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Pony Motor to Start Large Motor on Weak Supply

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EddyWirbelstrom

EddyWirbelstrom

Electrical
Feb 17, 2002
214
It is proposed to start a 1.3MW, 3.3kV asynchronous induction motor for a ball mill on a weak supply by using a pony motor to bring the motor up to say 90% of rated speed before energizing the motor. After the motor is energized and stable the clutch will engage. What transient current will flow when the motor is energized ?
 
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I would look at the torque cure for the motor and estimate the current at 10% slip.
If you mean the magnetizing inrush I am not sure.

Bill
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Jimmy Carter
 
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First of all, 90% of rated speed is less then required to overcome the peak of torque-speed curve and to enter the operation stable region. I think more then 95% will be better.
Second, I think the maximum current would be the current corresponding to maximum torque [approximate 2.5-3 time the rated].Some transient inrush current due to residual magnetic flux is possible, nevertheless is lower than noload transformer inrush current.
Regards
 
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It would depend on whether the pony motor has sufficent torque to start the other motor along with the load. Have you considered a vfd or even a soft starter or auto-transformer starting ?
 
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Magnetizing inrush does not change, you still have to establish flux, but it only takes a cycle or two. What a pony motor does do is reduce the duration of high starting kVA on the main system, which can reduce the transient conditions at the supply transformer which cause voltage drop. I have no idea how you would quantify that though, waross' suggestion makes sense to me from that standpoint.

But don't forget, unless it is an engine, the power for the pony motor has to come from somewhere...

I no longer have SKM Power Tools, but if I remember correctly they had an option for "assisted starting" mode in their Transient Motor Starting analysis software. That would imply that an algorithm exists to determine the affect it would have on voltage drop.



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You may need a VFD or reduced current start on the poney motor. That would still be a lot cheaper than a VFD for the larger motor.
Search this site. This topic has been discussed before.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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I have a simple question, why not drive the main motor to full speed with either a belt drive or by cranking a VFD up above 60 Hz?
If you energize the field winding before closing the breaker wouldn't the Synchronous motor generate line voltage so that you could close the main breaker when it's in step?
Roy
 
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Roy might have the best answer. This is most likely a fairly slow speed motor so pick a suitable speed matched pony motor and run it up to something like 61hz with a VFD to put you just above synchronous speed. The motor will likely produce more than rated voltage if full field is applied so you will have to find a field current that produces close to line voltage. Let the motor coast and then close the breaker when the motor is synchronized by the use of a synchronizing relay.

Check the synchronous motor data carefully. I have seen synchronous motors that still drew 350% of the FLA current when running at full speed before the field is applied. >200% of FLA is very common. So just having the motor near full speed to eliminate the high starting current may not have much chance at success.

 
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The most important reason that running up to speed before closing the field may be difficult is that the OP is trying to start an Asynchronous induction motor.
However I agree that the faster the motor is spinning, the less starting surge there will be. I suspect that you will still see the first 1/4 cycle magnetizing transient, and possibly some DC offset. Nevertheless, faster is better.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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Waross, Asynchronous, is that like, squirel cage motor?
The only motors that size I have seen on Ball mills had salient poles so I assume they are Syncronous. They start up with the rotor shorted out I think, I remember on one job the electricians screwed up the wiring leaving rotor open circuit. Several poles blew at start-up so we just jumpered around them as a temporary measure.
Roy
 
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Asynchronous motor is the term commonly used in the IEC world (note reference to MW rating rather than HP rating) for the piece of equipment commonly referred to as an induction motor in the IEEE/ANSI world.
 
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Correct me i I'm wrong Bill, but I took your comment to mean that, when starting a synchronous motor, you are using the amortisseur windings which are essentially an asynchronous motor. You can't really call it a squirrel cage motor per se because the rotor is still wound, just shorted during acceleration.


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jraef, I believe that the field (rotor) would be open during acceleration and that the amortisseur windings take the place of rotor bars in an induction motor. The amortisseur windings are always in place and always shorted.
 
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D'oh!
Yes, committed one of my own pet peeves, mixing WRIM and Synch. technologies. Hey, I just woke up...


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376
 
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David - they're induction motors here too. Have the last few contributors been hitting the bottle? [lol]

A correctly sized pony motor should get the main motor up to normal running speed give or take a few tens of RPM. They are only suitable for drive trains where the load can be mechancially decoupled through, for example, a clutch or a torque converter. The peak current will be very similar to the normal inrush as the magnetic field gets established, but the 'starting' current as the main motor pulls up to speed will be significantly less than LRC and will be drawn for a very short period.


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I just wonder how much you need to get a ball mill running? You have to get all the balls moving and they run backwards against each other. I suspect you might need quite a bit. We are talking 1,800hp loaded.

Perhaps someone with the same mill could be asked what the 'empty' load is.

If it is in the 500hp realm maybe a more useful method would be to add a genset that is used to reduce the network load just while starting. Perhaps it could then power the rest of the plant in emergencies?



Keith Cress
kcress -
 
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The original post mentioned a clutch so that the motor would be started unloaded.
Ball mills are a good candidate for synchronous motors. They may be over sized and over excited to correct the power factor of acres of small motors on the flotation cells.
I worked in one mill that used two 3000hp motors per mill.
Direct gear drive with two motors is not a good application for synchronous motors. We used wound rotor induction motors. They were started with liquid rheostats.
(The liquid rheostats would possibly have been a contender for the other thread looking for the maximum voltage and current ratings available in a load bank. LOL)
The power factor corection for the acres of flotation cells was provided by a pair of synchronous condensors.

Bill
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"Why not the best?"
Jimmy Carter
 
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Wasross is correct the motor starts no load then they supply air to the clutch through a flow restricton which allows the clutch to slip for a couple of seconds. Sometimes if the mill has been sitting for a while with mud in it the balls all stick together in a solid lump. Usually the operator will rock the mill a few times to try and break up the lump.
I'm sure it must be a synchronous motor that size.
Roy
 
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If this is actually a synchronous motor, I would buy a synchroscope and synchronize it like a generator.
Bring it up to close to synchronous speed and energise the field. Use the synchroscope and control the speed of the poney motor to match the line and close the breaker. Easy and quiet, and no surge or inrush. Just an increase from zero current to no load current as the pony motor is shut down.
For the drive experts:
Can a soft start be used for small speed adjustments at near full speed for a few minutes?

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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The OP clearly stated "asynchronous induction motor" on the first line of the opening post. I honestly can't see how anyone with a reasonable grasp of English could interpret that as meaning a synchronous machine. Shouldn't we be trying to help him solve his problem? If we really are trying to help mnewman then we are a long way off track with some interesting but ultimately pointless speculation about equipment that he doesn't have.

We can do better than this!

/rant


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