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Coupled 3 phase motor driving the same shaft

04/03/2022 1:31 PM

Hello guys. I got a weird set up (above pic) I only encounter 2 days ago. These are 2 unit 440V, 3 phase motor coupled to drive the same shaft, obviously in one direction with one motor actuated ahead by a delay timer seconds the other.

We got this equipment from a chinese manufaturer. Is this even a possible set up? I mean, to think this set-up is only quite possible if both motor poles are in complete synchronous phase with one another. Otherwise, the other motor cancels out the phase of the other at some point and can potential damage the utility provider.

I'd like to see what are your thoughts on this. Much thanks as always

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#1

Re: Coupled 3 phase motor driving the same shaft

04/03/2022 2:23 PM

Are those V-belts coupling the motors to the shaft?

If they are timing belts or chains, and the shaft positions are carefully lined up, it might be O.K., though unconventional to say the least.

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#2

Re: Coupled 3 phase motor driving the same shaft

04/03/2022 2:24 PM

I guess this is one way of reducing the starting current demand...I don't see a problem yet...

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#3
In reply to #1

Re: Coupled 3 phase motor driving the same shaft

04/03/2022 2:26 PM

Yep v-belts

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#4

Re: Coupled 3 phase motor driving the same shaft

04/03/2022 2:42 PM

It appears to me, that the motors are mechanically out of phase....180 degrees. So, just connect both motors to same power source, and electrically reverse one of them. I don't understand the need for delay timer. Assuming both drive trains are uniform.

What is the shaft driving?

Imagine both motors connected to the ends of one shaft. We used such a setup to test regenerative drives. Home brew dynamo.

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#5
In reply to #3

Re: Coupled 3 phase motor driving the same shaft

04/03/2022 3:17 PM

Assuming these are induction motors, it's probably not a big deal as long as they are fairly closely aligned.

If they are synchronous motors, then alignment becomes more critical because the "leading" motor will drive current rather than consume it.

https://en.wikipedia.org/wiki/Synchronous_motor

"Synchronous motors fall under the more general category of synchronous machines which also includes the synchronous generator. Generator action will be observed if the field poles are "driven ahead of the resultant air-gap flux by the forward motion of the prime mover". Motor action will be observed if the field poles are "dragged behind the resultant air-gap flux by the retarding torque of a shaft load".[1]

There are two major types of synchronous motors depending on how the rotor is magnetized: non-excited and direct-current excited.[3] "

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#6
In reply to #5

Re: Coupled 3 phase motor driving the same shaft

04/03/2022 3:48 PM

These are induction motors, not seen any vfd on site. Probably the best thing to do here I guess is to loosen up v-belt to set just enough traction to the pulley, so the 2 motors will find its own synchronization, right? No wonder, v-belts are used instead of gears and chains. Kinda more like the below. It keeps tripping the generator. Anyhow, these motors are the drivers of a concrete mixing paddles- one you would find in a concrete mix batching plant. I was summoned to test some of these equipment lately. So yeah, if id find phase failure error on the error logs of the genset, prolly cause by unaligned poles.

Thanks man.

Last thing, how do you aligned poles of an induction motor? without dismantling the cage?

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#7

Re: Coupled 3 phase motor driving the same shaft

04/03/2022 5:37 PM

As Brave Sir Robin says, if they are induction motors (and not synchronous motors), no problem. The induction motors do not have to be aligned and will share the load. Just make sure the phases are wired so that both motors are trying to turn the shaft in the same direction.

I am assuming from your drawing that the corresponding pulleys are the same size so that both motors are running at the same speed, a speed somewhat less than synchronous speed, depending on load.

The induction motor runs slightly slower than synchronous speed (field lines shown above).

https://en.wikipedia.org/wiki/Induction_motor

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#8
In reply to #7

Re: Coupled 3 phase motor driving the same shaft

04/03/2022 11:25 PM

Thanks, Rixter

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#9

Re: Coupled 3 phase motor driving the same shaft

04/04/2022 12:36 AM

If the motors & belts are similar, they will share the load reasonably well. I’d expect a little more belt wear on one as opposed to the other. Obviously motors are wired for opposite rotation. If the full load is equal to both motors, it will stress the first motor during start, probably more than a reduced voltage starter, but not a whole lot more. I’d try starting both together, and see if your lights flicker too much. If not, best not to sequential start them, as it delays the up to speed for one of the motors, giving you unnecessary rotor heating.

