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:

E_m = E_k + E_p

with:

E_m: mechanical energy

E_k: kinetic energy

E_p: potential energy

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.

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.

For more information, please refer to 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.