Regenerative Braking

Start with the Wikipedia article on regenerative braking.

Kiran: to date, with your 8 posts you have started 6 threads, and only posted a reply on one of them.

You will probably find that you will get better responses in future if you spend the few moments it takes to provide a bit of follow-up.

You are in danger of being ignored, like the child who repeatedly asks "why?" without really listening to the answers.

Hey John,

Thanks for your reply. I have joined CR4 just 1 months back. I found that this site is really well as some experts are replying extremely well. Some times I try to find the answer by studying lots of books but not getting it But this is really a nice way to get the answer here in short time from you guys.

Thanks and Regards

Kiran

I think, in regenerative mode, it is not the IGBTs of the inverter that would conduct, but the free-wheeling diodes connected across these IGBTs. These diodes would rectify the regenerative AC voltage into DC and feed into the DC Bus. Then by connecting a resistor (DBR) this regenerative energy is dissipated.

Hmmm... It's really quite complex but in a nutshell, its all relative!
Let's see if I can relate this in a condensed form. .

In order for an AC motor to regenerate, the windings need to be excited AND the speed of the motor must be greater than the frequency being applied to it. So normally (as in no VFD in use), a motor only regenerates as long as the load is "over hauling", spinning the motor faster than the synchronous speed as determined by the line frequency (and number of poles). So you cannot really have regenerative braking in that instance, because as soon as it works to feed power back onto the line, the motor slows and no longer generates. But people use this concept all the time as a form of load retardation, i.e. a motor put on a down-hill conveyor to keep it from running away due to the weight of the material on the belt.

Now add the VFD, which is re-creating a relative line frequency, and can do so on command. As the motor slows when a Brake command is given, the VFD lowers its output frequency while monitoring the motor speed, so the frequency of the power applied to the motor is always relatively lower than the relative synchronous speed of the motor, keeping it in a regenerative condition. Then, as mentioned, the free-wheeling diodes on the output transistors become a rectifier and move that energy off of the motor windings back into the DC bus. All modern PWM VFDs are capable of this by the way, it's inherent in the design. Most of the time a VFD can store and dissipate a small amount of that energy, larger amounts are often dumped off into a resistor bank and burned off as waste heat. The thing that makes a regen VFD different is in what is done with that energy.

After the DC bus charges to capacity, a sensor on the bus then uses transistors in the front-end of the VFD to re-fire that energy back into the line as AC (just like it does for the motor) at the same synchronized frequency as the line. So a Regenerative VFD must have transistors on the converter side in order to accomplish this. That is what sets them apart from a non-regen VFD. They are essentially two back-to-back VFDs from a hardware standpoint, although they don't need to vary the regen frequency and they do need line sensing to synchronize when regenerating, which is not otherwise necessary on simpler VFDs.

So Regenerative VFDs are usually around twice the cost of non-regen versions of the same VFD, and the option is usually only offered on top-of-the-line products because of the necessary added processing power in the controllers. As time goes on we will likely see a shift towards that cost difference narrowing as processing power increases and power device mfrs start integrating the components onto monolithic combination units. Regen drives are starting to catch on as a way of recovering excess energy that has heretofore been wasted in long spin-downs, braking resistors or friction devices. The elevator industry is chasing this concept big time. If you think about it, EVERY TIME and elevator goes up, it has to come back down. If you could recover even half of the energy it took to lift it, you can cut your operating costs dramatically.

Hello JRaef,

Thanks for your reply. This is really useful answer.

To tha same answer i would like to ask you again as follows.....

Are these transistors in Converter section fires in both direction ? OR what exactly is happening? because there is 2 condition... 1. It converter converts AC to DC in normal condition. 2. It also acts as inverter during regeneration(DC to AC). Actually what is happening .. i emagine that may be firing sequence of transistors are different in both cases !!!!

Regards

Kiran