Power Regeneration Back to Grid

If you have a VFD that is capable of absorbing regenerative power, it is probably not going back out onto the power grid.

VFD's usually convert AC power to DC power. Then the output signal is formed (usually from a preset data table using a microprocessor of some type) and amplified to the appropriate voltage using power stored in the DC power supply.

A regenerative device will simply dump power back into the DC supply rather than take power from the AC line, while it is available. So, the overall power draw is reduced. But, there may be exceptions to this design available. A lot is based on your specific application and capacity of the equipment or motor providing the energy stored in some form of kinetic energy or inertia.

First you have to know if you have what is called a "Regenerative" AC VFD, then you need to know what KIND of Regenerative (Regen) drive it is. The reply by NotYourOrdinaryJoe highlights the fact that the simple term "Regenerative" is not in and of itself an adequate description. There are Regen drives that only regen back into the DC bus as he said, so that the power can be utilized by other motoring drives connected to the same DC bus. These are however not inherently capable of putting the power back onto the grid.

But there are fully LINE REGENREATIVE AC drives now. What they do is essentiually use two back-to-back inverters so that once the regen energy gets to the DC bus, the front-end inverter then becomes a line inverter at a frequency fixed at and synchronized with the line source. So yes, that (but only that) version would put the energy back into the grid. If you do not have a full Line Regen capable VFD, you are NOT putting any power back into the grid. A quick way to tell on a Dynomometer is to look for a resistor grid. if you have one, that means it is burning off the motor regen energy into resistors that are connected to the DC bus, which means it is NOT a Line Regen drive.

As to measuring the AMOUNT of energy put back in, most of the fully Line Regenerative VFDs I have seen will come with kW and kWh metering. It's a bit tricky to measure power with high harmonic content, but the VFD will need to do most of it anyway for internal use, so displaying it to the user is a simple task at that point.

If you post the make and model of the VFD I can tell you in a heartbeat if it is line regen or not. There are not many that are.

Just so you know, a modern PWM AC drive works by converting the AC supply to DC with a rectifier (converter section), smooths the DC, then uses the IGBTs (inverter section) to recreate "psuedo AC" using PWM, Pulse Width Modulation. The inverter also includes flyback diodes that will act as a rectifier to allow regenerated energy from the load side to go back as DC to the bus, but it stays there and builds up untill one of 4 things happens:

1) the VFD trips on High DC Bus Voltage,

2) a Chopper circuit fires the excess energy into a resistor bank and burns it off as heat (called Dynamic Braking),

3) other inverters that are connected to that DC bus use it to power other motors, or

4) a regen front end was supplied that can put the energy back onto the line (grid) as I explained above.

But in all cases, DC is involved as an intermediary stage in the VFD process.