VFD through a slip ring?

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sii

sii

Senior Member
Location
Nebraska
I am trying to put together a proposal for updating some machinery at the plant I work for. One of our machines has four each 5, 3, and 2 Hp 240 VDC electric motors running through a slip-ring. This is the original design of the machine although we have replaced the original slip-ring. The 5 Hp motors we use cost about $2,000 each, and the smaller ones about $1,200. The DC controllers cost $2,600 apiece. I am unsure of the repair cost. Due to reasons beyond my control :mad: we tend to burn up a heck of a lot of motors and drives and as you can see that gets expensive. Not to mention getting called in on the weekend to replace one here and there.

What I would like to do is replace the motor/drive combinations that we have now with VFD's and AC motors. I have no experience with slip-rings beyond this particular machine and am unsure if there are any considerations I need to account for when running motor leads through the slip-ring. The manufacturer refuses to give any technical assistance on this question.

Facts:
Motor voltage: 480​
Distance from drive to slip-ring: ~25'​
Distance from slip-ring to motor: ~10'​
Rating of the slip-ring individual conductors: 600V/20 amp​

I plan to use input filters on the drives.

Can anyone tell me if there are any special considerations to take into account here. I can provide any other information that may be helpful.
 
what makes you think that the ac motor/VFD combination will be any more robust than what you have now? My experience has been that dc motors and dc drives will often take abuse for years that would kill an ac motor in a few hours.

the cost of the drives and motors you are having to replace does seem quite high. perhaps they are older versions that are no longer stocked except as replacement parts.

i would be spending my time trying to figure out why the things are failing and dealing with that rather than a means of replacing them with cheaper components.

I'd be inclined to run this by the VFD manufacturer for the slip ring question. a slip ring is basically a brush. it may not like the high frequency high voltage pulses that the VFD will be sending to the motor.

I suppose you could try one and see what happens for a year.

I think I would be looking at load chokes on the vfds to reduce the stress to the slip rings.
 
petersonra said:
i would be spending my time trying to figure out why the things are failing and dealing with that rather than a means of replacing them with cheaper components.
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To put it bluntly, the motors are failing due to the machine operators. They are constantly required to do setup changes on the machine which require frequent re-balancing of the load. They do not balance the load and the plant manager refuses to hold them responsible for it. Thus the :mad: in the OP. The abnormally high current from the regenaration created eats up SCR in the drive. There are no alarm circuits in the DC drive we use nor in any that I can find to replace it. If I use a VFD, I can interface with the PLC to cause an alarm to sound if the load is imbalanced. I can also require a personalized password to silence said alarm, giving me a way to say "this operator failed to do his job and I can prove it, what are you going to do about it?" All told, that is all secondary as far as I'm concerned.

Here is the primary reason. I'm not sure how most businesses work but all of the motor and drive repair/replacement comes out of our departments budget. Last year, on the 5Hp alone, we replaced 11 motors and 5 drives. That's $35,000 not to mention machine downtime at about $7/minute. The same number of AC units would cost me less than $10K. You get the point.

petersonra said:
what makes you think that the ac motor/VFD combination will be any more robust than what you have now? My experience has been that dc motors and dc drives will often take abuse for years that would kill an ac motor in a few hours.
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I know it works because I have done the exact same thing on other machines with excellent results. The only difference is that this particular machine requires the motor leads to pass through a slip-ring. I just have no experience putting VFD power through a slip-ring.
 
What is the reason for the motor power needing to go through the slip rings? Is it some sort of traction drive?

There are a number of problems doing that with AC motors powered by VFDs, because the interference due to any problems with conduction across the slip rings and brushes is going to cause imbalances and firing circuit problems. I would not expect the transistors to last very long at all, even if you manage to get the VFD to continue firing. I would imagine you will have the same problems with a new DC drive as well because most of them have gone to using PWM output, just like AC drives. Your DC drives are most likely older analog drives, so they will not have much of a problem with that issue (but obviously they do have some problem with it!).

I would look at a complete redesign of your system to take advantage of more modern technologies. For example in the days when this was built, the DC drive was likely big and heavy, leading to putting it off of the moving part of your machine and using the slip rings. Modern equipment is significantly smaller and lighter. So it wouldn't be a problem putting the AC SUPPLY to the VFD through the slip rings, then have a hard connection from the VFD to the motor. The VFDs or PWM DC drives would not have much of an issue with slip rings on the incoming AC power.

You might also want to look at the application with an eye for what is called a "linear motor". That is the leading edge technology for traction drive systems, especially at those kinds of HP levels.

This might be a good one to hire a consultant with recent drive experience for.
 
sii said:
If I use a VFD, I can interface with the PLC to cause an alarm to sound if the load is imbalanced. I can also require a personalized password to silence said alarm, giving me a way to say "this operator failed to do his job and I can prove it, what are you going to do about it?" All told, that is all secondary as far as I'm concerned.
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You can do all of that with a modern DC variable speed drive.
And have protection against excessive regenerative current.
Your easiest and cheapest route might be to replace the existing DC drives and they really aren't all that expensive. Recently, We bought some rated at 10kW (about 13 HP), four quadrant and they were under $1,000 each.
If you go for a three-phase VFD, will you have enough slip-rings?

Like Jaref, I'd be interested to know why you need to feed the motor through slip-rings. We make slip energy recovery variable speed drives for controlling large wound rotor machines. The slip-rings are often the most vulnerable component under fault conditions. We can design the rest of the circuit (including the SCRs) to withstand fault current without fuse protection. We always have high speed fuses in the feed from the rotor slip-rings.
That's not to say that you can't feed a variable speed drive from slip-rings.
We have installed a number of cargo hoist drives that are fed from overhead rails which is a similar concept.
 
