Why does the supply voltage rise when I decelerate the motor?
During deceleration, direction reversal or when the load turns the motor, the motor works like a generator. If the back-EMF it produces exceeds the supply voltage, current flows back into the source and the line voltage rises. Because switch-mode power supplies cannot sink current the voltage can climb quickly; a battery supply is more tolerant since it can absorb this energy.
The motor starts producing energy at every moment it is forced to turn by the shaft. This happens in three cases: fast deceleration, reversing direction while in motion, and the load dragging the motor (a load coming down, a platform travelling on a slope).
The energy produced has to have somewhere to go:
- Battery supply: A battery can accept current and absorbs the energy chemically. For this reason battery systems are relatively resistant to regenerative energy. Even so, opening the battery isolator switch during operation breaks this path and sends the voltage soaring.
- Switch-mode power supply (SMPS): It can only source current, not sink it. The returning energy charges the output capacitor and the voltage climbs quickly; the driver and the supply can be damaged.
Measures that can be taken:
- Lengthening the stopping ramp (KS250 soft stop, PT500 RMP trimmer) — it spreads the energy over time and lowers the peak value.
- Adding a DC link capacitor — it buffers short-term energy.
- Using a brake (dump) resistor — it turns the excess energy into heat and is the most effective solution in systems that decelerate continuously.
- Never opening the battery isolator while the motor is running.
The full topic: DC Motor Driver Selection Guide