Ac drives connect to ac induction motors, and provide nearly the same capability of adjustable speed, torque, and horsepower control. To obtain full motor torque at zero speed, a special type of ac drive must be employed (e.g., direct torque control or flux vector-type systems). Those types of controls are beyond the scope of this article and would be best understood after first reviewing the contents of this article.
If the fan can operate at virtually any speed required (below maximum motor speed), the air outlet damper can be "fixed" open. Using the fixed-speed motor starter method, the only means of varying the air out is by adjusting the air outlet damper.