> There is another way: when read EMF the motor acts as a generator. Loading >it with a variable resistive load (switched digitally using some transistors) >will decrease the EMF level as you wish. However that will brake the motor as >well, as high as the load becomes lower. Probably with 2 loads you can use two >different DC motors.
I'm not sure this will work very good. The magnetic field inside the motor does not change, it is generated by permanent magnets. The resistance of the windings is fairly small. The generated voltage is dependend of the strength of the magnetic field, the number of turns of the windings and the rotation speed. The load resistance influences the braking effect very much, but it almost does not affect the generated voltage. It's only a voltage divider between the resistance of the windings and the external resistor. Just some arbitrary little motors: http://www.nichibo-motor.com/proType_e.php?proType=13 You can find the resistance of the windings: divide the nominal voltage by the stall current. It's less than 10 Ohms for most little motors, sometimes only milliohms for RC racing cars or the like. If you don't want to brake the motor but to measure the EMF, you'll need more resistance. You can, of course, use an external resistor to make a proper voltage divider. If you're draining the current using a constant current source (Well, constant current sink in this case, but that's really the same :-) ) and an external resistor, you're in fact not dividing the voltage. The constant current leads to a constant voltage at the external resistor. In fact you're subtracting a known voltage from the EMF voltage you want to measure. This is a good method, I think! :-) Greets, Kiste -- You received this message because you are subscribed to the Google Groups "jallib" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/jallib?hl=en.
