REGENERATIVE ENERGY WHILE BRAKING BY VDF 2

[mash98]

Dear guys yesterday I have seen an equipment which sends back the regenerrative energy while braking through VFD back into the system, although it is very common in 4th quadrant dc drive application but strangely the same is not very popular in vfd.Can you guys tell me about the pros and cons of using such device especially when the energy price is going higher.

regards

 

[waross]

Initial cost and return on investment. A regenerative VFD is more expensive than a plain VFD and a lot of motor/VFD applications do not regenerate an appreciable amount of energy.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mash98]

Dear Waross the equipment I saw was a chinese cheaper version with one year warranty and the cost is roughly 400 us dollar suitable for a 5.5 kw inverter.The application I intent to use on long travel drive of overhead crane where currently we are using conventional dynamic resistor of( 2kw) for braking with a deceleration time of 1.5 sec and this braking is very frequent which raises the resistor temperature reasonably high.I want to use this unit on trial bases. First I will have to determine the current energy consumption for which I already have installed an hour meter and energy meter on this current application and record the energy consumption.Then we will have to measure the energy consumption on same amont of time with the equipment mentioned above and I think this way I will have some comparasion to decide to purchase such equipment on my other similar applications.Kindly give your valuable advice or comments.

 

[itsmoked]

A crane would be a typically good application for regenerative drives. "Cheap", however would not be a good thing when considering any VFD for crane service..

Keith Cress
kcress -

http://www.flaminsystems.com

 

[DickDV]

mash98, regenerative AC drives are not that uncommon. You just haven't seen them. The key reason for their existence is that they save more energy than the extra cost of the drive.

Their are other reasons. Maybe the heat thrown off from your present brake resistor adds heat to your building interior causing additional air conditioning load. Or, maybe there is a hazard associated with the having a red-hot resistor in the area.

It seems to me that the easiest way to estimate the value of the recovered energy is to put a voltage-time recorder on the existing brake resistor. Volts squared divided by the resistor ohms is watts. Divide by 1000 to get kw and multiple by the total time the brake is energized (in hours) to kwhr. That, times your cost of electric energy is the value of the braking energy.

Prepare to be disappointed on the savings! Unless your energy is very expensive, my experience is that a crane travel brake does not produce enough energy to make it pay.

 

[mash98]

Yes I am a bit unsure about the savings so I have put this thing up here to have your valueable comments.Yes cheap things should not be fit to cranes but as it is going to be fitted in long travel drive rather than hoisting so it is not that critical or hazard for any accident.
regards

 

[jraef]

In general, there are two methods of accomplishing "regenerative braking" with a VFD. One is "cheap", the other is more robust.

The cheaper way to do it is to just have a basic 6 step inverter connected to and triggered by the DC bus. When the DC bus voltage level goes higher than its design threshold, it begins firing into the line at a frequency fixed to match with (and synchronized to ) the line. This is essentially the same way your DB resistor system is working now, so the only difference is what is done with the excess energy. In the DB resistor system, it is burned off as waste heat, in the Regen system it is dumped back into the source. By the way, no need to go to China to get a cheap knock-off version with no history of usage, there is a company called Bonitron here in the US who has been making them for retrofit to existing VFDs for years.

http://www.bonitron.com/o-r-m3345.html

The more robust version is a lot more sophisticated. The difference is that instead of just a simplistic 6 step inverter with an output frequency fixed at the line frequency, it has another full blown PWM inverter on the front end. The advantage is, it can not only move excess power into the source, but can also detect and correct harmonics as well so that the VFD ends up cleaning up after itself (layman's version, it's a lot more complicated than it sounds). It also has the advantage of being able to accomplish regen at virtually any level of excess energy, not just when the DC bus voltage climbs, so it can affect braking even at low loads.

So the more robust versions are essentially two back-to-back complete VFDs; all they share is the DC bus section, and the commensurate cost is reflective of that. A fully regenerative AC VFD of this type is typically twice the price of one without it. But these are not typically a retrofit device; they are an option on some high-end VFDs that are built this way from the ground-up. ABB has been doing this for a long time very successfully, along with Siemens and a number of other major manufacturers. But if you already have a VFD, then this option would require replacing it and may be a cost burden.

A third method is what is called a "common DC bus" package where you have multiple AC motors, some of which will be regenerating while others are motoring. In this scheme, nothing goes back to the line, but is used by other connected loads and thus less energy is pulled fro the line, which also saves energy. but that doesn't apply to a single motor system such as yours.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[edison123]

Great post, Jeff.

 

[mash98]

Dear Jraef thanks for giving us so much valuable informations.
regards

 

[ScumPunk]

There is a third way, at least I don't recognise Jraef's first description as the same method....
The input bridge has an IGBT across each diode. This bridge is controlled so that the pair of diodes which are conducting at any one time have their complementary IGBT switched on. This means that if the DC bus goes "outside" of the mains, the power flow is back to the supply. No overvoltage trips, no expensive PWM control of the input bridge. The DC bus has a reduced capacitance (a few uF), so electrolytics don't dry out either. These drives are more expensive than conventional units, but the difference will certainly be much less than a "Full" AFE.
Cheers,
Mort.

 

[DickDV]

ScumPunk, which manufacturers are building VFD front ends that way?

 

[ScumPunk]

Hi, DickDV,
There are Siemens products which use this technology. The PM250 power modules from the Sinamics G120 range are the standard drive version, there are distributed versions for conveyors (G120D) and also inverters and motor starters for the ET200 range.
/end sales mode/
Cheers,
Mort

 

[DickDV]

Thx, ScumPunk. I'd not heard of this and will look into it further. Interesting!

 

[jraef]

Ah ha! So that's what they are doing. I even worked there (different division) and when they released that module for the G120's I asked what it was doing to regenerate, but they wouldn't (or couldn't) tell me.

Thanks.


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[ScumPunk]

I've seen a thesis describing the theory behind it, if I can find it online I'll post a link. The front end IGBT's are effectively switched at the line frequency (50-60Hz), by a hardware comparator-type circuit, and if you ask the IGBT module manufacturer nicely, they can even supply modules with big enough freewheel diodes to act as the rectifier diodes
Cheers,
Mort

 

[electricuwe]

If you want to understand the circuit Siemens uses in some newer products search for papers on "Fundamental Frequency Frontend", F³E.

 

[jraef]

Just FYI if you are searching using the text string, put a space between "front" and "end"


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[ozmosis]

A word of caution with PM250. You cannot use it where the supply impedance is high. So no line reactors and only suitable on a strong supply.
The lack of DC link capacitors creates a resonance level with high impedance supplies.
Apart from this, it is a good technology. Simple and relatively low cost compared to active front ends.