VARIABLE SPEED DC MOTOR DRIVE

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DC MOTOR THEORY:
Inducing a Force on a Conductor.
There are two conditions which are necessary to produce a force on a conductor:
i. The conductor must be carrying current.
ii. The conductor must be within a magnetic field.
When these two conditions exist, a force will be applied to the conductor, which
will attempt to move the conductor in a direction perpendicular to the magnetic
field. This is the basic theory by which all DC motors operate.
Theory of Operation (Figure 1), Left-Hand Rule for Current-Carrying Conductors
Every current-carrying conductor has a magnetic field around it. The direction
of this magnetic field may be found by using the left-hand rule for currentcarrying
conductors. When the thumb points in the direction of current flow,
the fingers will point in the direction of the magnetic field produced, as
shown in Figure 1.


Torque
Torque is defined as that force which tends to produce and maintain rotation. The
function of torque in a DC motor is to provide the mechanical output or drive the
piece of equipment that the DC motor is attached to.
When a voltage is applied to a motor, current will flow through the field
winding, establishing a magnetic field. Current will also flow through the
armature winding, from the negative brush to the positive brush as shown in
Figure 5. Since the armature is a current- carrying conductor in a magnetic
field, the conductor has a force exerted on it, tending to move it at right angles
to that field. Using the left-hand rule for current- carrying conductors,
you will see that the magnetic field on one side is strengthened at the
bottom, while it is weakened on the other side. Using the right-hand rule for
motors, we can see that there is a force exerted on the armature which tends to turn
the armature in the counter-clockwise direction. The sum of the forces, in
pounds, multiplied by the radius of the armature, in feet, is equal to the torque
developed by the motor in pound-feet (1b - ft).


Generator Action in a Motor
A generator action is developed in every (CEMF) motor. When a
conductor cuts lines of force, an EMF is induced in that conductor. Current to start
the armature turning will flow in the direction determined by the applied DC
power source. After rotation starts, the conductor cuts lines of force. By
applying the left-hand rule for generators, the EMF that is induced in the armature
will produce a current in the opposite direction. The induced EMF, as a result of
motor operation, is called counter electromotive force, or CEMF,


DC Motor Theory Summary
There are two conditions necessary to produce a force on a conductor: -
i. The conductor must be carrying current.
ii. The conductor must be within a magnetic field.
The right-hand rule for motors states that when the forefinger is pointed in the
direction of the magnetic field lines, and the center finger is pointed in
the direction of current flow, the thumb will point in the direction of motion.
The function of torque in a DC motor is to provide the mechanical
output to drive the piece of equipment that the DC motor is attached to.
Torque is developed in a DC motor by the armature (current-carrying
conductor) being present in the motor field (magnetic field).
CEMF is developed in a DC motor by the armature (conductor) rotating
(relative motion) in the field of the motor (magnetic field).
The function of the voltage that is developed in a DC motor (CEMF) opposes
the applied voltage and results in the lowering of armature current.
The speed of a DC motor may be changed by using resistors to vary the field
current and, therefore, the field strength.