What is regenerative energy?

What is regenerative energy

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What is regenerative energy? Where does braking energy go?

Regenerative energy is electrical energy produced when a moving load drives the motor. During a crane's controlled descent or the deceleration of a high-inertia machine, the motor can act as a generator. The resulting energy first reaches the drive's DC bus. What happens next depends on the equipment: it may be turned into heat in a resistor, used by another drive on the same DC system, or returned to the supply through suitable regenerative hardware. Generating electricity while braking does not, by itself, mean that the system recovers it.

When does a motor act as a generator?

In normal motoring, a variable frequency drive (VFD) sends energy from the supply to the motor. When a load drives the shaft and the motor produces braking torque, power can flow the other way: moving load → motor → drive DC bus. A suspended load turning a hoist drum, or a large rotating mass slowing under control, are examples. The distinction between speed and torque control helps explain why a deceleration command alone is not an energy recovery system.

Why does the DC bus voltage rise?

Power generated by the motor flows through the inverter stage into the DC bus. If the energy has nowhere else to go, the bus capacitor voltage rises. The drive may limit braking or trip to protect itself. The diode rectifier in many standard VFDs allows power to travel from the AC supply to the DC bus, but cannot send power back to the grid on its own. A very short deceleration ramp can make this particularly apparent under a heavy load; changing the ramp does not make the load's energy disappear.

Three destinations for braking energy

1. Braking resistor → heat. A braking chopper sends energy from the DC bus to a suitable braking resistor when the bus reaches the relevant threshold. This helps the drive brake, but does not recover the energy as electricity. Read about the mechanism under dynamic braking.

2. Common DC bus → another motor. In an appropriately designed multi-drive system, another drive motoring at the same time can use energy supplied by the braking drive. This is on-site reuse; it is different from sending energy back to the grid.

3. Regenerative unit / active front end (AFE) → supply. Suitable drive and grid-interface equipment can return energy to the AC side in a controlled way. The required equipment, protection, connection and permission conditions must be assessed for the project. Adding an ordinary braking resistor does not turn a VFD into a regenerative drive.

With DC injection braking, the drive applies DC current to the motor windings to produce braking torque; much of the heat can be generated inside the motor. It is not a way to export DC-bus energy back to the grid.

Does every elevator or crane descent regenerate?

With an elevator drive, the direction in which regeneration occurs depends on the car load and counterweight. A heavily loaded car descending, or a lightly loaded car rising, may drive the motor depending on the balance: not every descent regenerates. An energy estimate must consider the net unbalanced mass and losses, rather than using the car's full mass without accounting for the counterweight and other moving parts.

For crane drives, lowering a load and repeatedly accelerating and decelerating can present recovery opportunities. The mechanical brake that holds a load safely serves a different purpose from electrical braking by the motor. Safe Torque Off (STO) neither recovers energy nor mechanically holds a suspended load. Follow the manufacturer's brake sequence and the machine's safety design.

Worked example: lowering one tonne by ten metres

Consider an example hoist without a counterweight. Lowering a 1,000 kg load by 10 m releases about E = m × g × h = 1,000 × 9.81 × 10 = 98,100 J ≈ 0.027 kWh of gravitational potential energy. That is a theoretical upper limit; motor, gearbox, drive and regeneration losses leave less energy available for export to the supply. With resistor braking, the amount of electrical energy returned to the grid is zero, and much of the braking energy is dissipated as heat. For an elevator, using the full load in this equation without accounting for its counterweight would be misleading.

For a rotating machine without vertical motion, use a different estimate: ΔE = ½ × J × (ω_initial² − ω_final²). Here J is the total rotational inertia referred to the motor shaft, and ω is angular speed in rad/s. For a centrifuge or winder, examine the inertia and change in speed rather than mass times height.

Where does the energy go in an electric vehicle?

During regenerative braking, an EV's motor can act as a generator and send energy to the battery when conditions allow. That does not mean an industrial VFD can send energy to the grid; the two systems have different destinations for the power. Regeneration may also be limited when an EV battery is cold or already full. The actual electrical architecture determines where energy can go.

When is a regenerative system worthwhile?

Alongside the energy in one braking event, consider the number of events per day, braking time, peak braking power, and whether the recovered energy can actually be used at that moment. A shared DC bus needs a destination for excess energy when no other motor is consuming it. Grid regeneration calls for suitable drive and input-stage equipment as well as a check of site connection conditions. Motor and VFD losses rule out 100% recovery. When assessing a solution, consider the basic VFD selection parameters, duty cycle and manufacturer-rated braking capacity together, rather than relying on the motor's power rating alone.

Editor's field tip

When someone says a system has a “regenerative drive”, ask one question first: Where does the braking energy go: through a chopper and resistor, to another drive on a common DC bus, or back to the supply through a regenerative unit? A resistor in the cabinet usually means the relevant energy is being dissipated as heat. Base a savings estimate on the documented energy path, measured braking cycle and operating hours; avoid assigning a savings percentage solely because “the elevator generates electricity while descending”.

Frequently asked questions

Can every VFD return energy to the grid?
No. Power cannot flow back through a standard diode input on its own; suitable regenerative equipment is required.

Does a braking resistor save the braking energy?
No. It converts electrical braking energy into heat to help control DC bus voltage.

Why does a drive report DC bus overvoltage while braking?
The motor may be feeding energy into the bus faster than it can leave. Check the ramp, duty cycle, chopper and resistor, and any regenerative arrangement against the manufacturer's instructions.

Does an elevator regenerate on every downward trip?
No. Car load, counterweight, direction and losses determine the operating condition.