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Forging new generations of engineers AC vs. DC The production of and use of electricity is an expensive business.Therefore the most efficient use of the resource is required. The most efficient way of generating electricity is through the use of an alternator. Additionally, alternating current is the most efficient to transport over long distances. Direct current transmission is limited to the distance it can travel. DC Motor Field winding The production of and use of electricity in a motor is done through the use of an armature, field winding, brushes, slip ring or commutator for a DC motor. Brushes Brush holders Commutator Armature Sine Waves To generate or use electricity, the rotor of the generator passes through the magnetic field produced by the field winding. As the rotor travels through its 360 degree rotation the field is charged and discharged twice in different directions. This in turn, develops the pictured sine wave. Single Phase Sine Wave The pictured sine wave is a single phase. In a single phase generator the rotor is rotated 3,600 times per minute, or 60 revolutions per second. This rotation produces the 60hz line voltages used today. 3 Phase Generator A three phase generator has three generator windings equally spaced around the circle. The windings will produce output voltages 120 degrees out of phase with each other. 3 Phase AC Voltage The advantages of three phase AC voltage is more efficient distribution of power. AC induction motors are self-starting, and the AC ripple is easier to filter out when rectifying to DC. Motor Control Centers Motor control centers combine individual control units into standard modular structure joined together for a convenient control center in one location. The motor control circuit is usually supplied from a panel board or switchboard. Basically the function of the control center is to take the incoming power and deliver it to the motor loads that are being controlled. DC Motor Control Direct current (DC) motors are used in industrial applications that require variable speed control, high torque, or both. Speed of most DC motors can be controlled smoothly and easily form zero to full speed. They are used in many acceleration and deceleration applications. DC motors can be controlled by the use of a simple rheostat (variable resistor). Silicon Control Rectifiers(SCR) Silicon control rectifiers (SCR) control motors by controlling conduction time with a pulsed gate. A rheostat controls motor speed with resistance SCRs are smaller in size for the same rating, energyefficient by not wasting power within itself, and less expensive. For these reasons most DC speed controls use SCRs instead of rheostats. DC Advantages In addition to having excellent speed control, the DC motor is ideal in applications that call for momentarily higher torque outputs. This is because the DC motor can deliver three to five times its rated torque for short periods of time. AC Motors Alternating current motors are basically a constant speed motor. This is because the synchronous speed of an induction motor are designed for 60 hertz use. Unlike the speed of a DC motor, the speed of an AC motor should not be changed by varying the applied voltage. In fact, if the supply voltage is varied to an AC motor more than 10% above or below the rated voltage, damage may be done. Changing AC Motor Speed The frequency or number of poles must be changed to change the speed of an AC motor, two methods of speed control are available. Changing the frequency applied to the motor Reconnecting the windings to create a different number of poles.