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Running Two 3500HP Induction Motors for Load Sharing? 4

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Laplacian

Laplacian

Electrical
Jul 15, 2002
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For a hypothetical situation, would it be possible to connect two 3500HP induction motors (drive end into non-drive end on dual shafted motors) both with identical nameplate ratings to load share a 7000HP load?

Assume the final output shaft can handle the torque requirements. I've heard of this being done on certain applications, but smaller motors were used (500HP).
 
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I would say yes. The number one question is if the output shaft can take the mechanical load. Number two is that the grid will have to deliver a sizable starting current.

There is also a question about transient torque between the two motors when starting. Some also think that the cogging (mostly non-existing) makes an "electrical alignment" necessary. But I do not subscribe to that.

Gunnar Englund
 
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Thanks for the responses so far guys.

This would be used for a contingency basis only. Two new 7000HP motors, breakers, gearboxes, etc. are being installed for a major project to upgrade existing extruder systems.

Our system is plenty stiff enough (we can start a 10,000HP induction motor with no voltage issues at 13.2kv). The existing 3500HP motors, starters and captive transformers would be left in place. In the event of a 7000HP, 13.2KV motor failure, we would like to use the original two 3500HP motors as a backup.

We would definitely do a stress analysis on the load handling output shaft before implementing such a plan. My question was directed at the electrical issues only. We would start one motor at a time, then close the other starter to energize both motors. Just testing the feasibility of such an idea. The cost of a spare 7000HP is significant considering the existing equipment that will be replaced.
 
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Starting the second motor will be less stressful. You will still get a starting transient but it is smaller than the first one. If you got a stiff grid and the shaft and couplings can take it, I see no problems there.

Gunnar Englund
 
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Large extruders typically start unloaded; they have a separate auxiliary drive that purges the barrels at very low speed. Feed is not introduced until the main drive is up to full speed.
 
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Thanks Laplacian.

The OP mentioned stiff power, so seemed to me two 35's DOL in lue of the primary 70 would not make much difference.

I'm not very familiar with extruders. What kinds of things do you extrude that need 7000hp? I can picture aluminum extruders for heatsinks and ornamental molding and perhaps 24" terracotta pipe, but 7khp? Got a link to this kind of thing?



Keith Cress
Flamin Systems, Inc.-
 
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Most motors are designed with a seriously oversized driveshaft to handle severe bending loads from pulley driven loads, so I very much doubt if the shaft will ever be overstressed in torsion by coupling two togeteher. That assumes that the motors are truly double ended construction.

The motos will need to be identical, or at least have identical slip speeds at identical Rpm and load, if they are going to load share.

I would fit individual overload protection to each motor, and then parallel them up and treat them as a single motor for starting and running. The load protectors can be arranged so either can trip the main contactor should something go wrong.
 
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The important aspect for load-sharing would be identical torque vs slip characteristics as you suggested.

There is one aspect of load-sharing that I'm wondering about.

Consider very small initial difference in torque-slip characteristics due to manufacturing variations. Motor with higher torque at a given slip carries a tiny bit more load. That motor gets a little hotter and the other gets a little cooler. Change in rotor temperature affects torque-slip characteristic. Motor getting hotter will have higher rotor resistance and shift towards even higher torque-slip characteristic and carry even more of load while motor getting cooler shifts in opposite direction. Seems like in theory it could be an unstable situation which continues to grow until one motor carries all the load and the other carries none? I'm not sure. Has it been done before?

I don't see any other big issues. It is of course a special purpose design. It needs some study for lateral and torsional resonances of the assembled system among other things. Starting needs studied and specified to the motor supplier and if possible require testing or at a minimum calculations to determine that the starting duty planned does not result in unacceptable rotor temperature rise. (By the way, inertia generally plays a lot bigger role than load for starting severity....starting with no-load most likely doesn't come anywhere close to compensating for doubling the inertia to be acclerated).

Relaying might take you into some philosophical areas. I think most motors are designed to mechanically withstand locked rotor condition. If for some reason the load is jammed during start, can the center motor shift withstand twice locked rotor torque? Need to decide if shaft failure under that unlikely scenario is an acceptable risk or else specify as a design requirement that the motor will handle this without damage.


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I think my concern about twice locked rotor torque was unfounded. It can be avoided by not starting 2nd motor until first motor comes up to speed.

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I was backwards on the effect of resistance. Higher rotor resistance shifts the torque curve lower. That tends to provide a stabilizing (equalziing) effect rather than a destabilizing effect.

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As SNL's Roseanne Rosana Danna would say.......
"Never mind."

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Years ago, I worked on the installation of some large two motor drives.
These were autogenous mills in a concentrator plant at a copper mine.
Each mill was driven by two 3000 hp. wound rotor induction motors. There was a 10 meter diameter ring gear around the mill drum. A motor and reduction gear sat on each side of the drum.
There was no special electrical considerations in regards to sharing the load.
Starting was by means of liquid reostats in the rotor circuits.
There was a no-load contactor panel whereby any pair of motors could be connected to the liquid reostats. When the motors were up to speed, a shorting contactor at each motor closed the rotor circuit. The liquid reostats could then be used to start the next mill.
The reason that induction motors were used instead of synchronous motors was because of the difficulty of getting synchronous motors to share a load when direct connected.
Wound rotors were used to supply the high starting torques needed.
Each mill weighed about 300 or 400 tons, and had about 100 tons of ore sliding inside as they turned.
These are basically ball mills, but are so large that the weight of the ore will grind itself, without the need or expense of grinding balls.
I realise that this doesn't address the shaft issue, but given the similarity between squirrel cage motors and wound rotor motors, I doubt that you will have load sharing problems.
respectfully
 
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