So if I put a meter on the DC bus or watch DC bus voltage on the display and see 810V then I know I cant go higher than 5%. But if I see someting lower then I know I can increase the duty cycle as long at the voltage I'm seeing times the duty cycle is less than 4000W? What if the voltage is fluctuating?
V^2/R * duty cycle = average power dissipated in the resistor
As a test can I go all the way to 100% just to see if this lets the drive stop without tripping then adjust based on voltage levels, or am I risking burning the resistor by doing this even just one time.
I'm a bad one to ask. A couple of weeks ago I (unintentionally) dumped 80KW into a test motor during a locked rotor test. The smoke came out _very_ quickly.
Seriously, such a test is plausible, but not with considerably more design work. For example, do you have a datasheet for the resistor, and does it give the resistor's overload capacity or thermal time constant? The resistor should be rated for both its maximum _continuous_ dissipation (4KW) and also rated for different power levels for different time periods. If the resistor is rated for a _peak_ dissipation of 20KW, then I would not attempt a test at 100%, where the dissipation could easily be 80KW. On the other hand, if your resistor is rated for 100KW for 10s (or some such) then I'd be much more comfortable with such a test...just be ready with an emergency power off.
In your example above, what if the PWM time is greater than 1s? Does the calculations then change as I think they would? How do you find what the PWM time is for a given drive, does it state it.
The amount of energy dissipated in a single pulse is proportional to the PWM period. In my example, the PWM period was 1 second, and the 'on' time 0.05 seconds, so you got 4KJ per pulse, 1 pulse per second. Say we keep the same % duty cycle, but make the PWM period 10 seconds. The 'on' time is now 5% of 10 seconds, or 0.5 seconds. You dissipate 0.5s * 80KW = 40KJ per pulse, one pulse every 10 seconds. Note that the _average_ power dissipation stays the same, but energy in a single pulse changes.
During the 'on' period, the resistor is dissipating power and its temperature is rising. During the 'off' period the resistor is cooling down. If the 'on' period is too long, then the resistor will overheat and fail before it has a chance to cool down. If the PWM period is very long, the resistor 'sees' the full power. If the PWM period is very short, then the temperature change during one pulse will be very small, and (at least thermally) the resistor 'sees' the average power. 'Short' and 'long' are relative terms that depend upon the thermal time constant of the resistor, how long it takes to heat and cool.
I also want to verify that the brake is indeed coming on and the chopper is working. Is the best way to do this by measuring the voltage across the actual resistor during braking?
That seems like a simple approach. Remember that you will need a meter that can handle >800V DC, switched at 2 KHz.
Question: have you evaluated the amount of kinetic energy stored in the system that you are stopping?
-Jon