VFDs may be significantly less than the single-lot price.

 

[jbartos]

Suggestion: By responsible controls, a number of contactors can be reduced down to one per motor branch circuit. A selector switch turning on one contactor and off others appears to be a solution. However, the mechanical interlocks are important in case that any contactor becomes stuck in the close position.

 

[buzzp]

You could use electrical type interlocks by monitoring the current in each motor (about $60 US a piece for accurate drive current measurement or buy standard monitor and compensate for the error in the reading). This will verify the motor is indeed off before starting another motor. I am not that familiar with interlock issues with standards and such but I would do it electrically if at all possible, keeping in mind the fail safe strategy with relay contacts.

 

[etman]

We have been doing this since the '60's on spinning machines where we have up to 32 motors per inverter. These are synduction or perm magnet motors. Each motor has a manual starter and fused protection. 1-16 motors can be run at a time. As expected when we went to IGBT drives we started having problems that were eventually solved by a special filters. We have hundreds of ~ 1hp motors running this way. You just have to understand that if you have an inverter problem - they all go down. Of course they all run at the same speed - but that is what we want.

 

[jbartos]

Suggestion: If there is an inverter problem, there has to be another one to provide the inverter redundancy. In case of hundreds of motors, an extra inverter or two is the reasonable redundancy to have.