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Published byBarrie Malone Modified over 4 years ago

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ECE 4411 Resistance-Start Split-Phase Motor R = R ext

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ECE 4412 Graphical Analysis I aux decreases with increasing R ext angle α increases with increasing R ext Locked-rotor Torque “peaks” for an “optimal” value of R ext. Phase displacement angle α is between 25° and 30°.

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ECE 4413 Practical Resistance-Start Motor “Centrifugal” switch or TRIAC Closed (shorted) when the motor is at rest Opens when motor speed is 75% – 85% of synchronous speed

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ECE 4414 Practical Resistance-Start Motor Phasor Diagram at start-up

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ECE 4415 Torque-Speed Characteristic

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ECE 4416 Cutaway view of a Split-Phase Motor

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ECE 4417 Capacitor-Start Split-Phase Motor Develop a larger value of I aw sinα, and, hence, a larger locked-rotor torque Phase-displacement angle between 75° and 85°

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ECE 4418 Capacitor-Start Motor Phasor Diagram at start-up

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ECE 4419 Torque-Speed Characteristic Higher Starting Torque Same Running Torque as before

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ECE 44110 Permanent-Split Capacitor Motor Uses a permanently-connected auxiliary circuit containing a capacitor. Smoother and quieter operation than resistor or capacitor starting motor Speed control by autotransformer across the line, or external resistor or reactor (inductor) in series with the main or auxiliary winding (or both).

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ECE 44111 Permanent-Split Capacitor Motor “Permanent” Capacitor Speed control by autotransformer

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ECE 44112 Two-Value Capacitor Motor Large capacitor for starting Small capacitor for running Centrifugal switch main auxiliary

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ECE 44113 Example 6-2 Using the motor from Example 6-1, determine the capacitance required in series with the auxiliary winding in order to obtain a 90° phase displacement between the current in the main winding and the current in the auxiliary winding at locked- rotor and the locked-rotor torque in terms of the machine constant.

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ECE 44114 Example 6-2 continued From Example 6-1

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ECE 44115 Phasor Diagram

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ECE 44116 Modified Circuit

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ECE 44117 Impedance Diagram for Auxiliary Winding

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ECE 44118 Calculation of Capacitance

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ECE 44119 Locked-rotor Torque

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ECE 44120 Graphical Analysis Auxiliary winding current increases then decreases with increasing capacitive reactance (why?) Angle α increases with increasing capacitive reactance Locked-rotor torque “peaks” for the optimal value of capacitive reactance. The resulting phase displacement angle is approximately 75°

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