A method of constant airflow control for a ventilation system is
disclosed. The method includes various controls to accomplish a
substantially constant airflow rate over a significant change of the
static pressure in a ventilation duct. One control is a constant IRPM
control, which is primarily used in a low static pressure range. Another
control is a constant RPM control, which is primarily used in a high
static pressure range. These controls requires neither a static pressure
sensor nor an airflow rate sensor to accomplish substantially constant
airflow rate while static pressure changes. This is because these
controls use only intrinsic control variables which are electric current
and rotational speed of the motor. Also, the method improves the accuracy
of the control by correcting certain deviations that are caused by the
motor's current-RPM characteristics. To compensate the deviation, the
method adopts a test operation in a minimum static pressure condition.
Also disclosed is an apparatus for conducting these control methods.