Regenerative energy

An electric motor can act as a generator when gravity effects or deceleration drive the motor in generator mode and power is injected into the power line. The amount of regenerated energy is relative to the mechanical energy. Higher deceleration and higher inertia can cause more regenerated power. The regenerated energy can cause overvoltage on the DC link which poses a dangerous situation for all equipment on the shared bus. It is essential to prevent overvoltage for the system to work correctly. The regenerative energy can be either absorbed by a battery (e.g. in an AGV) or burnt by resistors (braking chopper or shunt).

Estimation of regenerated energy

The regenerated energy is related to the mechanical energy. Load, gravity and deceleration are important parameters to define the amount of regenerated energy:

with:

To estimate the DC bus capacitor and braking resistor:

  1. Calculate the amount of regenerated energy.

  2. Calculate the maximum capacitor using the overvoltage protection level.

  3. Calculate the dissipated energy.

  4. Calculate the amount of power that needs to be dissipated by the braking chopper.

Restoring or dissipating regenerated power

Restoring or dissipating regenerated power is a critical point in many applications where machines feed energy back to the DC bus.
It is easily possible to restore the regenerated power when batteries are used as a power supply or big capacitors can be used. This is an energy-efficient and cost-effective approach, but the component cost may outweigh the benefits.

Otherwise, it is essential to protect the electrical part against overvoltage by dissipating the energy with resistors.
Ideally, the DC bus capacitance can absorb the regenerated power, then no braking resistor or braking chopper would be required in the circuit.

Braking chopper

A braking chopper (or other shunt circuit) is used to burn the energy that can’t be recuperated. It is recommended to use SOMANET drives together with one or multiple SOMANET Braking Chopper 48V 500W.

The number of braking choppers depends on the amount of excessive energy (for peak power) and the frequency of regeneration (thermal load).

The SOMANET Braking Chopper is protected against overvoltage and overcurrent. In both cases of overload the chopper will switch off, which will lead to an overvoltage fault on the drive.

Attention: This can lead to dangerous situations if the robot isn’t able to decelerate as necessary.

The calculated number of choppers can be verified in the real system:

  1. check the regenerated max power in watching the 3 LEDs on the chopper board:

    • if one or two LEDs are flashing during braking, the number of choppers is sufficient. You have multiple choppers, you can give a try with one less.

    • if 3 LEDs are flashing number you have to check the max voltage with an oscilloscope if voltage is limited (e.g. by the TVR diode of the system)

    • if the drive is having a overvoltage fault, you have to add additional choppers

  2. check the thermal behavior of the drive by operating the system in the worst case scenario and check the temperature. If the drive is not switching off at this temperature, the system is well dimensioned.

Note: For optimal performance, connect the braking choppers to the closest possible point to the inverter terminals.