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
If you mean the magnetizing inrush I am not sure.
Bill
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"Why not the best?"
Jimmy Carter
RE: Pony Motor to Start Large Motor on Weak Supply
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
RE: Pony Motor to Start Large Motor on Weak Supply
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.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: Pony Motor to Start Large Motor on Weak Supply
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
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
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
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
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
RE: Pony Motor to Start Large Motor on Weak Supply
"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: Eng-Tips.com Forum Policies
RE: Pony Motor to Start Large Motor on Weak Supply
RE: Pony Motor to Start Large Motor on Weak Supply
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: Eng-Tips.com Forum Policies
RE: Pony Motor to Start Large Motor on Weak Supply
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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If we learn from our mistakes I'm getting a great education!
RE: Pony Motor to Start Large Motor on Weak Supply
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
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
RE: Pony Motor to Start Large Motor on Weak Supply
I'm sure it must be a synchronous motor that size.
Roy
RE: Pony Motor to Start Large Motor on Weak Supply
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
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"Why not the best?"
Jimmy Carter
RE: Pony Motor to Start Large Motor on Weak Supply
We can do better than this!
/rant
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If we learn from our mistakes I'm getting a great education!
RE: Pony Motor to Start Large Motor on Weak Supply
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
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
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
In the words of Rosanne Rosanadanna... "Never mind".
RE: Pony Motor to Start Large Motor on Weak Supply
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Pony Motor to Start Large Motor on Weak Supply
Roy