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

[CaseyK]

I once saw two dc drives with two motors operate on the same trolley bus bar rail. I could never figure out how it worked. The newer drive ran the newer motor and vice versus. No matter where on the bus the motors were connected. I still have that equipment, I'll have to hook it up some day and try to figure it out. Even though he told me never to try it.

My father set it up in the fifties. After becoming an EE, he went to work in Chicago - La Grange for GM's Electro-Motive locomotive plant. I spent two years there in the 90's. Neat place, lots of up to 1000 hp traction drive motors laying around.



Anyway, another wierd application.

regards.....casey

 

[JesperMP]

Have run 10 motors on the same VFD at intermittent duty.

When running several motors on the same VFD, it is recommended to use load reactors (don't know exactly why though).
Allways when using VFDs it is a good idea to protect the motors with PTCs in the windings. Your problem will be if there are such sensors allready in the existing motors. If not you will have to rely on the protection that the VFD provides. A thermorelay for each motor doesnt really provide protection if the motor is run at speeds that differ from the nominal speed.

 

[grs]

What about the programming of the drive? I was under the impression that to use more than one motor on a drive you had to force it to a volts/hz mode instead of sensorless or flux vector control. This may affect the type of drive you are looking at.

 

[DickDV]

You can choose any drive that has V/Hz (scalar) operation available. Many drives these days have sensorless capability with software selection of V/Hz. That's ok too as long as you choose V/Hz.

Regarding the motors, if they are class F insulation and the leads are not longer than 80 feet, you should be ok as long as you choose the lowest available carrier frequency on the drive. 1 kHz, 2kHz or 4 kHz is ok but 8kHz just could be too high. Rather than change out the three motors, you could better just put a 3% reactor in the drive output leads before you split the leads to the three motors. There is nothing gained by putting a separate reactor in each motor leg.

The suggestion to use thermal switches in the motor windings would be good for applications where slow speed operation occurs for extended periods of time but, in this case, it seems that the motors run up to full speed normally, using the inverter mostly for softstarting and soft reversing. Old fashioned overload blocks will be ok for that type of service.

 

[electrified3]

As everyone else has pretty much said already, should be no problem to hook multiple motors to on VFD, adjusting the current limit on the drive appropriately to the load it will now see but be sure to use the separate thermal overloads on the output to each motor. I have done this type of application before with one powerflex AB drive and 4 motors. We used individual thermal overloads and put them in series with the enable circuit to be sure to shut down all motors if any 1 motor overloaded, also well advised to have second contact on overload so if one goes out you can easily identify it, monitor on a plc or just use dummy lights.

 

[Sliver]

Thanks for the feedback everyone!
The great minds have been using a consultant (greater mind?) and he is coming in tommorrow and they have set up a meeting where I can ask some questions. I have many now. I have been told today that they intend to replace motors,gearboxes and wheels. No fluid coupling. Dick is right, there is no intention to run at lower than nominal motor speeds other than ramping. Cable lengths would be around 50' to the furthest motor. I will post again after tommorrow's meeting.

Brian.

 

[Sliver]

Well the meeting was very quick. The consultant hasn't recieved the go ahead on the project and didn't commit to any design decisions.
I relayed all of the points of concern I had (thanks largely to you guys) and I think he appreciated the depth of the problem, old mechanical issues included. When I get more news I will update this thread. Thanks again,

Brian.

 

[rdrast]

I run multiple motors off of one VFD for some conveyor lines. On soft-coupled lines, standard inverter duty motors work fine, just using seperate overload's and reactors for each motor.

On hard-coupled lines (several motors driving one belted load), I use synchronous motors. A bit more expensive, but they work fine.

In either case, you probably do NOT want to use a 'Vector' mode, and stick with plain old V/Hz modes.

 

[Sliver]

Update

The parts arrived a couple of weeks ago along with the drive for another low priority project. The drive selected for this project is 25 HP 1336 plus II, the other in identical A-B enclosure (about 8' high) was a 10 HP drive. Apparently one of the drives fell over when it was being unloaded at our receiving and 'someone' said no worries, it's only the 10 HP, good thing it wasn't the 25HP as we have a week to get this stacker back to production. When I went to dig the manual out of the enclosure I saw a little 10hp drive and all hell broke loose. They had returned the banged up drive to A-B service for checking over but they couldn't locate it. Anyway Monday the right drive made it back and Friday the stacker needs to be running.
We are installing new motors and reducers (no fluid coupling). Contractor doing the lugging and I get to do all the final connection, Drive programming, PLC wiring, programming, HMI programming. I asked for it and I got it. Thanks for all the help and if I get stuck you know where I'm coming first. I mentioned to the Electrical Engineer about setting up for Volts/Hertz again and he scratched his head and couldn't remember if that was right. I would put the recommendations of this board (DickDV especially) against any unsure engineer any day!

Just to keep you guys up on things, thanks

Brian

 

[DickDV]

Sliver, just be sure to set the drive up for maximum overload capacity. Multiple motors need all the current available for starting and a stacker (sounds like a power house coal pile stacker/reclaimer) needs all the short term overload capacity available to get over stones, coal, air hoses, etc laying on the guide rails. Well, maybe not air hoses but, you know what I mean. Maybe I should have said ice chunks.

This is Ontario, after all!

 

[Sliver]

pictures of stacker in this thread

http://www.plctalk.net/qanda/showthread.php?threadid=10579&highlight=laser


Thanks Dick, I thought you might like to see some previously posted pics of the limestone stacker (just scroll down a bit). Not too much ice, except where the roof leaks, but lots of DUST to plow through. Our Limestone bucketwheel reclaimer actually has plows. You won't believe one of the root causes to our stalling/overload problem. The original design was reduced voltage starting with series resistors, one per phase, 3 per motor. For 20 years (before my time in the electrical biz) the resistor shorting contactor had been removed and the traversing drives were running on reduced voltage/low speed/low torque. I told them that box on the floor that was always warm was probably still wired in but noone believed until we yarded out the old cables yesterday. Not that I mind getting a new drive to play with mind you, but if we had removed the resistor bank, installed new wheels and profiled the rails back to original specs we would be sailing. Oh well, supper break over, back to work,

Brian

ps: (for those who read the linked thread) No laser on the stacker yet, we haven't demolished a truck recently I guess.

 

[rdrangdal]

yes u can use

Dear sir,

Yeah u can use 3 motors, with one VFD, but VFD should be such that it should include 3 motors rating (eg. 5.5 KW X 3 = 16.5KW).

one more thing has to be taken care, each motor should have its own thermal overload, and drives overload rating should be disabled.

This is possbible with any make VFD, I have tried this with Yasakawa L&T VFD, which is working very satisfactorily.


regards

R D Rangdal
[email protected]

 

[mordred]

I would also include a line reactor

 

[Sliver]

Thanks

"one more thing has to be taken care, each motor should have its own thermal overload, and drives overload rating should be disabled."

each motor has seperate overload protection and Dick's advice was to set overload protection at max which is the same as disabling?

"I would also include a line reactor"

Yes Line reactor is included.
they even gave me a chopper module and braking resistor bank to play with.

I have a line side contactor that I have wired into the e-stop circuitry of my PLC and I just powered it up for the first time. Some fault showing but no sparks!

I have already bump tested my motors (across the line) and have all three horses pulling the cart in the same direction,

Well supper break over, back to work,

Brian.

 

[DickDV]

Sliver, what I intended to say is to set the overcurrent limit on the drive to the highest level available. The motor overload protection should be turned off.

These are two different parameters. Typically, the current limit is found in the Limits Group and the motor overload protection is found in Fault Functions.

Good luck!