View Full Version : Any experts on DC motors?

08-29-2006, 09:00 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?

08-30-2006, 06:50 AM

You'll probably want to at least have a look at any first year DC machines course textbook. One that goes along with the labs would be best.

1. You can get a reasonable estimate of the resistance by using an ohm meter across the motor terminals ( with no other power on, off course!). Take several readings, moving the shaft a few degrees each time. Average the readings. Other than the effects of temperature, R will remain constant.

2. To do a neat experiment, connect another motor to the shaft of the first one with a piece of plastic tubing. Put power on one motor and measure the voltage generated at the terminals of the other with a voltmeter. At a given rpm, what you're seeing is what the back emf would be if the motor was running with Vi applied.

3. Now, connect a small light bulb or resistor across the terminals of the driven machine. Now you've loaded the driver and you can calculate mechanical power, efficiency and a whole bunch of other things.

4. In step 3, if you measure the current going into the driver motor, you'll see the current go up as you load the second machine. More load means more torque required, more torque means more current drawn. To prove the proportionality, load the motor as you watch the current.

DC machines are great to work with because they need so few fudge factors in the basic equations.


Tom Sisk