Why do DC motors not work on AC?

[derek10]

Yes I now a DC motor is for DC (lol) but I don't understand why do they change rotation 50-60 times per second (vibrate) when connected to AC, I though that, like AC or Universal motors, the alternating current would "replace" the commutator?

Thank you :)

 

[Bystander]

why do they change rotation 50-60 times per second

How many commutator sections are there on the DC motor you're using? What's its rated rpm? How many times per second at that rpm will the commutator shift poles?

 

[derek10]

Thank you, the motor on the subject is a small one taken off an old Toshiba CD-ROM (which opens and closes its tray) so I cannot know its parameters, only I know It's DC and works on low voltages and can also act as a generator.
It's identical to the motor right of the 9V battery in this photo

But I also used bigger ones from old printers and showed the same behavior.

 

[jim hardy]

I'd guess you are using motors with permanent magnet field
When the field and armature currents swap directions together direction doesn't change.
When one reverses but the other one doesn't, direction changes.
Now-
You can't reverse the permanent magnet field.
So a PM motor that's suffered the misfortune of being connected to an AC source will try its best to follow its alternating armature current.
http://www.freescale.com/webapp/sps/site/training_information.jsp?code=WBT_MOTORCONTROL_TUT

 

[phinds]

Printers and disk drives use not just DC motors but the more limited DC "stepping" motors. Google "stepping motor" for more information.

 

[Windadct]

In general the two smaller motors - work by aligning the magnetic field created by the supply with the magnets in the motor - as the motor shaft turns, there are brushes in the motor that switch the polarity of the current through the winding to then switch the polarity of the magnetic field- i.e. so the two fields are never fully aligned, and the motor is continuously trying to get aligned and then runs continuously. . Once you get a better handle on how a basic DC motor works with DC - you will then understand why the motor vibrates when AC is applied. - Google DC motor video -- there are plenty.

 

[derek10]

Thanks again

Phinds:it isn't a steeper motor (those require a controller to run, right?) it seems to be a regular DC motor), it was the motor which opened and costs the tray door.

Windadct yeah the brushes /commutator changes polarity to "emulate" AC current, right? But if I run it from AC, wouldn't it bypass the commutator and run directly? I read that universal motors that work in both DC and AC work that way but I am not sure,

 

[phinds]

Thanks again

Phinds:it isn't a steeper motor (those require a controller to run, right?) it seems to be a regular DC motor), it was the motor which opened and costs the tray door.

Yeah, I think you're right, the door motor is not a stepping motor, it's the drive-head drive that is a stepping motor. I'm not POSITIVE about that (the door motor) though.

 

[Windadct]

Why would it bypass the commutator - it physically part of the motor? -- so now you have 2 "things" commutating - the AC signal and the Brushes... the outcome is not predictable because it is not built that way.

As for a universal motor - the permanent magnets ( think a fixed, no changing magnetic field) are replaced by a field winding. The filed winding (Stator) is connected to the same source feeding the rotor's winding, but the rotor winding still has a commutating brush... so when the AC signal "commutates" the polarity of BOTH the field and Rotor change... but as the rotor moves - and these filed begin to align ( similar to the PM DC motor) the brushes commutate the rotor, switching the polarity of the of it's field. I know it is confusing - but really it is like a logic problem - and there are two things changing all the time ( the AC source and the rotation of the motor ) Thus the name - it works with universal power ( AC or DC)

 

[derek10]

I'd guess you are using motors with permanent magnet field
When the field and armature currents swap directions together direction doesn't change.
When one reverses but the other one doesn't, direction changes.
Now-
You can't reverse the permanent magnet field.
So a PM motor that's suffered the misfortune of being connected to an AC source will try its best to follow its alternating armature current.
http://www.freescale.com/webapp/sps/site/training_information.jsp?code=WBT_MOTORCONTROL_TUT

Oops didn't saw your post, I think they are indeed permanent magnet ones, as they attract iron and such I think I am sort of understanding a bit.

So when an AC current is applied, it will "conflict" with the armature so it reverts a currently negative AC cycle?

 

[derek10]

Yeah, I think you're right, the door motor is not a stepping motor, it's the drive-head drive that is a stepping motor. I'm not POSITIVE about that (the door motor) though.

Yeah the spindle motor and the laser ones seemed to be steeper motors as they had more than two contacts (unlike the tray one which had only two) :)

Why would it bypass the commutator - it physically part of the motor? -- so now you have 2 "things" commutating - the AC signal and the Brushes... the outcome is not predictable because it is not built that way.

As for a universal motor - the permanent magnets ( think a fixed, no changing magnetic field) are replaced by a field winding. The filed winding (Stator) is connected to the same source feeding the rotor's winding, but the rotor winding still has a commutating brush... so when the AC signal "commutates" the polarity of BOTH the field and Rotor change... but as the rotor moves - and these filed begin to align ( similar to the PM DC motor) the brushes commutate the rotor, switching the polarity of the of it's field. I know it is confusing - but really it is like a logic problem - and there are two things changing all the time ( the AC source and the rotation of the motor ) Thus the name - it works with universal power ( AC or DC)

Whoa thank you, your post cleared many doubts specially when I compared with Universal motors and was easier to me to understand you than the wiki article, so I was right in my latest post and this is (in a basic sense lol) conflict between the AC and the armature :)