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Pony Motor to Start Large Motor on Weak Supply

Pony Motor to Start Large Motor on Weak Supply

Pony Motor to Start Large Motor on Weak Supply

(OP)
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 ?  

RE: Pony Motor to Start Large Motor on Weak Supply

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
--------------------
"Why not the best?"
Jimmy Carter

RE: Pony Motor to Start Large Motor on Weak Supply

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
 

RE: Pony Motor to Start Large Motor on Weak Supply

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 ?

RE: Pony Motor to Start Large Motor on Weak Supply

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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RE: Pony Motor to Start Large Motor on Weak Supply

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

RE: Pony Motor to Start Large Motor on Weak Supply

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

RE: Pony Motor to Start Large Motor on Weak Supply

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.

 

RE: Pony Motor to Start Large Motor on Weak Supply

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

RE: Pony Motor to Start Large Motor on Weak Supply

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

RE: Pony Motor to Start Large Motor on Weak Supply

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.

RE: Pony Motor to Start Large Motor on Weak Supply

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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RE: Pony Motor to Start Large Motor on Weak Supply

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.

RE: Pony Motor to Start Large Motor on Weak Supply

D'oh!
Yes, committed one of my own pet peeves, mixing WRIM and Synch. technologies. Hey, I just woke up...


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RE: Pony Motor to Start Large Motor on Weak Supply

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.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Pony Motor to Start Large Motor on Weak Supply

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 - http://www.flaminsystems.com

RE: Pony Motor to Start Large Motor on Weak Supply

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
--------------------
"Why not the best?"
Jimmy Carter

RE: Pony Motor to Start Large Motor on Weak Supply

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

RE: Pony Motor to Start Large Motor on Weak Supply

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

RE: Pony Motor to Start Large Motor on Weak Supply

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
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Pony Motor to Start Large Motor on Weak Supply

Scotty, First of all the original poster has never come back to confirm the motor type.
Secondly, I worked in mining (operation & design) for almost 20 years and never saw an asynchronous motor on a ball mill unless it was really small, certainly not 1,700 HP
Thirdly, the poster said he has a weak supply, I might be wrong but I think in that situation a synchronous motor is usually used to improve the situation.
Fourthly, we are all learning something.
Finally, although we may have lost the plot I'm sure somewhere along the way we threw in some ideas that mnewman can use.
Cheers
Roy

RE: Pony Motor to Start Large Motor on Weak Supply

(OP)
Thank you for your helpful posts.  Sorry for not responding earlier to confirm that the motor is asynchronous ( squirrel cage ), not synchronous.
Your posts have mentioned the two phenomena at play when starting an asynchronous motor ( or a synchronous motor without the field energized ) direct-on-line ( DOL ):
1.    The transient magnetising inrush current which persist until the magnetic field is established, for small motors is less than 1 cycle, for large 1.3MW motor ?
2.    The high rotor current and therefore high stator current due to high rotor slip which persists until the rotor slip is less than approx. 10%( peak torque on the motor's torque speed curve ).
The motor torque speed curves could be used to determine a speed at which to energize the motor such that the slip would be sufficient to supply torque for no-load losses, however what would the magnitude and duration of the magnetising inrush current be ?
What is a typical value of motor sub-transient reactance to use to calculate the the a.c. component of the magnetising inrush current ?  
The posts on the advantages of a synchronous motor for a ball-mill application are very interesting however we need to know if an asynchronous ( squirrel cage motor ) started with a pony motor can be used.
I think the SKM transient motor start software does not model transient inrush current, however I will check SKM's transient study program ISIM.    I have heard that SKM's latest version 6.5 has enhanced its transient motor starting program to include transients.

Cheers
Murray
 

RE: Pony Motor to Start Large Motor on Weak Supply

I misread or too quickly read your first post and did think you had posted synchronous there.

If you start with a pony motor you would just let that motor go right to full speed which will be much better than 90% of your main motor's rated speed. Assuming speeds of the two motors are matched of course. Then, you'll find the inrush will be a number of cycles. I'd put it as being somewhat similar to a loaded transformer but it probably won't last as long. Definitely enough to cause lights to flicker though.

If you can not allow any inrush at all on your power system then consider installing a soft-starter along with the pony motor. Supplying some current to the main motor during starting would lower the requirement of the pony motor and eliminate the magnetizing inrush currents.

 

RE: Pony Motor to Start Large Motor on Weak Supply

D'oh! I did the same. In fact, if it weren't for the fact that there is no editing provided in this forum, I would say it DID originally say synchronous and someone fixed it!lol

In the words of Rosanne Rosanadanna... "Never mind".

RE: Pony Motor to Start Large Motor on Weak Supply

Your motor may have 3% slip. Energizing at 90% speed will result in about 200% to 300% of full load current in addition to the magnetizing inrush. Take it to between rated speed and synchronous speed before energizing. if you can get to 90% speed, you can probably get to 98% speed.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Pony Motor to Start Large Motor on Weak Supply

I'm not sure why we are talking about slip. the main motor of a mill is usually low speed < 500 RPM. The pony motor speed will be much higher and require gearing down with a gearbox or belt drive. With the right ratio the Pony might be able to get the main motor very close to 100% even without a VFD. How do you plan on coupling the Pony to the main, via some sort of clutch or permanent connection?
Roy

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