Mechanics tricks and tips

To find the speed a DC electric motor is turning:

Determine the number of commutator bars on the armature. You may have to take some stuff apart to accomplish this step. It is helpful to tear apart and count the number of bars on every motor you change. Attatch current clamp with graphing meter to one wire of the motor. If you need help with the tools or techniques you can contact me for help. With the motor running, you should see humps in the pattern on the screen. Each hump of the pattern is one commutator bar. Set the timebase on the meter so there is at least one revolution on the screen. If there are 12 bars, capture at least 12 humps, or one revolution, on the screen. Determine how long it takes in milliseconds to get one revoution. Use the instructions with your meter to figure out this one. If you use a Snap On Vantage, I can explain it to you if you need help e-mail me. Divide 60,000 (ms in one minute) by milliseconds in one revolution.


60,000 divided by 400 ms to make one revolution would be 2400 RPM.

This trick is useful for checking blower motors and electric fuel pumps. If the current is higher than normal, and the speed of the motor is slower than normal, you have a motor that is working harder than it should be. This could be because it is working against an abnormal load (dirty evaperator or plugged fuel line). It could also mean the motor is worn out. If the bushings in the motor are worn, the motor will have to work harder to overcome this. That is why that rattling old worn out blower motor starts tripping the circuit breaker. When the fuel pump speed seems a lot faster than normal, you are sucking air. Put some fuel in the tank (don't ask how I know this). If you check motor current draw and speed along with the pattern it makes on the meter before they start to fail you will have an example of what good looks like. After you have checked a few you can begin to predict failures before they happen.

This can also be used to find cranking RPM of an engine. With your current clamp on the positive cable, disable ignition or fuel system and crank engine. Each compression stroke will make a hump. A six cylinder engine will make three humps per revolution. 60,000 divided by the time it takes to make one revolution equals the RPM. I like to have two revolutions on the screen to figure out the time and then multiply the result of dividing 60,000 by the time by two.


screen capture

The time for six compression cycles is .80 seconds (800 ms). Divide 60,000 ms by 800 ms. The result is 75. Multiply times two to get 150 RPM. If that seems confusing just measure three compression humps to get 400 ms. 60,000 divided by 400 equals 150 RPM.

Make sense? If not e-mail me at and I will try to help.

Steve Blankinship 2004