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

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°.

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

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

ECE 4415 Torque-Speed Characteristic

ECE 4416 Cutaway view of a Split-Phase Motor

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°

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

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

ECE 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).

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

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

ECE 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.

ECE Example 6-2 continued From Example 6-1

ECE Phasor Diagram

ECE Modified Circuit

ECE Impedance Diagram for Auxiliary Winding

ECE Calculation of Capacitance

ECE Locked-rotor Torque

ECE 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°