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#10

Re: Coupled 3 phase motor driving the same shaft

04/04/2022 2:47 AM

This is known as a "pony motor" setup, which allows for two speeds of the driven shaft. The motors are interlocked so that only one can run at a time. There are time delays involved, especially when slowing down. The motors are of unequal horsepower.

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#11

Re: Coupled 3 phase motor driving the same shaft

04/04/2022 5:16 AM

This setup was used often, long ago, when high HP motors were super expensive so two motors were used and they were half the HP of the load. Nothing wrong with this the motors will have been chosen to have similar % slip values so one motor shold not hog more of the load.

For lighter loads one motor can be turned off and then it will just add a small amount of frictional losses. Have seen this used in old time sawmills with overhead shaft drives to supply varying loads.

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#12
In reply to #10

Re: Coupled 3 phase motor driving the same shaft

04/04/2022 2:08 PM

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#13
In reply to #10

Re: Coupled 3 phase motor driving the same shaft

04/04/2022 7:20 PM

“obviously in one direction with one motor actuated ahead by a delay timer seconds the other.“ from OP

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#14

Re: Coupled 3 phase motor driving the same shaft

04/04/2022 10:53 PM

What are the approximate diameters of all the pulleys?
What are the hp/kw and rpm of the two motors?
What sort of load is driven by the common shaft?
Does the shaft rotate the same direction in each case? (Just double checking--looks like yes.)

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#15
In reply to #12

Re: Coupled 3 phase motor driving the same shaft

04/04/2022 11:07 PM

That is another valid use of the term "pony motor", but is not the same as I described, nor applicable to the present situation.

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#16

Re: Coupled 3 phase motor driving the same shaft

04/05/2022 6:31 AM

Been done before, it seems:
At first glance, it would seem the motors would just sync their speeds and there would be no problem. I'm guessing the motors are the same (brand, model, power, voltage, speed, etc.). If one motor shuts down, it would just be an additional load for the remaining motor.

I'm not sure why this is done (additional torque?). Why not just use a single, bigger motor?

This video is for a crusher. If one motor shuts down, the other motor can keep the crusher moving (at less power) until the incoming stream of rocks can be stopped. Having worked with grinders, I could see the advantage.

https://www.youtube.com/watch?v=iY4gwczyRj4

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#17
In reply to #10

Re: Coupled 3 phase motor driving the same shaft

04/05/2022 11:21 AM

What makes you think that is the arrangement? I can't see anything in OP's posts to suggest it.

I can't see anything wrong with it. I don't see a need to start one motor before the other, and depending on the type of load there may not be enough torque to get it away.

Many years ago I was involved with thickener mechanisms, where the rake was attached to a bull gear, which was driven by a pinion via a reduction gearbox. To get more torque at the rake mech, they tried adding an extra pinion/gearbox, but due to tolerance in the bull gear gear, first one gearbox, then the other, took most of the load, so both soon failed. To solve it, they used specially wound high-slip motors, 6-8% at full load, which worked. But I don't think the OP's set-up has a similar problem. If the motor slip characteristics are not identical they take slightly different loads, but not likely to be a problem.

Also if I remember right there are horizontal split-case centrifugal pumps with shaft coming out both ends, and a motor on each end.

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#18
In reply to #17

Re: Coupled 3 phase motor driving the same shaft

04/05/2022 3:43 PM

I meant that more as a possibility rather than necessarily the case. What made me think of it was that I have seen and used the two-speed arrangement many times. It is/was common on fan drives for condensers and cooling towers. Today a single motor and VFD might be more likely.

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#19
In reply to #6

Re: Coupled 3 phase motor driving the same shaft

04/06/2022 4:41 AM

Hi,

At risk of answering a question with a question;

  • do the two motors have the same rating and speed (what are they)?
  • Are the motors direct-on-line started?
  • Is the mixing vessel loaded when the motors are started?

This arrangement may simply be to avoid using more complex starting gear. the time delay flattens the peak current due to inrush

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#20

Re: Coupled 3 phase motor driving the same shaft

04/06/2022 6:11 AM

Before VFD's came in to the market it was standard practice to use "common field" configurations on twin motors.

Applications such as overhead cranes needed two motors driving at the same speed as they powered a crane up and down its rails with an identical motor on each end of the crane.

with the advent of VFD / VSD drives and shaft encoders the common field was not required.

as for the OP , we need more information about the intentions in that setup

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#21
In reply to #6

Re: Coupled 3 phase motor driving the same shaft

04/06/2022 6:52 AM

What type of starter do you have? What does the mixer supplier recommend? I would guess that load is close to constant torque, so with no VFD, needs DOL, and start both simultaneously. The generator needs to be rated accordingly, maybe it trips because it isn't big enough?

