Electric motors when forced to run develop a voltage that is proportional to their speed. For DC motors, the regenerative voltage is equal to the motor speed multiplied by the back emf constant (Ke).
The same regenerative voltage will be developed even if the motor is being rotated as a result from the voltage provided by a motor drive. The regenerative voltage of a motor cannot exceed the motor’s rated voltage unless if the motor exceeds its nominal speed. For example, a 48 V, 3000 RPM motor cannot have a regenerative voltage higher than 48 V unless if it is forced to rotate at higher speed than 3000 RPM.
Below is the electrical equivalent of a DC motor. On the one side of the circuit is the Motor drive, supplying the drive voltage. On the other side is the back emf of the motor and in between is the motor impedance. The motor impedance has a real part (resistance) and an imaginary (inductance) but for simplicity it will be considered as single impedance with units in Ohm.
When the motor drive is applying a constant voltage, the motor speed will start to increase and the same will do its back emf. At some point the motor speed will stabilize and the motor back emf will be slightly lower than the drive voltage. That way the motor current will be flowing from the drive to the motor. This operation is called motoring.
Consider now the case that the drive voltage starts to decrease, to make the motor decelerate. If the motor voltage is reducing faster than the motor’s back emf voltage, the current direction will be opposite, and the current will be flowing from the motor to the drive.
The current that will be returning to the battery through the drive can also make the DC bus voltage rise. How much the DC bus voltage (battery voltage) will rise depends on the capability of the battery to absorb the regenerative current. If the battery health is bad, then its current absorb capabilities will decrease.
Regeneration and MOSFETs
When the controller will detect that the DC voltage increase more than a defined level, it will disable the power stage. That way the current will be prevented from returning to the battery and the bus voltage will return to normal. As mentioned previously, normally the back emf of the motor will be lower than the battery voltage, so there will be no current flow through the MOSFETs body diode.
The only way that the current can return through the MOSFET’s body diode (When the power stage is disabled) is if the regenerative voltage is higher than the battery voltage. As mentioned this can happen only if the motor is forced to rotate at higher speed than the nominal (for example when a vehicle runs downhill). Only in that case, even if the controller MOSFETs are disabled they cannot prevent the DC bus voltage from increasing. In case that the DC bus voltage exceeds 60 V, the 60 V controllers can be damaged.