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Old Feb 07, 2005, 04:27 PM
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MSC Rödinghausen, Germany
Joined Feb 2001
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Gordon, thank you for posting all this information about real motors.

After writing the article I haven't taken the time to apply it to motors very often, instead I have followed the suggestions for good motors that others (including Gordon of course) have found out.

But I would like to add a few formulas here that are helpful to compare and make good use of motors. These are not new but fit nicely in here.

Firstly, a good rule of thumb for maximum input power of a motor is: as many watts as its own weight in grams. Look into the above lists and you will find that this is true for some of the motors that have proven to be working well, e.g. the M20-LV, the Didel 4.5 Ohm and the SuperSlicks red.

Secondly, since the maximum output power point is near half of the stall current, it is easy to calculate a good first guess for working voltage of any motor. You need the motor resistance Ri which is easy to measure at the motor terminals.

Stall current: Istall = Ubatt / Ri
Working point current: ~Iwork = Ubatt / 2 / Ri
Input power: Pin,work = Ubatt * Iwork = Ubatt^2 / 2 / Ri = motor weight!!!
==> Working voltage: Ubatt = sqrt( motor weight * 2 * Ri )
==> Working current: Iwork = sqrt( motor weight / 2 / Ri )

So, just by weighing a motor and measuring the inner resistance at the motor terminals, you get a good first guess for the working voltage and current. Of course you can refine this by applying the formulas from my article and get a better guess for the working point. To do so use the spreadsheet and vary the supply voltage until the motor takes as many watts as its own weight just at the maximum power point. The formulas will deliver the efficiency in this point. This way you can compare output power per gram of motor weight at the "natural" working point and are more fair to your motor.

Reards, Jochen
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Last edited by jberg; Feb 07, 2005 at 04:32 PM.
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