No idea about unaligned poles, or how to align them, but I doubt it's necessary.

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#22

Re: Coupled 3 phase motor driving the same shaft

04/06/2022 10:10 AM

I cannot for the life of me see a problem with this arrangement. It should work just fine......

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#23

Re: Coupled 3 phase motor driving the same shaft

04/06/2022 3:14 PM

A lot of missing details in the posted question (the devil is in the details)

Several of the comments have possibly answered the question already.

I would like to offer up another possible reason for this configuration.

It was not indicated if both motors have the same number of poles.

If these motors had different number of poles .. this configuration would allow for two speed operation. (assuming 60 hz 3 phase.... ) often a specific motor "frame" (size) can be purchased with any of these pole configurations.

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#24
In reply to #23

Re: Coupled 3 phase motor driving the same shaft

04/06/2022 3:39 PM

Minor point - your Speeds with rated load are for 60Hz, your synchronous speeds are for 50Hz.

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#25

Re: Coupled 3 phase motor driving the same shaft

04/07/2022 1:51 AM

Induction motor.....does the rotor truly rotate slower than the stator field.......or just behind it? Or does it just yo-yo a little behind the stator rotation, with load changes?

Is the rotor RPM less than the stator RPM....or just see-saws lagging position with the stator field?

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#26
In reply to #25

Re: Coupled 3 phase motor driving the same shaft

04/07/2022 3:02 AM

A 3 phase induction motor, runs slightly slower than the fields, you might call it "slip".

If the load is constant, the amount of slip is too.

It happens smoothly and completely unnoticeable to the user.

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#27
In reply to #23

Re: Coupled 3 phase motor driving the same shaft

04/07/2022 5:19 AM

Nice idea. There are two things to consider:

  1. The lower speed machine would have to be capable of turning at the higher speed. In effect it would have to have a continuous mechanical overspeed rating. It would not therefore be a standard machine design. If we are considering 2/4/6 pole machines the overspeed would be very significant and would impact on the rotating elements of the design such as the squirrel cage;
  2. The load is probably not linear so the rating of the high speed machine would be much greater than that of the lower. It would also have to overcome the windage losses of the lower speed machine rotating at the higher speed.

There is a lot we don't seem to know about this system so much of what is said is conjecture as well as speed(s) & rating(s) the age of the equipment and its location & environment would tell us a lot. Most of the contributors have implied that doing this today makes little sense when packaged motors and drives are available off-the-shelf; I can only agree.

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#28

Re: Coupled 3 phase motor driving the same shaft

04/07/2022 12:54 PM

A red car is traveling down the road at 50 MPH. A blue car is following 20 ft. behind the red car at 50 MPH. Is the blue car slower than the red car?

When the motor load increases, the rotor will slip a little. But then, doesn't it also advance some, in response to the slip? So, the rotor first slips, then it advances. It yo-yos BEHIND the stator field.

Isn't that what's going on here?

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#29
In reply to #28

Re: Coupled 3 phase motor driving the same shaft

04/07/2022 2:03 PM

The rotor turns a little (typically 3% at full load) slower than the stator field. I don't know about yo-yoing, or what that has to do with this thread.

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#30
In reply to #28

Re: Coupled 3 phase motor driving the same shaft

04/07/2022 4:06 PM

I can only ask "WHY?".

If the load is constant, the angle of the slip will be constant.

There is no force in the world that will cause oscillation as you describe.

Even if load changes, then the slip will also change, but not oscillate with a fixed load.

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#31
In reply to #30

Re: Coupled 3 phase motor driving the same shaft

04/07/2022 5:21 PM

He’s talking about 2 motors coupled, torque easily could oscillate at some frequency, as both motor drive line unlikely to be perfectly matched

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#32

Re: Coupled 3 phase motor driving the same shaft

04/07/2022 7:31 PM

Gutmonarch,

This is a reasonable application for the two motors, providing they have the same ratings (HP, speed). It is an indirect method for achieving "reduced voltage starting". Look up "part winding starting", in which a portion (typically 1/2) of the motor's windings are energized and after a short time delay the rest are energized. This will reduce the total inrush current while the load is getting up to speed. I believe the manufacturer knew what they were doing.

Continue learning--look up all the methods for reduced voltage starting (there are at least 6). See the differences between them and which ones are preferred for which applications.

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#33
In reply to #31

Re: Coupled 3 phase motor driving the same shaft

04/08/2022 2:53 AM

Please explain how!

