Ot pf525

I have a 3Hp Powerflex 525 600Vac drive equipped with an external DB resistor.

Does anyone know if it's possible to use DB and DC braking at the same time?

My customer wants to use the DB (with resistor) to stop the machine and then use DC to "lock" the rotor for a short time.

I'm presently using DB for stopping purposes.

Just how "locked" does he thing the rotor is going to get with DC injection? Assuming the drive is set up in SV mode I think you would be FAR ahead just leaving the drive enabled with a zero speed command.

But to answer the question, I believe the modes are mutually exclusive.

Keith

That's kind of what I originally thought .....and then I found something called DC hold, hmmmm.

They are mutually exclusive because of how they work. But you almost always use DCIB when you use DB anyway.

DB works by pulling motor energy off of the motor as electricity by turning the motor into a generator, which pushes the electricity back into the DC bus where it is shuntied into a resistor to be burned off as heat. But that suffers from the law of diminishing returns because as the motor slows, it contains less and less kinetic energy, so less and less braking torque is realized and at some point, the motor cannot generate enough electricity to feed back into the drive and the motor coasts. So once that happens, you cease the DB and trigger the DCIB to put DC on one winding, which makes the rotor line up to the now non-rotating magnetic field. DCIB will "lock" the rotor, but can only do do briefly, because it is heating up the rotor, yet the cooling system for it no longer exists. So once the rotor stops, the DCIB turns off. All of this is programmable in the VFD.

If you need to HOLD the shaft for safety reasons, you cannot use electronic braking of any kind, because if power fails, you have nothing. So you use a mechanical brake as a "parking brake", but use the electronic braking first so that the mechanical brake never wears out.

I misspoke above. Dynamic braking is a bus regulation mode. DC braking is a stop mode. They can operate together because they perform different functions.

The case jraef describes is true in V/Hz mode. Because the motor voltage is tied to output frequency you will get to a point where the motor is no longer magnetically fluxed enough to do anything. However, in SV mode, this point is extended much farther toward zero speed than in V/Hz mode. Sooner or later, though, you will get to a point where even that will fall apart.

DC Hold is not a magic bullet either, though. An induction motor is called an induction motor for a reason.The magnetic field on the rotor is created by current induced in the rotor bards from a changing magnetic field in the stator. If the stator field is fixed (DC applied) the magnetic field in the rotor will collapse, usually within 100-300 msec. After that, motion of the rotor would be required to induce more current in the rotor to regain a rotor magnetic field for the stator filed to interact with. DC Hold is a pretty loose hold and speed dependent at that.

Keith

I totally agree with what your'e saying Keith.

My understanding is the device tends to "wander" a bit after stopping using DB.

All they're after is a short (>500mSec) rotor lock to allow the next product piece to load properly.

Sounds like I should experiment with both DC brake with a time base slightly longer than the decel ramp, or if that doesn't fly, have the plc turn on DC hold for a short duration once the drive reaches zero speed.

Let me try this again. You just program the drive to engage the DC Braking (after a Stop command) once the output gets to less than 10Hz. Adjust that Hz setting according to performance observation. How it gets from full speed to that speed is irrelevant to the DCIB functionality, you can coast to there or use the Dynamic Braking. They are separate issues in the drive programming.

The drive can automatically turn off the DCIB when it stops, or hold it for a programmed amount of time.