Determining motor speed from PWM waveform

[tomcorker]

As part of a lab report I have to determine the speed of a motor from the PWM waveform.

The motor in question is a lego motor, of type at disassembled at http://www.philohome.com/motors/motor.htm. It has three coils on the rotor and a 4 pole permanent magnet stator.

This is the waveform I have:

You can see the PWM pulses, and in between there is the output waveform of the motor acting as a generator when no voltage is being applied. I should be able to calculate the speed of the motor from the time of one of the generator pulses (not the PWM pulses). I measured this as 3.1ms (you can check on the image).

I suppose my question is how many pulses should there be per revolution of the rotor?This is what I thought:
I know what I am seeing is effectively rectified 3-phase AC from the three coils, so there should be 6 pulses per complete cycle on one coil. There are 2 pole pairs, so there will be 2 cycles per coil per revolution, giving a total of 12 per revolution.
At 3.1ms per pulse, one revolution should be 37.2ms per revolution. The motor has an internal 14:1 gearing, so the output shaft should be rotating at about 0.5s per rev, or about 120rpm.
But what I actually observed was 4s per revolution at the output shaft (15rpm), so I'm out by a factor of 8.

Any thoughts?

 

[NascentOxygen]

Reduce the sweep speed until you are able to perceive a complete period of the waveform, i.e., the time taken before each nuance on the waveform repeats. You'll see what I mean if you try it. Each winding and brush gives a unique signature to its contribution to the emf and it is consistent.

 

[tomcorker]

Thanks - I see what you mean. I'll have a go when I'm in the lab tomorrow.

 

[NascentOxygen]

Synchronizing the CRO to display a steady repetitive signal will be a problem. The PWM pulses are not synchronized to the rotation of the shaft, are they? Can you capture a single sweep and examine that?

 

[DeeNos]

I would first like to commend the author for their thorough and detailed analysis of the motor's PWM waveform. It shows a good understanding of the principles behind motor operation and the use of PWM to control its speed.

Based on the information provided, I agree with the author's calculation of 12 pulses per revolution of the rotor. However, I believe the discrepancy in the actual observed speed of 15rpm versus the calculated speed of 120rpm could be due to several factors.

Firstly, the internal gearing of the motor could play a significant role in the observed speed. The 14:1 gearing could be affecting the motor's output speed, resulting in a lower than expected revolution per minute.

Secondly, there could be factors such as friction and mechanical resistance that are not accounted for in the calculation, which could also contribute to the lower observed speed.

Lastly, it is important to note that the motor's speed can vary depending on the load it is driving. This could also explain the difference in observed speed compared to the calculated speed.

In conclusion, while the author's analysis and calculation are sound, there could be other factors at play that could affect the motor's speed. Further experimentation and analysis may be needed to determine the exact reason for the observed discrepancy.