Upgrading Eddy Current Drives to VFD

[Robro]

Hi,

This is a new thread for my problem regarding Eddy Current Drives and VFD's.

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.


Reply by 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.


Reply by Tom Jenkins - 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.


We are going for option 1 which is to modify the motor. We are going to remove the eddy current clutch and replace the shaft of the induction motor with a longer one so it can connect to the load gear. The machine we are operating is a Press machine. And I understand that by controlling the speed of the Induction motor, the required torque will be affected. The motor will now also be strained because it is connected directly to the load. My question is, is it plausible for a direct connection from AC motor to the load and still have the same results as when it an Eddy current motor was used? Is the rating of the motor enough?

 

[JesperMP]

No experience with eddy current clutch drives, or press applications, but..

The motor will now also be strained because it is connected directly to the load.

With a VFD you will have a softer start, because you can set a current overload limit that will be much less than the current (and thus torque) compared to direct start.
If that is not enough, you can set a long ramp-up time that will make the startup even gentler, if your application permits.

We are going to remove the eddy current clutch and replace the shaft of the induction motor with a longer one so it can connect to the load gear.

If there is a gear between motor and load, you should worry more about the gear. An induction motor is very simple (=rugged) in comparison to a gear.

 

[Robro]

Hi Jesper,

Thanks for the reply. The press machine i'm working on does not have any ramp up time. It starts and ends abruptly with a push and release of a button respectively.

Yes, I am also wondering if there should be a gear box between. But as my boss said, we are just going to directly connect it to the load.

 

[JesperMP]

So the application is like very frequent start/stops, with full torque almost 'instantly'.

Not sure if you can get that with a VFD. I think not.
In the existing application, the motor is running constantly at full speed.
If you remove the clutch, the motor has to speed up and down rapidly. That will mean high currents for both speeding up and braking. So you would need a higher-end VFD, and possibly a motor rated for heavy duty load. And I doubt that you can get the same response time as with the clutch.

edit: Please describe how frequent and for how long the motor must activate.
edit again: How is the motor rotation converted to linear movement for the press ?

 

[JesperMP]

I was just thinking.
How does the motor activate the press ?
If the clutch is used to drive the press downwards, how does it move up again ?

 

[jrwb4gbm]

Snip.....
My question is, is it plausible for a direct connection from AC motor to the load and still have the same results as when it an Eddy current motor was used? Is the rating of the motor enough?

As JesperMP pointed out, you will not get the same results. Whether or not it will be adequate, only trying it out will determine that. We call that "Empirical Engineering" or the S.W.A.G. (Scientific Wild -ss Guess) method.

 

[maxketcham]

I have had experience replacing eddy current clutches with drives, better go to a larger HP motor and drive or your asking for trouble. The eddy current clutch should have ran a flywheel and your going to find that your motor was actually only loaded during start up and speed slumps when you activate the press. From experience, if you just replace the existing clutch with a drive you will loose the motor within a month of going into operation, If you can even get it to run the press at speed. the reason you will loose the motor is that when the system is running with a clutch the motor is running at full speed all the time and the clutch is used to apply the needed torque to the system. this allows the motor to cool after the brief 300% current spikes. removing the clutch and using a drive will mean the motor no longer runs constantly at base speed, so the cooling fans will no longer do their job. You can put electric fans on the motor, but from experience that just extends the life of the motor a little longer (3-4 months before failure) DO yourself a favor, resize the motor to 1.5 times minimum of what you have and make sure the new motor is cooled by a separate electric fan and is inverter duty. hope this helps

 

[jraef]

... The machine we are operating is a Press machine. And I understand that by controlling the speed of the Induction motor, the required torque will be affected. The motor will now also be strained because it is connected directly to the load. My question is, is it plausible for a direct connection from AC motor to the load and still have the same results as when it an Eddy current motor was used? Is the rating of the motor enough?

A) What do you mean by "press machine"? A printing or web press? Or a metal forming or punch press? We can assume, I hope, that this is not a hydraulic press, correct?

B) Regarding your statement "...the required torque will be affected." What do you mean by that? The REQUIRED torque is the torque REQUIRED by the machine. Nothing you do in the design of the torque DELIVERY system affects what the machine REQUIRES; it requires what it requires.

But if by that you mean the torque DELIVERY is affected, then yes, that is true, but maybe not in the way you are thinking. A motor driven by a VFD is capable of MORE torque than what you can get from an Eddy Current Drive (clutch) system. So to that end, you can ALWAYS replace an ECD with a VFD. Use a good quality VFD that is at the very least capable of Sensorless Vector Control, and your motor can be made to deliver full Break Down Torque (200-220% FLT) at any moment it is needed for acceleration or re-acceleration.

