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Group 6

Franz, Doug Hafner, Richard Pierce, David Smith, Kasper Komiskey, Jennifer Miller, Tracy

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Group 6

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  1. Introduction to Three-Phase Synchronous & Induction motors • The three phase motor is widely used in industry because of its low maintenance characteristics. • Due to the nature of three phase power, creating a rotating magnetic field in the stator of this motor is simple and straight forward. • When the magnetic polarity on pole 'A' reaches its peak, 120 electrical degrees later pole 'B' will reach its peak, and 120 electrical degrees after that, pole 'C' will reach its peak. Then the cycle repeats itself, and a rotating magnetic field is developed in the stator of the motor.

  2. Three-Phase Synchronous Motor Operation • The 'Stator' winding, when excited by a poly-phase (usually 3-phase) supply, creates a rotating magnetic field inside the motor. • The rotor winding, which acts as a permanent magnet, simply locks in with the rotating magnetic field and rotates along with it. • During operation, as the rotor locks in with the rotating magnetic field, the motor is said to be in synchronization. • Once the motor is in operation, the speed of the motor is dependent only on the supply frequency

  3. How an Induction Motor Works • The rotor current is induced by the magnetic field, instead of electrical connections • The statorof an induction motor consists of a number of overlapping windings offset by an electrical angle of 120° • When the primary winding or stator is connected to a three phase alternating current supply, it establishes a rotating magnetic field which rotates at a synchronous speed. • The number of poles and the frequency of the applied voltage determine the synchronous speed of rotation in the motor’s stator. • Motors are commonly configured to have 2, 4, 6 or 8 poles. Squirrel Cage Three-Phase Induction Motor Video

  4. Applications of Synchronous Motors Synchronous motors find applications in all industrial applications where constant speed is necessary. Low power applications include positioning machines, where high precision is required, such as robot actuators. Electric clocks. Record player turntables

  5. Applications of Induction Motors • Elevators, lifts, feed motors • Metal-cutting machine tools • Slow-speed vehicles, high-speed vehicles • Extruders • Back-pressure fans • Centrifugal pumps • Rotary piston blowers • Ball mills • Centrifugal mills • Hammer mills • Impact mills

  6. Advantages & Disadvantages of Synchronous Motors Advantages Disadvantages No speed adjustment, therefore cannot be used in variable speed jobs. Cannot start if under load. Starting torque has to be zero. It wanders slightly. If over-loaded will fall out of synchronism. Requires collector rings and brushes. Is not good application for frequent starting, or where strong torque for starting is required. • Operates at constant speed irrespective of load. • Electromagnetic power varies linearly with the applied voltage. • Can be operated at unity or leading power factor. • Efficiency of operation is usually high, especially in the low speed and unity power factor ranges.

  7. Advantages & Disadvantages of Induction Motors ADVANTAGES DISADVANTAGES Speed decreases as load increases, unlike Synchronous motors. The speed control auxiliary can cost up to ten times the cost of the motor Low starting torque, unlike Direct Current motors Single phase power is common while three phase power is less commonly available • Simple in construction: So large numbers can be constructed inexpensively • Extremely rugged construction  • Low cost : As construction is simple • Reliable : As it is extremely rugged • Low Maintenance : Only periodic dusting is required • Simple starting arrangement : Small motors can be started directly • High Efficiency : Full load efficiency is 85% to 94% • High Power Factor : Full Load power factor is 0.85 to 0.92

  8. Summing Up Three-Phase Motors • The precision of the synchronous motor and the variable speed of the induction motor, allow for a wide variety of applications between them. • Where constant speed is necessary, synchronous motor are best (ex: clocks and robotics), The variable speed offered by induction motors allow for different applications (ex: vehicles and milling machines). • Both types of three phase motors have advantages and disadvantages; but between the two types there is a broad range of industrial and commercial applications.

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