Eddy Current "clutch" drives

allscott · Jul 22, 2014

How common are these in industry anymore? We have a couple of them left and I currently have a 25HP Dynamatic that needs rebuilding and needs new controls for the clutch. Should I be looking at getting it replaced with a VFD or should I rebuild it?

Lancie1 · Jul 22, 2014

I haven't seen one in a plant in about 40 years. I would be surprised if you can find parts for the original model.

A Google search shows that Dynamatics are still made, and I am very surprised!

allscott · Jul 22, 2014

Lancie1 said:
I haven't seen one in a plant in about 40 years. I would be surprised if you can find parts for the original model.

A Google search shows that Dynamatics are still made, and I am very surprised!

You should come tour my plant, it's like a history lesson in Electrical Engineering.

I'm going to contact Dynamatics and see what they say as far as the cost of rebuilding the motor and clutch and replacing the analog controls with digital. My guess is it is from around 1980-1985.

I am going to assume they are going to want a fortune so I'll probably just replace it with a new motor and VFD.

Tom Jenkins · Jul 22, 2014

Eddy current clutches are essentially obsolete, having been replaced by VFDs. I understand from a plant operator that there is still a firm out there that repairs and services them, but I don't recall the company name - I think they are somewhere in Wisconsin.

VFDs are generally more efficient than eddy current drives, especially at lower speeds, so you might be better off replacing them. Another option is MagnaDrive, which makes a product similar to eddy current clutches but more efficient.

http://www.magna-drive.com/e/pdf/FGC.pdf

scameron81 · Jul 22, 2014

Many of our bridge cranes still have the dynamatic clutches from the 70's which control the trolley and bridge movement. The cranes with major problems though get upgraded to VFD controls.

DickDV · Jul 22, 2014

Many larger motor shops still repair eddy current clutches but, by the time you add in the poor efficiency, the cost of repair is is prohibitive. The energy losses are roughly proportional to the speed turndown times the motor kw. So, for example, assuming a typical constant torque load, a 30hp motor clutch running at half speed delivers 15hp to the load and wastes the remaining 15hp or about 10kw as heat. At today's electricity prices, that's pretty ugly unless it's winter time and your building needs the extra heat!

A VFD and premium efficient motor will often pay back the initial investment in 15-20 months in energy savings alone.

These numbers are not as ugly if the load is an increasing torque type like a fan or centrifugal pump but, even then, the VFD is generally the choice.

allscott · Jul 22, 2014

DickDV said:
Many larger motor shops still repair eddy current clutches but, by the time you add in the poor efficiency, the cost of repair is is prohibitive. The energy losses are roughly proportional to the speed turndown times the motor kw. So, for example, assuming a typical constant torque load, a 30hp motor clutch running at half speed delivers 15hp to the load and wastes the remaining 15hp or about 10kw as heat. At today's electricity prices, that's pretty ugly unless it's winter time and your building needs the extra heat!

A VFD and premium efficient motor will often pay back the initial investment in 15-20 months in energy savings alone.

These numbers are not as ugly if the load is an increasing torque type like a fan or centrifugal pump but, even then, the VFD is generally the choice.

It is a constant torque load with a 25 HP motor. I'm not sure there is actually anything wrong with the motor or clutch the problem I am having is it is occasionally "running away" ie: jumping up to max speed. The controller for the clutch is 1980's vintage or maybe even 1970's vintage so I don't even want to touch it. If I can get a digital controller for it at a reasonable price I will do that if not it will be a new motor and drive.

As far as efficiency this machine only runs about 20 hours a month so it is tough to justify it on a cost basis, but as of right now justification on a "it's broken" basis will do.

jrwb4gbm · Jul 22, 2014

The biggest problem as far as the "running away" condition on Dynamatic drives was usually caused by the center bearing trying to fail. It could also come from the SCR in the controller module overheating and near its end-of life. Some of the earlier units used a tube so SCR failure would not apply to them. However, the tube could short internally intermittently. As others have said, you should replace the Dynamatic units with a good VFD at failure time.

allscott · Jul 22, 2014

jrwb4gbm said:
The biggest problem as far as the "running away" condition on Dynamatic drives was usually caused by the center bearing trying to fail. It could also come from the SCR in the controller module overheating and near its end-of life. Some of the earlier units used a tube so SCR failure would not apply to them. However, the tube could short internally intermittently. As others have said, you should replace the Dynamatic units with a good VFD at failure time.

I wondered about that but honestly haven't seen the problem actually happen yet, I'm just going by what operators are telling me so I don't know for sure whether the problem is mechanical or or in the controller. I have been told that it runs fine in the morning for a few hours before they have any trouble with it and it's been hotter than blazes here lately so I am suspecting some sort of overheating problem.

jrwb4gbm · Jul 22, 2014

We used to pull all the Dynamatic drive motors annually at summer PM shutdown to just replace the center bearing and dip the windings to seal out moisture. They were used in wire drawing machines and the drives stayed damp from wire lubricant spray.

Edit:
Normally when the center bearing fails, the output shaft just locks to the rotor and runs at full speed the moment the power mains to the motor are energized.
If a cooling down period restores normal function for a few hours, the controller is probably the issue.

allscott · Jul 22, 2014

jrwb4gbm said:
We used to pull all the Dynamatic drive motors annually at summer PM shutdown to just replace the center bearing and dip the windings to seal out moisture. They were used in wire drawing machines and the drives stayed damp from wire lubricant spray.

Edit:
Normally when the center bearing fails, the output shaft just locks to the rotor and runs at full speed the moment the power mains to the motor are energized.
If a cooling down period restores normal function for a few hours, the controller is probably the issue.

I work in a wire and cable plant as well. This unit is used on a stripping machine but the two other clutch drives we have left are on wire drawing bobbin takeups.

Unfortunately the guy that knows about this older stuff has been diagnosed with terminal cancer so I am now on my own with this type of stuff.

Thanks for the help.

Eddy Current "clutch" drives

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