Ch 4

3-phase power is the standard power supplied by electrical power companies to almost all commercial and industrial locations.

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3-phase motors have a fairly constant speed characteristics and are available in a wide variety of torque characteristics.

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Each phase must have an unequal number of coils, so the total number of coils must be divisible by the number 3.

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While almost all 3-phase motors have very similar stator designs, the main difference between different types of 3-phase motors is the design of the rotors and windings.

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The end of a motor shaft is machined with a keyway to contain a bar-type key or with a circular slot that contains a half-moon key.

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For single-phase power, three sine curves placed 120 electrical degrees apart are used.

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In a wye-connected, 3-phase motor, the north or south polarity of the poles is determined by the direction of the voltage rating and the motor design.

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In a delta-connected, 3-phase motor, the current starts to change as the sine waves rotates through the cycle and the amount of current flowing through each coil changes.

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In a dual-voltage, wye-connected, 3-phase motor, each phase coil (A,B,C) is divided into two equal parts, by dividing the phase coils in two, eleven terminal leads are available.

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The direction of rotation of 3-phase motors cannot be reversed by interchanging any of the 3-phase power lines to the motor.

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