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The separately excited direct current motor
Motor armature and motor field are supplied from separate voltage sources.
The windings in the motor:
The armature winding: The armature winding is located in the movable motor armature and
armature current flows through it. The number of turns of the armature winding determines
the EMF of the motor.
The commutating winding: This is used to keep the area below the carbon brushes field-free,
to ensure good commutation.
The compensation winding: An anchor current causes the distortion of main flux and
therefore a reaction that corresponds to a field weakening. As this is dependent on the
magnitude of the armature current, for higher motor powers in particular direct current motors
are implemented with a compensation winding in the stator to compensate for this effect.
Without a compensation winding the following formulas can only be approximations. If, for
example, the motor torque is to be determined very precisely as a function of the armature
current, or operation without a tachometer with speed control via the EMF is to be performed,
a compensation winding is mandatory.
The compound winding: This winding is a stabilizing series winding and strengthens the
motor flux proportionally with the armature current. This increases the motor torque.
Stabilizing series windings are frequently to be found in old motors.
See under:
http://support.automation.siemens.com/WW/view/en/40871945
Armature winding, commutating winding, and compound winding are arranged in series and
then led outwards via terminals A1 and A2 for the customer connection. The effective value
for resistance Ra and induction La applies to all four windings collectively.
The field winding: Motor flux is applied via the current of the field winding, which produces
the EMF as a function of the speed and the torque of the motor as a function of the armature
current.
The commutator: The current to the rotating motor armature with its armature winding is led
to a commutator via carbon brushes. The commutator supplies the armature winding in the
correct polarity (electromechanical commutation) so that a torque is generated in the motor
as a function of the armature current using the flux in the torque.
The formula for the voltage is as follows:
Ud = EMF+ I * Ra + La * di / dt
I * Ra: Voltage drop at armature resistance of the motor as a function of the armature current
La * di / dt: Induced voltage in the armature induction of the motor that occurs on a current
change i during the time t.
For example, i = 500 A during time interval t = 10 ms, at an La of 1 mH:
A2
A1
La*di/dt
EMF
I*Ra
a
Ra
La
M
Ud
M: DC motor
A1, A2: Terminals, motor armature
F1, F2: Terminals, motor field
: Motor flux
I: Armature current, motor
EMF: Voltage induced in the motor
Ra: Armature resistance, motor
La: Armature inductance, motor
Ud: Terminal voltage, motor armature
