[Part 1 of this article provides an overview of regeneration - what it is and how it occurs.]
The motor as a generator
Figure 8 depicts an electrical vehicle climbing an incline moving forward (top) or in reverse (bottom). When the vehicle rolls downhill it will turn the motor pressed by the force of gravity so the motor will actually turn into a generator.
When the vehicle is climbing the motor will absorb current; when the vehicle reaches the summit the torque required will drop significantly and the motor current will drop practically to zero. When the vehicle rolls down the downhill slope the current will actually invert direction and flow into the controller.
The four quadrants
The figure below illustrates the four quadrants corresponding to the above situation of a vehicle moving forward and reverse either climbing up or rolling down.
For simplicity we have indicated only the current flow when the Power Bridge is in conduction, so the Top MOS ON and the corresponding Bottom MOS is OFF.
The currents are indicated only for the phase of the Duty Cycle corresponding to the pulse (ON); when the Duty Cycle is OFF the current flow is not indicated, but in this case both Top MOS are OFF and both Bottom MOS are ON, so the motor is shorted and the current flows through the two Bottom MOS.
The Forward/Reverse gear switch controls the Power Bridge, which in turn controls which side of the Bridge conducts so inverting the current in the motor by exchanging the polarity of the voltage across the motor.
The accelerator pedal controls the duty cycle, which can be zero (both Bottom MOS ON), 100% (DC current) or in between (pulsating).
While the vehicle rolls downhill the operator could keep gear and pedal in any position.
This means we have different situations where the motor regenerates. Specifically:
- If the operator releases the accelerator pedal then the duty cycle becomes zero, both Bottom MOS are ON, both Top MOS are OFF and the generator is permanently shorted. The gear position makes no difference. This corresponds to dynamic braking, where the generator dissipates energy on its own internal resistance developing a Counter Torque that has a braking effect on the vehicle. No regeneration takes place.
- If the operator keeps the pedal pressed half way and the gear Forward, there will be a duty cycle and during the pulse of the duty cycle (ON) the Power Bridge will conduct and the generator will discharge its current into the battery. The regeneration lasts only the time of the pulse in the duty cycle (ON). During the OFF time of the duty cycle the motor is still shorted being both Bottom MOS turned ON so dynamic braking takes place.
- If the operator keeps the pedal pressed half way and the gear in Reverse, the above situation still applies, except that the other half of the Power Bridge will conduct. Consequently the current will enter the battery from the negative pole, so there will be no regeneration.
From the above discussion results that the maximum regeneration is achieved with the pedal pressed fully forward so the duty cycle is 100% forward.
In this case the top left MOS and the bottom right MOS are permanently ON, and we have un-interrupted regeneration.