Q: Protecting against Reneration
When a motor is spinning faster than it would normally at the applied voltage, such as when moving downhill or decelerating, the motor acts like a generator. In such cases, the current will flow in the opposite direction, back to the power source.
It is therefore essential that the controller be connected to rechargeable batteries. If a power supply is used instead, the current will attempt to flow back in the power supply during regeneration, potentially damaging it and/or the controller.
Regeneration can also cause potential problems if the battery is disconnected while the motors are still spinning. In such a case, the energy generated by the motor will keep the controller On, and depending on the command level applied at that time, the regenerated current will attempt to flow back to the battery. Since none is present, the voltage will rise to potentially unsafe levels.
The oscilloscope capture below shows the voltage at the input of an SDC2160 controller connected to a power supply.
When suddenly stopping a motor that was running at full speed, the voltage quickly rises from 24V to nearly 60V. The voltage could rise even higher under certain conditions. When the motor is finallly stopped, the voltage that accumulated in the controller's capacitors starts decaying.
The controller includes an overvoltage protection circuit to prevent damage to the output transistors. The oscilloscope capture below shows the voltage at the input of an SDC2160 controller connected to a power supply with the overvoltage protection set to 25V.
The overvoltage protection circuit will cut off (float) the motor, quickly stopping the regeneration and voltage surge. Once the motor is floating, the voltage accumulated in the controller's capacitor will decay. When the voltage goes level below the overvoltage threshold, the transistors reactivate. If the motor is still spinning because of inertia, regeneration will resume and the voltage quickly rise again above the overvoltage limit. This cycle will continue until the motor has stopped. Because the power transistors disconnect, there is no braking taking place. The motors are freewheeling down.
If the motor inertia is very high, the voltage jump at the time the transistors turn back on can be very dangerously high. The overvoltage protection can therefore not entirely be depended upon to protect the controller and the power supply. This protection will work best when the overvoltage limit is set as close as possible above the supply voltage. For example 25V on a 24V supply system.
A safer technique and one that will cause the motor to brake instead of freewheeling is to place a resistive load in parallel with the power supply, with a circuit to enable that load during regeneration. This solution is more complex but will provide a safe path for the braking energy into a load designed to dissipate it. The diagram below shows an example of such a circuit.
The controller must be configured so that its digital output is activated when an overvoltage condition is detected. The MOSFET and brake resistor value must be sized according to the expected regeneration current that must be absorbed.
The oscilloscope capture below shows the effect of this circtuit.
With a resitor value of 1ohm, the power supply and regeneration voltage suddenly see a load of 25Amps, causing a rapid drop in voltage. The resistor is then quicly disconnected and the voltage rises as the controller's capacitors recharge. When the voltage rises again above the overvoltage threshold, the resistive load reconnects again, repeating the cycle until all the motor's kinetic energy is dissipated.