When I rotate the rod or the moving part of motor and probe its output through a multimeter (set on ac measuring) it shows around 0.50 volts.
When I set the multimeter on dc measuring, it still shows 0.50 volts. How is this this possible?.i have a mextech multimeter. When i slow the rotating of the motor the voltage falls in both the ac voltage and the dc voltage

  • \$\begingroup\$ you cannot have DC and AC at the same time .... if there is any change in the output voltage, then it is AC ..... what you may be seeing is AC with a DC offset \$\endgroup\$ – jsotola Jun 6 '19 at 5:28
  • \$\begingroup\$ What brand and model is your multimeter? \$\endgroup\$ – Bruce Abbott Jun 6 '19 at 5:49
  • 2
    \$\begingroup\$ @jsotola: "... if there is any change in the output voltage, then it is AC ..." Surely AC requires a change in polarity, sign or direction, not just a change in voltage. "... what you may be seeing is AC with a DC offset." Isn't that AC and DC at the same time? \$\endgroup\$ – Transistor Jun 6 '19 at 6:13
  • \$\begingroup\$ maybe my thinking is flawed .... my thoughts are related to what you would see on an oscilloscope screen ... DC would produce a steady trace .... anything other than a steady trace is AC .... as far as AC having to change polarity, that depends on the reference point that is being used \$\endgroup\$ – jsotola Jun 6 '19 at 6:38
  • \$\begingroup\$ Can someone please send the picture of them rotating the rod of motor probed with oscilloscope please \$\endgroup\$ – Jyotir Jun 7 '19 at 5:17

What waveform does the motor give when you rotate it?
If you know the waveform, you'll understand the multimeter readings.

For most (cheap) multimeters the DC measurement gives you the average value of the waveform, or the DC component and the AC measurement gives the RMS value of which the DC component is blocked, so, not the true RMS.

I made a setup up of 2 brushed DC motors, one driving the other.
I measured the back EMF of the driven brushed DC motor:

with the multimeter on DC: 4.0 V
with the multimeter on AC: 0.041 V
with the scope, mean value: 7.97/2 = 3.99 V
with the scope, RMS value: 7.97/2 = 3.99 V

(Somehow my DSO scope multiplies the readings with 2, no idea why.)

So, in my case the waveform hardly has an AC component and therefore the multimeter on AC measures no RMS as it blocks the DC component.

In your case: check the waveform with a scope and/or check how the multimeter is measuring the RMS value.

enter image description here

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There are two effects which may be contributing to your results.

Firstly, a DC generator does not produce smooth DC. The output is actually a combination of all the individual armature coil AC voltages, rectified by the commutator. It is similar to the output of a 3 phase rectifier, but with switching noise and flat spots where the brushes short between commutator segments.

This is what rectified 3 phase AC should look like (upper colored sine waves are the 3 AC phases, lower solid black waveform is the 'DC' output):-

enter image description here

And here's a typical output waveform from a small 3 pole DC motor, captured on an analog oscilloscope:-

enter image description here

A good multimeter should be able to block DC when measuring AC, so would measure this example as ~2.3V on DC and ~0.25V on AC. Expensive true rms meters may show both the DC and AC components, or have an option to select between AC and AC+DC. Some analog meters have a socket marked 'Output' which is connected to a DC blocking capacitor inside the meter.

However many cheap multimeters don't block DC on AC ranges, instead simply assuming that when measuring AC there is no DC component. When there is DC a cheap meter may show the DC value, or an even higher value if it uses a half-wave rectifier (which has lower average output voltage than DC, so the meter is calibrated to compensate for the voltage loss).

If you have a cheap meter that doesn't block DC then you can do it externally by connecting a suitable value capacitor (eg. 1uF) in series with one meter lead.

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  • \$\begingroup\$ "Firstly, a DC generator does not produce smooth DC [...] It is similar to the output of a 3 phase rectifier." So, how would you call the "generator" in my setup? It certainly is not "similar to the output of a 3 phase rectifier" \$\endgroup\$ – Huisman Jun 6 '19 at 9:29
  • \$\begingroup\$ If by "not similar" you mean that your waveform has very little ripple, I can't explain that (perhaps if I know more about your setup - motor type etc.). But then you can't explain why your scope reads double either... I had a Rigol DS1054Z for a while and while it was nice, I trust the analog scope more. Digital scopes don't have as good resolution and they can tell you lies if not set up properly. \$\endgroup\$ – Bruce Abbott Jun 6 '19 at 10:39
  • \$\begingroup\$ @Huisman your motor might have built-in EMI suppression capacitors which are smoothing out the waveform. Try adding a load across the output eg. 1k resistor. \$\endgroup\$ – Bruce Abbott Jun 6 '19 at 10:46
  • \$\begingroup\$ No, I used a brushed DC motor from E-drive (e-ds.com). It has no EMI suppresion. It has 11 commutator surfaces, maybe that explains. \$\endgroup\$ – Huisman Jun 6 '19 at 11:17
  • \$\begingroup\$ Anyway, my point is: I think the output of a DC motor is only "similar to the output of a 3 phase rectifier" if you have a 3 phase motor (e.g. a brushless DC motor). If there are more (or less) armature coils, the output will be different. \$\endgroup\$ – Huisman Jun 6 '19 at 11:19

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