Kirk Wallace wrote: > > What are the issues with choosing a PWM frequency? I would think that > efficiency would be one, where you would balance switching losses and > iron or copper losses? Evidence of this might be seen as higher motor or > amp temperatures? Lower frequencies might produce noise. I believe the > Pico PWM amp has a 25kHz to 100 kHz range? What would happen with lower > or higher frequencies? > Well, in my PWM servo amps, specifically, the output filters are tuned for 50 KHz, so that the PWM square wave is reduced to a small sine wave ripple on a DC voltage. I wouldn't increase the PWM frequency, as it increases losses with no benefit. lowering the frequency will > What are the symptoms of not setting the bootstrap parameter, like for > my parport setup? > On my DC brush PWM servo amps, the IR FET driver chips need these pulses to come out of their shutdown mode. The brushless servo amp does it by internal logic, so you wouldn't need this with those amps. The bootstrap option in the PPMC driver generates a 5% PWM in one direction for one servo cycle, then in the opposite direction on the next servo cycle, then it goes to normal operation. It does this every time the amps are brought out of estop. > Does the current limit resistor affect the amp performance or just set > the current at which the amp alarms out? > No, the current limit limits the current to protect the motor. Effectively, if you cause current to exceed the limit set by the resistor, then the on-time of the high side transistor is cut off. This limiting resets every PWM cycle. There is a second, fixed limit which causes the amp to go into fault state. These are two totally different current sense mechanisms. The fault sensing also will detect motor grounds, where the fault current would bypass the current sense resistor used for the limiting.
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