vfd on slip ring "wound rotor" motors

[southerner]

I am involved in converting a ball mill , which requires a large amount of starting torque, from a slip ring motor driven application to a vfd driven appl. . We have sent the motor to our motor "guy" to refurbish and interconnect the rotor windings.Is there any advice or similar applications out there that could be helpful.

 

 

 

southerner:P

[Guest]

Hello Southerner,

 

Using a slip ring motor and a VFD is generally fine thing to do, and as long as the drive is up to the job, you'll get good performance. However, slip ring motors that have been in service for a while tend to be of old construction. In particular the winding insulation is not usually good enough to withstand the fast changes in voltage that a VFD will inflict on them. I would recomend that you get a dV/dt filter for the output of your VFD.

 

Unless of course you want a new motor but the bean counter stopped you, in which case don't put the filter on and you'll probably need a new one in six months.

 

Cheers Niallnz

[vishy_44]

Hi

I had myself done the conversion of slip ring IM to VSD application but i have not done anything to the motor. I had simply shorted the motor at the rotor starter end and have connected the VSD. this was basically for the fan application and the KW range was from (110-170) and the voltage was LV. I do agree that there was an apprehension regarding the current factor but this had been incoporated in almost 8 machies and for the last 4 years are working fine. There had been one failure in case of a motor where the stator braiding at the overhang position has come out.

As far as the conversion of the slip ring to VSD mode for ball mill application is concerned, i feel that this is a bit tricky. Further i would like to know as to whether this will be running at full speed ar at any intermediate speed

 

Vishnu

vbsingh@raymondindia.com

[bob]

Hi,

 

I am quoting what Marke wrote on 7/8/05 concerning soft start of slip ring motors. "The other issue of course, is where does the slip energy go during start? With external resistors, we keep the bulk of the slip energy out of the rotor. With the rotor shorted, all the energy is dissipated in the rotor winding. This winding has a standard varnish insulation and is far more sensitive to the effects of heating than the standard cage rotor.

 

In any starting situation, it is important to ensure that a) the motor can develop enough torque to start the machine, and the rotor can absorb the slip energy.

 

I am concerned that in most situations, the torque available under shorted rotor conditions is not sufficient to start machines, and more particularly, that the slip energy will cause damage to the rotor winding".

 

Any comment welcome.

 

Bob

[marke]

Hello Bob

 

When you use a vfd on an induction motor, you never operate the motor under high slip conditions. The inverter is started at zero frequency and then increases up to the operating frequency, taking the motor speed with it. Therefore, there are no appreciable slip losses to dissipate in the rotor and it is no different from the normal operation of the slip ring motor at full speed.

 

Provided that the rings are shorted in a manner that prevents excessive arcing, the performance of the motor will be the same as the motor would be at full speed with the rings shorted.

 

Do not become alarmed or concerned about high slip characteristics, (start torque slip losses etc) when using a motor with a VFD. The motor only ever operates under low slip conditions with a VFD.

 

Best regards,

[jraef]

Bob,

In case you didn't catch it, the subtle difference is in that this discussion is about applying a VFD to a slip ring motor, Marke's earlier comment was in relation to applying a soft starter. There are significant differences in those technologies when it comes to starting a slip ring motor.

[bob]

Hi Marke,

 

I have another concern about the installation of VSD on old slip ring motor. It is known that old wound rotors were built with very low resistance value( very near to zero ohm) and my doubt is that will they be able to start for high inertia load if the rotor is short-ciruited.

Thanks for your usual prompt and excellent reply.

Bob

[marke]

Hello Bob

 

As you will never operate under high slip when using a variable speed driv e, the rotor resistance becomes almost irrelevant. If the motor is able to operate with the rings shoted, then it will work under variable frequency control.

Remeber, to Start the motor on a drive, we begin the drive at zero Hz and slowly increase the frequency. The motor remains almost in synch with the drive at all frequencies. i.e. the slip is very small. If there is an overload that the drive and motor can not cope with, the drive will shut down.

 

Best regards,

[jraef]

Again, that is a difference between using a VFD on a WR motor versus using a Soft Starter. Your concern, Bob, is quite valid when using a Soft Starter because the lack of rotor resistance when the rings are shorted means very high current with very low torque output. That problem however goes away when using a VFD as Marke said.

[marke]

When you are using a starter, the motor is initially at rest and the supply is at line frequency. The difference betwen the motor speed and synchronous speed is equal to the synchronous speed. We describe this as a slip of 100%. To develop torque under these conditions, there must be sufficient rotor resistance. The torque produced is dependant on the power dissipated in the rotor resistance. If the rotor has no resistance, there is no power dissipated (R = 0) and therefore no torque.

