Engineering Measuring rotational speed for Tachometer vs Oscilloscope

AI Thread Summary
The discussion centers on the discrepancy between rotational speed readings from a tachometer and an oscilloscope, with the tachometer showing 1930 RPM and the oscilloscope indicating 3857.91 RPM due to it detecting two pulses per rotation. Participants suggest that the oscilloscope may be misinterpreting the signal from a Hall effect sensor, which outputs two pulses for each complete rotation of the motor. Recommendations include adjusting the oscilloscope's gain settings to align the readings more closely with the tachometer. Additionally, there is a need for clarity on the type of motor and tachometer used, as well as the experimental setup. Accurate measurement of the signal period is advised to resolve the frequency discrepancy.
Special One
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Homework Statement
f = 64.3 [Hz]
w = 1930 [RPM]
Relevant Equations
w=2*pi*f
Question: Why does the oscilloscope double almost the exact value of rotational speed measured by Tachometer?

Rotational speed from Tachometer = 1930 [RPM]
Frequency of 1 period = 64.3 [Hz] which means 3857.91 [RPM]

The output waveform of hall-effect sensor is attached.
Can you have any explanation please?
 

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We need a link to the experimental setup.

A magnet has two poles, but it appears the oscilloscope is seeing only one polarity, so that should be OK. What is the hall effect sensor sensing? How is the magnet mounted?

What type of tachometer are you using?
Maybe it is locking to a harmonic of the rotation.

What type of motor is it?
At 65 Hz, it is probably not an AC induction motor.
The distributor of a 4 stroke motor, where there might be a Hall effect sensor, rotates at half the speed of the crankshaft.
 
Baluncore said:
We need a link to the experimental setup.

A magnet has two poles, but it appears the oscilloscope is seeing only one polarity, so that should be OK. What is the hall effect sensor sensing? How is the magnet mounted?

What type of tachometer are you using?
Maybe it is locking to a harmonic of the rotation.

What type of motor is it?
At 65 Hz, it is probably not an AC induction motor.
The distributor of a 4 stroke motor, where there might be a Hall effect sensor, rotates at half the speed of the crankshaft.
The answer is because it puts out 2 pulses for every rotation.
So, in order fix this problem we should change the gain in oscilloscope from
+60.000,000 to +30.000,000
Now we will be getting the exact value as it is in the tachometer.
 
Special One said:
The answer is because it puts out 2 pulses for every rotation.
What is "it", and why does it "put out" two pulses per rotation ?

Special One said:
So, in order fix this problem we should change the gain in oscilloscope from
+60.000,000 to +30.000,000
The gain of an oscilloscope sets the vertical scale, not the sweep rate.

I would not assume that the two pulses per rev are equally spaced. So you should measure the period of two cycles of the signal with the oscilloscope, then take the reciprocal to convert that to frequency.
 

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