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I have read that shorting out the terminals of a DC motor will quickly bring it to a halt due to the motor's own EMF. How would I be able to safely do this with some much inertia in the scooter and such large loads?
Yes, to brake a dc motor you just short the two terminals while power is applied.To accomplish what you want, you would probably want a microcontroller of some sort.what sort of sensors are you looking at?
Quote from: StAnkys on September 16, 2009, 01:30:12 AMI have read that shorting out the terminals of a DC motor will quickly bring it to a halt due to the motor's own EMF. How would I be able to safely do this with some much inertia in the scooter and such large loads?Whilel this is correct, I'm afraid it won't be enough to produce a considerable breaking effect, eg.: you'll do the shortcircuit and the scooter will keep on moving.However there's a different approach that will work: Use the same DC motor but with a reductor (gear ratio must be computed based on torque required to reduce your scooter load innertia) ! This will definitively work as a breakAlso a good idea is to do the motor short circuit via an adjustable resistor of a few ohms.
Could you upload some pictures with this motor?Regarding the resistors, 0 ohm is indeed more aggressive, but I was thinking of using some kind of handle to go from 20 Ohm -> 0Ohm proportionally with hand pressure or something. Not really necessary, since the magnetic field inside the motor does a good buffering job already and the breaking would be smooth already.Sure you can use a relay, I'm not sure I got your question right, but you don't need any special relay for this, just compute the current that will flow through the motor's windings and pick the relay accordingly. Or better: use a 10A relay and you should be on the safe side.
You could use a simple momentary switch for the brakes?I drew a basic diagram of how to connect it when using a relay. (see attached)While the brake is dis-engaged the two terminals will not short because the relay gate is open.When the switch closes the relay closes thus shorting the terminals of the motor.
just out of curiosity what is the purpose of shorting the terminals of the dc motor? i've generally come to believe that short=bad and that it would do nothing but damage your motor controller.
It would be very wise to incorporate a circuit that only allows the relay to shunt the motor when the throttle is fully released!And do use a relay with wolfram (tungsten) contact points or they may weld together.
Quote from: blackbeard on September 17, 2009, 12:54:55 PMjust out of curiosity what is the purpose of shorting the terminals of the dc motor? i've generally come to believe that short=bad and that it would do nothing but damage your motor controller. From what I've gathered...When a motor spins from an outside force or its own inertia, it acts like a generator. If you short out the leads, suddenly I would think that the electromagnets in the motor would want to repel rather than attract (spinning)...and voila, motor slows itself and the scooter, and when it stops, there is no longer any potential being "generated".
I don't know about this idea of shorting the motor to brake a scooter. The only place for the kinetic energy that comes from braking to go will be into the motor. With the motor circuits shorted, a lot of heat will need be dissipated inside just the motor and its casing. That may damage your motor, which is just air-cooled, if you have the brakes on for any length of time. I don't have much experience with this, but I worry about trying to get braking just by shorting. Regenerative braking, on the other hand, should work better. That turns the motor into a generator, with a load to take the energy from braking, rather than just shorting the motor. Even there, replacing mechanical brakes with electrical braking is not as simple and complete as it might seem. With a scooter, you might get away with it, since you will have your feet as a braking backup. Be prepared for things like locked wheels, though. I wouldn't try just shorting the motor to brake a car, that's for sure. But for a scooter, it may work. If you try it, good luck, and let us know how it turns out. I'll be interested.