KBPC-225D for 90V 22A brushed DC motor

[mcdanlj]

I'm thinking about variable speed control for my G0709. I had thought about 3Ø. But last night I had another idea...

I have an unused 3HP 90V 22A brushed DC motor (intended for a heavy-duty treadmill) for historical reasons. It looks like the KBPC-225D would be a good controller for this motor. Seems like a better idea than a treadmill controller transplant. I would have to make up mounts and probably new pulleys because I doubt the shaft is the same, but it doesn't seem obviously crazy. Has brake function, optional forward/brake/reverse switch. I would have to do an encoder-to-DAC for tachometer input but I would want something to drive a tach display anyway and adding a DAC output for analog speed feedback would be trivial if I'm already doing a digital tach display from optical encoder. That would be less expensive than a 3Ø + VFD with similar benefit. Am I missing a downside?

 

[markba633csi]

That is a lot of current- the KB unit is rated for it, but you might find it needs forced air cooling. Should work though. Need hefty wire
M

 

[warrjon]

The problem with brushed DC motors is they will slow down with load, so it may not be the best replacement for a milling machine, especially if you take heavy cuts.

I replaced the dead 240V single phase motor on my 4 x 6 bandsaw with a 220V DC treadmill motor and it works fine for that. But for a mill I would use 3phase or BLDC.

 

[mcdanlj]

That is a lot of current- the KB unit is rated for it, but you might find it needs forced air cooling. Should work though. Need hefty wire

The current (no pun intended) motor on the lathe is rated 2HP. This control unit is rated to 3HP continuous at up to 45⁰ and if it's that hot in my shop I won't be working in it. Given that it's washdown-safe, I really expect they designed it for free-air cooling with an appropriate engineering margin.

I'm aware of the need for heavy gauge wire. My previous conversion project sometimes pulls more than 200A.

☺

The problem with brushed DC motors is they will slow down with load, so it may not be the best replacement for a milling machine, especially if you take heavy cuts.

It's a lathe, not a milling machine. (I know griz has model numbers that don't tell you what kind of machine it is.)

This controller has load regulation via armature feedback, so at least in theory it should respond to load. It also has tachometer feedback that is intended to be more precise, and there's an optical encoder on the motor itself that I could read and turn into an analog voltage for speed feedback that could be more precise. I think the encoder wheel is pulse per 5⁰ IIRC; it's something like that, so it could react faster than the armature feedback, I think. Both are rated to ±1%; armature ±1% base speed (relative to potentiometer setting of max speed IIRC) and tachometer ±1% set speed (as set by the dial on the front). How quickly it responds when it's outside that ±1% I don't know.

Tachometer Feedback (J3) – The control is factory set for armature feedback which provides good load regulation for most applications. If superior load regulation is required, tachometer feedback can provide over 1% load regulation over a 50:1 speed range. If tachometer feedback is to be used, J3 must be placed in the "TFB" position and an external DC tachometer must be connected.

I expect I'd do two tachometers; one motor and one spindle since I wouldn't be doing away with the gearbox. The fastest gears in the lathe are already stepping 3600RPM from the motor down to 2000RPM on the spindle. I would use a different pulley since this DC motor is rated 4300 max RPM, so I would see less than ±1% on the spindle from stepping down at least 2:1 between pulleys and gear train.

Also, I could select the target RPM digitally with the same computer I have reading the motor RPM and command the selected RPM to the controller with another DAC. From the manual:

J7 - Signal Input Voltage – The output of this control is normally controlled with the main potentiometer. However, an Isolated analog voltage may also be used in place of a potentiometer. The control can be scaled for either a 0-5VDC or 0-10VDC by placing J7 in the appropriate position "5V" or "10V". The scaling can be further adjusted with the "Max" trimpot.

The controller has remote enable/inhibit to connect to the existing safety systems.

 

[markba633csi]

Yes that's correct, the KB controllers have feedback compensation. A dc motor by itself will slow under load.
M