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Thread: concept of braking in VFDs

  1. #1
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    concept of braking in VFDs

    I am new to VFDs and searched lots and lots in google regarding it. And the more I searched the more confusion arised. So my questions are
    What is regenerative braking?
    What is dynamic braking?
    What braking chopper circuit?
    What is DBR?
    and when, where, why and how above techniques is used?
    I have seen in my work place that some drives have external resistance and some dont. And some drives had external resistance initially like in cranes but later
    on it was deactivated for some reasons and now those drives runs with external resistance.
    And on more thing, if an ac motor is stopped and no current flows through it, then how it will acts as a generator.

    All these doubts I have is for induction motors which runs on AC supply.

    Please help me out.

  2. #2
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    Regenative is an electronic type of breaking, where the controller through electronics reverses the current, and is the best type in my opinion. Dynamic breaking uses an external resistor, but I have only seen this type with DC Drive Systems. DBR I Am Guessing is a dynamic break resistor. D.C. Drives also use Regenative Breaking.


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  3. #3
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    Quote Originally Posted by Abhi View Post
    I am new to VFDs and searched lots and lots in google regarding it. And the more I searched the more confusion arised. So my questions are
    What is regenerative braking?
    What is dynamic braking?
    What braking chopper circuit?
    What is DBR?
    and when, where, why and how above techniques is used?
    I have seen in my work place that some drives have external resistance and some dont. And some drives had external resistance initially like in cranes but later
    on it was deactivated for some reasons and now those drives runs with external resistance.
    And on more thing, if an ac motor is stopped and no current flows through it, then how it will acts as a generator.

    All these doubts I have is for induction motors which runs on AC supply.

    Please help me out.
    typically this is used when load inertia can overhaul the motor and cause it to generate power
    there are a few subject matter experts on the forum, hopefully they'll chime in (and correct me if I misstate anything)

    regen allows the motor to act as a generator and inject power into the grid
    dynamic dissipates that power into a resistor (DBR = dynamic braking resistor?)
    chopper, it is the circuit that allows the power produced by the motor to be directed to the resistor

    some drives require these techniques, some don't, all dependent on application and the load
    it the motor is stopped and no current flows it can't generate, it generates when the load is applying torque to it
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  4. #4
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    Quote Originally Posted by shull View Post
    Regenative is an electronic type of breaking, where the controller through electronics reverses the current, and is the best type in my opinion. Dynamic breaking uses an external resistor, but I have only seen this type with DC Drive Systems. DBR I Am Guessing is a dynamic break resistor. D.C. Drives also use Regenative Breaking.


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    Reversing current would force the motor to put out torque in opposite direction. Is effective breaking method but forces motor to do the breaking and to dissipate the excess heat that develops. The other methods are taking energy already existing in the spinning load and either putting it back into the supply circuit or dissipating that energy into another load - typically a braking resistor.
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  5. #5
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    DBR is actually i didn't mean it. Actually it was CDBR which is dynamic braking unit and not dynamic braking resistance. I have seen in some drives that first cdbr is connected and then external resistance. And in some cases external resistance is directly connected to drive. But how an induction motor turns into generator, and to become a generator magnetic field is required. and when supply is cut then no magnetic field forms. then how motor becomes generator.
    all this phenomena in ac motor how it is possible
    some concepts are missing that i dont know
    plz help
    And regenerative braking occurs when speed of rotor is higher than speed of rotating magnetic field in stator

    how it is possible

  6. #6
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    Generator and a motor are very much the same, just reversal of the basic functions.

