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.
