VFD, Ramped decel that is stopped by an external source?

I'm trying to better understand what actually happens to a motor when a VFD is decelerating it. And how it would act if physically stopped while still in the decel process.


If I had a fly wheel spinning, and set the VFD to coast to stop, the fly wheel would just spin, slowly losing speed until it stops. Something like an external brake could stop the fly wheel just working against the inertia of the flywheel.


But what if you are applying an external brake to a flywheel that is still in an active decel ramp? If it is a decel that is longer than what the brake would take to stop the load, would the VFD's deceleration ramp be working against the braking action? If the brake could stop the wheel in 20 seconds, but your VFD had a 60 second deceleration.


These are just hypothetical situations, no real world numbers. I don't have a fly wheel with a VFD and a brake. I'm just trying to think of an actively decelerating vs coasting load both having an external brake applied. Or any other sort of action that would be counter to the spinning motor's load during that time.

A good motion controller and drive can ramp down the motor no problem. We, our customers, do it all the time.

Peter Nachtwey said:

A good motion controller and drive can ramp down the motor no problem. We, our customers, do it all the time.

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But if something external to the motion controller and drive enters the equation and tries to stop the load faster than the set decel ramp, will the drive work against that motion and still try to "power?" (for the lack of a term on my end) the motor thru the new additional load and keep the new load slowing down on the original slope to zero speed? Or will it adjust to the new load stopping time and stop trying to decelerate the load, ending the decel ramp sooner than programmed?

Most VFDs have options for regenerative braking on heavy loads.

NetNathan said:

Most VFDs have options for regenerative braking on heavy loads.

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Unfortunately none of the drives I use seem to have regenerative braking. Mostly just Dynamic Braking and DC Injection. What I've quickly read on it suggests that regenerative braking is a higher end option on drives. At least in the PowerFlex series I am familiar with.


But I'm more curious about how the drive reacts to the motor being stopped (or trying to be stopped) by an external force faster than the decel rate it has been set to.

Do you mean if you set the VSD for a 60 second decel time for example will the vsd be applying power until the decel time has completed?

chelton said:

Do you mean if you set the VSD for a 60 second decel time for example will the vsd be applying power until the decel time has completed?

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Yes. If it was set to a 60 second decel but something else is trying to stop the load faster, will the drive still be applying power for that 60 seconds at that fixed decel ramp.

chelton said:

Do you mean if you set the VSD for a 60 second decel time for example will the vsd be applying power until the decel time has completed?

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Yes, I believe this his question. An interesting one.
The drive receives a stop command (or equivalent) and starts its declaration which is set for 60 seconds. At the same time the holding brake is activated (not sure how, denergize I guess).


I think the drive should manage the brake on and off but still what would it do, when would it release the holding brake, during deceleration or after it's finished.

If the motion is stopped and the controller is trying to do a controlled ramp down then the torque limit will eventually be reached and the drive will fault. If it is just coasting then there will only be a bang. The saws in a "bucking" line that cut trees to 20 ft length or smaller are mounted rails and powered by VFD and motors so the length of the cut sections of the log can be varied. There may be 4 saws on rails. One to trim each end and 2 or more to cut a long log into smaller sections. Normally this is not a problem as the drive and motor can move the saw to the desired position and they are usually quite far apart. This is using a controlled ramp down. If power goes out the saw will coast on its rail until it hits a stop of some sort unless a brake is used. If the saw hits another saw while powered the torque limit will be exceeded and the drive will fault.

kalabdel said:

At the same time the holding brake is activated (not sure how, denergize I guess).


I think the drive should manage the brake on and off but still what would it do, when would it release the holding brake, during deceleration or after it's finished.

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In this hypothetical, the brake or other external stopping source is an unexpected event or failure of the system. The VFD would normally be allowed to Decel thru it's process and stop the load that way.

Peter Nachtwey said:

If the motion is stopped and the controller is trying to do a controlled ramp down then the torque limit will eventually be reached and the drive will fault. If it is just coasting then there will only be a bang.

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So if the drive doesn't reach the torque limit and fault, it could keep the load moving thru the external stopping force until it decelerates the load as normal? If what ever attempting to stop the load is below the torque limit of the drive?

Tharon said:

In this hypothetical, the brake or other external stopping source is an unexpected event or failure of the system. The VFD would normally be allowed to Decel thru it's process and stop the load that way.

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https://www.kebamerica.com/blog/application-notes-vfd-brake-control/
Thanks.


I had a bit of a read in reference to a holding brake and it's one hell of an algorithm.

But I now I understand your hypothetical external force is very different.

kalabdel said:

I had a bit of a read in reference to a holding brake and it's one hell of an algorithm.

But I now I understand your hypothetical external force is very different.

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Ya, I'm just trying to better understand what the drive is actually doing during a deceleration ramp. To get a better understanding of what can occur during unexpected situations while Category 0 and Category 1 stops are active.

Tharon said:

Ya, I'm just trying to better understand what the drive is actually doing during a deceleration ramp. To get a better understanding of what can occur during unexpected situations while Category 0 and Category 1 stops are active.

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Category 0 would be an instant removal of power so no deceleration would be in effect.
Category 1 is a controlled stop in which it has been identified that the deceleration time for the machine removes the risks that could cause injury.

Tharon said:

So if the drive doesn't reach the torque limit and fault, it could keep the load moving thru the external stopping force until it decelerates the load as normal?

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Not as normal. The motor may not be able to follow the intended trajectory. When decelerating there must be an opposing force or torque. The motion controller would normally be applying a negative torque but if the opposing load is high the motion controller would apply a positive torque in an effort to stay on the trajectory. If the opposing force is low the motion controller would probably just decrease the amount of negative torque since some opposition is from the load.

If what ever attempting to stop the load is below the torque limit of the drive?

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Probably not much immediately but the torque that is applied in effort to overcome the obstruction comes from current so the armature would heat up quickly. Motors don't like obstructions. This is where hydraulics work better than motors.