The only issue with sizing the VFD is in how LONG the drive will be expected to make the motor deliver that much torque. Typical heavy duty (CT) rated VFDs are going to be capable of that for 3 seconds or so at that PEAK torque output, but up to 60 seconds at 150% of FLT, which is still a lot in terms of re-acceleration capability and more importantly, probably as much as the motor can safely deliver anyway.

C) The point about continuous slow speed operation is an important one, especially in light of your decision to re-use the existing motor from the ECD. That motor was likely selected based upon the fact that it would be running at full speed, regardless of load speed, so because they are less expensive it is likely fan cooled, such as TEFC or ODP construction. For running at low speeds continuously, you will want a fan corced cooling option for that motor.

D) But... this comes back to Issue A); what do you mean by "press"? If it is a punch press or metal stamping press, then most likely the ECD was used as a way to engage the load electronically on demand, with a flywheel actually being used as a form of mechanical energy storage. If that is the case, replacing the EDC with a VFD is NOT a valid strategy, unless you can live with reduced production rates. In a stamping / punch press application, the motor and flywheel spins continuously, the user engages the clutch, the clutch couples the flywheel to the load, presses, then releases, at which time the flywheel is almost immediately brought back up to full speed because the only load is the flywheel itself at that point. Now the press is ready for action again. You cannot adequately duplicate that action with a VFD, because it will have the load (press) coupled all the time. How do you accelerate that for one operation?

If however, it is a PRINTING / WEB press, that's totally different. If the EDC was being used to maintain a constant torque on a web so that speeds matched as roll diameters changed, then the VFD is a TOTALLY VALID replacement.

 

[maxketcham]

think that's what I already said.

 

[DickDV]

No, maxketcham, that is not what you just said. First, there is no need to oversize the motor (jraef clearly states that you can get more out of the motor with a VFD compared to a ECD). Second, if the application is a continuously-stroking stamping press, then cooling is rarely an issue because the motor is unloaded about 1/3 of the stroke cycle and most stamping presses are not designed to run really slow. 30-60 strokes per minute would be typical. A better less expensive way to increase cooling would be to size the belt sheaves so the motor is running about 80hz when the press is running at max speed.

Now, if the machine is some other kind of press, then the above may not apply. It almost sounds from the description given by the OP that the ECD is used to control the start/stop for single stroke operation or something similar. If changing to a direct drive induction motor would result in the motor being started and stopped in rapid fire style, then, as jraef stated, a VFD is not the answer.

We really need better information from the OP in order to answer with precision

 

[maxketcham]

DickDV, that may be true with the newer motors, I've been working with drives since the first drives came out in the 70's. You are right we do need more info, but here is one thing you need to understand, when was the last time you saw an eddy current clutch in a new application? most likely the clutch is 40-50 years old, and the motors they made back then may have said 5hp but I guarantee they could typically put out 150% more torque in that day than the new state of the art "Inverter" motor. I've seen and had a whole lot of engineers fail to realize this in my day, and size a drive and motor that would fail because they didn't understand that one fact. I am assuming, and yes that is bad but it's what I have to go on at this time, that he has a 1960's or earlier Eddy current clutch such as made by warner in the day. The thing about an e3ddy current clutch is you can run the motor and couple it at a certain torque to pick up the flywheel. And you tell me what happens if you resize the pulleys, seen that done before too and it didn't work, so tell me why? Because if you go to a smaller pully at the motor to a larger pully at the flywheel you need recalculate your torque requirements. I suggest you look at http://www.motorsanddrives.com/cowern/motorterms7.html particularly the high inertia section. that would be a typical punch press, very high inertia during start up. If it's a printing press, then he might be able to get away with just changing the drive, depending on whether it is a web press or a sheet fed press. he may have problems with the web press.

 

[DickDV]

max, I really am not much inclined to argue on this forum but I can say that I have been doing machine upgrades for 26 years and have probably converted 30+ old (even very old!) automotive duty stamping presses from eddy current clutches to modern VFD's and high efficiency motors.

I have never needed to increase the hp from the clutch nameplate and I routinely use overspeed to improve starting torque and motor cooling and reduce load-induced speed error. It might feel comfortable to increase hp but it is not needed if the new system is properly optimized. And, no, you do not have to recalculate torque when using overspeed. The reason is that, if you increase the pulley drive ratio by a given percent and then increase the motor top speed by the same percent, the torque in the overspeed range will be the same as before the ratio increase. You will need to study the torque output characteristics of an induction motor in field weakening to understand why that is true. In the end, the ratio change simply shifts the motor speed up over the whole speed range. The flywheel doesn't know the difference. That's because the torque you lost in overspeed is recovered with the higher ratio.

Check it out. I think you will see that that is true.