 

When the motor is operating at full speed, (close to synchronous speed) we do not require any rotor resistance to develop torque. The difference between the rotor speed and synchronous speed is small (low slip). The lower the rotor resistance, the lower the slip at full load, or the closer to synchronous speed the motor operates.

 

If you look at the torque curve of an induction motor, there is a torque peak close to full speed. If you are operating below this maximum torque, you are operating in a high slip mode and the rotor resistance is important to develop torque. If you are operating above this torque peak, then you are operating in a low slip mode and the rotor resistance is not important in developing torque.

Although the torque curve is usually printed of torque against speed, it should infact be printed as torque against rotor current frequency. If the frequency of the supply changes, the torque curve stays the same, but the zero axis moves up and down. ie take a 60Hz motor and operate it on 50Hz and the zero axis moves to where the 10Hz point is on the 60 Hz curve.

If we reduce the frequency down low enough, the zero axis will shift to the right of the maximum torque point. This is what happens when we use a variable speed controller. We begin the drive at zero Hz and ramp the frequency up. This way, the motor always operates to the right of the torque maximum and is always in the low slip mode so the rotor resistance is not needed.

 

This is one of the concepts that differentiates the applications engineering of drives and starters and causes many issues for drives engineers trying to understand and apply starters.

Perhaps I should create a web page on this??

 

Best regards,

[kamb]

Hi all,

 

I am going to be involved in a project to change the Statovar control

systems to VFDs for motor control of cranes; and I noticed that there

have been some discussion about this experience.

 

My questions are:

1)Is that a good practice to change the speed control of slip ring motors to

VFD?

2) What factors should I consider during design regarding to the motors,

drives, cables, etc.

 

Thanks a lot for your help.

 

Regards,

Babak Kamali

 

[marke]

Hello kamb

 

1)Is that a good practice to change the speed control of slip ring motors to VFD?

Yes, the use of VFDs to control the speeds of motors is much more accurate and efficient than the use of rotor resistors. The rotor resistance control the speed torque curve, not the speed. The final speed is determined by the intersect of the motor speed torque curve and the load speed torque curve. As the load torque changes, the motor speed will also change.

Rotor resistors alter the slip of the motor and produce high slip losses. At half speed, the power dissipated in the rotor resistors is equal to the power produced to the load.

 

2) What factors should I consider during design regarding to the motors, drives, cables, etc.

For normal applications, you can apply a VFDto a slip ring motor by shorting out the rotor (no resistors) and applying the VFD to the stator as per a normal induction motor.

When you apply a VFD to lifting devices such as cranes and elevators, there are a number of additional things to consider. The brake control must be done such that the brake can only be released when the drive is producing sufficient torque to control the load. If the brake is released as soon as the drive begins to produce an output voltage, there is a very real danger that the load will begin to drop and the vfd may loose control.

It is normal to use a closed loop vector control system to ensure that accurate control is achieved.

In essence, use a specific VFD that has special software and set up for lifting applications. Many drive manufacturers have special application software for this aplication.

 

Best regards,

[kamb]

Hello kamb

 

Yes, the use of VFDs to control the speeds of motors is much more accurate and efficient than the use of rotor resistors. The rotor resistance control the speed torque curve, not the speed. The final speed is determined by the intersect of the motor speed torque curve and the load speed torque curve. As the load torque changes, the motor speed will also change.

Rotor resistors alter the slip of the motor and produce high slip losses. At half speed, the power dissipated in the rotor resistors is equal to the power produced to the load.

 

For normal applications, you can apply a VFDto a slip ring motor by shorting out the rotor (no resistors) and applying the VFD to the stator as per a normal induction motor.

When you apply a VFD to lifting devices such as cranes and elevators, there are a number of additional things to consider. The brake control must be done such that the brake can only be released when the drive is producing sufficient torque to control the load. If the brake is released as soon as the drive begins to produce an output voltage, there is a very real danger that the load will begin to drop and the vfd may loose control.

It is normal to use a closed loop vector control system to ensure that accurate control is achieved.

In essence, use a specific VFD that has special software and set up for lifting applications. Many drive manufacturers have special application software for this aplication.

 

Best regards,

 

Hi Marke,

I had seen your valuable comments to other participant and I ahve to say that they are very useful.

I appriciate the time that you put to reply to me and others.

Thanks