one VFD driving 3 motors? Has anyone any experience with similar setups?

Background:

We have a crushed stone stacker, running on rails, three 7.5 hp traversing drive motors fed from one size 3 reversing starter, 3 thermal overload blocks on load side of VFD protect each motor.
The stacker has been experiencing overload trips at time of reversing direction. (my observations indicate problem with worn down rails and wheels but that is another issue..)
The great minds have come together and decided that we will install a VFD in place of the starter. I haven't heard of one VFD driving multiple motors. I may not be involved in the engineering but I will be called in the middle of the night to troubleshoot the system.
Just looking for a heads up,
thanks,

Brian.

(we have been using A-B 1336 plus II drives, but I won't know what I get until I get it.)

If there are mechanical issues causing one or more of the motors to trip then using a VFD will not eliminate the problem. The mechanical problems should be thoroughly investigated and eliminated.

VFD's are used with multiple motors in many situations but personally I would prefer individual drives. THe cost of one 25HP (or 22.5) drive could be significantly more than 3-7.5HP drives. Troubleshooting could be less complicated.

While I haven't seen a 1336 running multiple motors I have come across GPD vfds that ran up to 12 motors on the same drive. Essentially each motor is geared so they all have the same speed. And as you increase the freguency they will all increase equally. We ran that system for about two years but as we had zero problems with it. I never had to become overly familiar with it.

thanks for the response,

I think that when the mechanical problem persists, I will be asked to 'tune' the drive somehow to compensate and I have no experience with trying to program a VFD for multiple motor loads. I have done everything but drag the 'great minds' out to the stacker and have them feel the metal burs along the rails where the weight of the stacker wheels has been hammering them for 35 odd years.

rant off..

Would it be as simple as programming the drive to treat the three 7.5 hp motors as a 22.5 hp motor?
You would still require seperate overloads for each I would think.

here is a pdf file from http://www.yaskawa.com/ It will provide some insight as to how to size for multiple motors.

Sliver

I have used one 25Hp 1336Plus II with two 7.5HP SEW Eurodrive gear motors in the past. Because of mechanical limitations we had to use two motors, but since the two were very tightly tied together mechanically we decided to use one VFD.

It worked out very well but you will want to make sure that you still have the overload protection between the VFD and the motors since the electronic overload built into the drive will be useless.

In my case we just set up the drive for the total full load current and treated it as one motor.

I think from what I am reading that the issue of having tight mechanical linkage between the motors is going to get us. Maybe that's contributing to our overload problem, always one drive tripping out (occassionally) when reversing direction.
Each motor currently drives a fluid clutch, gearbox, seperate wheel. Each wheel would have different down pressure, slip threshhold, diameter due to wear.
I can see one motor taking all the load and the other two along for the ride. They (the great minds) mentioned that they would be removing the fluid clutch, using the VFD to ramp up and down, but I don't think that is going to be enough to prevent new problems let alone resolve old ones.

thanks for the PDF mordred and for the confirmation Sekeely,

Brian.

For God's sake, don't take out the fluid clutches!

If DickDV can chime in he can explain the background theory. Basically you have three motors "coupled" to a common load. The motors won't be truly identical, and will tend to run at slightly different rpms for the same load. If they are forced to operate at the same speed, which is true unless one end of your stacker can move at at a different speed than the other end, the motor that tends to run fastest will be dragged down in speed and tend to take more load.

Having the fluid clutches in there will let the motors slip relative to the load and each other and tend to balance the loading better.

This topic has come up before on the site, but I don't remember the description. A little creative searching should turn up something.

Multiple motors - one VFD????

The slip clutch is a definate requirement. They act as a differential to the motors!!

When running place hand on the 3 clutches. The warmer one is the motor that is slipping due to unequal load.

SHEEEEESH!

Rod (the CNC dude)

Is it OK if I use an infrared thermometer instead of my hand?
I might have to change my handle to Lefty!
These fluid clutches get up to 1800 rpm pretty quick...
I'll check to see if they REALLY want to eliminate the fluid clutches, thanks for ammo guys!

vfdssssss

We use combinations of 4 - 5HP and 8 - 7.5HP motors on one drive.
I have worked with Allen Bradley Plus and Plus II, Impact and Toshiba
G3 and G7.
Some problems to watch out for, the drive must be oversized by a factor recomended by AB. 1.1 is a common factor and it is too low.
Your application determins the sizing.
Use thermal OL on each motor individually. I wouldnt use breakers.
Do not use contactors on the line or load side of the VFD. A fused disconnect is best.(Use the run command on the drive to start motors.)
Put in both line and load reactors.
Dont forget to have some fun. Sounds like a good project
Bruce

Sliver said:

for 35 odd years.

Click to expand...

That's what I see as a red flag. Sounds like to me the great minds at your company don't want to listen to you. If this has been a working setup for that long, they need to ask themselves what has changed. Somehow though I have a feeling you aren't going to convince any of them.

Comment:

Sounds like the egg heads are trying to compenate for years and years of wear. So back to Engineering Econ 101. Is the unknown cost of contunuely attempting to compensate for mechanical problems by newfangled controls really going to be less then just fixing the problem and K.I.S.S.-ing the sysetm?

Factors include:

1. Cost of new system,
2. Cost of mechanical restoration/repair,
3. Possable cost of having to do both if you start with item 1. (what's the probability that new system wont fix actual problem)
4. Lost production profits in all three cases above.

My guess the numbers are going to tell you to fix what's really broke.

Running three motors on a single drive is fairly common these days and the only extra features necessary are separate overload blocks for each motor and the drive output amps sized for the total of the three individual motor FLA's plus 10% extra for each motor beyond the first one. So, in this case, the motor nameplate FLA's added up and then add 20%. The drive output must be capable of this ampacity continuously or you will get overcurrent faults on starting.

The problem with the existing setup is that, except for the fluid couplings, you are basically plug reversing the motors to reverse the machine. With a drive, you will get a controlled decel ramp and controlled accel ramp even if plug reversed.

And, for me, I would take the fluid couplings out. It is true that they would serve to even out the torques if the motors differ significantly in slip but that shouldn't be. If all three motors have identical nameplates, the torque should divide equal enough to not have problems.

You could do this job with SCR-chopping soft-starters but the inrush currents would be at least three times higher on start and there would be very little cost savings at these low hp's. I would recommend the VFD.

I am in complete agreement concerning the drive on the load. It will improve your operation and reversing a VFD is much easier on the motor than any other way to do so.

Since I doubt your existing motors are inverter rated, you may want to think about getting some load reactors (essentially line reactors between the VFD and the motor) to lessen the dv/dt on the motors.

http://www.ecmweb.com/mag/electric_line_reactors_vfds/

The article above is written by the VP Sales & Mktg for MTE, which makes reactors. The information, though, is good.