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Running Two 3500HP Induction Motors for Load Sharing?
4

Running Two 3500HP Induction Motors for Load Sharing?

Running Two 3500HP Induction Motors for Load Sharing?

(OP)
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).

RE: Running Two 3500HP Induction Motors for Load Sharing?

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
www.gke.org

RE: Running Two 3500HP Induction Motors for Load Sharing?

Another 10,000 foot-pounds thru the shaft?

I doubt off the shelf motors would like this.

The load driving motor... might just give you the 'shaft'.

Others will come along though and confirm or refute this.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Running Two 3500HP Induction Motors for Load Sharing?

(OP)
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.

RE: Running Two 3500HP Induction Motors for Load Sharing?

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
www.gke.org

RE: Running Two 3500HP Induction Motors for Load Sharing?

Won't there be a problem trying to start a 7000hp load and the inertia of the the second motor all with one 3500hp motor?  I would think it would be better to DOL both simultaneously.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Running Two 3500HP Induction Motors for Load Sharing?

Good point. The first motor will have to deliver a lot during start. Rotor heat will be lower if both are started simultaneously.

Gunnar Englund
www.gke.org

RE: Running Two 3500HP Induction Motors for Load Sharing?

(OP)
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.

RE: Running Two 3500HP Induction Motors for Load Sharing?

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.- http://www.flaminsystems.com

RE: Running Two 3500HP Induction Motors for Load Sharing?

Ha!  Lost track that you are  the OP.hammer

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Running Two 3500HP Induction Motors for Load Sharing?

(OP)
HDPE and PP; we have synchronous motors for this up to 24,000HP, 1200rpm.

RE: Running Two 3500HP Induction Motors for Load Sharing?

(OP)
http://www.coperion.com/(wa2gvb45snn3af55s0ihwsi2)/Default-L1-3-506-30-611.aspx

RE: Running Two 3500HP Induction Motors for Load Sharing?

Interesting!  Thanks for the link Laplacian.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Running Two 3500HP Induction Motors for Load Sharing?

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.

RE: Running Two 3500HP Induction Motors for Load Sharing?

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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RE: Running Two 3500HP Induction Motors for Load Sharing?

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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RE: Running Two 3500HP Induction Motors for Load Sharing?

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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RE: Running Two 3500HP Induction Motors for Load Sharing?

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

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Running Two 3500HP Induction Motors for Load Sharing?

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

RE: Running Two 3500HP Induction Motors for Load Sharing?

electricpete,

I liked your monologue on rotor temperature and load-sharing. An excellent example of keyboardthinking.

Gunnar Englund
www.gke.org

RE: Running Two 3500HP Induction Motors for Load Sharing?

I rather liked it too, it is the demonstration of an honest and sincere thinker.

  

  

RE: Running Two 3500HP Induction Motors for Load Sharing?

I was amazed by the full multi-sided conversation... held.. by himself! laughtears

Multiple personalities discussing a problem.. A potent, effective mutation.

But I didn't like the "start one then the other" conclusion arrived at after the starting inertia point.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Running Two 3500HP Induction Motors for Load Sharing?

(OP)
Thank you guys for your valuable input.  It is so nice to bounce ideas around in a forum with so much knowledge available.

RE: Running Two 3500HP Induction Motors for Load Sharing?

What's the overload rating of the motors, and what is the running characteristic of the load (extruder)?

For example, a rolling mill motor will have up to 250% frequent overload rating. The shaft and coupling must be able to take this, with a safety factor.

I would suggest that you consider starting by using only the motor which is nearest to the load, so that you limit the starting torque that is applied to the final motor coupling.

RE: Running Two 3500HP Induction Motors for Load Sharing?

Just a general question: is it typical to start a 7000hp motor DOL(or 2 x 3500hp), even on a 13.2kV network?

RE: Running Two 3500HP Induction Motors for Load Sharing?

Yes. If the grid isn't too weak.

Gunnar Englund
www.gke.org

RE: Running Two 3500HP Induction Motors for Load Sharing?

(OP)
DaveScott,

There are multiple levels of overload protection for the load.  First, the controlling PLC trips the entire process if load is above 125% instantly.  Second, there is a Desch friction coupling between the motor and gearbox that will break loose at 150% torque instantly, also tripping the process.  
The motor overloads are much slower, but are typical of class 10 curves.  The motors have a 1.15SF and the load is smooth and easy to control.

sed2developer,

Yes, it is typical in a large ground based industrial facility.  For smaller plants, offshore, or temporary sites, not so typical.

RE: Running Two 3500HP Induction Motors for Load Sharing?

I’m glad to provide some entertainment.   

Good point Keith – there are disadvantages of starting motors sequentially as well (much tougher starting duty for the first motor).  Whatever the starting approach, it needs to be determined up front and associated requirements incorporated into the moto specification.

It sounds like the coupling will provide some mechanical protection for locked rotor condition during starting.  (By the way that scenario is not all that remote - we had a 7000hp 2-pole motor that bound and tripped during starting  after having been idle for a long period without manual rotation.)

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Running Two 3500HP Induction Motors for Load Sharing?

Laplacian,
It looks like you don't have much to work with. I assume that you aim to replace the friction coupling, because it won't take the torque from two motors?
The motors themselves are only designed for 115% short term, but nowhere near 200%, which is what the shaft must transmit.

I don't know the mechanical layout, which I suppose must be such that the motors can be placed in series. Have you considered putting the motors in parallel, with a gearbox? You could use two pinions with one gear (Perhaps the gearbox would cost more than a replacement motor!). Just a thought.

RE: Running Two 3500HP Induction Motors for Load Sharing?

I used to work at a fossil fired power plant at a large government owned utility which had a start up boiler feed water pump on a 500MW unit using this configuration. Two 3500HP motors were connected in tandem to the pump. The pump it self was only about one half the physical size of one of the motors. The motors were 4160V. This unit was built in ~1970. The unit aux. system was weak. While I was there one of the two unit auxilary xfmrs failed a second time during starting of this pump.  

RE: Running Two 3500HP Induction Motors for Load Sharing?

In Florida at least two different fossil plants have two 3500 HP, 3580 rpm induction motors coupled in tandem to move a 7000 bhp boiler feed water pump. The motors are started one at a time under no load and engaged later to the load by a fluid clutch drive.
To share the load both motors must have a close speed-torque curve, for that reason the manufacturer should know the application to provide the matching curves, shaft extensions and strength to handle the torque.

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