Motor Loading

I would start with the manufacturer, maybe the motor that has been spec'd into the job is't quite right. What kind of motor are you using? is it fan cooled? I tend to think that putting extra load on the motor would only be a band-aid as a properly spec'd motor should not have any problem.

If the turntable is abruptly slowed, the overrunning load can "back drive" the motor. A VFD with regenerative braking feature (or similar terminology) might be needed.

You can put a magnet close to the disc. That will induce potential in the disc in proportion with its speed, the distance to the disc and how powerfull is the magnet. By variation of this parameters, you can vary the load.

GA

If you absolutely have to resort to braking the disk this would be a good way to do it.

It might be worth looking at magswitch:-

http://www.magswitch.com.au/

That way you fix the unit solidly in position and use a control mechanism to rotate the "switch" to apply the necessary braking.

You could introduce a load with a wheel held against the disc by a spring mechanism or maybe a cam type mechanism.

Have you set the motor current, V/F pattern, accel/decel and stall prevention parameters right? If yes no reason to overamp or overrev. Or is it overvoltage? On the latter increase decel time.

There is a (very slim) chance the VFD switching frequency has something to do on a very light loaded motor. If all else fails try changing it.

What about adding a governor either temporarily or permanently to prevent overspeed

Most VFDs allow you to control the ramp up rate and the ramp down rate. Slow down the ramp down rate. I>e. Make it take longer to stop. That way the motor will not cause an overvoltage situation in the VFD. You mentioned overamps in your post and this is solved the same way - slow down the ramp when stopping the motor.

I think there is a confusion here:

- Are you saying OVERAMPS to mean Over-speed or over- Amps (too much amps?

This is because your comparison with a centrifuge blower does not stand: If there are no restrictions on a centrifuge blower, it does not increase speed, but drives more air, which will make more noise to make you feel it is revving up. It actually slows down and take more Amps because it gets loaded. On the other hand, if you restrict the inlet or outlet, it will rev-up and take less current.

In your case, if the current is increasing, it means that the motor is under rated for the load and you will need a bigger one (in terms of power output).

if the speed is uncontrollable to your liking, you need to tweek the VFD parameters and maybe use a DC brake when slowing down etc as recommended by others. You should be able to control the speed with a VFD ...

Thanks for making the point. I've been scratching my head since I first read the question.

As you say, if the motor is drawing too many amps, there is too much load on the motor (unless it is a bad motor with some mechanical or electrical problem--even then, over amps is an indication of a problem).

And yes, you can probably cut down the amps by slowing down the motor / driven equipment which will result in less load on the motor.

I was going to mark this OT, because LAA_Lucke and others made the points, and I'm just sort of echoing / thanking him, but it really isn't OT, is it.

Typically- when a motor driving any load "over-amps", the motor is seeing a load higher than the capacity of the motor.

Your reference to an "unloaded" centrifugal fan is exactly what happens- the fan is trying to move more pounds of air in a given time frame than the motor is capable of supporting- restricted flow brings the world back to equilibrium.

I think your motor is too small for the load presented- run the numbers on the mass being spun and the center of mass and the distance traveled by that center of mass over time (at 1000 RPM).

One HP is 550 Lb-Ft per second. Your 42" dia. disk likely has a center of mass somewhere around 30" radius (calculations are available- I chose not to look them up). If the disk weighs, say, 10 pounds and you are spinning it at 1000 RPM, then the "work" is 10 pounds x (2.5 feet x Pi) x 1000 / 60 = 1309 Lb-Ft per second. Add about 5% for the added "load" of friction with the air and another 10% for bearing friction losses yields 1511 Lb-ft/sec, or 2.74 HP. You would need a 3 HP motor to drive this scenario.

Run the numbers.

Gentlemen,

You are correct, but what puzzles me is that if load is a spinning Aluminium disk, the load is accelerating it up to speed - after that there are only bearing losses (and perhaps gearing losses).....Is it driving something else?

We are not told - as with many posts we are commenting in the half or total dark!

A little more information would be helpful to be able to post a really helpful reply.

If there is no load other than accelerating the disk, a small motor should be fine, the starting circuit then simply needs to be modified to current limit until the disk is up to sufficient speed to bring the motor within its design load limits at full voltage.

