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

From: Bob Weber ([email protected])
Date: Thu Nov 09 2000 - 15:46:23 PST

Erwin,

The back EMF is a well-defined voltage proportional to the speed of the
motor. It will decrease to zero as the motor stops. A spinning DC motor
will generate this EMF and no current needs to flow in either direction.

If the motor is connected to a source with a voltage greater then this
EMF, current flow into the motor will increase. If the motor is
connected
to a source with a voltage lower then this EMF, current flow into the
motor will decrease, and can actually flow from the motor (motor acts
like
a generator).

The inductance of the motor can be viewed as a discrete inductance
connected to the back EMF generator of the motor. When a current is
established in the inductance, it will not drop to zero in zero time.
When
the switch transistor is turned off, the clamp diode takes up the motor
current until the energy in the winding inductance is dissipated. This
happens relatively quickly (the motor will still be spinning), and the
voltage will then 'pop' back up to the back EMF of the spinning motor. A
capacitive or RC snubber is often placed around the clamp diode to
reduce
the voltage slew rate and dampen out ringing from parasitic LC
resonance(s) when the motor is turned off.

Bob W.

-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of SEOW,ERWIN-SP
(HP-Singapore,ex1)
Sent: Wednesday, November 08, 2000 8:34 PM
To: [email protected]
Subject: [SI-LIST] : DC Motor question

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?