Servo Back-EMF Braking — Why Your Driver Gets Hot
Servo back EMF braking does not show up in most spec sheets, but it should. Push against a servo or let gravity back-drive it and the motor turns into a generator. That energy needs somewhere to go. If your driver board cannot handle it, that somewhere is straight into heat, and eventually into a thermal shutdown.
Where the Back-EMF Comes From
Back-EMF — back electromotive force — is the voltage a spinning motor generates. Even with no command signal, any external force turning the output shaft makes the windings produce voltage. With a brushed servo, spin the motor and you get voltage on the terminals. Nothing complicated. With a brushless servo, the three-phase windings pump current backward through the MOSFET body diodes straight into the DC bus.
The faster the external force spins things, the higher that back-EMF voltage climbs. Once it exceeds the bus voltage, current flows backward into the driver. That is when components start cooking.
What Heats Up and Why
High-inertia industrial loads create constant back-EMF. Picture an AGV steering against a loaded pallet, or a robot joint holding position against gravity. The servo pulses current to fight the load. Each time the load pushes back and wins a little, the motor fires a voltage spike back at the driver. The MOSFET body diodes and DC bus capacitors absorb it.
Now if the driver lacks enough bus capacitance or a braking resistor, that absorbed energy turns straight into heat in the FETs. Thermal protection trips eventually and the servo shuts down mid-operation. I have debugged “random shutdowns” on more machines than I can count. Back-EMF heating caused at least half of them.
Braking Resistors Are the Real Fix
Proper industrial servo drivers include a braking chopper circuit. When bus voltage passes a set threshold, a resistor across the DC bus burns off the excess. That resistor becomes the heat sink instead of your MOSFETs. This is real regenerative braking, and it works.
If your driver skips this feature, you can add external bus capacitance. A 1000-4700uF low-ESR electrolytic right at the driver input soaks up short spikes. For sustained regeneration — an AGV rolling downhill, a lowering crane load — you need an actual braking resistor or a four-quadrant driver that pumps energy back into the battery. Capacitors alone will not save you there.
Three Questions for Your Supplier
Not every industrial servo handles back-EMF the same way, so ask. Does the driver have regenerative braking? What maximum bus voltage triggers the protection? Does it provide an external braking resistor terminal? If all three answers come back “no,” budget for external capacitance or accept that thermal shutdowns live in your future. A servo back EMF braking strategy belongs in the design phase, not the troubleshooting phase.1