Tachometer to Siemens PLC input

KatSpike

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Join Date
Sep 2023
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Michigan
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I have a fan motor with a tach out. How do I connect to a Siemens PLC ?
I have attahced the wiring diagram from Motor Manufacturer.I was thinking 24v to grey, wht to plc pnp input, (high speed module?) I only want to verify it's running

Fan Motor2023-09-22 092127.png
 
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I wasn't sure if that was a 0-10V output, or if that was for me to supply 0-10v for speed control. That was my first thouight to go to analog input module.
 
Last edited:
I wasn't sure if that was a 0-10V output, or of that was for me to supply 0-10v for speed control. That was my first thouight.


My thought is that if it's just a motor, it won't have speed control built in. That would be part of the VFD. Motors do relatively often come with encoders/tachometers/etc as feedback to the PLC.
 
The text "SPEED SETTING WITH FIXED OR VARIABLE RESISTOR" hints at that it does have an integrated speed controller. Or maybe it is just a simple DC motor.
I think that the fan receives up to 10V, anything less than 10V reduces the speed. The resistor and the PWM output is just another way to generate a voltage less than 10V.

The real question is what is the frequency of the tacho output. If it is not so high, you can possibly connect it directly to a regular PLC input. If it is relatively high, then you may have to use a dedicated counter card.
 
Assuming the fan outputs one impulse per revolution, that will give you an impulse every 20 ms.
Assuming the pulse and pause are symmetrical, you need to sense the signal at ~7 ms or better. So you need an input that updates its inputs for both on and off transitions faster than 7 ms.
The PLC must update its program scan faster than the 7 ms, or the input must trigger a hardware interrupt.

edit: So you need to check if the above assumptions are correct.
 
https://puhallinverkkokauppa.fi/files/pdf/53/L02-A1.pdf?


To be pedantic, I suspect OP actually wants to verify that (cooling?) air is flowing; detecting motor rotation may be good enough as a proxy but it is not, strictly speaking, a guarantee of same. That said, assembling a reliable airflow telltale sensor is a non-trivial exercise, and detecting motor rotation is probably the next best choice.

I worked with a group in a nuke plant managing computers and other electronic hardware for a while; one experienced engineer reflexively held his hand by equipment fans' exhausts every time he entered a room.
 
My thought is that if it's just a motor, it won't have speed control built in. That would be part of the VFD. Motors do relatively often come with encoders/tachometers/etc as feedback to the PLC.


welp, nevermind, maybe I should read the dang print next time....
 

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