Quote:
It all hinges on the "off" PWM current. We have a low FET "on" and the rest "off". Any current flow must now pass through a protection diode. As long as the diode can conduct, the current is heading toward a steady state value of -Vemf/R (assuming positive current during the "on" PWM drive). If the L/R is short with respect to the PWM period, the current decays quickly and is zero for most of the off phase. So the average current for the whole pulse is determined by the area under the "on" pulse which = (12-Vemf)/R*duty. This would be the case for the slow Victors and we would expect a nonlinear response equal to my old coast formula.
For the JAG, the L/R is large relative to the PWM period. If the current cannot decay to zero during the "off" phase, it will continue to accumulate until the average current stabilizes. Since the current never gets below zero we can consider the problem as the superposition of a steady state Vemf and a pulsed 12v. And this leads us to the linear equation = (12*duty -Vemf)/R .
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Here are the time histories I promised to support my argument. They were made assuming the high side switching model.
There are two plots for the .3 duty case: one for the 884 and one for the JAG.
These plots were made with the attached LABVIEW Hbridge time history vi and I think Eric will find it goes a long way to help explain whats going on. The sim allows you to follow each pulse as it accumulates ....but its like watching paint dry...so I added a "pre_init" that runs the sim to steady state current and then turns on the plots. I am by no means an accomplished LABVIEW programmer so I chose it as practice ... It would be great if some real programmers tidied it up. But , I'm real happy with how it works.
If Al doesn't at least try the program I'll be disappointed.
Have fun all and I welcome any comments to help improve the model.
PS if the configuration settles to something else, I can easily update the program.