    Motor uses magnetic fields to create motion. Those magnetic fields don't instantly disappear when when you interrupt the supply circuit. The fact you still have motion and magnetic fields gives you a generator, maybe not an optmized generator but a generator nonetheless. Connecting a load (the braking resistor) to is is a way of pulling some of the energy from that generator, since you only have the inertia of the load and are not putting in any new energy, loading it will only slow it down until the magnetic fields reach a certain point where it can't generate any more output current. If a high enough inertia load you may still need to inject reverse current at that point to help continue decelerating the load.
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  7. #7
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    Quote Originally Posted by Abhi View Post
    DBR is actually i didn't mean it. Actually it was CDBR which is dynamic braking unit and not dynamic braking resistance. I have seen in some drives that first cdbr is connected and then external resistance. And in some cases external resistance is directly connected to drive. But how an induction motor turns into generator, and to become a generator magnetic field is required. and when supply is cut then no magnetic field forms. then how motor becomes generator.
    all this phenomena in ac motor how it is possible
    some concepts are missing that i dont know
    plz help
    And regenerative braking occurs when speed of rotor is higher than speed of rotating magnetic field in stator

    how it is possible
    In the normal motoring mode the Variable Frequency Drive Puts out a frequency which produces a rotating field in the motor which is slightly faster than the shaft speed. It is this difference, called slip, positive slip in this case, that results in the torque that drives the load. Now, if the motor is driven faster than the rotating field produced by the VFD the slip is reversed, and the torque is reversed and you get braking rather than motoring.

    Typically, this only happens when you are reducing the speed of a high inertia load more rapidly than it would just coasting down.
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    And this is which type of braking, regenerative or dynamic?

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    Quote Originally Posted by Abhi View Post
    And this is which type of braking, regenerative or dynamic?
    It applies to both. The difference is just in where the energy from braking goes. With dynamic braking it is dissipated in a resistor. The usual arrangement is a chopper IGBT which switches on a resistor across the DC link when the DC link rises above a predetermined level. Essentially the stored energy in the load is wasted. If deceleration isn't frequent this is the normal arrangement. Some VSDs come with the braking IGBT fitted as standard whether or not it gets used.

    Regenerative puts the energy back into the supply. This is a bit more complex. In a standard VFD the input bridge converts AC to DC. For regenerative operation there is a second bridge that converts DC to AC. So, instead of dumping the DC braking energy into a resistor it is fed back into the supply via that second bridge.

    The AC to DC is known as conversion. The DC to AC is known as inversion.

    Regenerative braking is obviously more energy efficient but more expensive to manufacture. It is used where there is frequent braking and in situations where there is an ovehauling load. This not uncommon in some production applications such as paper making machines.
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  10. #10
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    Quote Originally Posted by Abhi View Post
    I am new to VFDs and searched lots and lots in google regarding it. And the more I searched the more confusion arised. So my questions are
    What is regenerative braking?
    What is dynamic braking?
    What braking chopper circuit?
    What is DBR?
    and when, where, why and how above techniques is used?
    I have seen in my work place that some drives have external resistance and some dont. And some drives had external resistance initially like in cranes but later
    on it was deactivated for some reasons and now those drives runs with external resistance.
    And on more thing, if an ac motor is stopped and no current flows through it, then how it will acts as a generator.

    All these doubts I have is for induction motors which runs on AC supply.

    Please help me out.
    In all of the above braking techniques, when you want to stop the motor QUICKER than it will coast to a stop, you do NOT remove energy from it, otherwise as you said, it will NOT act as a generator because there will no longer be magnetic fields in it (other than some relatively insignificant residual magnetism in the iron).


    So the first step in doing this with the VFD, since you have total control of the frequency it applies to the motor, is that when a stop command is given, the drive LOWERS the output frequency to where is is BELOW the rotor frequency. Now the motor stator windings are energized so they have magnetism and that then passes into the rotor, but the relative frequency of the stator is kept lower than the relative frequency of the rotor. That then creates a situation wherein the motor is now running in "negative slip" and the kinetic energy of that spinning mass becomes the "prime mover" of the induction motor which has just become an induction generator. The energy of that generator is now going to flow through the transistors back into the VFD and charge up the DC bus.


    Meanwhile, the VFD is monitoring that DC bus voltage and if if begins to drop, the microprocessor continually lowers the output stator frequency to keep that motor in the state of being an induction generator. So that answers the last part of your question first, but it sets up what happens next. The VFD must now finish the process of "transmuting" the kinetic energy in that rotating mass into some other form of energy, because you cannot violate the First Law of Thermodynamics, that energy is not created or destroyed, it is only moved from one place to another. In this case you are moving that kinetic energy from that moving mass into some other place and that “place” is the difference in the systems you asked about.

