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Author Topic: DC Motor Electric Braking (Large Motor)  (Read 6114 times)

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Offline StAnkysTopic starter

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DC Motor Electric Braking (Large Motor)
« on: September 16, 2009, 01:30:12 AM »
I would like to make an electronic brake for an electric scooter so that it does not require a mechanical brake.

Specifications:
24 volt DC motor
350-500 watt (15+ amp nominal under forward motion)
brush motor, reversible, 2-lead (+ and - only)

I would like to use a button, sensor, resistor, or some other non-mechanical means of activating the electronic brake.



Method:
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?

Offline SmAsH

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Re: DC Motor Electric Braking (Large Motor)
« Reply #1 on: September 16, 2009, 02:26:46 AM »
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?
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Offline radhoo

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Re: DC Motor Electric Braking (Large Motor)
« Reply #2 on: September 16, 2009, 03:31:24 AM »
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?
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 break

Also a good idea is to do the motor short circuit via an adjustable resistor of a few ohms.

Offline StAnkysTopic starter

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Re: DC Motor Electric Braking (Large Motor)
« Reply #3 on: September 16, 2009, 06:04:17 AM »
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?

The throttle is controlled through an FSR already.  If I could somehow operate a brake through a second one, I'd be very pleased.

However, I specifically want to avoid the use of a microcontroller for a number of design reasons that I'll spare you right now.

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?
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 break

Also a good idea is to do the motor short circuit via an adjustable resistor of a few ohms.

I'm looking to be able to bring the scooter to a relative halt from ~15mph in 20-30 feet...significant braking is needed, but nothing that will necessarily lock up the drive wheel.

Gearing down the scooter would negatively affect cruising speed, and I'd rather avoid the complexity.

What would using an adjustable resistor do?  It would allow variable braking force (good!), and can be done with a FSR (good!), but wouldn't it brake less aggressively than a zero-ohm short?

Offline StAnkysTopic starter

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Re: DC Motor Electric Braking (Large Motor)
« Reply #4 on: September 16, 2009, 06:05:50 AM »
How would I get the terminals to short?  Is there some kind of relay-switch I can use to accomplish this?

Offline radhoo

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Re: DC Motor Electric Braking (Large Motor)
« Reply #5 on: September 16, 2009, 09:36:02 AM »
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.

Offline StAnkysTopic starter

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Re: DC Motor Electric Braking (Large Motor)
« Reply #6 on: September 16, 2009, 02:35:38 PM »
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.


The motor is a MY1016 350W 24v DC brushed motor currently.  Here is an example:
http://www.monsterscooterparts.com/24-volt-350-watt-motor-razor-dune-buggy.html

I'm trying to figure out a way to control braking with feet rather than hands.  I'm moving toward a hands-free electric skateboard design.

I think I'm still a little perplexed at how this short-out would physically connect...would it essentially make an "H" on the motor leads, where the center connection is a relay of some kind that bridges the + and - leads when braking is desired?  If so, how might I keep this bridge from occurring while juice is flowing to the motor (braking while trying to accelerate)?

Anyway, I appreciate the continued input!

Offline SmAsH

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Re: DC Motor Electric Braking (Large Motor)
« Reply #7 on: September 17, 2009, 06:23:39 AM »
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.
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Offline StAnkysTopic starter

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Re: DC Motor Electric Braking (Large Motor)
« Reply #8 on: September 17, 2009, 06:42:48 AM »
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.

That is how I was visualizing it, thank you for clearing it up for me!

Do you think shorting the motor would have any negative effects on the speed controller? 

Offline Soeren

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Re: DC Motor Electric Braking (Large Motor)
« Reply #9 on: September 17, 2009, 09:12:51 AM »
Hi,

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.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
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Offline blackbeard

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Re: DC Motor Electric Braking (Large Motor)
« Reply #10 on: September 17, 2009, 12:54:55 PM »
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.
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Offline StAnkysTopic starter

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Re: DC Motor Electric Braking (Large Motor)
« Reply #11 on: September 17, 2009, 02:24:04 PM »
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.
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".

