VFD Regeneration Explained: Everything You Need to Know

At its most basic, an electric motor is essentially an energy conversion device. It takes power from a source, whether AC or DC, and converts it into a mechanical torque to drive a load. But did you know that power in an electric motor is bi-directional? The motor’s torque can be converted into electrical energy and sent back to the VFD controlling the motor; this is called power regeneration.

Below, we’ll look at the basics of a standard VFD versus a regenerative drive, what causes regeneration to occur, how to mitigate any issues caused by the excess energy in the circuit, and how regeneration could be saving you money.

Basic Principles of a Standard VFD

Your standard VFD works in three stages.

1. Rectifier Stage – The input rectifiers allow the AC current provided by the grid to flow in, and they convert the power from AC to DC. The flow of energy is uni-directional here.
2. DC bus – A capacitor bank acts as a buffer to the energy flowing through the circuit, helping to smooth the rectified power.
3. Transistor Stage – The output insulated-gate bipolar transistors (IGBTs) act as an electronic switch. Using pulse width modulation, the IGBTs switch the DC power on and off to create a highly controlled AC current.

Current can flow from the ac power sourcer into the VFD but cannot flow back into the grid with a standard VFD. However, the flow of current at the transistor stage is bi-directional as AC power alternates directions. This means that power generated by the motor and sent back to the VFD can flow into the device but cannot flow back to the power grid.

Basic Principles of a Regenerative Drive

While similar to the standard VFD, regenerative drives replace the input rectifiers with IGBTs. These IGBTs act as the input rectifiers but provide the opportunity for bi-directional power flow. So, when the motor generates power, it flows into the ac driveand then to the grid. When this generated power can flow back to the power grid, it reduces the overall energy consumption of your application and lowers total energy costs.

What Causes “Regeneration”

In an AC induction motor system, certain situations cause the motor’s rotor to spin faster than the winding frequency, sustaining a magnetic field that generates power back into the line. Let’s take a look at an elevator as an example to understand further.

In an elevator system, power is delivered to the VFD and flows out to the motor. The motor takes the electrical energy and converts it into a rotational torque that lifts the elevator. As the elevator gets higher, so to does the gravitational force. When the elevator is lowered, the gravitational force produces an extra load on the motor’s shaft, causing the rotor to spin faster than the winding’s frequency and turning the motor into a generator. The generated power is sent back towards the VFD.

Alternatively, when you shut off power to the motor, the rotor continues to spin as it gradually slows down. Power is generated by the still rotating rotor and is sent back to the VFD. In a regenerative drive, the power is put back into the grid to help reduce energy costs.

VFD Regenerative Applications

1. Machines that lift and lower loads, such as hoists, cranes, and elevators.
2. Applications that require highly controlled torque, such as web handing systems, test stands, and tension unwinders.
3. Cyclical applications like centrifuges for larger masses that need to start and stop quickly.

What if You Don’t Have a Regenerative Drive?

As noted above, the standard VFD’s rectifier stage only allows for uni-directional flow. As such, when power generation occurs, there is nowhere for the excess energy to flow. Instead, the energy will continue to build up within the DC bus capacitor bank until an overvoltage failure occurs. Beyond regenerative drives, there are a couple of other ways to deal with the excess energy in the circuit; braking resistors or VFD systems in DC bus connection.

Braking Resistors

Braking resistors are a brake chopper circuit and a resistor together in series, used to help dissipate excess energy from a system. When the DC bus stage reaches a specific voltage threshold, the brake circuit closes, sending the current over a large external resistor that turns the electrical energy into heat. As the operator, you must take care to help dissipate this heat to avoid any failures that it can cause.

Some drives come standard with a braking resistor, and some need to have an external one added to the circuit.

VFD Systems

Two or more standard VFDs can be connected together in a circuit, connecting the individual VFDs’ DC bus stage. Putting the connection at the DC bus stage creates a common DC bus that allows the two separate VFDs to share power. So, when regeneration occurs in VFD, the motoring VFD can pull power from the DC bus rather than from the grid.

The power here is never returned to the grid; it is shared between the VFDs controlling different motor systems.

Summary

As an energy conversion device, a motor generally takes electrical power and turns it into mechanical power in the form of rotational torque to move a load. But there are situations where the motor can take mechanical torque and turn it into electrical energy, sending it back to the VFD. This power regeneration is optimal for a few different applications, but it benefits you most with its energy-saving ability. When power can be sent back to the grid or shared through a DC bus connection, your total energy consumption is lowered, creating cost savings for your business.

Curious about how VFD regeneration could help your business? Get in contact with one of our technical experts today.

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