# Re: [SI-LIST] : DC Motor question

From: Ole Segtnan ([email protected])
Date: Thu Nov 09 2000 - 04:11:35 PST

Hi Erwin,
I used to design motor drives in my previous life ;-), so I can comment on this.

I am not sure I interpret your drawing correctly but it looks like the "EMF" is
the internal voltage generated by the motor when it rotates. Don't confuse this
"EMF" with the "back EMF" of an inductance! The "back EMF" occurs whenever you
switch off a current through an inductance, and in your case it is actually
generated by the "Armature winding", so it's not either or. Can be very harmful
- see below.

When the switch is opened at t=0, the inductance of the system ( The "Armature
winding" in your drawing) will try to preserve the current. This gives rise to a
voltage pulse ("back EMF") across the motor, with polarity opposite to that of
the driving supply voltage. The rightmost diode in your drawing will clamp this
voltage to a safe value and dissipate the energy stored in the magnetic field of
the motor.

This is the dominant behaviour of your system at t=0. Be sure to select a diode
that can handle the peak current of these pulses. You should also pay attention
to induced voltage in adjacent conductors (crosstalk) because of the edge rate
of the current decay.

If you were to remove the clamp diode, the reverse voltage would rise until
something was broken,- most likely the switch transistor. If nothing broke, the
reverse voltage would easily reach tens of times the supply voltage.

The energy stored in the motor's magnetic field is: W=1/2 L I**2, where L is the
inductance of the "Armature winding" and I is the steady current through the
motor at t<0.

The "EMF" in your drawing is a DC voltage proportional to the motor speed. When
the switch is closed (t<0) it should be slightly smaller than the driving
voltage Vdc. When Vdc is disconnected (t=0), the motor will continue to rotate
for a time T because of the moment of inertia. T is normally from tenths of
secons to several seconds (depeending on the motor's size, mass, damping, ...),
and much longer than the "back EMF" pulse described above. The "EMF" is still
proportional to the speed, so it simply decreases to zero as the motor speed
does.

Hope this can shed some light on motor drive design!

Ole

```--
Ole Segtnan                    Phone: +46 (0)70 267 0475
Ericsson Radio Systems         email: [email protected]
Torshamnsgatan 30B
SE-164 80 Stockholm
"SEOW,ERWIN-SP (HP-Singapore,ex1)" wrote:
> Hi all,
>
> I have a question about DC motors.
>
> Lets say I have a DC souce connected to a DC motor. The motor is running for
> a while, what happens when I disconnect the switch at time t=0? Please see
> attached figure.
>
> Which is the dominating factor of current flow? The armature inductance or
> the back EMF?? If I neglect back EMF, the circuit will be easily explained.
> Do back EMF come into this picture?
>
>
> Erwin Seow
>
> email: [email protected]
>
>   ------------------------------------------------------------------------
>                 Name: motor.jpg
>    motor.jpg    Type: JPEG Image (image/jpeg)
>             Encoding: base64
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