multi motor single drive setup

I have an ideal situation that would allow me to do something with a drive i've heard about but never have done. So i'm on here asking the plethera of knowledge here what the opinions are. I have 5 5Hp motors of identical nameplates that currently run off of 5 separate starters. I'd like to upgrade the panel and put one drive in there to run all 5 motors. Any speed changes we always make to all 5 motors so the situation is really ideal for this. I know that certain AB drives support this i just would like to know the PROPER way of implementing this. Doing this upgrade will save a lot of money in energy costs over the long term.

Thanks in advance.

Search the forum for the derating formulas, or confer with RA Tech Support.
Do keep separate overload protection for each motor (required).
Do not add loads while the drive is in operation...in other words, only pull in the output contactors with the drive stopped. I have seen folks get away with dropping them out during run, but I didn't like that either...I suspect is was a reason for some high failure rates.

Don't do it because you never got a chance to do it before.
Only do it to make more money by improving the process.
Realize that 5 5hp VFDs unlocks even greater potential and fewer headaches and should, in most cases, be the next phase of enhancement.

Okay. Will do. I've heard about not engaging a contactor with a drive already under load and what it can do (fireworks?). I think that I might go with 5 5Hp drives like you suggested. That was my first thought. Have you done something like this?

This 1 drive upgrade has the potential to cool our product much faster without any mechanical changes. The PLC can more accurately control the duration of product to cooling jacket contact with a single or multiple VFD setup instead of starters. Also, this application is started and stopped multiple times a day so the current pull out of just one of the systems can get large, and we have 4 sets.

Here's a FAQ about it from Siemens site:

http://support.automation.siemens.c...lib.csinfo&lang=en&objid=14390055&caller=view

Kalle

beanpole_ecc said:

I think that I might go with 5 5Hp drives like you suggested. That was my first thought. Have you done something like this?

Click to expand...

First thoughts often end up being the best decisions.

I have not done something like this, but I have supported a handful of them. Its all fine and well until one of 6 contactors develops an intermittent problem and the technician is down half a day trying to locate the source of the problem. All he knows is the drive stopped and you have 5 OL contacts wired in series...

Just the basic premise that all relays fail, and the bigger they are the harder they flame out makes me dislike adding multiple contactors to a design.

If there is any advantage to keeping 4 of the motors operational while one has faulted or failed, then go with individual VFDs.

If the panel is laid out nicely, and some troubleshooting guidelines are followed, I think it could be just fine. One system I had to support was done that way, using one drive to any one of four different ballscrew motors that were only operated during product changeovers. It suffered from mismatched sized motors and a single OL setting so it would cook the little motor if there was a mechanical bind, until it's MPCB would trip which then had to be annunciated with another pair of wires and input point and reset manually....

Another system with fans, was great as far as function and reliability, but when any problem occurs, you lose all three fans for at least the first occurrence. So, in my opinion,. they all suffer from reduced flexibility and reliability. Single VFD multi-motor systems will increase troubleshooting time and errors, too.

Thanks for your time okiepc. Just wanted to touch base with some others in the field to see past experiences, weigh all options, then act on facts.

KalleOlsen said:

Here's a FAQ about it from Siemens site:

http://support.automation.siemens.c...lib.csinfo&lang=en&objid=14390055&caller=view

Kalle

Click to expand...

That is excellent information applicable to any VFD mfg. Thanks.

Sequentially adding multiple motors to a VFD drive is not uncommon. The key is to size the drive for the motors that are being added to the existing load based on the x6 inrush current of an AC motor and size the drive accordingly.
For example 5 HP drive is capable of instantaneous loads of 150% [fairly typical with current drive technology] then it can momentarily drive 7.5HP. So if you are powering a 1 HP motor from a 5HP drive you can readily add a 1HP motor to the load [1HP + 6x1HP = 7HP] without tripping the drive.
Now if you have 2 x 1 HP motors attached and running from the same 5HP drive you now only have 7.5-2=5.5 momentary HP remaining. 5.5 / 6 = 0.92. You might get away with adding a 1HP motor but a 0.75 works just fine.

Have used this formula/approach for over 25 years with NO problems.

As long as the five motors all run together or, if the number of running motors is always changed during a stop, I would use only one drive with each motor fed thru an overload relay with a normally closed control contact. String up the five overload contacts in series and set up an external fault loop on the drive digital inputs.

As to sizing the drive, two ways to definitely NOT size it is to add up the hp's (5 x 5 = 25) and choose a twenty five hp drive. The drive will be too small. Or, add up the FLA's on each motor and size the drive that way (about 8 amps x 5 = 40amps). Again, the drive will be too small and you will experience nuisance overcurrent faults.

There are several good ways to size the drive and I find the one mentioned above by Bits N Bytes as a good one. The one I like to use is to add up the FLA's on the five motors and add an extra 10% for each extra motor. Five motors is four extra so that would be an adder of 40%. So, 8 x 5 x 1.4 = 56amps.

One last thing, with more than three motors you need to add fusing or, better, a fused disconnect in each motor lead set. This is to protect against fire and catastrophic damage in one defective motor when such a comparitively large size drive is supplying the current.

Otherwise, this sounds like a good application for multi-motors.

dickdv and bits n bytes

i'm glad you posted those replies. i think a lot of people including myself just think you add up the motors and size the drive like that. that's very good information.