# baldor vfd brake comtrols..



## JRaef (Mar 23, 2009)

Calectrical said:


> I have an issue with the dynamic brake controls on a h series baldor vfd is there ant way to use the brake without the dynamic brake controls.


Depends on what you want to do. Dynamic Braking (DB) in a VFD is a way of converting the kinetic energy of the rotating mass into some other controllable form. So when implemented in the VFD, the DB system keeps the motor windings excited, but monitors the speed and lowers the output frequency of the VFD output so that it is always below the rotation speed, which makes the motor into a generator. The motor / generator than pumps energy back into the VFD in the form of excess voltage on the DC bus. The DB chopper circuit them "pumps" that excess energy into a set of resistors and burns it off as heat. Without the DB controller and/or chopper circuit, that energy has nowhere to go. So to that end, no you cannot "use the brake without the dynamic brake controls", because that IS the brake!

But VFDs also have other forms of braking. The other universal and very simple one is DC Injection Braking, or DCIB. This is primarily used as the "clean up" of the braking cycle started by the DB system, because f you think about it, the braking energy in the DB system is getting progressively less and less as the load slows down, and eventually there is nothing left. So at some point, the VFD stops bothering with DB and switches on the DCIB system. DCIB pumps DC current into the stator windings of the motor and creates a stationary magnetic field, so the rotor then "follows" that to a stop. But DCIB does not remove the kinetic energy from the motor, it stays IN the motor and heat. This limits the amount of Braking that can be done this way. That's why it's typically only done at the very end of the braking cycle when there isn't a lot of kinetic energy remaining anyway. 

BUT... some drives allow you the option of using the DCIB at any point, not just at the end of the DB cycle. You have to understand the ramifications in terms of heating up the motor of course, but in a pinch t may be possible. To get an idea of how often you can do this, look at the "Hot Starts per hour" rating of the motor and cut that number in half, because using DCIB is the same as starting the motor from a heat standpoint.

Some advanced Vector Drives may also have a 3rd braking system called "Flux Braking" where the kinetic energy is also converted to DC bus energy, but instead of pumping it into a resistor bank, it increases the flux current in the motor itself, essentially severely decreasing the motor efficiency temporarily so that it can burn off the excess energy in itself. So from that standpoint, it's kind of a hybrid of DB and DCIB in that it uses regeneration as the tool, but the heat remains in the motor. Not all Vector Drives have this feature though, and I'm not sure about the Baldor. But I would suspect that if your "use the brake without the dynamic brake controls" statement means something is wrong with the VFD, flux braking is probably out as well.


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## rick279 (Nov 9, 2010)

Calectrical said:


> I have an issue with the dynamic brake controls on a h series baldor vfd is there ant way to use the brake without the dynamic brake controls.


No. The brake transistor on the VFD supplies excess voltage to the resistor when it is required. If this is a new application you may not have programmed the VFD properly for the brake. If the transistor has failed, you'll need to either buy a new VFD or an external brake module which gets wired to the DC bus.


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