Measuring rotational speed for Tachometer vs Oscilloscope

[Special One]

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?

 

[Baluncore]

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.

 

[Special One]

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.

 

[Baluncore]

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 ?

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.