That is a very challenging application. It appears that each arm will, at some point in the cycle, be in the heating stage. From what I see there, that would likely preclude the ability to mount any electronics on the arm itself, so running the motor power through the slip rings does appear to be the only way to do it.

But I don't think using a VFD and AC motors is going to solve your problems. As I said earlier, the PWM output of the VFD will not like the slip rings. On top of that, you will increase the number of failure points; with DC you have 2 slip rings per motor, with AC you will have 3 per motor.

If I were you, I would take this application to a slip ring supplier and ask them to recommend the best technology available to meet this challenge. There may be similar machines that they will have worked on where they solved these problems. I have used Wampfler in the past, they are extremely knowledgeable and helpful.

Good luck.
 
Jraef said:
On top of that, you will increase the number of failure points; with DC you have 2 slip rings per motor, with AC you will have 3 per motor.
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Unless they are permanent magnet machines DC might require four slip rings. Two for the armature and two for the field.
 
Besoeker said:
Unless they are permanent magnet machines DC might require four slip rings. Two for the armature and two for the field.
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Plus a ground, plus feedback?

I have had sleepless nights from slip-rings, and they can be a nightmare. I would side with the others that Modern DC drives would be the way to go. I was working on a AB 1395 last night, that I'm pretty sure is very outdated, it has all the features you want and more. The only draw-back I see to DC is motor maintenance.

I can also say the with the right slip-ring you can do it with AC. In-house we run two 10kw? drives, 24dc, and Interbus through the same slip ring. It works fairly well. (14 slip-rings, 5 years of service, no problems, with no upkeep, no dropped I-bus packets) HOWEVER I did not engineer it, and spins slowly and intermittently.

Keep an open mind, there are other ways to get what you want.

Regards,
Doug S.
 
Have you considered...

Have you considered...

To put it bluntly, the motors are failing due to the machine operators. They are constantly required to do setup changes on the machine which require frequent re-balancing of the load. They do not balance the load and the plant manager refuses to hold them responsible for it. Thus the in the OP.

If balance is the issue, have you considered a vibration transducer. Some models are even wireless, to avoid slip-ring issues. I might consider a large red flashing light and horn combo if the load is out of balance. This could also be used to stop a process after a time using a simple timer or PLC program mod.

You could even track the number of balance issues within a range ect... Perhaps a type of product has a higher vibration than another, this could pass the cost in predicted loss along to its customer.

As a high point you will be able to identify the true cost or a poor operator.
 
VinceS said:
To put it bluntly, the motors are failing due to the machine operators. They are constantly required to do setup changes on the machine which require frequent re-balancing of the load. They do not balance the load and the plant manager refuses to hold them responsible for it. Thus the in the OP.

If balance is the issue, have you considered a vibration transducer. Some models are even wireless, to avoid slip-ring issues. I might consider a large red flashing light and horn combo if the load is out of balance. This could also be used to stop a process after a time using a simple timer or PLC program mod.

You could even track the number of balance issues within a range ect... Perhaps a type of product has a higher vibration than another, this could pass the cost in predicted loss along to its customer.

As a high point you will be able to identify the true cost or a poor operator.
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Good points Vince.
I have a problem with this as a whole.
If the operator can break the drive system then maybe the drive system is flawed.
 
The load here is an arm mounted on a 10" shaft, and offset about 7' from center that only turns at ~2.5 RPM so vibration is not the issue. The arm goes through a 35 minute oven cycle, then two 35 minute cooling cycles. I cannot turn the motors off mid-cycle because then the parts will be lost as well as possible mold damage. There are thousands of these machines across the world exactly like this one in various sizes. Very little has changed throughout the years. The drive system is not flawed, it's just that the equipment is outdated. After looking around over the weekend, I have found a couple of DC drives that will fit in the allotted panel space. A Baldor equivalent to the one we use now is only $500 compared to the $2600 we're spending now. Maybe I will give a couple of them a try. Thanks for all the input.

I think I am gonna keep researching the AC thing though, I have software that I use to monitor all of the VFD's I have over the plant network. That's pretty nice. I also never thought of putting current transducers on the load and triggering an alarm if the current goes too high too many times in a cycle (or something like that). Depending on the mood I'm in I may even make it so they cannot silence the alarm!:D
 
sii said:
The drive system is not flawed, it's just that the equipment is outdated.
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I was considering that in the light of this comment you posted earlier:
"The abnormally high current from the regenaration created eats up SCR in the drive."
I don't know how old the equipment is. I have been involved with the design, manufacture, testing and commissioning of DC drives since the late 1960s - yes I am very old. I was born more than a decade before thyristors were invented......:D
Protection has always included current limit, instantaneous electronic overload protection, IDMT, high-speed fuses co-ordinated with the SCR I^2t rating, phase loss, and incorrect phase rotation.
The regeneration current for four-quadrant applications would also be limited by the current limit circuit so should not permit sufficient current to destroy the SCRs. If the current rating of the drive is too low to provide adequate braking, then the voltage rise could be enough to take out the SCRs but that is then a drive rating problem with it not being matched to the most onerous duty it could see in service.
In short, if the operator can break the drive, it is a drive or drive application problem. In my opinion.

sii said:
After looking around over the weekend, I have found a couple of DC drives that will fit in the allotted panel space. A Baldor equivalent to the one we use now is only $500 compared to the $2600 we're spending now. Maybe I will give a couple of them a try. Thanks for all the input.
Click to expand...
Good. Make sure they are comfortably rated for the duty and can handle regeneration.
 
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