bulkhead

08-29-2006, 10:00 AM

This isn't really related to anything, but I thought that this would be the place to get some answers.

I'm trying to do some simple experiments with electric DC motors that are physics-related. By simple, I mean just measuring power input with two multimeters(voltage and current) and using a phototransistor and stamp to measure RPM, all done with no motor load (since I can't measure that). Anyways, after doing some experiments and searching the internet, I have a few basic questions that I would like some answers to.

1) Can the back emf voltage (Vemf) generated by a motor be calculated using the data that I have? The equation I have is Vi - Vemf = IR, but what I am not sure about is, is the R value the resistance measured at the motor terminals at rest?

1) a) For that matter, is a motor's resistance constant (assuming same temperature) no matter it's speed?

2) Is it true that a motor's speed is directly proportional to the voltage, and that the back EMF is also directly proportional to the speed, so they are all proportional to each other?

3) Finally, the statement that "current is proportional to the torque load on the motor," does that ever hold so, and if true, does it require a constant voltage?

I'm trying to do some simple experiments with electric DC motors that are physics-related. By simple, I mean just measuring power input with two multimeters(voltage and current) and using a phototransistor and stamp to measure RPM, all done with no motor load (since I can't measure that). Anyways, after doing some experiments and searching the internet, I have a few basic questions that I would like some answers to.

1) Can the back emf voltage (Vemf) generated by a motor be calculated using the data that I have? The equation I have is Vi - Vemf = IR, but what I am not sure about is, is the R value the resistance measured at the motor terminals at rest?

1) a) For that matter, is a motor's resistance constant (assuming same temperature) no matter it's speed?

2) Is it true that a motor's speed is directly proportional to the voltage, and that the back EMF is also directly proportional to the speed, so they are all proportional to each other?

3) Finally, the statement that "current is proportional to the torque load on the motor," does that ever hold so, and if true, does it require a constant voltage?