Also please reference any online papers explaining and supporting your thoughts.

Just saying something is so, is far far away from an explanation of or fact.

If motors REALLY oscillated under such conditions, they would be often damaged and breaking down daily, let alone what they were trying to drive.....as the forces that you are talking about, if true, would be huge.

You would also hear, VERY LOUDLY, if such oscillations were ever present!

But they are simply not there....

Look here, which is quite extensive, the problem is not even mentioned, I wonder why:-

https://electrical-engineering-portal.com/troubleshooting-winding-problems-three-phase-electric-motors

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#34

Re: Coupled 3 phase motor driving the same shaft

04/08/2022 4:03 AM

Heavens to murgatroyd, I didn't mean to start an argument. If two opposite turning motors are connected to a common shaft, wouldn't the slip on one be an advance on the other? Would a strobe light reveal slip on such a set up? Could this be an anti-slip configuration?

I was just asking questions and wondering purpose about such a set-up.

I apologize if my curiosity and possible explanations upset anybody. Please pardon me.

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#35
In reply to #34

Re: Coupled 3 phase motor driving the same shaft

04/08/2022 4:56 AM

Actually, you were basiically making statements, incorrect ones at that.....

Read what you wrote from the point of view of someone else.

You may have meant to say something else, but sadly you did not achieve what you wanted to say apparently....done it myself over the years, that is why I always now read through what I write, to make sure that it is self explanatory and accurate for someone else....

The two motors in this case, with one running in one direction to the other, would in most cases be identical builds (but I feel that may not always be true, as long as speed isthe same or very close too, but knowledge on that point I do not have!), but one would have two of the three phase connections reversed, I assume at this time that you know why that is done.

If I am wrong, and it is not obvious for you why, just ask me or any other of the "'lekkies" here. It is a simple change, that is known well and made when needed.

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#36
In reply to #35

Re: Coupled 3 phase motor driving the same shaft

04/08/2022 7:27 AM

The OP has gone rather quiet! More details requested in #19 and #21, but nothing back.

To go back to the thickener application in my #17, in that case the bull gear rotated quite slowly, around 15 minutes/rev, depending on tank diameter. So (before mods) each pinion would take high load for several minutes. On OP's job, at motor speed that's much less an issue, so it should be OK even with chain or gear drive, though V-belts better.

Also as jmueller says, presumably the supplier has done it before.

A lot of off-topic-ing on this thread.

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#37
In reply to #34

Re: Coupled 3 phase motor driving the same shaft

04/08/2022 4:54 PM

It appears that you do not understand what "slip" is!

Have you forgotten your studies, or have you never studied electrical engineering at all.

Slip is related to load. The heavier the load, the greater the slip. But if i remember correctly, it maxes out at about 3% or so..

A 3 phase rotor will never become completely synchronous (no slip!), even with no load, as the difference in angle of "slip" is needed to produce torque.

It is rather difficult to train someone in such things via the web....

Two identical motors, running on the same shaft, taking their power from the same source, even though one motor runs in the opposite direction to the other, needed so that they both rotate the shaft in the same direction, will "split" the amount of slip, equally.

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#38

Re: Coupled 3 phase motor driving the same shaft

04/09/2022 12:00 PM

There is something that puzzles me with this proposed two-motor configuration. The mechanical load presented to just one motor during startup puzzles me. Look at this nominal torque vs slip curve.

If one motor's starting torque is sufficient to spin the mechanical load up to speed then why energize a second motor later? I guess this implies that the mechanical load will change significantly from time to time after reaching operating speed but as the curve shows the breakdown (aka pullout) torque of just one motor is nominally multiples of the stating torque. If that mechanical load does change enough that a single motor might not have enough pull-up torque to overcome the mechanical load changing a bigger motor instead of paralleling a second motor seems more cost-effective. No anticipated belt slipping, and no additional mechanical parts are necessary.

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#39
In reply to #38

Re: Coupled 3 phase motor driving the same shaft

04/09/2022 1:44 PM

OP said 2nd motor starts within seconds of first, so more like reduced voltage starter, with lower accel torque, not up to speed either motor when energized

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#40
In reply to #39

Re: Coupled 3 phase motor driving the same shaft

04/12/2022 9:13 AM

I agree with rwilliams. As said earlier, this is an application that is very similar to a part-winding starter on a single motor. This approach may be unusual but is not wrong. Study the torque/current/speed curves for a motor on a part-winding starter. This application will have virtually the same curves and performance.

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