If there is an additional load the motor may be too small, or if the additional load is light enough the approach mentioned above (startup current limit) may be sufficient.

A startup current limit may be as simple as one(single phase) or three (three phase) Motor start NTC Thermistors of suitable rating, Or it may require something more complex - this is application dependent.

For sure there is something else loading the motor. Who is going to design an alu disk anjust spin it and look at it? He does not want to talk about the rest...

Otherwise, you are righ. Once the speed is reached, the load will be minimum for a plain disk. Bearing resistance etc.

Yes,

What the additional loading may be is unimportant, however its characteristics are...also the disk may be performing some optical or other reference function where the additional loading is negligible and it is there to provide a stable reference for 'something'.

So a disk spinning by itself may have a purpose .... I am trying to assume nothing!

It would be good if the OP states more clearly where the alleged overload is occurring; while ramping the disk up to speed, or while slowing it down. The mention of flywheel effect suggests that the problem is while slowing the disk.

The steady-state load of a spinning disk is small (unless the bearings are crappy). This is all about acceleration/deceleration. Longer ramp times on the VFD can help.

An abrupt slowdown is analogous to "plugging duty" on a motor, which is the severest type. It can entail surge surrents equal to or greater even than starting current. Various magnet and brake arrangements might work, but they are kludgy compared to a regenerative-braking VFD.

The equivalent radius of a 42" diameter disk is about 15", not 30". No weights have been given, so it is impossible to estimate the acceleration/deceleration loads. This is another case of woefully insufficient OP data.

Can you please be more specific about your "variable speed drive with an ac motor" . Is it a decent programmable VFD or you are joking on us ? A lot of people give time on your question and you don't seem to care to respond. Why should we?

"Induse load on the motor" Pfffffff.....

The motor turns on and brings the 1/4" thick 42" dia. wheel up to speed. It is an ac motor with a vfd that has been programmed and tuned. It struggles to come up to speed in the fact that the motor is always wanting to run faster than the ramp up that we have set in the program. As we was testig the process and we would vary the speed, when the speed setting was changed, it would always overshoot

When the motor is turned off, the disc will spin for several minutes before it stops.

The motor kicks out after 1-2 hours of running time.

There is no load on the disc, it does have an important purpose, but that is not the topic of this discussion. Sometimes the wisdom is in being silent and listening and not always having to speak.

Well now we know.

It has been suggested that you should use the regenerative braking with the VFD. Some come with it included, some others you need to request or fit separately.

The issue is that because of the inertia, if you try to change the spedd down, it will take a long time to comply!

Also, if you are overshooting while going up to speed, then something is wrong with the programing of the vfd: there are several parameters to look at when adjusting the ramps, including the maximum frequency, etc.

Check the braking accessories.

Also, you should stop the motor using the Enable / disable inputs on the VFD and not cutting off the power from the VFD or the Motor directly. This will destroy the VFD eventually...

Re: It struggles to come up to speed in the fact that the motor is always wanting to run faster than the ramp up that we have set in the program.

I just don't understand that sentence. How do you know the motor wants to run faster than the ramp up set in the program?

Watching the hertz reading on the drive, there is a random/routine (if that makes sense) fault that blinks on when it is coming up to speed or you change the speed setting - i believe it is a 501(?) fault code. when it flashes back to the hertz display the reading is always much higher than the setting and then it backs down and it will settle at the setpoint. Our electrical person has worked on the settings to get it to just come up to speed in a reasonable time without over shooting the setting and without taking way too long to get there. Yet to find the right set-up for that.

Except V/F, ramp-up and down times, braking and Overvoltage prevention there are other parameters also to be 'tuned' friend. I strongly suggest you take a good look at the programming manual. Look for "stall prevention during acceleration" and "stall prevention on constant speed run" or something equivalent according to VFD manufacturer. They must both be enabled, and also there must be a multiplier for each to be set. In normal applications is propubly set to 1, but with an application inertia 500+ times over the one of the motor, which is not "usual" it should be tampered up also. Or deal with a bit more experienced applier. Tuning special parms can be tricky. Perhaps if we knew the VFD model somebody could advice. Good Luck.