Hi ,
My question may seem basic but is really hard for me to understand, maybe it is not as simple as it appears ? I built an oscillating digital circuit in school. Do not remember too much but it just "flashed "on and off. Considered an A/C signal for most purposes. My issue now is I have a rotating motor and it is 100% D/C fan from a vehicle. I turned it into a generator and measure 1 volt rotating it manually. ( increases voltage and I as speed increases) as far as I got so far. What I do not understand is how it can produce a D/C and not varying A/C emf? I have taken many apart and know the components but what makes a/c motors so different from d/c motors. For instance an A/C motor I experimented with did not have permanent magnets and generates zip , zero emf when spun. Not much use for motors if I do not know how to power them. Thanks for info.

Note: I am using a simple multimeter for measurements -no oscilloscope available, if I connect and it "reads" d/c then I am on the correct range, or try the a/c switch.

 

What I do not understand is how it can produce a D/C and not varying A/C emf?

The commutator switches the winding(s) according to the rotation angle.

 

In your particular case, the motor will produce DC when rotated since if you examine it carefully it will have a set of carbon "brushes" fitted at one end. These "brushes" form a sliding contact with the "armature' of the motor/generator. The armature consists of a number of coils of wire wound around the armature and the coils ends are connected to a set of copper bars in a specific pattern and these "brushes" make contact with the coils via the bars called a "commutator". This is basically a rotating switch allowing current to flow into/out of the coils as they rotate. A magnetic field generated by a current in the fixed "pole" windings reacts with the magnetic field generated by the armature coils and if the unit is supplied with power it become a motor and if spun by a "prime mover" it will generate electricity. All generators generally supply AC but by using a "commutator" as a switch this can generate a DC current.
Pure AC motors are somewhat different as they rely on the positioning of the winding to provide a magnetic field which rotates and causes and induced current in the rotor which then tries to follow the fixed field making the rotor "rotate".
You can find a lot more detailed explanation of this whole issue on the web or by studying same.

 

I like buxton's answer but not sure it was on your level, so I'll reiterate:

Inside a DC motor you have several coils which are cutting through magnetic fields created by permanent magnets. We know that when you move a wire through a magnetic field (or magnet through a coil of wire) it generates a voltage. Since the thing is spinning in a circle, a sinusoidal wave is generated. Several sinusoidal waves are generated in the DC motor simultaneously; one for each sector of the commutator. the wave in each sector will peak right as it comes around to the brush assembly. so when you connect your meter to the brush assembly, you are reading the peaks of a bunch of mechanically rectified AC peaks, which is effectively DC.

You can't read anything on your induction motors because there is no magnetic field inside. The induction motor relies on induction; that is, when you apply voltage to it, a rotating magnetic field induces a current in the rotor coils, and this induced current creates its own magnetic field, which opposes the rotating magnetic field from the applied voltage, which results in movement. So unless you're applying voltage, you're just spinning a ball of copper and that doesn't get you anything.

 

sometimes yo can read voltage on ac motors when turned manally, residual magnetism left in the armature can induce voltage in the field coils. I use that as a simple field check for shorted field windings, on a 3 phase motor, a little voltage across each field that dosnt have a shorted turn. if no voltage at all, probably no residual magnetism, or has been off too long and it has faded.

 

The last post touched on residual magnetism.
This is how Edisons original Dynamo and now most wound field DC generators work.
If a DC motor/generator has a wound field, at first there is no field to produce a generated voltage, except that usually there is some small residual left in the field, When the armature is rotated, this small residual generates an output voltage which is then fed back to the field and the generator pulls itself up by its bootstraps.
On new generators or others that have lost their residual, this field can be 'flashed' with an automotive battery to (re)produce the residual.
Max.

 

and the most common reason to lose their residual magnetism is bad brushes or comutator arcing.

 

So glad I posted the question and thanks for the answers. In most diagrams of a basic motor in text books and whatever it just shows the coil cutting the magnets N -S field poles so this really helps in understanding that the commutator does switch the voltage ( also having polarity ) negative and positive. But it also seems interesting that you can not directly "generate " a DC voltage then without switching.

 

But it also seems interesting that you can not directly "generate " a DC voltage then without switching.

As a magnet pole moves past a coil it generates one voltage polarity as it approaches the coil and the opposite polarity as it recedes from the coil. Hence the need for switching.

 

...Which also could be said technically speaking that although DC is applied, it is an AC operating device.
Each armature winding voltage alternates.
Max.

 

...Which also could be said technically speaking that although DC is applied, it is an AC operating device.
Each armature winding voltage alternates.
Max.

Did you really mean to say that? The polarity in a DC motor does not change as it rotates. It just moves/switches from one armature coil to the next.

Whether the field uses electricity or permanent magnets for its magnetism, it always keeps the same polarity. Its the moving or switching ofthe opposite polarity in the armature coils that makes the motor rotate. Not an "AC" voltage in the armature. The north pole of the field is always attracting the south pole of the armature and viceversa.

 

At 180° the current through the coil is reversed. Or alternating every half revolution (transitions a N pole then a S pole)
This maintains DC at the commutator brushes.
Max.

 

But it is still not an AC current. It is still DC, the commutator has just physically changed positions due to rotation. One brush is positive DC and the other is negative DC. AC goes through "0" volts, changing negative and positive, but you know that.

 

Looking at it purely form one winding point of view.
I just meant to imply that the current through the relative coil alternates, all you would need to do would be to replace the com segments with a 2 slip rings and you would have an alternator.
Max.

 

I see you guys are making a debate of this.
I am going to put my location in my profile. I am in S. Florida , Pembroke Pines.

 

Could not find my own post. Sure you have been through it. I am trying a bookmark in browser and copy subject (write it down) in notepad.
Principle of electromagnetism is that their respective wave fields always travel perpendicular to each other - occur together and propagate at the spped of light. One important fact is that the field must be cut at an angle perpendicular to change it somehow ie magnetic field to a electric field and hence voltage and current in the wire. But the type of current, A/C or DC is not a factor because it is all just charges traveling around a circuit.

 

Could not find my own post. Sure you have been through it. I am trying a bookmark in browser and copy subject (write it down) in notepad.
Principle of electromagnetism is that their respective wave fields always travel perpendicular to each other - occur together and propagate at the spped of light. One important fact is that the field must be cut at an angle perpendicular to change it somehow ie magnetic field to a electric field and hence voltage and current in the wire. But the type of current, A/C or DC is not a factor because it is all just charges traveling around a circuit.

Is this loosewire?

 

Thinking out loud- sorry for the extraneous information. I found the search tool though, I was looking for something to isolate only my posts and view it. No problems bro.