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.

  • \$\begingroup\$ It seems to me that the vehicle should coast on power failure. A secondary braking system under drive control should bring the vehicle to a safe stop in a safe location. \$\endgroup\$ – Transistor Jul 11 '17 at 13:28
  • \$\begingroup\$ @Transistor that is only true if the vehicle is NOT Autonomous. With a driver the decision to brake can presumably be trusted. If it's a smart vehicle, no power = no smarts = STOP ASAP! \$\endgroup\$ – Trevor_G Jul 11 '17 at 13:48
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    \$\begingroup\$ Didn't think of autonomous. Pity the guy behind then and the passengers! \$\endgroup\$ – Transistor Jul 11 '17 at 14:00
  • \$\begingroup\$ The vehicle is not manned in any way. With "power failure" I mean the event in which there's a complete or partial loss of power that can extend to the logical sub system and then any active control over vehicle is lost. \$\endgroup\$ – weirdgyn Jul 11 '17 at 14:33

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.


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.


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.

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    \$\begingroup\$ Current limiting would of course mean even less braking. \$\endgroup\$ – Chris Stratton Jul 11 '17 at 14:19
  • \$\begingroup\$ @ChrisStratton correct, at higher speeds braking would be reduced. At some point, maximum braking would be achieved as the vehicle slowed. \$\endgroup\$ – Trevor_G Jul 11 '17 at 14:37
  • \$\begingroup\$ @Trevor I agree with your considerations but I think that it can be a valuable help into mitigating some other dangerous behaviour. \$\endgroup\$ – weirdgyn Jul 11 '17 at 14:37
  • \$\begingroup\$ @weirdgyn indeed.See Add 2 \$\endgroup\$ – Trevor_G Jul 11 '17 at 14:41
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    \$\begingroup\$ I bet that a proper FMEA would show that a mechanical brake that is separate from the motor system is needed. There are options for brake assemblies that can be sandwiched with the motor. This would provide your system with a separate brake system and thereby help you meet market safety standards - I'm not sure which ones you are subject to but I'd be surprised if they didn't require something like this. \$\endgroup\$ – Smith Jul 11 '17 at 15:19

This is dynamic braking. In 0 speed does nothing, on lower speed stops the motion. But on high speed that will heat and destroy the motor. At very least you need a power resistor to waste regenerated energy on it. Pain in your neck, bht like Transistor said, vehicle should keep moving, but not under power.

  • \$\begingroup\$ Overheating issue is a good addition though may or may not be an issue. Currents involved should NOT be any greater than flooring the accelerator on an equivalent hill, which the motor should be able to take anyway. However, if the driving circuitry includes current limiting features to limit stall currents the addition of similar circuitry in the braking circuitry would be required. \$\endgroup\$ – Trevor_G Jul 11 '17 at 13:45
  • \$\begingroup\$ I think you are confusing voltage and current. Since speed is gained by the drive, voltage indeed will not be higher than bus voltage. But current is only limited by coil and wires resistance, so in many cases all system will burn. \$\endgroup\$ – Gregory Kornblum Jul 11 '17 at 13:49
  • \$\begingroup\$ Yes if it's a complex system I agree. If it's a simple... "Apply battery voltage and go" type system the loads would be identical though, assuming no run-away. I would also use a current limiting load, not a just simple resistor bank, the latter would significantly reduce the braking affect at lower speeds. \$\endgroup\$ – Trevor_G Jul 11 '17 at 13:52
  • \$\begingroup\$ I don't know why you assume that. \$\endgroup\$ – Gregory Kornblum Jul 11 '17 at 14:04
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    \$\begingroup\$ Ah, ok. Now i get it. So technically without acceleration profile he will burn the motor on startup. \$\endgroup\$ – Gregory Kornblum Jul 11 '17 at 14:35

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