Powerflex 40 and Dynamic Braking Resistors

I don't think I understand how to use the Dynamic Braking options for the Powerflex 4/40 drives.

I have a Powerflex 40, and a properly sized Dynamic Braking resistor. I'm trying to quickly slow down a high speed motor when I drop out the Run/Forward command.

If I set the Stop mode to Ramp, I can quickly slow it down, but if I go too fast, I get an F05 alarm, Overvoltage.

I installed the braking resistor on the BR+/- terminals, but I do not believe I understand how to set A080, A081, and A082. The protection, DB Level, and DB Time options.

If I set the Stop mode to DCBrake, leave the A081 to the default Drive Rated A x 0.05, and set the A082 to 4.0 seconds, the drive turns off, and the motor just coasts.

My motor is a 380V 3 phase, 300HZ, 18000 RPM, 4.0A full load.
My Powerflex 40 is rated 480V 6.0A

I think my problem is I don't understand the Dynamic brake settings enough to comfortably play around with the settings. The User Manual for the Powerflex 40 doesn't explain the meaning of the settings very well.

DC Braking stop mode means DC current is applied to the motor windings which can provide a very fast stop or a holding force after a ramp stop. This method can tend to heat the motor windings and can result in overcurrent conditions and motor failures if the stop needs to occur with much frequency.

This has nothing to do with dynamic braking which is only used to regulate the DC Bus (hence the confusion).

To use the dynamic braking resistor, you will want to set the stop mode to ramp, choose the deceleration time you need, and will only be concerned with setting A082 for the correct value to match your dynamic braking resistor.

If you want to inject DC into the motor windings to hold position, then set A080 and A081 for the amount of DC current and the duration in seconds, otherwise set the time to zero (default).

If you can't get a fast enough decel without going into a bus overvoltage fault, then increase A082, but be sure your resistor doesn't overheat and you aren't exceeding its rated duty cycle.

Seems like a messed a few parameters up in my first post. Was just going by memory. A082 is the DB Resistor Selection.

I think I understand it well enough now.

How can I determine what DC Current levels I should use if I choose to use DC Braking?

Tharon, you cannot use DC Braking and dynamic braking at the same time. This is due to the simple fact that you cannot put DC and AC current thru a motor at the same time. Therefore, DC Braking can only be used if the drive is set to Coast to Stop. That's not what you want.

Setting the drive to Ramp to Stop makes the motor follow the selected decel ramp every time, regardless of loading, so the stop time is consistent. The motor, if overhauled by its load, becomes a generator and sends the braking energy back to the drive where it accumulates in the DC bus capacitors. The brake chopper monitors the bus voltage and, when it gets too high, diverts most of the excess energy to the resistor. That's what you want. Of course, there are always limitations. First, you cannot stop so fast that the drive current limit is exceeded (it works both ways). Second, you cannot exceed the thermal capacity of the resistor.

OkiePC mentions DC Hold which can be a very useful thing if you need to hold the motor in place once it's stopped. You can use this feature with dynamic braking because the DC doesn't go to the motor until the motor is stopped and the AC is turned off. Don't confuse this with DC Braking tho. DC Braking takes the spinning motor down to a stop but, as I said above, it cannot be mixed with AC.

Just a word about resistor sizing. The wattage rating of a resistor is its continuous rating. For infrequent stopping where the resistor starts out completely cold, you can actually nail it with ten times it's wattage rating for a few seconds and the resistor will not be harmed. This works very nicely for E-stop, for example, where it rarely is used but, when it is used, you want to slam stop. Of course, you are still limited by the current limit in the drive but, if it's high enough, you can get away with a relatively small resistor and still get a powerful stop.