Eddy current drives

[tomneth]

Hi all,

I have a press machine with an eddy current drive that is presently operating with the motor at 460 vac 60 Hz 3 phase. It is a 15 HP drive. This machine is destined to go overseas where the power is 380 vac 50 Hz. The control board on this takes 115 vac and states 47-62 Hz so that is not a concern, but what about the motor? We're running this at a low enough speed that I'm sure I can just turn the pot up to match existing speeds. Does anyone have any experience with eddy current drives? What will likely happen to it when we lower the voltage and frequency by 20%

 

[Tom Jenkins]

Eddy current drives are a very old technology, and they have largely been replaced by VFDs. To the best of my knowledge an eddy current drive can not have an output speed greater than the input speed. If you put your motor on 50Hz current the best you can get is 83% of the existing max speed. You also need to make sure the motor can handle the torque and horsepower required for the load at 380 VAC 50 Hz. As to the eddy current drive itself, I strongly suggest that you contact the manufacturer for assistance.n It has been a long time since I've been involved with a true eddy current drive, and I'm not sure what effects using different voltage and frequency will have.

 

[randylud]

Ah, the old days!

As Tom said, the old eddy current clutch or drive is really out dated but be that as it may. The motor needs a 7.6 V/Hz ratio and 460/60 and 380/50 both give the 7.6V/Hz. But, at the lower voltage a corresponding increase in current will result to provide the horsepower. So, motor heating is going to be a problem if you use the motor near name plate. As for the clutch circuit itself, there should be no problem. The magnetic field will still vary and that is what engages the clutch. I would recommend that the customer in Japan change the motor out once it is in their hands. Man, it has been a very long time since I have even seen a drive like this one. Brings back some good, and bad memories.

 

[rsdoran]

For those not familiar with eddy current drives

Eddy Current drive is a combination of a constant speed AC induction motor, coupled to an eddy current clutch. The function of an eddy current clutch is to transmit a controlled torque from the induction motor, (prime mover) to the load (driven machine). The medium of torque transmission is a controlled magnetic field, generated by a stationary field coil. A thyristor base electronic controller is used to convert a relatively small amount of AC power into DC power in order to excite the field coil.

Basically what happens is varying the level of excitation to the field coil controls the output speed of the eddy current drive.

Verify this situation with motor/drive manufacturer to be certain.

 

[tomneth]

Thanks for the replies guys. From what I know and what I've gathered my only concern will be whether the motor itself can handle the increased amperage that will be required and the resulting increased temperature. I have contacted an eddy current drive expert and I'll see what he says. I think its going to come down to how much risk and reliability the company is willing to pay for.

 

[DickDV]

Actually, tomneth, you may not be in as much trouble with this as it may seem. As was pointed out, the motor at 380V 50Hz will deliver the same torque as it does at 460V 60Hz due to the constant 7.6V/Hz ratio. And the eddy current clutch exciter is already rated for 115VAC 50Hz so that is taken care of.

What is different is that the motor is turning 17% slower due to the frequency reduction. Note---the same torque available but at a slower speed. This means that 17% less horsepower is available and that the motor cooling fan will be somewhat less effective.

If you analyze your load requirements based on torque rather than horsepower, most likely you will find that you are ok at the lower speed. And, for motor cooling, unless you are pushing the motor right up to its design limits, the drop in fan output will probably not be a problem either.

Since most eddy current systems are built with the motor and clutch in one assembly, just changing out the motor is not really an option. It sounds to me like you are going to be ok with this.

Just a note about these eddy current clutches. As rsdoran mentions, they pass load torque directly back to the motor while reducing the speed. They cannot increase speed over the motor shaft speed and do not increase or reduce torque. They simply drop motor speed more or less based on the exciter coil's magnetic intensity.

In view of the above, you can see that, at bottom, these devices are simply massive energy wasters (the larger ones are even water cooled). So, if you have a 15hp motor and you are loaded to rated motor torque and the clutch is reducing speed to one-half the motor speed, you are wasting one-half the motor hp as heat. You can do the math but, in my area, electricity runs about 8 cents per kwhr for larger industries. At that rate, each motor hp that runs 24/7/50 uses $400 of electricity per year. The above system is wasting 7.5hp which calculates to $3000 per year in wasted energy. That would pay for an AC drive in less than 18 months in energy savings alone.

Do I need to say that converting presses from E.C. clutches to AC drives is a large part of my work?!!!!

 

[Robro]

Eddy current motor

Hi,

I'm a controls engineer and is currently in need of advice. This may be off topic but if I have to replace the eddy current clutch drive with a vfd. I have a strong feeling that I also need to replace the Eddy current motor but my boss said we can still use said motor. Any thoughts? BTW, the motor is 45 kW and they already bought the VFD which is also rated at 45 kW.

Best Regards,
Rob Bernardino

 

[jrwb4gbm]

Years ago (40+) when the center bearing locked up (failed to turn) the output shaft would run at 100% of motor RPM. To use your existing motor I would suggest replacement as the 1st option, supply the eddy current clutch with 100% excitation voltage as the 2nd option, and weld or otherwise lock up the center bearing as the 3rd option.

 

[Tom Jenkins]

Hi,

I'm a controls engineer and is currently in need of advice. This may be off topic but if I have to replace the eddy current clutch drive with a vfd. I have a strong feeling that I also need to replace the Eddy current motor but my boss said we can still use said motor. Any thoughts? BTW, the motor is 45 kW and they already bought the VFD which is also rated at 45 kW.

Best Regards,
Rob Bernardino

In most cases the motor used with an eddy current clutch was standard. Check the motor nameplate. Most of the time you can use an existing motor with a VFD as long as it has Class F insulation or better and has a 1.15 service factor.

 

[jdbrandt]

Eddy Current drive is a combination of a constant speed AC induction motor, coupled to an eddy current clutch. The function of an eddy current clutch is to transmit a controlled torque from the induction motor, (prime mover) to the load (driven machine). The medium of torque transmission is a controlled magnetic field, generated by a stationary field coil. A thyristor base electronic controller is used to convert a relatively small amount of AC power into DC power in order to excite the field coil.

Basically what happens is varying the level of excitation to the field coil controls the output speed of the eddy current drive.

Verify this situation with motor/drive manufacturer to be certain.

What's next? Wound Rotor Motor questions using a Flow Matcher???

 

[Pablox]

I looked up this thread, because I never heard of eddy current drives. When I read the title I assumed it was some new technology to rival VFD's.

 

[Tom Jenkins]

I looked up this thread, because I never heard of eddy current drives. When I read the title I assumed it was some new technology to rival VFD's.

There is actually a new technology, kind of sort of like the old eddy current clutch. It has uses in lower hp pumping and similar applications, particularly vertical pumps. If harmonics are a big concern it can have an advantage over a VFD.

http://www.magnadrive.com/

 

[jraef]

What's next? Wound Rotor Motor questions using a Flow Matcher???

If you were attempting to comment on Ron Doran's post, hire a medium. He passed away in 2007...

 

[Tom Jenkins]

If you were attempting to comment on Ron Doran's post, hire a medium. He passed away in 2007...

Yes, but robro reopened it yesterday. He should have started new thread, of course, but there it is.

Ron is still missed on this forum.

 

[Robro]

Thanks for the reply! I should open a new thread.