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Pdelcast
RC-Monster Titanium
 
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01.14.2011, 11:54 PM

Let me see if I can explain--

The ripple voltage graph shows the difference between a voltage sample that is taken in the middle of the "on" PWM cycle, vrs a sample taken in the middle of the "off" PWM cycle.

The PWM (or throttle) runs asynchronously to the commutation cycle... so don't get the two confused.

So, for example, if the battery voltage sags to 6V when the motor is switched on, and rebounds to 8V when the motor is switched off (which happens 12,000 times a second when not at full throttle) then that would be 2V of ripple, and the graph would show 2V.

The data log shows the battery voltage, but it's averaged.

Usually the ripple voltage will drop a LOT when at or near full throttle, or when at or near zero throttle. And it will be the worst at or near 50% throttle. This is because the pack voltage either drops (near full throttle) to the minimum point, or rises (near zero throttle) to the maximum point.

A good place to be is at less than 5%-7% of pack voltage for the worst-case ripple voltage.

An OK place to be is 7%-10% of pack voltage for worst-case ripple.

10%-12% is marginal, and is risky.

If ripple voltage is above 12-15% (or more) of pack voltage at any point, it's likely that the battery isn't up to the task, and you run the risk of damaging the battery and the electronics.

The graph shown here shows a worst-case of about 2.2V of ripple on a 34V pack -- about 6.5%-7% ripple. Not a bad place to be.

PLEASE NOTE -- RC Cars are pretty hard on the system during acceleration, so very quick excursions above the "marginal" point is OK -- as long as the system doesn't spend any significant time above the "marginal" or "risky" range, it will usually be OK.

Hope that makes sense!

Patrick


Patrick del Castillo
President, Principle Engineer
Castle Creations
   
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