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How Variable Speed Works -1-

You can divide the world of electronic motor drives into two categories: AC and DC. A motor drive controls the speed, torque, direction and resulting horsepower of a motor. A DC drive typically controls a shunt wound DC motor, which has separate armature and field circuits. AC drives control AC induction motors, and - like their DC counterparts - control speed, torque, and horsepower.
Application As An Example Let's take a brief look at a drive application. In Fig. 1, you can see a simple application with a fixed speed fan using a motor starter. You could replace the 3-phase motor starter with Variable Frequency Drive (VFD) to operate the fan at variable speed. Since you can operate the fan at any speed below its maximum, you can vary airflow by controlling the motor speed instead of the air outlet damper. Figure 1. Simple VFD / Fan Application
A drive can control two main elements of a 3-phase induction motor: speed and torque. To understand how a drive controls these two elements, we will take a short review of AC induction motors. Fig. 2 shows the construction of an induction motor. The two basic parts of the motor, the rotor and stator, work through magnetic interaction. Figure 2. AC Induction Motor Construction
A motor contains pole pairs. These are iron pieces in the stator, wound in a specific pattern to provide a north to south magnetic field (Fig. 3). Figure 3. Rotor and Stator Operation