# Replacing slipring with induction motor



## kevinsimba (Aug 7, 2014)

i am running a Ball Mill using a 750HP , 4 pole ,slip ring motor at 3.3KV. i want to upgrade the system to a squirrel cage motor and VSD at 525Vac. What motor size can i use as replacement ?


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## Bootss (Dec 30, 2011)

Anyway you can post pictures of your project for a more comprehensive analysis of your question?


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## John Valdes (May 17, 2007)

Match HP, RPM, and Voltage.
Pick a true torque drive (VFD)

This is done all the time. Matching base speed (RPM on nameplate) has to be the exact same to be able to acheive the same amount of torque.
Since this is one beast of a motor, I would get the drive and motor provider involved.


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## Vintage Sounds (Oct 23, 2009)

Why would you want to change to a much lower voltage on a motor that large ?


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## AK_sparky (Aug 13, 2013)

kevinsimba said:


> i am running a Ball Mill using a *750HP*


I would personally replace it with a 750HP.


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## JRaef (Mar 23, 2009)

It may not be as simple as a 1:1 HP replacement, you really have to analyze the torque-speed requirements of the Ball Mill before making that decision. A VFD on a standard induction motor is good at making sure the motor can supply a CONSTANT level of torque regardless of speed. But WR motors are capable of providing higher than standard Design B motor torque at some speeds, but the nameplate "HP" rating given to it is typically only the HP at full speed with a shorted rotor, which may not be the peak shaft torque being used. So someone needs to look at the resistance steps used to accelerate and control the mill, then relate those to the complex torque curves that the motor puts out at various rotor resistance levels. Then what you would do is to find the HIGHEST possible sustained torque needed by Mill from the motor being used now, use that to calculate the HP at that specific speed, then that becomes your motor HP that you start with for the VFD. This is because the VFD, although capable of providing CONSTANT torque, and brief moments of ADDED torque, cannot make the motor produce more than rated torque for more than a minute or so. If you pick a motor that cannot give you the necessary sustained torque, even if you don't need it ALL of the time, then it stalls and fails.

And by the way on a Ball Mill, the highest peak torque requirement from the machine is usually at starting from a resting position, because it has to immediately start lifting it's balls. Been there, done that...

Example of a WRIM torque vs speed vs resistance "curve", emphasizing that it is not one curve, it is multiple different curves depending on the resistance steps.


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## CYoung (Apr 19, 2013)

JRaef said:


> start lifting it's balls. Been there, done that...


Insert joke here.....


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## Jhellwig (Jun 18, 2014)

JRaef said:


> It may not be as simple as a 1:1 HP replacement, you really have to analyze the torque-speed requirements of the Ball Mill before making that decision. A VFD on a standard induction motor is good at making sure the motor can supply a CONSTANT level of torque regardless of speed. But WR motors are capable of providing higher than standard Design B motor torque at some speeds, but the nameplate "HP" rating given to it is typically only the HP at full speed with a shorted rotor, which may not be the peak shaft torque being used. So someone needs to look at the resistance steps used to accelerate and control the mill, then relate those to the complex torque curves that the motor puts out at various rotor resistance levels. Then what you would do is to find the HIGHEST possible sustained torque needed by Mill from the motor being used now, use that to calculate the HP at that specific speed, then that becomes your motor HP that you start with for the VFD. This is because the VFD, although capable of providing CONSTANT torque, and brief moments of ADDED torque, cannot make the motor produce more than rated torque for more than a minute or so. If you pick a motor that cannot give you the necessary sustained torque, even if you don't need it ALL of the time, then it stalls and fails.
> 
> And by the way on a Ball Mill, the highest peak torque requirement from the machine is usually at starting from a resting position, because it has to immediately start lifting it's balls. Been there, done that...
> 
> Example of a WRIM torque vs speed vs resistance "curve", emphasizing that it is not one curve, it is multiple different curves depending on the resistance steps.



Good information. This also shows why it is important to properly select components that will work together to produce the desired result. While it is easy to replace an outdated or obsolete motor setup with an off the shelf drive and squirrel cage motor you have to take into account the characteristics about the old setup that warranted its selection in the first place. Squirrel cage motors lack in some areas that other styles are better in. 

Even just adding an inverter to squirrel cage motor can cause problems if one doesn't take into account that a machine might not have a liniear tourqe requirement in relation to speed.


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## Bootss (Dec 30, 2011)

Jraef
your analysis is needed on synchronous motor thread in motor (motor is in cement plant) section posted within the last week.
:thumbup::thumbup::laughing:


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## John Valdes (May 17, 2007)

arminkeller said:


> I would personally replace it with a 750HP.


