1 vfd and three motors

The 1 vfd running several motors caught my attention. I have a question concerning this. Is it possible to use 1 vfd to control say 5 identical motors, only 1 motor at a time will be running, here is a rough schematic of what I am talking about


[VFD1]--------[contactor1]------[motor1]
[VFD1]--------[contactor2]------[motor2]
[VFD1]--------[contactor3]------[motor2]


Get the picture?
If this would work is it not good practices?


www.PowerhouseAutomation.com

As long as the contactors don't open and close while the VFD output is ON, I don't see a problem with your idea.

Essentially, it's the same as physically moving the wiring from one motor to another, just a heck of a lot easier and quicker!...

beerchug

-Eric

eastkodakguy said:

The 1 vfd running ...

Get the picture?

Click to expand...

Yeah I got the "picture"; and the pun. Your customer must have an itty bitty wallet. I hate when that happens.

We did a similar arrangement in Shanghai, China on one of their sewage treatment facilities.

We had two VFD's and 10 motors on dual identical systems. As the flow rate increased (12 million people can flush a lot of toilets), we'd bring another motor online. We'd have one motor on the VFD, to control the deep well level/flow rate, then when that drive increased past synchronous speed, we'd sync it to the line, putting the first motor directly on the line, then bring up the next motor on the VFD for flow control once again.

Eventually, we'd have up to 4 motors on the line, with a single motor on the VFD for control. As the deep well level decreased, we'd slow the control motor until we took it completely off the line, then once again synchronize the VFD to another running motor, taking it from the line back to the VFD for control.

Each set of 5 motors looked like your drawing.

Very Interesting.

Would you need to derate your contactor if you are going to use it on the output of a drive?

We never derated the contactors that I am aware of. There was no need as we used "old" technology that had a sinusoidal output (due to filtering). The motor never knew it was on a drive, for all intents and purposes.

Interlocks

eastkodakguy
what you are asking about is a good practice with some precutions one of the is the interlocks between your contactors either electrical or mechanical interlocks to ensure that only and only one motor is on line at a time otherwise your inverter may be damaged or a least trip when switching from one motor to the other also a small time delay between deenergizing one contactor and energizing the other will not be a bad thing.
good luck beerchug

Thanks all

This isn't for a customer, but it could be an option if the right application came along. I was just curious about it, Again Thanks All

Regards
Dallas Smith

I have heard of derating contactors, but never have personaly.

I normally use GE or Square D Nema rated starters. While at GE, we built inverter bypass panels for a major drive manufacturer, without any contactor or starter problems, but they were fully rated nema, probably a solid 3 to 1 or better safety facot.

I would definitely over size IEC starters.

regards.....casey

Contactors & VFDs

I am not sure about other drives, But Allen Bradley & Toshiba Say right in the manuals not to use contactors on either side of the drive. The frequency and voltages produced by the drives can make very large arcs and burn the contacts. A solid state switching device will work but will cost as much as the VFDs. There are some oil filled contactors that will work, But they are very large and not fun to work with.
An engineer friend keeps telling me ( cheap Scott ) "If you do the job cheap it will cost you twice as much, do the job right and it will be cheaper in the long run".
Bruce

To utilize the drive in an inverter bypass panel, you would have to have a contactor on each side of the drive. I think I will have to get a bypass panel literature or spec sheet from each of them to see how they handle the switching.

I could see arcing on the drive output at very low frequencies, but...

regards.....casey

Contactors

Hi Casey,
All of our bypass systems are on a selector switch. The drive has to be stopped, and someone has to go into the MCC and switch over. A interlock is used to make sure mistakes are not made. The concern is drive life can be shortened by opening the load side when running. Only one machine in our mill has one of these pannels, and we never use it, reducing production that much is not worth the effort. This system was ordered as a package and the pannel was included. I know its done everywhere, most mills here do it, but I dont like it. The trend is changing, as drives get cheaper. Some now stock one of each size they use in the electrical distributer wearhouse on a deal that they get the drive whithin a specific time frame. The distributer gets more buisness this way. One such sales person keeps showing up at our mill with the offer, you never know, someday we may be doing this.

Bruce

Don, They have flush toilets in China now?

I have shared two motors from the same drive before, ONCE! It basically sucks . I'm not that cheap anymore.

2 on 1

I built a tumbler two years ago that makes new concrete paving blocks look old in a matter of about 2 min...it uses two 7.5 hp motors feed off of one 20 hp drive to rotate the big drum,one at each end...with two seperate overload units to protect the motors and the drive...the nice thing about this is they both run at the same speed everytime, all the time without any extra circuitry.

David

Sawmills are full of these.....

This isn't exactly what you want to do, but here's one way a drive runs a group of motors. At my last job we had large Toshiba drives controlling groups of motors that ran the line that moved the logs through the saws and chipping heads. Lets say we have 6 motors. They were all constantly driven as a group to control log speed (small logs faster, large logs slower). The drive fed a series of overload blocks and if a motor was overloaded, it would trip out and stop the drive.