Can a series wound DC motor be used as a brake?
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    Default Can a series wound DC motor be used as a brake?

    Curious if a series wound 12 volt DC motor with 3 terminals can have DC current applied to act as a light holding brake like is done with AC motors?

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    Short the brushes together and push current through the field.

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    Quote Originally Posted by Garwood View Post
    Curious if a series wound 12 volt DC motor with 3 terminals can have DC current applied to act as a light holding brake like is done with AC motors?
    "Light" only if seriously undersized.

    IF so arranged, all-too DAMNED well!

    My Milwaukee M12 cordless hand drill does that hard-braking automagically when yah let-off the trigger. Bugger ALSO chronically unwinds the keyless chuck off the back of the outer sleeve's inertia doin' it, too!

    "Binary" engagement, y'see. Pain in the anatomy in that case.

    Depending on your "needs", shades of grey or adjustable braking level could require some electronic fiddling and diddling.

    All mechanical toe-brake bar has more inherent finesse.

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    I have the same problem with my DeWalt. More often than not the chuck comes loose on that damn thing because it stops so fast. And it's got the ratcheting chuck with carbide jaws too, so it's not like it isn't getting a good tight bite. I've gotten adjusted to it eventually and learned to slowly let off the trigger.

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    A company that rented billboard space had a problem with their boom trucks. There was no provision for retracting the telescoping boom except to point it straight up and release the mechanical brake. As the boom retracted, it would pull cable off the winch, running the motor backwards. The operators were supposed to ride the brake to keep it from going too fast, but they often didn't and the motor would throw the windings out. These were series wound DC motors, similar to automobile starter motors. We found that we could connect the field and armature together and the motor would reach a speed where it generated enough brake to maintain a constant reasonable speed. When we released the boom, the motor would speed up, then slow back down as the magnetic flux developed. It would not hold a position because it needed to turn to produce braking current.

    Bill

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    Similar application, but I'm looking to keep an already stopped motor from spinning.

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    Quote Originally Posted by Garwood View Post
    Curious if a series wound 12 volt DC motor with 3 terminals can have DC current applied to act as a light holding brake like is done with AC motors?
    The problem with your simplistic approach is that you need a way to monitor motor commutation. In other words, a feedback device.

    AC motors controlled with a VFD in vector mode accomplish this quite well but the feedback comes from the magnetic flux in the motor windings.

    You have the choice of using an electro-mechanical brake or else going to need to use a more complex motor controller.

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    Quote Originally Posted by Ziggy2 View Post
    The problem with your simplistic approach is that you need a way to monitor motor commutation. In other words, a feedback device.

    AC motors controlled with a VFD in vector mode accomplish this quite well but the feedback comes from the magnetic flux in the motor windings.

    You have the choice of using an electro-mechanical brake or else going to need to use a more complex motor controller.
    I've seen several AC motors that use a simple dumb rectifier circuit to put DC into one winding which stops the motor from spinning right away and stops it in exactly the same spot every time. This setup is used on toolchangers a lot. No VFD. No feedback.

    I want to do the same thing, but I don't care about position accuracy and I don't need a rectifier because we already have DC here. I'd imagine I need a way to limit current (I can figure that out). The motors are essentially reversible starter motors. The windings are .100" thick copper.

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    Quote Originally Posted by Garwood View Post
    I've seen several AC motors that use a simple dumb rectifier circuit to put DC into one winding which stops the motor from spinning right away and stops it in exactly the same spot every time. This setup is used on toolchangers a lot. No VFD. No feedback.

    I want to do the same thing, but I don't care about position accuracy and I don't need a rectifier because we already have DC here. I'd imagine I need a way to limit current (I can figure that out). The motors are essentially reversible starter motors. The windings are .100" thick copper.
    Yes, you should be able to do that, and that simply.

    If you need to trade money for savings in time, just vet it with a 4Q DC Drive as can make it do whatever it CAN do, either direction, at any rate you choose, no relays or contacts. They can even sense "Zero rotation command has been accomplished". 4Q SSD drives even have a specific output signal to tell yah so.

    Much the same as well-equipped VFD can do with THEIR tribe of motors, whether you need ALL their features or not.

    Your time is short and has other demands on it, the "overkill" may even end up cheaper than "R&D" of a cheap and simple way.. that detonates the odd FWB or turns a contactor into an arc-light show...'til you get it right!

    DAMHIKT

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    There is a difference in motor design between AC and DC.

