Measuring Motor Activity

[leoEB]

Hey everyone,

I want to measure the activity i.e. the rpm of an electrical motor from outside.

What ways are there to accomplish this?
I know I can use a current clamp, but that doesnt work for me, because the AC-cabel is not in reach.

Is it possible to get the rpm by measuring the vibration on the motor-casis?

Thank you in advance for your Help !

 

[spudboy488]

If there's a flat on the motor shaft, you could use an appropriate proximity detector to detect the flat and count that. You can also try adding an encoder like mechanism to the shaft to count pulses. Ideally, your motor wouldn't have too much vibration and it might be relatively random.

 

[andre_luis]

I know I can use a current clamp, but that doesnt work for me, because the AC-cabel is not in reach.

Current measurement is an indirect approach and is not reliable at all, as it can vary depending on whether the motor is in a steady state or undergoes a variation in mechanical load. The ideal way would be the direct measurement of the positioning, either by tachometer or to obtain greater precision within a rotation, with an Encoder as mentioned above.

 

[BradtheRad]

Is it possible to get the rpm by measuring the vibration on the motor-casis?

A microphone picks up sound from the motor. Or, a coil is an inductive detector. Amplify the signal and feed to a frequency counter.

You can count pulses during a window of time (say, 1/10 second) which gives an idea of motor speed. It's a relative indicator. That much may be satisfactory.

If you really want to measure rpm, then you must determine how many pulses make a rotation. If I were doing it I'd use an oscilloscope to examine pulses in order to find any unique marker that repeats once per rotation.