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Thread: How Do Dynamic Motor Brakes Work?

  1. #1
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    Question How Do Dynamic Motor Brakes Work?

    Lately I have been installing VSDs on a number of 5 HP 3 phase grinders. Along with the VSD we install an optional resistor that gets wired to terminals labeled dynamic brake on the VSD. The grinder without the brake would coast for at least a minute or two. With the brake it stops in a couple of seconds max.

    What exactly is happening when the VSD changes from run to stop?

    Is the motor being used as a generator and dumping the power into the resistor?

    And if so why only one resistor for a three phase motor, I would have expected at least two or three?

    Does the VSD put power into the motor to keep the field working as a generator?


    Go easy, I am no electric motor expert. I can wire them fine, just don't know how they work.
    Last edited by iwire; 02-06-08 at 03:46 PM.

  2. #2
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    Bob.,,

    I will explain real quick and simple way with the dyanamatic braking system


    for most AC motor it is common to inject a small DC current in one of the two leads [ it will work both single and multi phase motors ] and what it do is make a magatanitc feild to slow down the rotor because of injected DC source act like counter electromagatic force to slow down the motor much quicker.

    with the VSD it is easy to do this by feed little DC current to the AC motor but tricky part is to timed to how far you can inject it and when to stop injecting the DC current in the motor [ senice most VSD useally have somekind of feedback so they can able to read how far it will before it stop feeding the DC current.]

    but with AC motors without VSD's this get little more trickier with this but it done often on it. they have a interlock switch on motor contractor to sense if the power is on or not when the motor is coasting it will inject the DC from power supply to slow down pretty fast but they have build in timer to stop suppling the power to the AC motor

    for the DC motor it pretty straght foward it will become generator and used the load bank to slow them down pretty fast as long you are not doing the plugging it [ plugging mean reversing the connection very fast when still running ]

    Hope it will help you.

    If need more expain or other let me know i may find something to show ya

    Merci, Marc
    Marc
    master electrician
    Wisconsin and Paris France

    "Pas de problème, il marche n'est-ce pas?" (No problem, it works doesn't it?)

  3. #3
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    Mark,
    I don't think that dynamic breaking and DC injection are the same thing. Most of the drives I have worked with can do both, but you have to add the resistor for dynamic breaking.
    Don
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  4. #4
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    Don, Yes that true you can add the resistor to adjust the amout you want to use on the dyamatic braking system.


    I know the crane system often used this type of braking system.

    [ i will find a link related to this info later and i will add to this fourm ]

    Merci, Marc
    Marc
    master electrician
    Wisconsin and Paris France

    "Pas de problème, il marche n'est-ce pas?" (No problem, it works doesn't it?)

  5. #5
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    Bob Lets see if I can help you understand..The analogy I will use is a diesel engine when they want to sow down they can use a device called a Jake break..It is a device that use the compression on the engine to slow down the engine that is generating the compression by changing the valve useage..Now the dynamic brake is very similar..the motor is spinning in a lets clockwise direction and you disconnect the electrical source and now you have a generator..the drive now takes the power and applies it to a different magnetic field and is injected back into the motor..This process generates allot of heat the resistor is how the excessive heat is disipated..The process is actually more complex but I think this is an easy and simple explaination..I am sure we will now hear more on the topic..

    edited to add some more info..

    I actually believe the style of braking you have discribed would be load braking..I believe you have described a machine that creates mechanical energy and the drive is probabully designed to do load braking..I believe that is why you are inserting the external resistor is to aid in the disipation of mechanical energy..I believe there will more in depth answers..I know Jraef has a very good understanding of this..
    Last edited by cschmid; 02-06-08 at 08:33 PM.
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  6. #6
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    Some of the dynamic brakes that I have worked on (printing presses) were even water cooled. The resistors darned near short the motor leads, and they can get quite hot. The bigger one's have a water cooled radiator.

  7. #7
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    here is a link to Allen Bradley on ac brake basics

    http://literature.rockwellautomation...p004_-en-p.pdf

    http://www.tecnaut.com.br/utilidades...t001f-en-p.pdf

    I hope this helps
    Last edited by cschmid; 02-06-08 at 11:18 PM.
    Life is temporary, heaven is forever. live life like it is your only chance to make a difference..

    to do nothing is the surest way to achieve nothing..

  8. #8
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    You were right, Dynamic Braking is all about regeneration.

    Any AC motor can become a generator under the following conditions:
    1) The motor stator field is excited.
    2) The motor is spinning faster than the applied power frequency used to excite it (overhauling).

    When you have a VFD you are controlling the frequency to run at different speeds. But then when you want to shut down, you also now have the ability to alter the excitation field frequency as well. So during braking, the VFD is keeping the motor excited so that it keep generating no matter how much slower it is getting.

    Then, the power it is generating is pumping back through the VFD transistors into the DC bus and charging the bus voltage higher and higher. At some point, the bus voltage would climb too high and damage the components. So a voltage monitor is put on the DC bus and when a threshold is reached, it fires a 7th transistor that pumps that excess DC power into a resistor to be burned off as heat. That is why you only need 1 resistor; it is DC going to it. The bigger the resistor the more power it can dissipate, but you are still limited to the power rating of the transistor used to fire into it. In mdshunk's example of a printing press where there is a lot of inertia to overcome and/or the braking can be constant in order to control web tension, the transistor is custom designed for the job. But for most common machine applications, the drive will come with a transistor sized the same as the power transistors, so that is why you will see the braking torque limited to 100% of the motor torque.

    It's called "Dynamic Braking" (DB) because the amount of braking torque that can be applied is dynamically changing as the load decelerates. In other words, the braking energy is a function of the kinetic energy in the spinning mass and as it declines, so does the braking capacity. So the faster it is spinning or the more nertia it has, the harder you can apply the brakes to it, but as it slows, you run into the law of diminishing returns and at some point, you actually have NO braking power left. That is why in almost all VFDs they will use DC Injection Braking (DCIB) at the very end. The problem with DC injection is that it traps the kinetic energy inside of the motor, specifically in the rotor where it has trouble getting out as heat. So the ideal package is to use the DB to get most of the energy out of the load, then finish it off with DCIB when it's just creeping along.

    An alternative to using DB resistors is what is called Line Regenerative (a.k.a. Regen) Dynamic Braking. It is the same, except instead of firing the excess DC power into a resistor as heat, it pumps it through another inverter on the front end as power back into the utility supply. That's great for those continuous load situations like printing presses, but is a quantum leap more expensive because you are essentially using 2 VFDs to do one job.

    Hope that helped.

  9. #9
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    Thumbs up

    Thank you all, I have a much better understanding of how it works.


    Quote Originally Posted by Jraef
    Hope that helped.
    Yes very much, I appreciate the added info on the DC injection.

    The machines are actually large belt grinders and do not roll that easily due to the soft wheels the belts run on. They seem to rely on only the dynamic braking. When stop is pressed they rapidly slow down and then when almost stopped coast to a stop.

    They also have other large conventional grinders that are just grinder wheels connected directly to a the motor shaft, I have to assume they are using DC injection as they stop very quickly, almost like a mechanical brake has been applied.

    The motors also groan pretty loud during the braking, I assume the motor could be damaged with short cycles?

  10. #10
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    DC injection?

    I have dealt with numerous dynamic braking systems for motors with VSD (including a 30 MW motor), but I am not familiar with DC injection. It has to saturate the core or back steel, but how does this dissipate the energy from the load inertia?

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