VFD and Stopping Distance 1

[Servo54]

I have a 20Ft conveyor moving "feathers"(On a conveyer belt that has a low coefficient of friction).This Motor has to stop right away with no coasting at all.This Belt is stopped by a sensor that in my opinion is to close to the 4th part ready to be picked.When this happens the 5th part is seen by the sensor a quarter of a inch away from the 4th part yet it shingles onto the 4th part by the time the belt stops.
We changed out the DC brush motor with a VFD controller and Motor set up.I set the brake to Ramp and set the Decel on 1/10 sec(Lowest I could go)
This VFD Motor has not improved are situation.Would Dynamic Braking help with such a light load??(This conveyor is ultra light with 1/2HP AC Motor)
Personally I think part of the problem with the Delay in Stopping(Some swear we are coasting) is related to The PLC Recieving that input from the sensor and sending back the output to stop.
Are we going in the wrong direction here??
Thanks,
Servo54

 

[itsmoked]

If it has stop 'right now' possibly you should use a brake instead.
It could be mechanical, electromechanical, or wholly electrical(eddy current). Then the motor goes off, brake goes on, belt stops,(now). Or let the motor attempt to brake but apply the brake too.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Servo54]

When I first ordered this VFD I was under the impression it would have dynamic braking(Which we thought was the solution).After receiving the drive we found out the Dyn braking was a add on option.After contacting the sales rep who contacted a engineering who was not on site at our plant to see the application mind you.The engineeer had told our sales rep that with such a light load Dynamic braking was not going to help."Is this true??I have come up empty handed searching for this on the web.
Thanks,
Servo54

 

[itsmoked]

I am not a drive guy like some people around here so take this with a grain of salt.

I would expect dynamic braking for larger heavier loads like loaded, inclined, conveyor belts.

I was also under the impression that dynamic braking is built into most drives but that it requires the addition of external resistors as the drive cannot just swallow a bunch of kinetic energy.

It also sounds like you procured a scalar VFD and you should have gotten a vector VFD as they can be used for positioning as opposed 'speed' control.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Dynamic braking with VFDs is the transfer of kinetic energy from the moving mass back into the motor as magnetic flux, which is converted back into electrical energy in the VFD and then converted into something else, usually heat in a resistor. You are asking a lot of a VFD to do all that in 100msec. The magnetic field in the AC motor can take 1/2 that long just to collapse when you cut power to the stator. A quick and dirty rule-of-thumb is that you cannot stop a motor electronically any faster than it takes to start across-the-line. So if when you started it X-Line it took 1 second to get to full speed, the fastest you can expect to stop it will be 1 second. It just takes a certain amount of time to create magnetic flux in an inductive environment. Your VFD supplier or his tech support people should have been aware of that.

itsmoked is right, the best way for you to accomplish that kind of stopping speed is with a mechanical brake.

http:/

http://www.jraef.com

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[itsmoked]

Consider something like this.

http://www.idicb.com/electmag.htm

http://www.idicb.com/mtrbrake.htm

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[LionelHutz]

You likely are coasting. By using the lowest time possible the drive will ramp the frequency down really quickly and then shut off the output. However, the motor won't follow the drive because it can't decel that quickly so it ends up coasting.

Try increasing the decel time a bit at a time until you find the quickest decel possible. If there is a low frequency voltage boost setting then try turning it up a bit to get more torque as the motor slows down.

If there is DC braking you could try it too. Try setting the drive up to DC inject for a short period of time at 0 speed after the drive has finished the decel. Then, the motor may not keep up with the drive but the DC comes on and finishes the deceleration.

 

[ozmosis]

I would have thought that if the drive is not stopping the load in the time set that it would trip on over-voltage. I would double check your method of stop command in the VFD. Most drives have the choice of either decelerated stop (controlled) or coasting stop (uncontrolled) and if it is a controlled stop and the deceleration time is accepted by the ramp parameter, the only reason it should not stop it (normally) is if the regenerated energy is too much (as described by jraef), the DC link voltage rises very quickly and the drive trips on overvoltage.
If your drive is tripping, then this will point you towards other methods of stopping. If the VFD has a brake chopper (additional transistor on the DC link) to dump the regen energy then you can fit resistors to absorb this and get hot. If no brake chopper is included (not all VFDs have them)then you may need to look at getting an external one or going the route of mechnical brake on the motor.

 

[CJCPE]

Servo54:

Could you quantify your requirements? What is the speed of the conveyor? How far can it be allowed to move after a stop command is received?

 

[DickDV]

DC braking is a poor choice because the stopping distance will vary depending on load inertia.

Ramp to stop is the controlled and predictable stop but only if the drive anti-trip provisions are turned off for High DC Bus. With this function left operating, rather than fault, the drive alters the decel ramp. The result is an unpredictable stop time and stop distance.

In order to use ramp-to-stop, you simply have to have enough snubbing capacity on the DC bus to avoid High DC Bus faulting.

Actually, with the stop times being discussed, a mechanical brake piloted by the drive Run/Stop relay seems a better choice, based on the limited knowledge I have of the application.

 

[waross]

I see two distinct and seperate issues here.
1. We don't know how long the PLC is taking to procwess a stop command.
2. We want to stop the motor very fast, but we are not sure how fast.
I suggest first checking the time it takes for the stop signal to get to the VFD. If the processing time is excessive, no amount of braking will be sufficient.
I assume that an output on the PLC is connected to an input on the VFD. Connect a suitable light bulb to this PLC output.
Connect another light bulb to the input of the PLC that iniciates the stop sequence. It should be readily evident by observation if there is an excessive signal processing delay.
The answer to processing delay problems is to arrange the circuit so that the detector interfaces directly with the motor controller. That is, bypassing the PLC.
If braking is still needed after it is determined that there are no significant delays in signal processing time, then you may be well to consider a clutch-break assembly to control your conveyor and leave the motor running. To this end, you may find a solution on the website suggested by itsmoked.
respectfully.

 

[DickDV]

Very good points, waross.

 

[cbarn24050]

too bad you changed the dc motor, a 4 quadrant drive would have given you good braking.

 

[marbr2005]

Precise automatic positioning control would require a closed loop control that includes encoder, regenerative braking or snubber resistors, vector control for the VFD.
A cheap suggestion would be a slow-down sensor changing VFD speed reference, a stopping sensor and a mechanical brake.
Reduces the average speed but gets better positioning.