How to monitor AC motor speed

[bill_c]

I want to monitor the speed of a heat pump condenser fan motors (Typical outdoor 220V 1Ph capacitor run motor without electronics, about 800 to 1100 RPM).
  1. sensor must be reliable.
  2. sensor must survive and operate outdoor with sun, rain,  snow, or ice, and temperature (about -20F to 120F) .
  3. sensor must not interfere with the operation of the motor. (like fall into blade)
  4. sensor should be cheap and easily adapted to any of these type motors.
  5. I will also be monitoring indoor evaporator fan and it would most likely use the same as above.
What sensor or method would work for this?  Measuring volts and amps doesn't really tell me speed and would make it difficult to detect the extra slip from bad bearing or internal rust buildup.
I will also be measuring temperatures, pressures, voltage, and possibly compressor current.

[soldar]

It seems to me you can just use an optical sensor to measure the blades rotating. If the blades overlap then you can set the sensor at an angle.

[TimFox]

Another way to monitor the fan blades going by is to use an inductive proximity sensor, usually a hermetically sealed unit.
Brief description:  https://www.ia.omron.com/data_pdf/guide/41/proximity_tg_e_6_2.pdf
Examples:  https://assets.omron.com/m/2da718de55d407e8/original/D116-E1-02_E2B_Datasheet-pdf.pdf  (speeds for this series range from 1 to 2 kHz).

[bdunham7]

Are we talking about a research project involving existing installed or current production units or are you intending to incorporate the sensors into a new or adapted product?

[Benta]

Use an automotive ABS sensor and a toothed wheel on the shaft.
It won't get much more reliable, the sensor will take all the abuse you can throw at it.

[bill_c]

@soldar: I have used optical sensors on machines, just not sure how well they would work outdoors with day/night + dirt/water
@TimFox: Inductive may work, although the leading or trailing edge of slanted blade may not be enough for some sensors, may have to make a toothed wheel instead. Ebay has new Omron sensors for $9 with free shipping from China, wow that Omron stuff sure has come down in price....
@bdunham7: This will be adapting to existing refrigeration systems attempting to get early warning of failure or that it has failed.
@Benta: ABS sensor checks all the boxes and well proven in worse conditions with the only (small)downside is having to make the toothed wheel to fit the shaft.

Thanks to all. I will try both inductive types soon.

[soldar]

@soldar: I have used optical sensors on machines, just not sure how well they would work outdoors with day/night + dirt/water

I was under the impression this was a home experiment, a one time thing. I definitely would not recommend an optical sensor in any commercial machine for sale.

[Benta]

@Benta: ABS sensor checks all the boxes and well proven in worse conditions with the only (small)downside is having to make the toothed wheel to fit the shaft.

Thanks to all. I will try both inductive types soon.

You'll find cheap gear wheels for a couple of dollars from China. They don't even have to be precise. They just need to be ferrous.
Note that there are two types of ABS sensors on the market: plain inductive (passive) and Hall that need a power supply. You decide.