How to Make a Tachometer and Motor Dynometer

by DaveJr

To calculate the power of a small D.C. Motor you need to know the Stall Torque and the Maximum RPM.

To find the maximum RPM of the test motor, make a disc approximately 1” in diameter that will fit the armature shaft snugly. Use a magic marker to blacken half of the disc.

Connect the motor and a DVM to a variable power supply. Under fluorescent lighting, increase the motor’s RPM slowly from zero and watch the disc spin. The first time the pattern on the disc stops turning is at 3600 RPM, the second time it stops turning is at 7200 RPM.

Let the motor run for a few seconds at 7200 RPM and check the Voltage on the DVM. The motor I tested was at 3.05 Volts.

Divide 12 by 3.05 and multiply by 7200, this gives the Max RPM of 28327 at 12 Volts.

The motor and disc are then inserted in a Dremel. The motor leads are twisted together to use the dynamic brake action of the test motor. A “C” shaped clip attaches a piece of Nylon string to the motor case, then it is passed through the scale from the bottom. A heat sink clip is attached to the end of the string to secure it to the scale. The scale is clamped to the work bench.

The Dremel is held under the scale and the power is increase slowly to apply tension to the string. Allow the string to wrap around the motor a couple of times and make sure the motor is flat so the string is on the round contour of the motor case. Keep the string vertical to the scale and increase the speed of the Dremel to 7200 RPM.

Switch off the Dremel and allow the test motor to cool back to room temperature.

Hold the Dremel under the scale and make sure there is no tension on the string, then turn on the scale. Turn on the Dremel and check the disc to make sure the Dremel is at 7200 RPM, then read the load on the scale.

The motor I tested made 40 g/cm at 7200 RPM.

Divide 28327 by 7200 and multiply by 40, this gives the torque of 157g/cm at Max RPM.

Multiply 28327 by 157, divide by 400,000, this gives the power of the motor, 11.1 Watts.

My testing has shown that with the Slot it type motor (FC-130), the torque at Max RPM is approximately the same as the Stall Torque.

Testing SCX Motors using this method has shown that, as the Stall Torque is increased from motor to motor, the Dynamic Braking force increases at a greater rate than the Stall Torque.

Measuring the RPM with this method has shown a slight variations on different days of testing when compared to the actual RPM measured with an oscilloscope. The only variable would be the Duty Cycle of the A.C.

This method of testing Slot it type motors has shown that it is close to the actual measurement arrived at using my motor dynamometer.