Short circuiting motor leads for emergency stop braking

[AndyVercauteren]

Hello,

I'm working on an application where a permanent magnet DC motor is used to power an electrical vehicle.

For safety purpose I want to short circuit the motor leads when an emergency stop is activated.

Can someone tell me whether I can do this (just short cicuiting the motor) or do I have to provide a braking resistor to dissipate the power during short circuit.

If I have to use a resistor how can I determine/calculate the exact resistorvalue?

Thanks for any help,

Andy

 

[gordonl]

You will have to use a braking resistor, you would probably burn out the motor if you didn't because of the large current. The value of the resistor will depend on the voltage and short time current rating of the motor, and for watts, the length of time you'll be braking.

 

[electricuwe]

I guess if you short circuit the motor the tyres are more prone to be burned than the motor but I would recommend an braking resistor too.

 

[dydt]

shorting the output of a DC motor for an E-stop is not advisable. you will subject this motor to a high electrical and mechanical stress which will eventually shorten its life. you have to use a "Dynamic Braking Resistor" the value of which can be determine by contacting the motor manufacturer or searching some good websites.

have you thought of using a clutch/brake system?.. by re-designing some circuitry you can activate brake system to work as an E-stop. i don't know how you want the system to work..i hope you can tell us more details

dydt

 

[shorehamtv]

Yes you will need a resistor, we have been working on regen / dynamic braking for a railway and it gets very difficult. the resistor has to be very high power 500w ++ depending on motor type, vehicle weight etc. We have been using 1.5 ohm 500w resistors on a 2 tonne 120v train but it seems a bit like trial and error!. check out

http://www.cressall.com.

Regards
John

 

[electricgti]

Shorting the motor of an electric vehicle would be an extremely dangerous event. When would you activate this emergency brake? If the motor is over-reving, your control electronics are probably already gone, so your system to short the motor wouldn't work either. If you need to stop quickly, real brakes would work better. Shorting the motor would simply lock up the tires, and locked tires will take longer to stop than just brakes. Use an ABS system if you are very concerned about stopping in time.
-Nathaniel

 

[geezer]

The thermal mass of the motor iron may possibly save the winding insulation, but the magnetic field generated by the winding during braking may de-magnetize the field magnets in your motor. This will show itself as a faster running motor (but with less torque) with the same applied armature voltage.

The dynamic brake resistor value should be sized to provide a current less than the peak rated current of the motor when the motor back EMF voltage at full speed is applied to it. Since we are dealing with a temporary load, the resistor should be pulse rated. This will allow a lower wattage rating than would normally be implied by the current through the resistor. The relative frequency of braking would be limited by the wattage rating of the resistor.