Two motors on one drive or separate drive for two motors

Plc_User · Feb 9, 2013

I have a doubt about an upgrade of a machine.
On the old machine two parallel belts (not touching eachother and not mechanically coupled) both have a motor, but both motors are driven by one frequency converter (no encoders are used. Now we will upgrade the whole machine.
We want both belts to run the same speeds.
Should using a separate frequency for each motor be much more speed accurate then one drive for both motors?
Using encoders for closed loop control is not our intention, we want to do it open loop anyway, only question is one or two drives.
Has someone experience in this issue?

NetNathan · Feb 9, 2013

If you want to always run the motors at the same speed, and motors are same type, then 1 VFD will wok well.

Mark Buskell · Feb 9, 2013

I have a system with 3 steamer fans being controlled by one drive. This is a pretty common practice on a lot of systems where close is good enough. It's been running this way for a number of years.

jrwb4gbm · Feb 9, 2013

If speed accuracy or zero difference between the parallel belts is required, I would suggest using direct mechanical coupling between the two belts and one motor/drive large enough to power both belts. If close is good enough, then two motors on one drive large enough to power both drives as you stated in your 1st post should be OK. If your speed must track some other part of the machinery then an encoder is almost a necessity.

milldrone · Feb 9, 2013

Plc_User said:
We want both belts to run the same speeds.
Should using a separate frequency for each motor be much more speed accurate then one drive for both motors?

This will sound counter intuitive but two ABB ACS800's in DTC mode will give much more accurate results.

PeterW · Feb 10, 2013

As long as the motors are individually protected after the VFD.

KalleOlsen · Feb 10, 2013

milldrone said:
This will sound counter intuitive but two ABB ACS800's in DTC mode will give much more accurate results.

Pick up your lorgnette, Peter

OkiePC · Feb 10, 2013

Plc_User said:
I have a doubt about an upgrade of a machine.
On the old machine two parallel belts (not touching eachother and not mechanically coupled) both have a motor, but both motors are driven by one frequency converter (no encoders are used. Now we will upgrade the whole machine.
We want both belts to run the same speeds.
Should using a separate frequency for each motor be much more speed accurate then one drive for both motors?

If it ain't broke don't fix it. The drawback to a single drive in this case is going to be that your motors must match exactly to ensure the speeds match, and the loads need to be light or balanced.

Using two drives without feedback I think would be worse for synchronizing the belts.

I can't believe I am doing this, but in this case I say keep the single drive multi-motor setup...for the sake of maintaining their current level of synchronicity...in general I am opposed to multi-motor drive arrangements.

DickDV · Feb 11, 2013

Look at the nameplates on the motors. The nameplate rpm's (hopefully they are the same but, if not, take the lower rpm) need to be subtracted from the synchronous speed which, in your 50hz world, would be 1000rpm, 1500rpm, or 3000rpm depending on the number of motor poles.

That number (for example, 1500 - 1450 = 50rpm) is going to be your worst case speed error between the two belts. That would occur only when one belt has no load and the other is loaded right up to the motor's full load amps. If the belts are always loaded about evenly, then you can expect maybe 10-15rpm difference in speed which is .67 - 1.0% error. If that is ok for your process, then nothing else needs to be done.

That note above about individual overload blocks for each motor is important, however. And, the drive should be sized so it can supply continuous amps equal to the two motor FLA's added together plus 10%.

If you need less error, come back to us. You've got lots of choices.

Plc_User · Feb 12, 2013

DickDV said:
Look at the nameplates on the motors. The nameplate rpm's (hopefully they are the same but, if not, take the lower rpm) need to be subtracted from the synchronous speed which, in your 50hz world, would be 1000rpm, 1500rpm, or 3000rpm depending on the number of motor poles.

That number (for example, 1500 - 1450 = 50rpm) is going to be your worst case speed error between the two belts. That would occur only when one belt has no load and the other is loaded right up to the motor's full load amps. If the belts are always loaded about evenly, then you can expect maybe 10-15rpm difference in speed which is .67 - 1.0% error. If that is ok for your process, then nothing else needs to be done.

That note above about individual overload blocks for each motor is important, however. And, the drive should be sized so it can supply continuous amps equal to the two motor FLA's added together plus 10%.

If you need less error, come back to us. You've got lots of choices.

Thanks for your detailed info!

Rakesh Kumar · Feb 12, 2013

Dear sir,

We are using 1 belt conveyor and 1 chain conveyor running with different speed. We are using AB PLC with VFD. How to control both speeds using ENCODER??? And pls suggest suitable PLC and suitable Encoder???

rootboy · Feb 12, 2013

We've done it with four motors on one VFD. We had to add a load reactor, but other than that it worked fine. The motors were mechanically coupled (once the skillet was on all four motors) and there was no problem with speed matching.

BASIC ELECTRICAL TEC · Feb 12, 2013

I too have set up a system using six fractional horsepower motors on one Allen Bradley VFD. Then a set of six VFD's, so thirty-six motors total. That's been in service for about 8yrs now, running 24-7 in a sand screening application. Very reliable.

Two motors on one drive or separate drive for two motors

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