Parallel Motors 1

[vennivivi]

Hi,

We are in the process of installating two 2,0 MW 6,6 k V electric motors mounted on the end of a common shaft to drive a stone crusher. The two motors are identical as far as full load slip is concerned.What are the issues other than mechanical-alignment, vibration..-that need to be considered. I have come across the term phase alignment.
Any comment.

Grundig.

 

[itsmoked]

The motors should share the load uniformly. Don't you need VFDs that maintain torque balance between the two or one motor can hog the load. Are you using VFDs?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[edison123]

Make sure the direction of rotation of the motors do not oppose each other.

 

[vennivivi]

Thanks for comments. The motors have got identical slip and so should share the load equally. I am looking for other issues.

Thanks

Grundig

 

[Skogsgurra]

Yes, there's also the "phasing myth". I have witnessed a guy "phasing" two induction motors by letting them run uncoupled and then couple them where they happened to stop when switching off - a completely random "phasing" that does no harm and no good.

I think that this myth has its roots in the necessity to phase align coupled synchronous motors. An obvious need.

Since there is no preferred angle between rotor and stator field (the slip changes the angle all the time), there is no need to "phase" the motors. At least not any that I have come across during several decades of work with motors and drives.



Gunnar Englund

http://www.gke.org

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[oftenlost]

I would be surprised if these two motors "shared the load" equally. If these are two off-the-shelf motors not specifically designed to be operated in tandem, I would guess that small manufacturing differences would allow one motor to hog the load and the other to go along as a cheerleader and backup when the worker motor goes out on disability.

 

[Skogsgurra]

Hello oftenlost,

Have you got any facts to back such an assumption?

The drooping characteristic of an asynch motor is one of the good reasons why two motors with identical slip will share the load.

Load "hogging" does not occur since any tendency to "hog" the load would result in more heat in that motor, which increases slip so that an equilibrium is created. I do not understand why such myths as "phasing" and "load hogging" circulate among professional engineers.


Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[vennivivi]

Hello Skogsgurra,

You are right.Spot on !Have you any literarture or link for the myth phase alignment. I hvent got any clue about it !

Grundig

 

[jraef]

I have yet to see a tandem motor situation (without a VFD) that does share the load totally equally! The difference is usually not much, but not equal either. The best I have seen is 1% delta between the two motors, the worst is around 4%, most are around 2% (running amps). I saw one where the delta was in excess of 60% even though the motors were identical, but I traced it to poor connections in one motor.

Gunnar is correct from a theoretical sense with regards to drooping characteristics making them share the load, but in practice I don't think that truly "identical" slip values exist. My guess would be that natural differences in materials and construction probably make motor response slightly different, even if the tested slip appeared the same in an unloaded condition. Still, 2% differential is nothing to get worried about.

I did a project least year with several VFDs on 800HP crushers with tandem motors (2 x 400HP on each). We had the VFDs for their intended purpose (changing finished product size), but instead of one x 800HP VFD we used 2 x 400HP, with one as a speed follower from the control system and the other in torque follower to the first. We managed to keep them within 1% delta at any speed; worked great. The side benefit we discovered later is that we were able to accomplish that with dissimilar motors as well! That proved to be a big benefit to the end user because they could get back up and running without having to find matched motors.

Twin VFDs for 2MW MV motors however would likely be hard to justify from a financial standpoint if variable speed was unimportant. The only kind of crusher I have seen be successful with speed modulation is Vertical Shaft Impactors (VSI). Jaws, HSIs, Rollers, Hammer Mills and Cones (Gyratory) do not benefit from changing speed on the fly.

Never heard of "phase alignment", sounds like a red herring to me. Belt tension is critical, motor lead connection is critical (as I mentioned above), bearing wear is critical, mechanical alignment is critical (I always recommend laser alignment), and making sure that the location of each motor is such that they are balanced in the structure, otherwise torque on the frame can pull them out of alignment. The nature of the beast is that even slight differences in those things can set up a load imbalance that can escalate quickly. On the ones I've done without VFDs, I put current sensing relays on each motor and look at the differential. If it climbs too much above what it is at commissioning, I would set up an alarm to have them go look at all of those issues above.

