I'm into a preliminary design of a eletrical vehicle that uses BLDC permanent magnet motors for motion (3 phases 36V 300W hub motors in wheel). For safety compliance this vehicle needs to provide a breaking mechanism that stops the vechicle when there's a power failure. My first choiche will be using reverse-action brakes/clutches that will triggers in case of power-loss unfortunately I have no room around motor and wheels.
I'm thinking about another solution that uses relays.
My idea is to place an SPDT relay (with coil bound to the primary power source) between motor driver phase output (relay NO) and BLDC motor phase (relay COMMON)
all relays NC will then be connected togheter (trough a resistor?).
- In normal operations relays will be forced into NO position allowing drivers to handle motors.
- In case of power loss all relays will return to their NC position and thus all phases will be shorted togheter. If the vehicle is then forced to move (because a slope or something pushing at it) BEMF will react in opposition to the motion.
I don't think this idea is something new I will only know if someone sucessfully used this or if there are hidden dangers/pitfall into my approach.
Best Answer
This type of system is fairly common but you must remember that this is a reactive system. It will not HOLD the vehicle but rather, will retard it's speed.
That is, the amount of brake force generated will depend on the speed of the vehicle. When stopped, there is ZERO brake force.
A vehicle parked on a steep slope will continue to run down the hill, all be it at a much slower and fixed rate. As such this mechanism can in no way be called an "emergency STOP" system.
ADDITION:
Current limiting would also be prudent to prevent the motor from burning out. This would be especially a problem if the vehicle is on a really steep slope. At higher speeds this would of course cause braking to be reduced. At some point, maximum braking would be achieved as the vehicle slowed.
ADDITION 2
For emergency stop though it should be considered supplemental braking at best. Some sort of mechanical braking system capable of stopping the thing with or without the assist would still be a MUST-HAVE. In the end you need to decide if the extra cost and reliability reduction of the effort is justified.