low PWM rates on ESCs



I think it was Gordon McComb who responded in a recent thread something to the effect that "unfiltered PWM works better" which started/continued a heated discussion on the topic. I just found it:
In the thread "Control electronics power - need for isolation?" Gordon McComb wrote:

I wanted to make the following comment in that thread, but my memory of reading about the model railroad speed controller was too hazy. I think it's come back to me well enough now to post it:
ISTR a Railroad Modelling magazine with an ad or review for a power supply/speed controller which had as one of its features an unfiltered/pulsing DC output. The reason given was that at low voltage the engine would sit there because the power wasn't enough to get over the starting friction in the motor. The pulsing voltage gives a higher peak voltage and current into the motor, starting it turning at a lower average voltage than pure filtered DC voltage, and so the speed would be controllable down to a lower speed. For the ESC's, I presume the pulses of the lower-frequency PWM are long enough to get the motor turning at low duty cycle, whereas a higher frequency PWM at the same low duty cycle would appear to the motor as a low DC voltage, too low to overcome static friction. I don't know how PWM frequency would affect efficiency, but that's apparently not the only consideration. If it's important that your motors run and their speed respond somewhat linearly to PWM at a very small fraction of max speed, you apparently need to look into this. Or have a speed detect on the motor used in a closed-loop operation - but then if friction is too much, it may go into a jittery start-stop operation at low speed (the control loop overshoots to compensate), and lower frequency PWM control may solve the problem.

----- http://mindspring.com/~benbradley
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Ben Bradley wrote:

There is a fundimental difference between "hand held" control of a motor, and computer control of a motor. This is a subtle point in the above discussion that I think was missed.
You don't want your power circuit to provide full power when it is not needed. A power crcuit delivering full power is less efficient. A constant current (at about 50%) capacity going into a motor, is more efficient than full power at a 50% duty cycle.
While a pulsed full power into the motor makes for a good human control (it feels "linear"), it makes for very inefficient motor driving. You spend too much of the time at the least efficient end of the motors power curve. Driving high current into the motors, limited by coil resistence, and creating heat. Driving at a more constant current, well below peak, the motor creates less heat.
As for "start" torque needed, that is one of those things that PID controllers are designed to provide. You don't need to over drive the motor on a constant basis for low end torque, you just increase the power delivered to the motor at startup and back it off when you near your speed target.
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Ben Bradley wrote:
...........

Gordon
applied to

full
Thanks Ben, I just read through that thread. Very different opinions, I think. Despite what mlw was saying regarding efficiency, it seems none of the ESC manufacturers consider using LC-filters between the ESC and the motors, so the argument is somewhat moot regarding what is more efficient and what is not. However, putting everything together sounds like the lower PWM-freq gives better low-speed control and better low-end torque in the monster current motors, like used in model cars and planes, but as mlw says, this is probably more advantageous regards human control than than possibly regards computer control, as none of the ESCs has PID built-in anyways. In model-control situations, the loop is being "closed" remotely by the human, rather than locally by the computer/controller. =============>

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the last few comments are the primary issue. At lower PWM-freq, you have longer pulses for the same % duty-cycle compared to higher-PWM freq, and this is apparently more important for getting the low-#turn, hi-amp motors to control well and produce good torque at low speeds. At least, this is what I am concluding from these discussions.
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