Pony Motor to Start Large Motor on Weak Supply

[EddyWirbelstrom]

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 ?

 

[roydm]

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

 

[EddyWirbelstrom]

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

 

[LionelHutz]

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.

 

[jraef]

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!

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

 

[waross]

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

 

[roydm]

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