    If the drive has a "DC brake chopper" in it, that is a 7th transistor that is connected to the DC bus. In "Dynamic Braking", aka "DB", the drive's microprocessor (mP) is ALSO monitoring the DC bus level for being too HIGH, not just too low. If the DC voltage gets beyond a certain threshold, the mP then fires that DC chopper transistor to allow DC energy to flow into a connected "Dynamic Braking Resistor" or "DBR" that is typically mounted outside of the drive somewhere. That DBR then transmutes that excess energy on the DC bus (from the motor/generator) into thermal energy (heat) in that resistor to waste it.

    If your particular VFD was not built WITH its own DC Chopper (7th) transistor, fear not because there are people whole make EXTERNAL DC choppers that can be added onto a VFD by simply connecting to that DC bus, provided they give you easy access to it via terminals. Most do. The external DB "modules" are often sold WITH the resistors built in as well. In those modules, the DC bus sensing system for controlling the chopper is now part of the module, not the VFD, but the VFD mP still has control of the output frequency to keep the motor in a state of being a generator.

    In either of the above cases, the limitation of this system is in the limits of the hardware; how much current can the chopper transistor handle safely, and how much heat can the resistor dissipate without burning itself up. These issues combine into what is called the "duty cycle" of the braking system; how fast can it transmute that energy, AND how often, meaning how much time must it rest in between cycles to cool off. When you find DB resistors that have been disconnected in the field, that is usually the result of someone not having taken the time to run the proper calculations and one or the other of these hardware limitations was exceeded.

    Another negative aspect of DB is in the “dynamic” aspect of it. It is called dynamic because the amount of braking energy is constantly changing, because the motor and load is slowing down. This then also means that as it gets to a low point, there is less and less braking power, so it suffers from the law of diminishing returns and has a hard time finishing the job at the very end. Friction usually takes care of that for you but if not, then you need another form of braking. If there is a mechanical holding brake, people usually use that since there is very little kinetic energy left by then and low areas on the mechanical brake. If not, most VFDs also offer what’s called DC Injection Braking (DCIB) that is triggered at low speeds to finish the job. DCIB puts DC into the AC winding, creating a stationary magnetic field that pulls the rotor field into alignment with it, stopping the load. DCIB however has the problem of converting the kinetic energy into thermal energy INSIDE of the motor, so although you CAN use DCIB to stop it at any time, it’s unadvised since in a VFD, you usually have the option of moving that energy out of the motor with DB. I will warn you however that a lot of people mistakenly interchange the term DB with DCIB, they are NOT the same.


    "Regenerative Braking" is different. It starts out with that same process of keeping the motor being a generator, but differs in how that kinetic energy is transmuted. The difference, referred to as an Active Front End (AFE) drive, is that instead of just having a simple diode bridge rectifier connected to the line side as a "one way" road for energy going in, it has a second inverter used as the rectifier, so it can be a "two way street" for electrical energy. So when that DC bus upper threshold is reached, instead of dumping energy into a resistor to be wasted as heat, it uses that line side inverter to synchronize with the line frequency and allow the excess energy to flow BACK into the AC line source, recovering it for use by other loads in your system. Because there is no direct thermal issue involved, it can do this all day every day, limited only by the same general limits of the motor and drive capacity, which should of course match each other. The only real down side of Line Regen braking however is that you are in essence buying two drives for one motor. But more recently, mfrs have come out with "low harmonic" AFE drives so that instead of JUST line regen braking, the AFE is ALSO mitigating the harmonics, so you can kill two birds with one stone and avoid that harmonic mitigation cost, making this a wiser choice, especially for larger motors.

    Again if you do not already have a Line Regen capable drive, there are companies that make add-on line regen modules, but they (so far) do not provide the option for simultaneous harmonic mitigation.

    I hope that cleared it up for you.
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