Regenerative Braking

If you want something to stop quickly as the motor does on most saws. To reduce possible injury.

To use the generated current as a electric fork lift does. It uses the generated current by placing some charge back on the battery. Extending run time.

It's hidden from Anonymous Posters.

Use Google.

Regenerative brake - Wikipedia, the free encyclopedia

The justification comes through energy savings.

Regenerative Braking is a method of recovering kinetic and gravitational energy that is normally lost in friction braking.

Imagine a flywheel attached to the shaft of an induction motor. As the motor is spun up the electrical energy (power * Time) is converted to the rotational energy of the flywheel (.5iw^2).

If you were to brake this flywheel using friction braking the energy would be converted to thermal energy in the brake pads and dissipated in the surrounding air. In this case it would approximate the coefficient of dynamic friction between the brake pad or shoe and the rotor X the applied force X translational displacement of the pad or shoe along the rotor. In rotational terms it would approximate the braking torque times the angular displacement.

In transportation processes where the prime mover is a heat engine, such as a conventional reciprocating engine, the total throughput efficiencies probably do not exceed 30 percent. That is, it takes about 3.3 units of combustion energy to perform one unit of applied work.

It then follows that for each unit of kinetic and gravitational energy that can recovered through the process of regenerative baking then 3.3 units of combustion energy is saved. In transportation or other processes that include numerous stop and go or lift and lower cycles it offers a significant energy savings.

Here are some links to an article that explains "The Three Fundamental Efficiencies of Hybrid Technology."

http://www.bestsyndication.com/Articles/2006/c/carter_mark/031206_hybrid_cars.htm

or

http://cr4.globalspec.com/thread/37460/The-Three-Fundamental-Efficiencies-of-Hybrid-Technology

Or as applied to a switching locomotive.

http://myprogressiverailroading.com/myprogressiverailroading_blogs/b/gavilan/archive/2009/07/26/hybrid-switch-engines-when-will-we-see-the-adaption-of-the-basic-principles-of-efficiency.aspx

Braking involves the transfer of energy, the kinetic energy energy contained in the mass of a moving object into something else so that the object stops moving.

In the case of an induction motor, braking involves transmuting the kinetic energy in the rotating load into some other form of energy, i.e. heat, or electrical energy. When using VFDs on induction motors, you have several options for doing this.

DC Injection and Flux Braking transmute the converted energy of the moving load directly back into the motor as heat. That has negative effects on the motor if done too much.

Dynamic Braking transmutes the energy into heat by turning the spinning motor into an induction generator, pulling the energy off as electricity, then dumping it into a resistor to be burned off as heat. It is very inexpensive to do this and although it cannot "finish the job" because the braking power is dynamically reducing as the load slows, you can finish it up with DCI or Flux braking when there is very little left, so the motor is not stressed. However if you must provide braking continuously, or at least beyond the ability of the resistors to dissipate the radiant heat, then the resistors can fail.

Regenerative braking starts the same way as Dynamic Braking by turning the induction motor into an induction generator, but instead of transmuting the energy again into heat, it directly pumps the electricity back into the source at the source line frequency. To accomplish this, a VFD must have what is called an "Active Front End" because instead of just passively rectifying AC to DC for use in motoring, it now has to re-invert the excess DC energy coming off the motor into AC again and push it back into the Line. That means the VFD is in essence, TWO VFDs; one for motoring, one for braking.

To justify the significant (2X) cost increase, you must need to accomplish braking continuously or repeatedly at a high duty cycle, where Dynamic Braking would not survive. Attempts to justify it based on energy savings are often misrepresented. You cannot "spin your meter backwards" and sell that energy back to the utility in most cases, because just like with Solar Grid Tie Inverters, there must be what is called "Net Metering Agreement" with your utility, i.e. meters that read in reverse. A Net Metering Agreement will always come with requirements for significant protection systems above and beyond what normally comes with a VFD. For one thing, this is to prevent a regenerating motor from doing so after a power failure so as to avoid killing utility line workers. 99.99% of the Active Front End VFDs being sold based on this energy savings principle are NOT inclusive of the necessary protections to actually get a direct revenue reduction from the utility for braking your motor. If however you have additional loads that can use the energy from YOUR side of the meter, then it can reduce the amount of energy those loads draw WHILE the motor is braking. But in general the amount of kWh saved is grossly over stated. You cannot recover any more energy in braking a load than it took to accelerate it, often only seconds worth.

The real justification for using Regen Braking is what I stated first; continuous or high duty cycle braking needs that would be too much for less expensive options.

As JR said, to dissipate the energy produced by braking a high inertia load, dynamic braking just isn't practical.

I was faced with a little problem, a 40T centrifuge that had to start and stop up to 15 times per hour. By using regeneration the drive would be pushing 110KW back in to the distribution system when braking. 500A @ 220V DC takes a bit of getting rid of any other way.
I did alter the system later to increase the braking time and so reduce the mechanical shock on the drive train, but there was still the same energy to dissipate all be it over a longer period.

TonyS:

"I was faced with a little problem, a 40T centrifuge that had to start and stop up to 15 times per hour. By using regeneration the drive would be pushing 110KW back in to the distribution system when braking. 500A @ 220V DC takes a bit of getting rid of any other way.
I did alter the system later to increase the braking time and so reduce the mechanical shock on the drive train, but there was still the same energy to dissipate all be it over a longer period."

Wow, what an interesting challenge!! This deserves a thread of its own. I wonder how many different approaches it would generate?

One day I'll get around to writing it up.

As it is, the plant is now a brown-field building site.

The principle of extended braking is now standard in nearly every large diameter ductile iron pipe casting machine throughout the world.
I'll admit, my first trials were a disaster. Imagine a washing machine full of molten iron, it's the nearest I can get to describing the trial runs.