Offline SmAsH

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Re: DC Motor Electric Braking (Large Motor)
« Reply #12 on: September 18, 2009, 02:31:08 AM »
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.
I have to strongly agree with this post, maybe just an extra relay that closes when the throttle is off?
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.
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".
You pretty much have it! The back energy created by the motor spinning counteracts the motors motion thus slowing it down until it stops, and will continue to do so while the motor is shorted.
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Offline blackbeard

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Re: DC Motor Electric Braking (Large Motor)
« Reply #13 on: September 18, 2009, 10:21:09 AM »
oh very cool! i never thought about it that way but it makes complete sense
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Offline Daanii

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Re: DC Motor Electric Braking (Large Motor)
« Reply #14 on: September 18, 2009, 12:42:17 PM »
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. 

Offline StAnkysTopic starter

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Re: DC Motor Electric Braking (Large Motor)
« Reply #15 on: September 18, 2009, 12:51:53 PM »
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. 

I would of course rather have regenerative braking...but my speed control is not reversible, and I can't find any custom wiring information about how to make my own.

Might you have some insight?

Offline Daanii

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Re: DC Motor Electric Braking (Large Motor)
« Reply #16 on: September 18, 2009, 01:43:20 PM »
Normally the motor controller is wired to do the regenerative braking.  Here your controller seems to be mainly just a potentiometer.  Is that right?  

As I said, with a scooter, you may be able to get away with just shorting the motor.  You don't have a big load to brake, so you won't have a whole lot of heat to dissipate and the brake will not need to be on a long time.  

On the other hand, you've got a small motor.  The kinetic energy from braking will need to dissipate in that motor on only the load from the resistance of the wiring.  That just seems like a burnt-out motor waiting to happen.  

Another problem may be your wheel locking up.  You'll have just abrupt braking, on or off.  Say you want to go down a hill, and need to brake a little, but want mostly to coast.  You can't, by just shorting the motor.  The brakes are either abruptly on or off.  Short the motor going down a hill, and I'll bet your wheel locks up.  

You can do gradual braking, of course, with regenerative braking, just like you can with mechanical brakes.  I guess you could wire up a regenerative braking circuit.  My apologies -- I don't have the knowledge or experience to suggest one that could work for your scooter.  I'll see if I can find anything on the Internet.  
« Last Edit: September 18, 2009, 03:18:22 PM by Daanii »

Offline Daanii

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Re: DC Motor Electric Braking (Large Motor)
« Reply #17 on: September 18, 2009, 02:12:18 PM »
Looking a little further into this, regenerative braking makes little sense for a scooter.  You would not get enough battery recharging to make it worth while. 

But putting a resistor in the circuit rather than just shorting the motor might make sense.  That's dynamic braking.  There are special dynamic braking resistors used for that to dissipate the heat from large electric motors, like in a railroad locomotive.  www.engineersedge.com/instrumentation/components/dynamic_braking_resistors.htm 

Obviously, with a small motor like yours, you would not need a big setup like that.  But I think you would want to have a resistor designed to take the heat, rather than put the heat in your motor.  The heat problem may be more theoretical than real, though, so you may want to try it. 

I'm still looking for a dynamic braking wiring circuit that may help.  You would probably want one with a potentiometer in it to vary your braking from soft to hard.  Even then, you would still need a mechanical brake in some cases, although your feet might work. 

Offline Soeren

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Re: DC Motor Electric Braking (Large Motor)
« Reply #18 on: September 18, 2009, 07:00:40 PM »
Hi,

Shunting the motor won't block the wheel and it won't burn the windings, after all, you can only brake for so long before you come to a stand still and the power that has to be dissipated is proportional to the speed, which is diminishing during the action.
Adding resistors will only make the brake less efficient.

Usually, you make it so when you engage the brake handle/pedal fully, there's a mechanical brake augmenting the braking action, as otherwise, you can't make a fast stop.
.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

 


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