    You can use the DC motor as you intend to slow rotation down but the braking force is a function of the motor current going through the braking resistor. As the rotation rate decreases, the braking force will decrease to zero at zero rpm.

    AC braking using DC injection operates by the armature position in essence locking into a winding position. You will have a braking force at zero rpm to max motor speed. With AC injection, you also have to be careful to not cause winding overheating so you either need to turn ovff the DC injection after Stop is achieved or else use some additional controls to reduce the DC current to a tolerable level resulting in a descrease in the braking force.

    The problem with your approach is, unless you are going to use tach feedback, the motor controller has no way of knowing if the motor armature is rotating forward, reverse, or stationary.

    You could achieve what you are trying to do with an AC motor using vector mode control. This would accomplish everything you are trying to do with the DC motor that you can't.

    If you still do not understand why I am telling you that you can;t do it the way you are trying to do it, then you need to study up on AC/DC motor design and theory. Then you will understand why.

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    Quote Originally Posted by Ziggy2 View Post
    There is a difference in motor design between AC and DC.

    You can use the DC motor as you intend to slow rotation down but the braking force is a function of the motor current going through the braking resistor. As the rotation rate decreases, the braking force will decrease to zero at zero rpm.

    AC braking using DC injection operates by the armature position in essence locking into a winding position. You will have a braking force at zero rpm to max motor speed. With AC injection, you also have to be careful to not cause winding overheating so you either need to turn ovff the DC injection after Stop is achieved or else use some additional controls to reduce the DC current to a tolerable level resulting in a descrease in the braking force.

    The problem with your approach is, unless you are going to use tach feedback, the motor controller has no way of knowing if the motor armature is rotating forward, reverse, or stationary.

    You could achieve what you are trying to do with an AC motor using vector mode control. This would accomplish everything you are trying to do with the DC motor that you can't.

    If you still do not understand why I am telling you that you can;t do it the way you are trying to do it, then you need to study up on AC/DC motor design and theory. Then you will understand why.
    Forgot to mention, it also has to work under water. That kind of rules out electronics.

    I guess I need to figure out exactly what these motors are. They just say series wound 12V 2.5KW. They have 4 terminals on them and are reversible hence why I thought I might be able to stop one with some current applied to one field coil.

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    "Stopping" is easy.

    " Holding" can be done with DC motors, and also with AC motors, but it takes more "stuff", and both can run into cooling issues. You apply counter-torque, basically drive the motor opposite to the torque that is trying to turn it. You do need to know if/when/how much it does turn, so that the counter torque can be adjusted to balance the input torque.

    What percent of max torque does the motor need to hold against?

    How much of the system is under water?

    If possible, what IS this doing?

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    I have several patents on this subject. Unfortunately, the search system has changed since I last looked and the only ones I could turn up didn't apply here. I think part of the problem is that my name is different on some, some with my middle name and some without. If anyone really wants to see them, I will keep trying.

    Anyway, the situation is that you cannot lock either a normal AC or DC motor by a simple application of voltage somewhere. The reason is that the motor must turn at least a little bit to generate the braking current. I had systems that would stop accurately but not lock and some that would lock, but the latter were permanent magnet armature synchronus motors. In those the armature magnet poles follow the rotating field produced by the stator and to lock it, you simply stop rotating by feeding it DC instead of AC. You can hold a normal motor at a point by sensing position and using a servo amplifier to drive it back when it goes off position, but even then, there has to be an error to activate the servo. Note that your CNC machines hunt one increment.

    Bill

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    Quote Originally Posted by Garwood View Post
    Forgot to mention, it also has to work under water. That kind of rules out electronics.

    I guess I need to figure out exactly what these motors are. They just say series wound 12V 2.5KW. They have 4 terminals on them and are reversible hence why I thought I might be able to stop one with some current applied to one field coil.
    There is a motor type that will do that nicely, called a "hydraulic motor".

    Bill

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    Quote Originally Posted by 9100 View Post
    There is a motor type that will do that nicely, called a "hydraulic motor".

    Bill
    LOL!

    Aye.. and some have scales and fins. Others tentacles and smell vaguely of ammonia.

    Unfortunately, the only ones as are readily trainable to commands tend to train their handlers right back out of sport.. train a person with an advanced degree in Marine Biology to tote plastic buckets of fish and do tricks?

    Will also spend all too much time shagging or masturbating when bored of captivity.

    Yah. Dolphins. They just do that.

    Must have a different set of priorities than Iron and Copper?

    Go figure.



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