I have an electric lawnmower that I custom built. After years of reliable service, it needed a complete teardown and rebuild. It’s powered by a 2HP induction motor and one of the features I wanted to add to it was an electronic brake. That way the blade stops quickly whenever the dead man switch is released.

Motor brakes already exist but they are very expensive so I figured I would try to roll my own.

DC injection brakes are a common type of motor brake. They basically work by disconnecting the mains power (120VAC in this case) and injecting a pulse of low voltage DC power (24VDC in this case) into the motor windings. This creates a non-rotating magnetic field which stops the rotation of the stator. It has to be powerful enough to stop the motor but without causing damage to it or the driven load.

This is what I came up with. The dead man switch cable pulls the primary switch. Primary switch closes the primary relay, delivering 24VAC to the contactor and turning the motor on.

When the dead man switch is released, the primary relay shunts power to the timed relay(white), momentarily backfeeding 24VDC to the motor and causing it to stop. It actually works well.

There is a small risk that if the contactor were to stick, it would let the smoke out. But, the rectifier and transformer would take the brunt of that failure and as cheap as those are to replace, that’s an acceptable risk.