Handling Regenerative LOads in VFD's 6

[DAS99]

I am a mechanical engineer with an application using a large counterbalanced oscillating conveyor running at up to 420 Cycles per minute. We currently use Allen Bradley VFD for the 25HP drive but it trips out on DC Bus overload after maybe 30 seconds

On advice from the conveyor manufacturer, we tried a 40HP VFD on the 25HP motor but experienced the same overload. Our solution has been to oversize the VFD and include a dynamic braking resistor to absorb the regeneration loads, although this has been a successful solution, it is expensive and is a nuisance as we have to mount the resistor external to the control panel due to the heat generated, and, the resistors are only available in an open enclosure for heat dissipation which is not suitable for all areas we locate our equipment



Can anyone advise if there is a more suitable VFD than Allen Bradley for this application, and if not whether there is an alternative economic solution to the braking resistor.

REgards

David

 

[Skogsgurra]

I think that the problem is the "counterbalance". If you reduce the counterbalance so that the inverter never needs to brake but always to supply power to the conveyor then you can avoid the need for the braking resistor.

Or shorter: It is better to supply power to the conveyor all the time than to brake the power away in an external resistor part of the time. And I think that you can have that situation if you make the counterbalance smaller.

Or I could be entirely mistaken - especially if the ratio between "up" and "down" is high. But I think it is worth considering.

Solutions with regeneration are usually not economical for periodic or intermittent loads at low power levels. They also have tripping problems when connected to grids with transients.

 

[ozmosis]

I don't think the problem is the type of drive, it is the management of the energy. Other makes of drives would have the same issues as A-B and similar costs I would say. As you have already invested in a larger A-B drive, changing to another one would just incur more cost.
As skogsgurra indicates, a further investigation on the actual application may be more beneficial. Also, I would tend to consult the drive supplier rather than the conveyor manufacturer for a better solution.
It may also depending on the type of AB drive you have. Some may have improved control of the DC link that could modify automatically the deceleration time or dump some of the overhauling load energy internally in the drive to keep the volts dc down. I don't work for AB but I know their applications dept is pretty good and their on-line forums have some good information regarding applications out of the norm.

 

[cbarn24050]

Hi, an alternative option is a DC motor with a 4 quadrant drive. More expensive and more maintainance but all the regen power is returned to the mains supply so you will save on electricity costs. You would need to get your calculator out.

 

[jraef]

DAS99,
Yes there are some VFDs that do specifically have solutions for this. One is PDL Electronics out of New Zealand in their Elite Series drives (also available from Motortronics as their ALX Series in the US). It has an application macro available that allows it to essentially detect the oscillating load and alter the output frequency pattern to avoid the regeneration in the first place. It is used on vibrating feeders for the rock crushing industry all of the time. Toshiba G5 drives also had this feature, unfortunately Toshiba has started switching people to their newer series which does not have it any longer.

"Venditori de oleum-vipera non vigere excordis populi"

 

[DickDV]

The problem is simply that there is a little braking energy in every cycle of the vibratory crankshaft or cam. Most drives have processors that do not detect the top of the cycle fast enough and end up motoring too far past top center. The result is that the drive has to brake hard to get back to proper speed and therein lies the problem. That the AB drive operates properly with the addition of a brake module is clear evidence of this.

The solution is a drive with very fast processing capability. The ABB ACS800 is, in my prejudiced view, the best in the field for this type of application. I use it frequently for stamping press drives without any additional braking hardware and also on printing presses that have an oscillating pressure plate or platen. Both of these have identical motoring/braking profiles and I have complete confidence that this type of cyclical regen will not be a problem.

But, don't trust me. Get your local ABB distributor or rep to get a properly sized ACS800 demo drive and try it. I strongly suspect that you won't let the test unit leave your plant once you see it run.

 

[Marke]

Hello DAS99

There is a lot of good information in the above posts, but there is one other option to consider.
Some drive manufacturers manufacture active front end drives which will dump regenerated energy back into the supply rather than into a braking resistor. This eleminates the heat problem, but is more expensive.

There should not be a need to oversize the drive unless there is a considerable period of time operating at greater than 100% shaft torque.

Increasing the drive size will not reduce the trips due to regenerated energy.

You must either use the drive to track the motor speed and reduce the regeneration, (some drives have a volage limit that will increase the drive frequency,) or dispose of the regenerated energy. This is achieved by the use of a braking resistor, or active front end.
To increase the braking power, change the braking resistor.

Best regards,

Mark Empson

http://www.lmphotonics.com