Re: [SI-LIST] : DC Motor part 2

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From: Ole Segtnan (ole.segtnan@era.ericsson.se)
Date: Fri Nov 10 2000 - 01:04:30 PST


Hi Erwin,

First of all let me apologize for a mistake in my previous mail: I thought
"back EMF" meant the induced reverse voltage in the armature winding when it
is switched off, but I can see from the other postings that "back EMF" is the
common term for the internal generated voltage in the motor. My mistake,-
blame it on my bad english..:-((
If you substitute "induced voltage" for "back EMF", the text is correct.

Regarding your new question: Yes, the last (negative) spike must be due to the
back EMF (The voltage generated by the motor spinning).

Now I have a question: Where is the return path of the negative current?
Looking at your schematic in the first mail, I cannot see anything that could
conduct it, but maybe there are more components?

I hope I haven't caused too much confusion with the "back EMF" issue!

Ole

"SEOW,ERWIN-SP (HP-Singapore,ex1)" wrote:

> Hi all,
>
> Thanks for answering my questions about the DC motor. Attached is a sketch
> of the current waveform that I am seeing in the motor.
>
> 1)The first spike is due to the low resitance in the motor (No back EMF)
> during start up.
> 2)The current settles to an almost constant value when it is turning.
> 3)When the motor is tured off, the armature's inductance will cause the
> cureent to flow in the same direction for a short time until the energy is
> dissipated.
> 4)Does this mean that the negative current spike that I see is due to the
> back emf of the motor?
>
> Appreciate the help!
>
> Erwin Seow
> email: erwin-sp_seow@hp.com
>
> ------------------------------------------------------------------------
> Name: current.jpg
> current.jpg Type: JPEG Image (image/jpeg)
> Encoding: base64

--
Ole Segtnan                    Phone: +46 (0)70 267 0475
Ericsson Radio Systems         email: ole.segtnan@era.ericsson.se
Torshamnsgatan 30B
SE-164 80 Stockholm

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