I'm doing a RPM counter for an used electric generator I bought. I found out that it has already had some sort of RPM counter attached to the end of the generator shaft.

This counter a steel box which contains 4 sets of magnets wound by copper wire. All of these sets are in series connection. The output of the box is two copper wires. The input is a large gear with 8 teeth, which connected to the generator shaft.

http://s6.postimg.org/cm6m5cp6p/DSC0421.jpg
http://s6.postimg.org/xhswglldt/DSC0452.jpg
http://s6.postimg.org/vopzsa075/DSC0456.jpg

My questions are:
1) Is this a RPM counter ?

2) If it is, why did they bother to have 4 sets of magnets to do the job. What is the concept of this kind of RPM counter ?

Could anyone please answer this for me ?

 

It has been a while, but that looks like a Tachometer. It will give a voltage output rather than counts still represents speed though.

 

Thank you for your reply. I think this kind of revolution meter have sensitivity of 8 PPR (since the gear has 8 teeth). Is it right ?

 

I think not.

This kind of device will have a voltage output, the faster it turns the higher the voltage will be.

 

Dang Khoa,

You can try rotating the shaft with a frequency counter connected to the "tachometer" leads. A digital VOM with a frequency sensor should work just fine. If it will read frequency, particularly at low RPM, then it's likely many if not most frequency counters/displays will also detect the pulses and you can use it as you wish.

"Early" RPM detection was done with tachogenerators, which measured a voltage that was proportional to speed. Early "electronics" weren't so good at counting pulses, so that's why tachogenerators were used; it was relatively easy to scale a meter to display speed as a function of voltage, and for control systems to use voltage as a function of speed.

There's nothing to be lost by trying to measure pulses (counts) with a frequency counter and seeing if it will work. I think a lot of tachogenerators were AC generators, but, again, it wasn't always so easy (or cheap) to count pulses per RPM with electronics as it is today.

Hope this helps! Please write back to let us know what you learned!

(P.S. If the output can be "counted" digitally, I would agree--it would likely require 8 pulses per revolution for proper scaling of the pulses for speed indication.)