Copyright Goodheart-Willcox Co., Inc. 254 Essential Electrical Skills for HVACR: Theory and Labs Note that one wire is used to produce two magnetic poles. This is accomplished by wrapping the wire clockwise around one iron core and counterclockwise around an opposite iron core. These iron cores are called stator poles. Thus, the stator shown in the figure has two poles and is called a two-pole motor. The direction of the supply current during the first half of the cycle induces the north and south magnetic fields in the poles with opposite polarities. Current flows into the bottom pole and induces a magnetic field that yields in a south pole. Since the wire is wound in the opposite direction on the top pole, a north pole is produced. In the sec- ond half of the cycle, the poles change polarity due to the change in current flow direction. For the rotor to rotate, its magnetic poles must interact with the stator poles. An ac induc- tion motor uses a rotor called the squirrel cage rotor. This rotor has a similar appearance to a hamster or squirrel cage and wheel. Other types of rotors are wire wound and must be connected to power, while others are made from perma- nent magnets. The collapsing of the stator mag- netic field induces a current in the rotor. The stator and rotor behave like a transformer—the stator is the primary, and the rotor is the second- ary. This also means that the induced current from the rotor is in the opposite direction of the current in the stator. See Figure 13-4. A large current is induced in the rotor since there is low resistance from the bars and shorting rings. The rotor current induces a magnetic field with a polarity that depends on current flow direction. In Figure 13-5, a rotor is placed within energized stator fields. This results in the rotor’s magnetic poles attracting the poles of the stator for most of the sine wave cycle. This motor can- not start automatically. It would require physi- cally spinning the rotor to get it started, which is impractical. Instead, in order for the motor to start and in the same rotor direction each time, a second set of poles is required. The second set of poles must be out-of-phase and have opposite polarity from the first set of poles. This creates a rotating magnetic field. The rotor then follows the rotating magnetic field. Generating opposite polarities is deter- mined by wrapping the stator pole in opposite directions. To create two out-of-phase mag- netic field poles, two out-of-phase currents are required. To generate out-of-phase currents from single-phase power, the windings must have different dc resistance and inductive reac- tance resistance. The main winding is called the run winding and has low dc resistance but high inductive reactance resistance. The auxiliary winding is called the start winding and has higher dc resistance but less inductive reactance resistance. This is because the run Stator Rotor Stator current Shorting rings Induced rotor current Bars Stator current N N S S Goodheart-Willcox Publisher Figure 13-4. The stator induces a current in rotor similar to a transformer. 1 N S S N N S + – Wire wrapped clockwise Wire wrapped counterclockwise 2 S N – + Goodheart-Willcox Publisher Figure 13-5. A rotor is placed within the stator and the opposite poles attract, resulting in the rotor not moving.