WRIM Config as a SCIM and Started with a VFD Issues

[Barry1952]

Looking to see if anyone has had experience with shorting out the rotor and starting the motor with a MV VFD.
The existing application is with a high inertia load (25MW MG set) that the motor has to bring up to 600 RPM via this WRIM. There are 6 armatures on this common shaft for which 2 are the main MG set. Once up to speed, the WRIM is disconnected from the supply and the rotor spins freely.
Interested in knowing the issues that may arise on making a WRIM work like a SCIM. Things like the drive able to produce enough brake away and acceleration torque as well as the rotor winding having limitations as it is not a true SC design for the high rotor currents.
More details can be provide if helpful.

 

[DickDV]

Using a VFD with the rotor leads shorted can work and will produce high torque on start if sized large enough for the corresponding starting current. It will be expensive.

Since you don't seem to need variable speed, you might want to evaluate using an SCR-type MV softstarter. These will not produce as much torque-per-amp as the VFD but it might produce enough to get the shaft to break away. You don't need maximum torque---just enough torque! And, a softstarter will be much less expensive in the MV range.

One other thing: sometimes it makes an old WRIM work better when a little resistance is left in the rotor circuit. These can even be mounted on the rotor if there is clearance and the brushes can be removed, same as if the leads were shorted. This gives the motor a bit more slip or looseness and the torque will generally be higher with lower current. It sounds like this motor is used for spin-up only so a little less efficiency is not a great loss.

 

[Barry1952]

Thanks DickDV
Forgot to note that we do need speed control to get the MG set up to 575RPM and then gradually up through 600RPM to synch the machine on line.
We have considered the thought of keeping some resistance in the rotor circuit to gain a higher torque but are not sure if it is needed. Preference is to short the rotor leads to eliminate the brushes.
The million dollar question is "how would one know that the torque capabilities of a SCIM is enough?" The original design was around a WRIM primarily (I think) for its high starting torque and now we go with a reduced starting torque with a VFD.
If indeed it does break away, perhaps the acceleration time is going to be longer and the motor unable to handle the added thermal stress.

I guess my bigest concern is "How does one know what the limitations may be to simply put a VFD on a WRIM and short out the rings?" It seems that one would almost need the motor design info to safely determin this (which is rarely available). If it is not a high inertia load, then there may rarely be an issue to go this route. It would be a bad thing to find out at commissioning time that it just doesn't have enough in it to do the job. That was why the WRIM was fitted.

 

[jraef]

The VFD will be able to limit the thermal stress on the motor. In electro-mechanical or soft starting methods, the big problem is that with the rotor shorted, the slip is very high in the beginning. So you get high current without creating a lot of torque. But with a VFD the slip in controllable as part of the overall V/F control capabilities. It sometimes does mean slightly more current for longer than it would be rated for if it were a true SCIM motor to begin with, that is why Dick mentioned over sizing the VFD. But you should be able to get full torque at any speed, just like with a basic SCIM motor.

http://www.lmphotonics.com/slipring.htm

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[Drivesrock]

I have used VFDs on low voltage WRIM many times - just shorting out the slip-rings. Best results are with flux vector drives where the drive measures the motor model and therefore takes into account the reduced rotor resistance.

Another solution could be a 'Rotor Drive'. This is basically a low voltage variable speed drive connected via a contactor to the rotor slip-rings and could have an active front end connected back to the HV mains thru a step up transformer. You still need an external rotor resistance though for starting - connected via another contactor. At first, you start the motor with the external rotor resistance. At close to asynchronous speed the rotor volts will low enough to now connect the LV drive and disconnect the resistors. The rotordrive can now control the speed of the motor loading the rotor by extracting energy and returning to the mains. I know of one drive that can take the motor supersynchronous by injecting energy into the rotor. When working in a small speed range below and above the synchronous speed of the motor where the rotor volts will be low the rating of the rotordrive will be something like 25% the rating of the motor - therefore much cheaper than a full blown HV VFD.
Typical application of the rotordrive would be a pump or a fan with limited speed ranges.
What's the rating of your motor? How often do you start the MG set?
Maybe the manufacturers of these 'rotordrives', if this drive still exists to buy, could comment on its suitability for this application?

