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Technology |
| AC Motors While there are only three general types of DC motors, there are many different AC motor types. This is because each type is confined to a narrow band of operating characteristics. These characteristics include torque, speed, and electrical service (single-phase or polyphase). These operating characteristics are used to determine a given motor’s suitability for a given application. What Makes an AC motor Different from a DC Motor? In a DC motor, electrical power is conducted directly to the armature through brushes and a commutator. An AC motor does not need a commutator to reverse the polarity of the current , as AC changes polarity “naturally.” Also, where the DC motor works by changing the polarity of the current running through the armature (the rotating part of the motor), the AC motor works by changing the polarity of the current running through the stator. (the stationary part of the motor). The many types of AC motor may be split into two main groups: single-phase and polyphase. Single Phase: A single-phase power system has one coil in the generator. Therefore, one alternating voltage is generated. Single-phase motors are generally motors with KW ratings of one or below. (These are generally called fractional KW motors.) They are generally used to operate mechanical devices and machines requiring a relatively small amount of power. Types of single-phase motors include: shaded-pole, capacitor, split-phase, repulsion, series (AC or universal) and synchronous. However, the single-phase motor is generally not used because it is inefficient, expensive to operate, and is not self starting. Three-Phase Three-phase or polyphase motors run on three-phase power. A three-phase power system has three coils in the generator. Therefore, three separate and distinct voltages will be generated Types of three-phase motors include: induction (squirrel-cage or wound), rotor types, commutator, and synchronous. The Squirrel Cage Induction Motor Induction refers to electrically charging a conductor by putting it near a charged body. Induction Principle The principle of the induction motor was first discovered by Arago in 1824. He observed that if a non-magnetic metal disk and a compass are pivoted with their axes parallel, so that one (or both) of the compass poles are located near the edge of the disk, spinning the disk will cause the compass needle to rotate. The direction of the induced rotation in the compass is always the same as that imparted to the disk. You can prove it to yourself if you like. Mount a simple copper or aluminum disk and a large compass on a vertical stem, so that each may be rotated on its own bearing, independently of the other. Spin the disk, and watch the compass needle. There is no more effective way to demonstrate the principle of induction. Applying the Induction Principle to AC Motor So, how do we apply the concept of induction to a motor? Recall that the AC motor works by changing the polarity of the current running through the stator (the stationary part of the motor). The stator plays the role of the metallic disk described above. A rotating magnetic field is established in the stator. The conductor, called the Rotor, “follows” the rotating magnetic field by beginning to rotate, just like the compass needle described above. The induction motor uses a rotor of a special design. It resembles a cage used for exercising squirrels. This is why it is called a squirrel cage rotor. The rotor consists of circular end rings joined together with metal bars. Note that the metal bars are placed directly opposite each other and provide a complete circuit within the rotor, regardless of the rotor's position. Rotors normally have several bars, but only a few are shown here for clarity. Squirrel cage motors are usually chosen over other types of motors because of their simplicity, ruggedness and reliability. Because of these features, squirrel-cage motors have practically become the accepted standard for AC, all-purpose, constant speed motor applications. Without a doubt, the squirrel-cage motor is the workhorse of the industry. The Squirrel Cage Induction Motor has certain advantages over the DC motor. · There are only two points of mechanical wear on the squirrel cage motor: the two bearings. · Because it has no commutator, there are no brushes to wear. This keeps maintenance minimal. · No sparks are generated to create a possible fire hazard. |
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