I have a DC motor and the sticker says its rated for 24Vdc and 3700rpm.

There is an AC tacho inside it which I am using to measure the speed of the motor.

So when I run the motor with a 24Vdc supply and measure the AC tacho output I get a frequency of around 900Hz. So I am correct in assuming there is a gearbox inside the motor that is speeding the tacho up?

And if so, is it accurate enough to calculate the gearbox ratio based on the motor speed and AC tacho frequency? I want to use it to calculate the motor speed?

Would appreciate any help, thanks everyone.!

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See image of oscilloscope with motor running full speed and no load

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    \$\begingroup\$ Doudtful. To get an answer, most of the following should be provided: Picture of motor. Sticker. Manufacturer. Part number. \$\endgroup\$ Nov 29, 2020 at 22:12
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    \$\begingroup\$ More likely the tacho has 14 or 16 poles and thus produces AC at 7 or 8 times the frequency a 2-pole tacho would. \$\endgroup\$ Nov 29, 2020 at 22:20
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    \$\begingroup\$ the end cover may be easy to slide off \$\endgroup\$
    – jsotola
    Nov 29, 2020 at 22:27
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    \$\begingroup\$ I think they are telling you that both the amplitude and frequency vary with speed. So 1000 rpm / 1.5V = 666.7 rpm/volt. At 2000 rpm it will be 3V, etc. Also, 1000 rpm / 200 Hz = 5rpm / Hz. In other words, the tach signal goes through 12 cycles every rotation. (200 * 60 / 1000 = 12). \$\endgroup\$
    – mkeith
    Nov 29, 2020 at 22:49
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    \$\begingroup\$ @mkeith ah ok I understand now. Thanks everyone for their help \$\endgroup\$
    – David777
    Nov 29, 2020 at 23:41

1 Answer 1


No it definitely does not have a gearbox. From other pictures I've found it appears that the tacho is implemented as an extra magnetic disk on the back of the motor that is picked up by the tacho coil. This disk has 12 sets of magnetic poles on it, so for every rotation of the motor shaft 12 pulses will be generated.

So when the motor turns at 1000RPM this corresponds to (1000/60)=16.67 rotations/second of the shaft, and multiplying this by the number of poles on the encoder disk (12) this gives you a 200Hz output signal at that speed.

Note it is highly unlikely that a motor that size actually has 12 rotor poles. It will be either 2 or 4.

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    \$\begingroup\$ Well, that style of can motor is only going to have 2 or 4. There's brushless model airplane motors that are significantly smaller than that which have 8 or more poles. But it's a significantly different market than a can motor. \$\endgroup\$
    – TimWescott
    Nov 29, 2020 at 23:38
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    \$\begingroup\$ Most likely the voltage of the tach signal is also proportional to speed. This creates the opportunity for measuring speed using voltage (if you rectify it and feed it to an ADC). Don't take my word for it, though. Test it out to make sure it works. \$\endgroup\$
    – mkeith
    Nov 30, 2020 at 2:58

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