How to track the speed (rpm) of an AC induction motor fan without using a microcontroller

Thread Starter

Preneshen_g26

Joined Feb 29, 2020
4
Good day everyone,

I have a design project to complete that requires my group and I to build a speed control of an induction motor. My responsibility is the speed measurement unit which requires me to track the speed of the fan in rpms and display the speed and temperature on a multiplexed seven segment display. I have found some research that uses IR leds and a phototransistor but i am having trouble finding a suitable implementation and circuit diagram.

Any help would be greatly appreciated.
Thank you.
 

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LesJones

Joined Jan 8, 2017
2,639
In my understanding the term "designing " means that YOU create the circuit diagram rather than just using someone else's design. I suggest designing a counting circuit circuit that counts the number of pulses from the IR photosensor in a fixes amount of time. If that time was chosen to be 60 seconds divide by the number of blades on the fan (Assuming the fan blades were being detected by the sensor.) then the count would be the RPM value.

Les.
 

Delta prime

Joined Nov 15, 2019
336
I hope this does not go under the category of homework or I'll be in big trouble!! For RPM you can go infrared you could go hall effect as far as temperature you can go thermistor, other techniques would be voltage to frequency converter frequency to voltage converter. I have no problem personally with providing you with a complete schematic but you have to choose which arena you would like to showcase your work. It's a team of four right? let's hear from all four of you. Your ideas your research. And I'll slap it all together in one big schematic. Correction :We will make one big schematic all five of us. But you have to be quick about it because right now it's under the forum "general electronics chat" In this forum I can help you if it gets moved then I can no longer help you.
 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
20,694
If a AC induction motor in a fan application then most likely either a shaded pole motor or a PSC type motor, if so then a simple Triac rpm control can be used, for rpm implementation you should only need a one-per-rev indicator such as a photo detector etc.
Also appears you have to design a LV DC SMPS?
Also ambient temp is to be measured and used for rpm control.
I see it is a Homework project.by the description.
For rpm control for a start see Fairchild AN-3006 and AN-3003.
Max.
 

MrAl

Joined Jun 17, 2014
7,591
Hi,

Use a frequency counter with LED and LDR or solar cell, divide the frequency measured by the number of fan blades.
You'll need to design circuitry to do the division however.
 
Last edited:

WBahn

Joined Mar 31, 2012
25,760
Hi,

Use a frequency counter with LED and LDR or solar cell, divide the frequency measured by the number of fan blades.
You'll need to design circuitry to do the division however.
No need for the circuitry to do any math. Just choose the counting period so that the number of pulses in that time period equals the RPM.
 

Duane P Wetick

Joined Apr 23, 2009
428
Your project should be divided into two parts; determining the fan's speed first, and second, controlling the temperature. A simple bang-bang on-off thermostat comes to mind for temp. control. For fan speed, a Hall-effect sensor, and 2 small magnets mounted to the motor plate 180 degrees apart will give fan RPM (N & S). One pulse per revolution fed to a LED counter, scaled to give RPMs.
Cheers, DPW [Everything has limitations, and I hate limitations.]
 

Thread Starter

Preneshen_g26

Joined Feb 29, 2020
4
In my understanding the term "designing " means that YOU create the circuit diagram rather than just using someone else's design. I suggest designing a counting circuit circuit that counts the number of pulses from the IR photosensor in a fixes amount of time. If that time was chosen to be 60 seconds divide by the number of blades on the fan (Assuming the fan blades were being detected by the sensor.) then the count would be the RPM value.

Les.
Thanks for your reply.
i have done some research and i create a design so im just struggling to calculate the rpm.
i used a piece of tape to interrupt the sensor on the fan
 

Thread Starter

Preneshen_g26

Joined Feb 29, 2020
4
I hope this does not go under the category of homework or I'll be in big trouble!! For RPM you can go infrared you could go hall effect as far as temperature you can go thermistor, other techniques would be voltage to frequency converter frequency to voltage converter. I have no problem personally with providing you with a complete schematic but you have to choose which arena you would like to showcase your work. It's a team of four right? let's hear from all four of you. Your ideas your research. And I'll slap it all together in one big schematic. Correction :We will make one big schematic all five of us. But you have to be quick about it because right now it's under the forum "general electronics chat" In this forum I can help you if it gets moved then I can no longer help you.
Thank you for the reply.
I already have the schematics for my part and so does my group members. so with regards to the temperature i'm just receiving the digital signal from the speed reference unit.
just having troubling calculating the rpm and figuring out how to time the counts correctly to give me rpm.
 

Thread Starter

Preneshen_g26

Joined Feb 29, 2020
4
If a AC induction motor in a fan application then most likely either a shaded pole motor or a PSC type motor, if so then a simple Triac rpm control can be used, for rpm implementation you should only need a one-per-rev indicator such as a photo detector etc.
Also appears you have to design a LV DC SMPS?
Also ambient temp is to be measured and used for rpm control.
I see it is a Homework project.by the description.
For rpm control for a start see Fairchild AN-3006 and AN-3003.
Max.
Thanks for the reply, i will check it out.
 

WBahn

Joined Mar 31, 2012
25,760
Thank you for the reply.
I already have the schematics for my part and so does my group members. so with regards to the temperature i'm just receiving the digital signal from the speed reference unit.
just having troubling calculating the rpm and figuring out how to time the counts correctly to give me rpm.
What's the problem you are having with calculating the RPM?

How many counts does your set up produce per revolution of the motor?

What resolution do you want on the measurement?
 
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