Why two induction motors don't run at exact speed

kalabdel · Jul 1, 2017

Hello gentlemen,

I know this from experience and I' interested in the theory behind it. Let's say we have two identical drives and motors with the same parameters (open loop) , why do they not *always* run at exact same speed.

Thanks
Kal

g.mccormick · Jul 1, 2017

Different torque against motor, different slip, small difference in windings and assembly? My guess.

GaryS · Jul 1, 2017

I don't know what drive you are using but from the post you have both setup as open loop drive this will allow the motor to slip change to open loop vector and use the u
Auto tune features you will find the they will hold a better speed match

kalabdel · Jul 2, 2017

GaryS said:
I don't know what drive you are using but from the post you have both setup as open loop drive this will allow the motor to slip change to open loop vector and use the u
Auto tune features you will find the they will hold a better speed match

Thanks GaryS and g.mccormick.

They vary, on one machine there are Unidrive M200 driving conveyor belts and sponge belts and on another machine there are Lenze 8200 doing similar job.

I gather from your replies that all things being equal they should run at the same speed, nothing changes over time and with continuous use (they run over 20 hours almost none stop) that would cause them to differ in performance (I'm thinking heat).

Thanks
Kal

Tom Jenkins · Jul 2, 2017

Identical only exists in textbooks.

Induction motors need slip to provide torque. Slip varies with torque load. Tolerances and manufacturing deviations will undoubtedly result in slight variations in slip.

Are you judging based on actual motor speed measurement or on conveyor speed measurement? If the latter, remember that belts slip, and that wear and tension affect the amount of slip.

If you want identical motor speeds you need to change to synchronous motors.

kalabdel · Jul 2, 2017

Tom Jenkins said:
Identical only exists in textbooks.

Induction motors need slip to provide torque. Slip varies with torque load. Tolerances and manufacturing deviations will undoubtedly result in slight variations in slip.

Are you judging based on actual motor speed measurement or on conveyor speed measurement? If the latter, remember that belts slip, and that wear and tension affect the amount of slip.

If you want identical motor speeds you need to change to synchronous motors.

Thanks Tom. My first post of everything being identical was hypothetical as I wanted to know if there are factors that I was not aware of like length of wire, heat,slight differences in batch of motors,etc. In reality the drive and motors are identical but the gearing and load are different which means that the parameters are different to match the speeds. For some processes that's not close enough.

How about closed loop control? I am thinking of adding encoders to two motors and use VFDs with motion control functions to synchronize them. I've looked at Emerson M700, Danfoss FC302 and Schneider ATV930 and lastly Yasawa's G7C. Each has functionality for Master/Slave. Not sure though how close they will be, I know the Danfoss dealer stated +/- 1 RPM and I guess I will not know for sure till I try.

Much appreciated

Kal

DickDV · Jul 2, 2017

The rule I've always used with motor shaft encoder systems is "one pulse per revolution plus or minus one pulse". Not sure if I could defend the wisdom of that but it seemed reasonable at the time.

Keep in mind that error has plus and minus as well. As a general rule, when a motor is driving the load, the error will be negative and when the load is overhauling the motor the error will be positive.

kalabdel · Jul 4, 2017

Thanks Dick.

Kal

Peter Nachtwey · Jul 4, 2017

"Each has functionality for Master/Slave. "
This is not the best way to run two motors synchronously if you are controlling both of them. They should both be geared to the same virtual master.

GaryS · Jul 4, 2017

I agree with Peter the best way is to do Master Slave configuration. but they still need to be capable of matching speed. a good Flux Vector rive will do that and a lot more
they can be setup to do Digital Line Shaft Sync it will speed match to the exact speed

Good luck

DickDV · Jul 5, 2017

No, Gary, they will not match to the EXACT speed. As with all control systems, there is error, sometimes very very small, but still error.

As I mentioned above, even with encoder-based systems, there is some small error.

Another issue not often mentioned when speaking about error is whether the error is cumulative or not. Cumulative error can make or break an application even if the error rate is acceptably small.

GaryS · Jul 5, 2017

check out electronic line shaft it's been around for many years a proven technology
as you said all system have errors how you define and control the errors that's important
in this case with conveyors that error will be huge before you will even see it.
if you can't get it work sake for help
you would be surprised what I can do.

James Mcquade · Jul 5, 2017

several reasons.

1. different drive manufacturers.
each drive may have the same components, the characteristics of diodes, resistor values, transistors, capacitors are different for each component
2. different motors, even though they are the same model. I characteristics of the wire, varnish, stacking plates, stacking factor, bearings is different for each.
3. gearing and load on each motor

regards,
james

kalabdel · Jul 13, 2017

Thanks a lot guys.
I spoke to a few suppliers and the most confident was the rep from Yaskawa. He recommended A1000 with line shaft custom software,
two encoder cards and two encoders one on the master motor which has a different drive and one on the follower motor with the A1000. The pulses from the master encoder determines the speed. According to the manual "The follower can match its position (phase angle) to the master within several quadrature encoder counts."

DickDV, what is an example of the error being cumulative? I think of it as a set point for the speed and the follower trying to match it.

Thanks
Kal

ganutenator · Jul 13, 2017

I assume that you can't drive them w/ the same shaft, aka, like my girlfriend.

sorry for that. just trying to get application details. two motors for one conveyor?

Why two induction motors don't run at exact speed

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