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

I think I understand.

Part of me wants to experiment on a stopping load now. See how it reacts to different external events. Something hitting the spinning load, something impeding the stopping load, the stopping load hitting something, etc.

Thanks for all the info.

You should be OK if the motion controller detects a following error and sets the control output to 0. Then the armature won't heat up but the load will not be pushed anymore.

If you have a normal ramp stop (not coast), it is a helpful step to start the motor, and then simply cut the drive connection...could be a Cat 0 STO action, or just power off.
THEN, time how long it takes the motor to coast to a full stop...for example: 120 seconds.
NOW, you have an idea of the longest setting of decel on a ramp stop...120 seconds.
Depending on your dynamic braking, and the drive's ability to regenerate, you could then try, perhaps, 60 seconds in a ramp stop...and monitor the drive DC Bus voltage, and any other events during the ramp (decel hold, etc.).
NOW, if you keep doing this, you'll know the natural minimum decel value.

I concur with jdbrandt.
That is how I do it. We have cooling tower pumps for an ammonia chiller system with a combined lift of 70 vertical feet. If we were allowing the VFD to decelerate the pumps incorrectly we would get the occasional overvoltage or motor stalled fault. This was happening with different manufacturer's drives. AB, Siemens, ABB, and Johnson Controls. The problem was resolved after timing how long the pump takes to come to a stop by disconnecting the line side power and timing it. That time was input as the decelerate value. The VFD current was monitored and if the current exceeded a defined setpoint then the routine opened the relay between the motor and VFD, and closed again after the VFD was no longer outputting. It has been a few years since we have experienced any VFD faults or alarms due to the decelerate configuration.

jdbrandt said:

If you have a normal ramp stop (not coast), it is a helpful step to start the motor, and then simply cut the drive connection...could be a Cat 0 STO action, or just power off.
THEN, time how long it takes the motor to coast to a full stop...for example: 120 seconds.

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I hope it doesn't hit anything during that time.

Well, of course.
The assumption is valid for a PUMP or a FAN. Not so much for a hoist, or a driven load on a horizontal surface.
careat emptor, YMMV. etc.

Your question is simple enough, and you are on the right track.

Just think of the output of the ramp generator in the drive as the 'real' speed reference, and know that the drive will attempt to keep the motor at that reference, unless the protective functions over-ride it. So, if it can, the drive will run the motor through the braking torque if it can, in order to follow the ramped reference.

It is not unusual to have a machine which has 2 different stopping sequences, such as normal and emergency stopping.When you apply the mechanical brake, you should either disable the drive and allow the friction brake to stop the load, or switch the decel ramp to minimum and use the braking/regen power of the drive to assist the brake (without any positive torque).

jdbrandt said:

careat emptor

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Let the buyer be consumed by an automobile?

I can say for a fact. that when a crane driven by a VFD slams into a stop block or another crane..... it will continue to attempt to push through until the ramp down time is done lol

IanM8040 said:

I can say for a fact. that when a crane driven by a VFD slams into a stop block or another crane..... it will continue to attempt to push through until the ramp down time is done lol

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You bring up a good point. What you say is true if the motor is using only speed control instead of position control. In position control the motor will keep trying to get to the commanded position, the integrator will windup, the armature will over heat if a fault doesn't shut down the motor.

Tharon said:

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?

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Yes, the drive WILL attempt to match the programmed Decel profile and essentially "fight" the external force.

Understand that Decel is almost the OPPOSITE of Braking, in that you use a controlled Decel when you want the deceleration to take LONGER than the natural coast to stop time. So if you set the Decel for 100 seconds, and the motor would coast to a stop in 50 seconds, the VFD will KEEP energy on the motor to PREVENT it from stopping faster than your Decel time. If you apply an external force to the motor to try to stop it, the drive will STILL keep trying to artificially extend the deceleration.

For most VFDs if you have that situation, you can program an input to disable the Decel when an external Brake is applied. So when you command the external Brake, you also signal the VFD to Coast so that it is not fighting you.

Peter Nachtwey said:

You bring up a good point. What you say is true if the motor is using only speed control instead of position control. In position control the motor will keep trying to get to the commanded position, the integrator will windup, the armature will over heat if a fault doesn't shut down the motor.

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absolutely. I've used lots of different setups on VFDs when it comes to movement and positioning. The point I made there was pretty much for the jokes sake, because of course it will drive through a given load if it's capable, and then trip out if the load is slowed down faster than the drive is trying to move it.

I think the OP just hasn't quite wrapped his head around that without some kind of Encoder/position feedback to the drive, the drive has no idea what the load is doing except for knowing that there is or is not some kind of force acting upon it.

In crane systems and other elevator type systems using VFD/encoder combos on regular motors (non servo). certain VFD can be set up to maintain their position while waiting for a mechanical brake to function, or simply hold the load suspended through gearing and "floating the load".

just like others, if we were to try and stop that load while it was decelerating, it would continue to want to control the load on it's own until stopped, unless there is some background programming with position feedback, I don't see the ability to manually stop a load and have a VFD respond accordingly by dropping it's output hz to match the physical braking action (ala, Robotic helper arms working next to people with touch safe operations enabled, where they will literally either slow or stop operations at the slightest change to their normal operations/loads)



but that's just my limited experience, I'm no guru on these.