 

[ScottyUK]

Hi Dick,

Is this 'rotordrive' what I know as a static Kramer Drive or something different? It certainly sounds similar.

I think there are few manufacturers still out there. I seem to recall that Brush at Loughborough were in this market at one point although whether they still are is a different question.


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[LionelHutz]

A WRIM works by using resistors which move the breakdown torque peak to a different speed. Higher resistance moves the breakdown torque peak towards 0 speed and lower resistance moves the breakdown torque peak towards full speed. You can also add enough resistance to effectively move the breakdown torque peak to a negative speed.

So, in theory, the motor could use the breakdown torque to accelerate from zero speed. However, in practice the resistors are typically sized to begin with the breakdown torque peak in the negative and accelerate with a lower torque. For example, the first step may accelerate the motor from 0% speed with 150% torque (and about 150% current) to 25% speed at 100% torque (and about 100% current). Then, the next resistor step is added, the torque jumps back up to 150% and the motor accelerates further.

So, the motor may be required to produce a torque of 150% or possibly even more as it accelerates. If you wanted to account for the worst possible case then record the stator current during the start and size the VFD to be capable of the highest current you record (ignoring the switching transients) which corresponds to the highest torque the motor was producing. The problem is that this method is likely very conservative and will likely result in a much higher rated VFD than required.

Personally, I would pick a VFD that matches the motor with a 150% for 60 second overload rating. The reason being that the load is just a big spinning inertia. I'm guessing, but more than likely the WRIM runs below rated current once you get this load up to full speed and you only see higher currents when the load is accelerating. So, if that is the case, then a VFD that matches the motor should be fine. The load should start like a fan load which is very easy to accelerate. It may take a long time but it's still an easy load. Picking a VFD with a decent short term overload capability gives you the ability to produce some extra breakaway or accelerating torque if it is necessary.

 

[Barry1952]

I would like to thank all that provided me feedback on this thread.
The major concern I have is still the issue with the Available Torque when the motor is configured as a SCIM.
I realize that the V/Hz aspect of the MV VFD will provide much better torque than if the motor was just set as a SCIM and FV started.
The issue was to understand that the WRIM had a specific purpose in the existing design so much so that its NP for HP states "Special" and under duty it states 5" ON - 20" OFF - 5" ON. We cannot exceed that so we will need to determine the acceleration time and torque requirements to ensure the VFD can do the job.
So much is stated for WRIM motors most suitable for starting high inertia loads and how it is less harsh on the power system (which is another factor to be considered).
The Torque Profile of the WRIM starting method with a liquid resistor may not be allowed to vary too much from the profile able to be produced with a VFD. As speed control is required to synch the MG set, a soft start is not suitable.
So in the end, we will have to work with the mass and time to accelerate to gain confidence that retrofitting with a VFD is capable of doing the job. As for this type of application, I was concern on a solution of simply shorting out the rotor and applying a VFD is going to work for all situations. VFD's may not always be the first and fastest solution is all I was thinking and this is mainly due to all the literature (and plain theory) of the differneces between WRIMand SCIM.
Thanks again to all.

 

[DickDV]

A WRIM may exist simply to limit inrush current when starting a high inertia load. Starting torque demands may be very low in that case.

In your case, the challenge is to determine the breakaway torque and then determine if the shorted rotor WRIM will produce that much or more.

If you can breakaway the load, I would expect that acceleration torque will be adequate.

So, bottom line: How much breakaway torque is needed and do you have enough. If you are a little short, leaving just a little resistance in the rotor circuit mounted on the rotor so there are no brushes should do it.