One of the most easy ways to damage a motor driver component or system is through unintentionally-induced overvoltage. Overvoltage can cause damage to system components due to violating the absolute maximum ratings of those devices. This FAQ will discuss how overvoltage occurs and methods to mitigate it.
A motor can act either as a method of power delivery from electrical sources into mechanical torque or as a generator taking mechanical torque and converting it back into electrical energy. Because of this fact, a motor may cause an *increase* in supply voltage under some circumstances. When the motor is acting as a generator, the induced current in the system needs to be handled properly or an overvoltage event may occur.
How overvoltage occurs in motor drive systems
- Coasting a motor to decelerate it: when you coast a motor, the mechanical/rotational energy of the motor is converted back into electrical energy (voltage & current). In coast mode, the power stage body diodes conduct current back into the supply from the motor resulting in supply pumping.
- Responses to fault conditions coasting the motor: In most fault cases, the default response is to turn OFF all MOSFETs to prevent any possible damage to the system. While this works well to prevent any cascading fault conditions due to high current or other common effects, turning OFF all of the MOSFETs results in a motor coast. If the motor was spinning, it will decelerate through coast and return all of its energy to the supply, causing supply pumping.
- Motor being back-driven: A good example of this would be an e-bike. When the e-bike is rolling down a hill the motor is acting as a generator and causing the supply voltage to increase. Some e-bike systems use this mode of operation to bring regenerative energy to the battery to recharge it. In any motor drive system where the motor can be externally forced, if the energy generated by the motor is not controlled, that can cause an overvoltage event.
- Not having enough bulk capacitance: When driving a motor with a variable load, it is common for the motor to cause minor supply pumping as it is spinning. While this is normal, it needs to be accounted for when designing the system. If the motor drive circuit does not have enough local bulk capacitance, this effect can cause significant supply pumping.
How to mitigate overvoltage in a motor drive system
- Use more brake than coast: Adjust the deceleration to include both “brake” and “coast” rather than just “coast” to mitigate some of the back-EMF. When operating in brake mode, current is not allowed to recirculate into the supply and cause supply pumping. However, some motors may generate excessive current when asserting a brake.
- Decrease the rate of deceleration: Limit the maximum possible deceleration in your algorithm to not apply a drive signal significantly less than the back-EMF. This will reduce the power generated by the motor while it is decelerating.
- Add more bulk capacitance: Adding more bulk capacitance at the motor will cause the voltage to rise slower during supply pumping. Therefore, the maximum voltage that the supply rises to will decrease with additional bulk capacitance. While this does not eliminate the supply pumping, it can keep the amount of supply pumping at a manageable level.
- AVS Technology: Some devices which integrate motor control algorithms also integrate anti-voltage surge technology, you can read more here: LINK
- Add external clamping circuits: Add an external pulldown (brake circuit). This circuit can be employed to “short” the motor externally and dissipate the motor power. A clamp circuit can be activated automatically by an analog feedback circuit or controlled by a microcontroller.
Thanks,
Matt
