DC Motor Speed

[coryroth]

Have gotten stuck on a problem at work. I have a small DC motor (specs: 6,000rpm, 1.5v, 220mA, 3-pole) located, inside of a product and have no visual access to. The only thing I have access to are the positive and negative terminals. Question is, how the heck can I measure the estimated RPM of the motor without cracking the item open? I've read a few ways here and there, but it's all over my head. I've tried reading the frequency with a multimeter, but that comes out to about 8kHz which can't be right.

Any help or pointers would be awesome!

 

[Audioguru]

If the motor vibrates slightly because it is out-of-balance then it will produce a hum sound. If it spins at 6000RPM then the hum will have a frequency of 100Hz. Use a microphone that feeds an audio amplifier and measure the frequency with your multimeter.

 

[Alec_t]

Connect a low-value resistor in series with the motor as a current sensor and check the voltage across the resistor with a scope. If it has a periodic component, that will be related to the rpm.