First one needs to understand what the electronic braking does and how it is applied. When one sets a braking time, it sets the slope of the braking rate based on the time (say 2 seconds) and the maximum Hz (say 60Hz). If you are running the motor at 30 Hz it will stop in 1 second, and at 15 Hz in 0.5 seconds. It is not 2 seconds for any Hz.
Ah, interesting, I didn't know that. It's based on the number of line power cycles vs time. Thanks!
There is also how the braking is applied, so there is linear and S-Curve, and these can effect the slope of the braking during deceleration. An S curve will have a steeper slope at the center, this may exacerbate an over voltage error, I find a linear slope on braking to give the most consistent stopping. The Invertek VFD offers no control over how the braking is applied. If you get an over voltage buss error then you are braking too fast. If you are spinning a big chuck at high speed then it may cause an over voltage error and the VFD will go into a free run mode with no braking. Higher end VFD's have parameters which will prevent this from happening by adjusting the braking rate if too high so an over voltage error does not occur. Use of DC injection does not effect braking until very low speed, so it is used to bring high momentum systems to a stop at the end of their braking.
I think I only got the over-volt error when I was running it without the braking resistor. I haven't seen it recur, even when stopping quickly from pretty high speed.
I might want to play with DC injection, it's a function on the Invertek (P-32, P-58 and P-59) - or maybe not, see my parting comment below.
I think what I was interpreting as faulty braking was just that its effect drops off at very low frequencies, when the motor is still spinning, albeit slowly. With a heavy chuck, it tends to "coast" a bit. I tried taking the chuck off, and the spindle on its own didn't tend to coast as much.
It turns out I was running the drive at a very low frequency (only 20 Hz), thinking I was being cautious by not having things spinning so fast. But at such a low RPM, I was likely close to the regime where electrical braking doesn't work as well. As noted below, it works pretty well at 60 Hz.
(An unimportant point, but I think I now understand another behavior I observed: If I program a very short stop time, it can actually take longer for the motor to stop turning than with a somewhat longer one. My interpretation is that the ramp-down is so fast that the chuck still has more kinetic energy left when the frequency drops below the effective control range.)
That's interesting about the more advanced drives letting you control the braking curve, and having the smarts to back off in an over-voltage condition; it makes a lot of sense, and I can imagine there are a lot more complex options that can be programmed for special industrial uses. This is definitely a basic application not needing all that, but it's interesting to read about
P-04 should be something like 2 seconds. P-24 allows a second ramp time (fast stop) input, but this only applies if it is set to a time less than P-04, so say 1.5 seconds or maybe 1 second. Braking resistor for 2 Hp unit is 100 ohms, 3 Hp unit is 50 ohms. There are different trad offs as to the resistor size, but maximal braking is achieved with a lower resistance, but less frequent repeat stops. This is outlined for other VFD drives.
Wow, thanks for going so far as to look up the Invertek manual!
Yeah, mine is a 3HP unit, running a 2HP motor. The resistor I bought is 47 ohms, which I figured was close enough. I didn't remotely need the wattage, but a 400W unit was cheaper than less burly ones. (I could have gone a lot cheaper with a regular power resistor, but I wanted the isolated case for safety.) After multiple fairly high-speed stops, the resistor is still cool to the touch.
Teco L510 does not support an external braking resistor. The emergency stops and braking for power loss are usually programmable, but depends on the VFD. In general, although the Invertek is relatively easy to setup, I do not use them for lathes because of limited programmable inputs as well as very limited programming ability.
I'm glad I got the Invertek then. It sounds like Tecos might be more programmable, but I like having the braking resistor and it's already way more programmable than I need
Bottom line on my issue, I was running the drive at such a low frequency that the electronic braking wasn't very effective and left a lot of momentum in the spindle and chuck at the point that the frequency dropped so low that it lost control of the motor. At 60Hz it stops pretty close to the time programmed (whether in P-4 or P-24) but still has some residual momentum. I might try experimenting with DC injection for the Fast Stop, but proceed carefully. The Invertek programs the DC current as a percentage of running current, though, and I'd think even 100% current (~9 amps) for less than a second couldn't do too much to the unit - and I'd start with the minimum of 20%.
OTOH, maybe I should just leave well enough alone. Stopping the motor in ~1 second is pretty darn fast, and I don't want to over-stress the gearing by having the motor slam everything to an instantaneous stop.
Thanks again for taking the time to leave such a complete and detailed answer, I really appreciate it!
