Eddy Current Drive Efficiencies - Info Required

[woodglass]

I am interested in replacing a number of eddy current drive systems in our plant with VFD's and new motors. The load type is constant torque.
Can anyone please point me in the right direction on the net where I can find usefull information on the efficiency of eddy current drives ?.
One of our vendors states that the maximum efficiency of an eddy current drive system is 50-60% maximum. I think this is a somewhat bold statement since I know that the efficiency varies dependant upon the slip in the eddy current coupling, so, surely when the slip is near zero, the efficiency of the drive system must be closer to 100% than 50-60%.
I would like to better understand the situation so I can accurately calculate a payback period for the proposed replacement system.
Regards,
woodglass...

 

[Skogsgurra]

There is more to losses in an eddy current coupling than slip. There is also the magnetizing current.

I wouldn't think it is so high that best efficiency is down to 60 %, however. But, since slip never can be 0 %, you will always have losses in an eddy current coupling. And, as you correctly say, proportional to slip if you have a constant torque load.

I wouldn't hesitate at all. Even if you will need to install filters, shielded cables and - perhaps - motors with a better insulation system. Plus do something about bearing EDM, you will always benefit from a change. The way energy prices are developping, VFD is always good.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[CJCPE]

The minimum slip of an eddy-current drive is probably not less than 3%, but it could be 5 or 6%. The friction windage and control power adds another 2% or so. With a constant torque load, the slip losses go up linearly as speed is reduced so you need to estimate the average operating speed to calculate payback. The average operating speed is the key bit of information.

The control and windage losses go down with reduced speed. New motors will also be more efficient than the existing motors. With most eddy-current drives, the motor is either flange mounted to the clutch or mounted in the same housing. Sometimes it is possible to pull the clutch parts out of the housing, replace the clutch with a shaft coupling and keep the motor, but that seems like a lot of work to keep a motor that is less efficient and is not designed for use with a VFD.

You can probably figure out the minimum slip from the EC drive nameplate, but you need to estimate the average speed and the actual operating torque, so you might just as well estimate everything. Unless the drive is operating at maximum speed most of the time, the payback is going to be so short that you shouldn't need more than a credible estimate.