


FourQuadrant Analysis of DC Motor Control
Operation of a bidirectional DC motor drive is best analyzed
by using a four quadrant rectangular coordinate graph.


The horizontal
axis is armature voltage (V to +V relative to a common zero
volts). The vertical axis is armature current (+I for clockwise
motor rotation, and I for counterclockwise rotation). Beginning
in Quadrant 1, as the armature voltage is increased, the motor’s
armature current increases, which increases the motor’s
RPM and thus delivers more torque to the load. Transfer to Quadrant
4 operation usually occurs when the applied armature voltage
is reduced in an effort to slow the motor’s speed. Load
inertia causes the motor to rotate at a speed greater than the
armature current caused by the applied voltage. Consequently,
the motor begins to function as a generator, producing an armature
current in the opposite direction. Armature current actually
reverses, relative to the applied armature voltage. The more
quickly quadrant transition is sensed and accounted for by the
DC drive electronics, the more accurate and efficient the speed
regulation. As the load reduces the motor’s inertia and
slows the motor’s speed, motor torque returns to the same
polarity as motor direction, and Quadrant 1 operation resumes.
Quadrant 3/Quadrant 4 operation is the same, for counterclockwise
rotation.



