Quadrant Operation of Drive?

using regenerative braking drives can generate energy!
how is this practically incoperate this arrangement? i am using rexroth frequency converter fe on a downslope conveyor.

Some drives may have the capability of regen but I have not seen one. Usually a "Regen" unit is required for the drive line up. have a read.... http://www.bonitron.com/m3713dm.html

Please advise if this is an AC or DC system. The term regen braking is applicable to both types of motors.

QUOTE rexroth frequency converter UNQUOTE

The term above implies AC and VFD to me.
What is being conveyed and does load change much?

Need clarify on whether AC or DC and size of motor and variations in load ie conveyed material.

Would also be nice to know what you have in mind ie where the power generated is going to go?

Dan Bentler

Yup, you're right Dan. So, its AC.

In order to regenerate or save the braking energy, an extra inverter is necessary which is facing backward. Or, to say it another way, it gets its input from the DC bus and inverts that excess energy back into the AC supply synchronously at 60hz.

It only operates when the DC bus voltage rises higher than a set limit. A DC bus voltage higher than normal indicates that the motor is being overhauled or pushed turning it into a generator and sending power backward to the drive. The drive's output IGBT's would not normally pass this energy back to the DC bus but the presence of flyback diodes in parallel with the IGBT's makes the power show up on the DC bus as rising voltage (the capacitors are being charged up with the excess energy).

Dick
Rough numbers /estimate only please. What is comparative cost of VFD with braking resistors versus the "backward inverter" to feed power back to AC line? My guess would be approx 50 to 75% more??

PRIT Another way to take advantage of power generated by dynamic braking is to use braking ressistors. Heat from resistors could be used to heat something else ie office etc etc. Would have to allow for surges and have a heat sink you could always dump the resistor heat to would be main consideraton / drawback.

Another way is to tie two VFD DC busses together. One when doing dynamic braking generates to DC bus which feeds the other motor(s??) DC buses. This would be ideal when one motor is braking "generating" and other motor is driving ie "motoring". Matching size and ensuring one motor is motoring I think would be the main considerations here. Also what if one unit breaks down?

Dan Bentler

Dan

Sounds like DickDv is speaking of a active front end or AFE unit which is the 3 phase to dc power supply and has the required components to push power back to the ac side.

We have many of them at my facility and they are arounf 80% -90% more expensive than having a 3 phase to dc power supply with a braking chopper and central braking resistor in place to burn off the excesss regen power.

We have both setups. Most of ours are 10 - 20 drives on the bus so the chopper and brake module does not get cycled much because in our setups 5 large motors are almost always in regen to some degree while the other 5-15 motors are motoring or pulling a load and use up the regen power made by the 5 motors in regen.

Dan

What you describe in post 6 is pretty much what we have on many lines just 1 large buss with all drives coming off of it.

1 ac/dc inverter / power supply and 1 chopper module connectedto 1 large resistor.

But rarely does the resistor cycle. A very fast change of regen power like a large sudden increase can cause problems with this setup though as the chopper module can not cycle that fast in many cases and that is why the solid state nature of the AFE is also better because it reacts so much faster in addition to saving energy.

Kid

Thanks those numbers are just what I was looking for. Thanks also for corroborating what I said

Dan Bentler

A link to the components we use http://ab.rockwellautomation.com/Drives/Common-Bus-Power-Supplies

That 80-90% premium is drifting down but its still well over 50% if you look at parts price only. Adding a brake chopper and brake resistor to a VFD involves some labor and space so the installed cost of the regen unit (all self-contained) is not quite that much more. But still in the 40-50+% range.

Dick

90% was being conservative but my numbers are based on allen bradley hardware and that makes a lot of difference. AB is known for being a lot of things but low cost is not one of them.

For certain setups you could use one rectifier unit, a common DC bus and inverter units. If one drive is breaking, the 'spare energy' is feeded to the other drives from the DC bus.

Kalle

You are right, PLC Kid. AB always gets their price and then some. And that applies for any service or support you might need too. But the product is usually solid and their ability to communicate is superior.