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Originally Posted by sciguy125
Let's say that we backdrive a motor clockwise. Would the back EMF be the same polarity as that which we would need to apply to the motor leads to make the motor turn clockwise? Or does backdriving it reverse the polarity? I want to cite Lens' Law, but I'm not sure if it completely applies here.
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Lenz's Law does apply here. A motor that develop's positive back-EMF when backdriven clockwise will also spin clockwise when positive voltage is applied at its terminals, assuming the mechanical load allows it to spin.
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Originally Posted by sciguy125
On a side note, I noticed that the MOSFETs are rated for 64A. Where does the Victor's 40A limit come from? It's not the power limit because at 40A, it's only 19.2W and it's rated for 94W.
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Check out Fig. 9 on page 5 of the
IRF3103 datasheet; it shows maximum drain current vs. case temperature. If your cooling system can hold the case temperature at 25 degrees Celsius then you can operate the FET at 64 Amperes drain current. At higher case temperatures, the maximum drain current decreases; e.g., at about 115 degrees Celsius case temperature the maximum drain current is 40 Amperes. Thermal analysis of FET installations can get complicated; however, in general the datasheet current rating is going to be higher than the current at which you can really operate the FET in a cost-effective system.
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Richard Wallace
Mentor since 2011 for FRC 3620 Average Joes (St. Joseph, Michigan)
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since 2003
I believe in intuition and inspiration. Imagination is more important than knowledge. For knowledge is limited, whereas imagination embraces the entire world, stimulating progress, giving birth to evolution. It is, strictly speaking, a real factor in scientific research.
(Cosmic Religion : With Other Opinions and Aphorisms (1931) by Albert Einstein, p. 97)