A *4 pole* 750 HP motor.


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## RICHGONZO1 (Mar 5, 2012)

Agreed, HP is not interchangeable between applications. Wound rotor types with a step starter, induction with a freq drive and synchronous with a clutch. We have all 3 and I must say wr for that size of motor for us seems to be the least troublesome and least expensive for repair. The drives are the Cadillacs and the syncs are the hybrids and the wr are the corollas.. Once they are engineered, they will run with little maintenance on the electrical end at least.

I'm no motor genius or drive expert like jraef, but I seen what has failed many times over and the wr for us seems to be trouble free for the most part. Just my 2 cents and I'm only speaking on what I've seen. Maybe the wr was engineered correctly, but it seems like there are fewer parts and the parts are not near as expensive. When we blow a diode card on a freq it's 20k and you have to change all 3! Not to mention the maintenance, everyday!

The synchs have solid state controllers with a lot of components a typical electrician struggles to troubleshoot, let alone program.

The wr only energize a starter for the stator, cut in and out resistors in an oil bath for a determine time using regular starters on the rotor, then once up to speed, it's a regular induction motor. It's easy to monitor the steps and check resistors. Change the oil once and awhile and you got a plain ole induction once up to speed.

I may be totally missing the mark here, but we run ball mills on all 3 and we get called on the wr's way less than all the others.


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## Bootss (Dec 30, 2011)

Don't the wound rotor motors have brushes and then they drop out at a certain speed?I guess they have slip rings and not commentators.

I have about 5 years induction motor repair and rewind experience .We didn't do too many round rotor motors just induction motors like up to 200 - 300 HP. Seems like the larger motors were easier to wind.Started in 1974 right out of high school ,good times.


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## RICHGONZO1 (Mar 5, 2012)

Ours is 4160v and the stator induces the rotor to about 1500vac through a slip ring to the resistor bank. The motor starts with all resistors in parallel, each contactor "step" has a contact in an oil bath to each resistor bank, each step takes like 5 seconds until all the resistors have been removed and the rotor is shorted. Ours have around seven steps. 

Yes you have to check the brushes once in awhile, but we changed all ours out in February and they still have a half inch sticking out of the holder. We change the oil out or run through a filter press every 2-3 years, depending on oil analysis, pretty simple. Think ours takes about 500 gal of shell diala, but I would have to check my notes.


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## Bootss (Dec 30, 2011)

Where did you get your electrical training rich?


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## RICHGONZO1 (Mar 5, 2012)

Took an apprenticeship at a power plant in my home town


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## RICHGONZO1 (Mar 5, 2012)

I might add ball mils are very unique, depending on ball charge, plate wear and tph plus many other ever changing process variables. You need an engineer dude! Or jraef, I think I would take jraef, if he isn't an engineer, over most engineers!

I know step starters are out of date, new technology comes with higher expenses though.

I love our medium voltage freqs, but I hate them in the same sentence.

And why on god's earth would you lower the voltage on that high of HP? If it aint medium voltage cable then I would upgrade the service as well.


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## Bootss (Dec 30, 2011)

RICHGONZO1 said:


> Took an apprenticeship at a power plant in my home town


sounds like you can't beat that deal.I wish I would have had a more formal training I had to do go through a junior college program kind of do things on my own.


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## RICHGONZO1 (Mar 5, 2012)

Lep said:


> sounds like you can't beat that deal.I wish I would have had a more formal training I had to do go through a junior college program kind of do things on my own.


Seriously though, most my training was OJT. The power plant was great for training, but It ran like a top. Most trouble calls that were really interesting happen once every 20 years. A power plant electrician can't touch much and when he has to, the heat is on!

I learned the most from old timers who didn't give a **** about engineering and only cared if the plant ran or not. Thus, process is the place for aspiring electricians to learn at. I went to a processing plant shortly after my apprenticeship ended and it was all fires. That's where you learn the most.

All in all, you see everything in a power plant, but you don't work on it that much. I knew I loved this trade because you never stop learning and if you do, then you're just lazy!


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## KennyW (Aug 31, 2013)

Do you have existing transformer capacity and somewhere to feed the 750hp low voltage drive from? Also need a feeder of some sort. And big cables of course. 

That's also large enough to potentially have harmonic implications with respect to the rest of the power system. 

Personally I'd keep it at 3.3K and use an MV VFD. You can most likely re-use the existing cables and feeder breaker etc. 

If it's currently in operation it's easy enough to datalog the current and get a reasonable guess on torque requirements.


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