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Thread: concept of braking in VFDs

  1. #11
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    You have been given lots of good information by people that are better schooled than me.

    But I’ll add something to consider. When the load is such that the motor is acting as a generator you have a few options. One, you can waste that energy in a resistor bank. Cheap but not very efficient. Two you can add costly parts and send it back into the electrical grid.

    However there is a third option that becomes appealing if you have enough of these loads. That being that you feed the drives with DC voltage. This could be one large rectifier bridge supplying few/many drives....or it could be one oversized drive supplying another drive’s DC bus for a motor used in the same machine. This way you are keeping all your self-generated power for your own use. Just another way to skin the cat that wasn’t mentioned. But it requires a little more engineering.

    Considering the questions you ask and the loads you mention I’d think it’s worth looking into.

  2. #12
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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
    Those comments address the fundamental concept on dynamic and regenerative braking, and I concur.

    I will address OP's concern that pertains to-- (verbatim) . . . “how an induction motor turns into a generator, and to become a generator magnetic field is required.. . . and when supply is cut then no magnetic field forms, then how motor becomes a generator.”

    It doesn't become a generator when referring to a pure induction motor. You can spin it all day long and it is not going to become a generator.

    A generator needs excited magnetic field or permanent magnet to make electricity.

    Both braking modes require that the motor (drive) is connected mechanically/electrically for dynamic or regenerative braking to occur.

    When the motor is actually disconnected (which is what OP is alluding to) . . . either by mechanical coupling or contactors. . . no (electrical) braking mode is going to happen.

    That is the prevailing dilemma.

    In a nutshell. . . dynamic braking in a pure induction motor is not possible while coasting in the absence of incoming power.
    Save for a synchronous motor which needs field excitation or permanent magnet to (re)generate power.

    Put another way:

    Either of the braking mode will happen when the motor is coasting to stop while still connected to power line. . . not when it is totally disconnected. (cut off as OP stated)

  3. #13
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    Quote Originally Posted by Besoeker View Post
    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.
    wow great man
    i got lots of knowledge and doubts cleared
    i think this explanation will not get on internet or text book
    i m very much thankful to u for giving me such clear concept

    thanks a lot again

  4. #14
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    Quote Originally Posted by Jraef View Post
    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.
    u are awesome
    no words for you
    u just elaborated it perfectly dear
    i think u all guys are very much experienced and have clear concept

    i think i have done some good deeds in past so i landed here in this forum

    thanks once again

  5. #15
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    i have seen R-, R+/DC+ and DC- terminal in drive which we have in our organisation. so my question is that there are three terminal for connecting resistors. then which terminal should i connect resistance directly or connect CDBR unit directly. and does every drive has these three terminal? if this drive has chopper circuit inbuilt? if not then CDBR are used for dissipating extra energy?

  6. #16
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    Quote Originally Posted by Abhi View Post
    i have seen R-, R+/DC+ and DC- terminal in drive which we have in our organisation. so my question is that there are three terminal for connecting resistors. then which terminal should i connect resistance directly or connect CDBR unit directly. and does every drive has these three terminal? if this drive has chopper circuit inbuilt? if not then CDBR are used for dissipating extra energy?
    The resistor only uses two leads. The "R" designation is for the resistor" DC+ and DC- are terminals that connect to the DC bus that is between the input rectifier and the output inverter. You apparently have one terminal that is common to both.
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  7. #17
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    And every drive mfr provides different terminal designations, you cannot assume anything.

    A very important aspect of VFD construction is what is called a "pre-charge circuit" in the power section. When a VFD is initially energized, the charging current of the DC bus capacitors, even though of very short duration, is potentially as high as the available current in the system, i.e. the Available Fault Current at the terminals. To avoid having that cause damage to the diode bridge and the capacitors themselves, something has to limit that initial current inrush, and that is the pre-charge circuit. That can be done in several different ways but the most common method is to use a current limiting resistor in the DC circuit, ahead of and in series with the capacitors. That only needs to be there for a second when the drive is first powered up, so there is then a contactor/relay in parallel with that resistor that bypasses it a second later. This is important because when a drive mfr puts terminals on the DC bus, it's imperative that they explain to you, in the installation instructions, whether or not those terminals are AHEAD or BEHIND the pre-charge circuit. So for example if the DC terminals are there so that you can tie the DC busses of several drives together to share loads, but the DC bus is meant to be fed by a common rectifier that has it's OWN separate pre-charge circuit, those DC terminals would be upstream of the drive's internal pre-charge. If the terminals are DOWNSTREAM of the pre-charge, connecting something else to them then is bypassing that drive's pre-charge and could cause damage. DBR terminals are usually like this because you don't need to worry about this issue if your only goal is Dynamic Braking. So in your example, I'd guess that the R+ and R- terminals are for the resistors and therefore downstream of the pre-charge, but the DC+ and DC- terminals are upstream for connection to a common DC bus sharing system. But this is a GUESS, the only people who know for sure are the ones who made that VFD, so never assume anything. RTFM.
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  8. #18
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    Quote Originally Posted by Jraef View Post
    And every drive mfr provides different terminal designations, you cannot assume anything.

    A very important aspect of VFD construction is what is called a "pre-charge circuit" in the power section. When a VFD is initially energized, the charging current of the DC bus capacitors, even though of very short duration, is potentially as high as the available current in the system, i.e. the Available Fault Current at the terminals. To avoid having that cause damage to the diode bridge and the capacitors themselves, something has to limit that initial current inrush, and that is the pre-charge circuit. That can be done in several different ways but the most common method is to use a current limiting resistor in the DC circuit, ahead of and in series with the capacitors. That only needs to be there for a second when the drive is first powered up, so there is then a contactor/relay in parallel with that resistor that bypasses it a second later. This is important because when a drive mfr puts terminals on the DC bus, it's imperative that they explain to you, in the installation instructions, whether or not those terminals are AHEAD or BEHIND the pre-charge circuit. So for example if the DC terminals are there so that you can tie the DC busses of several drives together to share loads, but the DC bus is meant to be fed by a common rectifier that has it's OWN separate pre-charge circuit, those DC terminals would be upstream of the drive's internal pre-charge. If the terminals are DOWNSTREAM of the pre-charge, connecting something else to them then is bypassing that drive's pre-charge and could cause damage. DBR terminals are usually like this because you don't need to worry about this issue if your only goal is Dynamic Braking. So in your example, I'd guess that the R+ and R- terminals are for the resistors and therefore downstream of the pre-charge, but the DC+ and DC- terminals are upstream for connection to a common DC bus sharing system. But this is a GUESS, the only people who know for sure are the ones who made that VFD, so never assume anything. RTFM.
    I think his confusion is he has three terminals - one of them marked R+/DC+. To me that means R+ and DC+ are internally tied together and they just provided one common terminal to be used for both functions.
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  9. #19
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    in this pic i want to ask that both provision is given for external resistance and cdbr ?
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  10. #20
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    Quote Originally Posted by Abhi View Post
    in this pic i want to ask that both provision is given for external resistance and cdbr ?
    As J said, it varies depending of manufacturer. To me it seems like one includes the chopper IGBT and the other doesn't. But a call to the manufacturer/supplier might be in order.
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