 

[SteveWag]

I have used two motors driving the pinion gears on a clarifier drive ring gear in cases were ring gear tooth load was limiting. The second motor/gear box allowed torque to be doubled. On startup, I never saw a motor running current difference greater than typical. These were 2 to 5 HP motors.

Steve

 

[Marke]

I have used two motors driving a common shaft many times with no problems. It is very important that the slip characteristics are the same. We always use identical motors to ensure equal operation. We normally soft start one motor and full voltage switch the second motor when the load is up to full speed. This requires a lower current than soft starting both machines at the same time.
Never heard of phase alignment with induction motors, but it is a requirement with synchronous motors.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[gepman]

I have never used tandem motors but if they are both on VFD's then I would be especially concerned with shaft currents. Usually on crushers there is a very large inertial load and a soft start is required. For medium voltage motors on rock crushers I have used (don't laugh) within the last six months liquid rheostats on wound rotor motors with good success and they are much cheaper than MV VFD's and much more robust. I have also seen them used very recently on an 8000hp car shredder (similar to a rock crusher). I would be concerned about starting both equally either with a VFD or a liquid rheostat and like Marke's idea of starting with one motor and then switching the other motor on.

 

[vennivivi]

Hi all,

Thanks for the excellent comments. There is an argument that travelling waves in the air gap of two tandem motors oppose each other and create standing waves and when the windings are subjected to the latter , stresses cause the windings to fail.This was the advice that a consultatnt on a site gave me. He claimed that he witnessed damages to MV motors.

Any guess.

Grundig

 

[Skogsgurra]

Travelling waves? Standing waves? At 50/60 Hz? That's nothing but crap and confusion. You better get another consultant. I am embarassed to hear things like that.

It is true that the flux in the motor travels around the air gap and it is possible that someone might confuse that with the travelling wave phenomenon in a cable with steep switching edges. But the fact that you CAN confuse the two is no excuse to carry such misconceptions to the open.

The "phasing" I have witnessed in an application with a four-wheel driven supply car for coal in a cokes work. Two VFDs and two motors on each VFD. These guys had a problem with vibrations (caused by bad rail alignement mostly) and they told me it was necessary to phase-adjust the motors after so and so many hours. They did that by loosening all couplings, run the motors and stop them. After that they tighened the couplings again. It didn't seem to help a bit. But they were convinced that this was a necessary maintenance procedure. They probably still do it. Many "home-brew" procedures like this out there. Some consultants pick them up and propagate them. This site is a good place to weed such things out.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[oftenlost]

Gunnar,
You could very well be right about the changes in slip due to differential loading
My observation was based on shop dyno testing repaired motors. We have noted that the speed at which the motors assume full load can vary by three or four RPM. For example, a 500 HP four pole we tested recently was loaded to 500 HP at 1787 RPM compared to a nameplate of 1784. We loaded it up to 115% and the RPM changed 3 RPM. A sister motor did not assume full load until 1784 and changed 4 RPM at 115% load. It would appear that those two motors would have a current imbalance of 15% or so at full load.

 

[Skogsgurra]

That's interesting, often!

I have done some work with "silent drive" with sensorless motors acting on dryer felts in paper machines. We use many parallel motors for that and I have never had any problem with load sharing. A 15 % current difference is rather much.

I think that a pair of 2 MW motors are "more equal" (Orwell) than a pair of identical 10+ kW motors (air gap changes and other tolerance related characteristics are easier to control in a large machine than in a small one).

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[vennivivi]

Hi Skogsgurra,

I tend to agree with you. However, how far the concept is true for synchronous motors ?

Grundig

 

[ScottyUK]

Synchronous machines have no slip. If they are running from a common supply the rotors would have to be mechanically aligned very carefully so their fields were in equivalent magnetic orientation relative to their respective stators. Achievable, but a lot of effort.


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Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[Skogsgurra]

Yes. That kind of closes the circle.

As I said above, it is probably the need for careful phasing of coupled synchronous motors that has "spilled over" to asynchronous ones and created the myth.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...