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PSC MOTORS & BACK EMF

PSC MOTORS & BACK EMF. A PowerPoint presentation by Terry L. Robinson 2006. Back EMF … …is the voltage-generating effect of an electric motor’s rotor turning within the motor. …read between Common and Start terminals. …applied to Run Capacitor on PSC motor circuit. R. RUN WINDING.

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PSC MOTORS & BACK EMF

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  1. PSC MOTORS & BACK EMF A PowerPoint presentation by Terry L. Robinson 2006

  2. Back EMF… …is the voltage-generating effect of an electric motor’s rotor turning within the motor. …read between Common and Start terminals. …applied to Run Capacitor on PSC motor circuit.

  3. R RUN WINDING VOLTAGE INDUCED IN ROTOR 330 VAC 120 VAC 0 VAC C RUN CAP 370 VAC Motor fully loaded pulls 8A VOLTAGE INDUCED IN START WINDING S START WINDING

  4. Fully loaded… …motor is under all of the load that it is designed to handle. …as if the panels are off of the furnace and no filter in place. Back EMF generated at highest current.

  5. R RUN WINDING VOLTAGE INDUCED IN ROTOR 120 VAC 0 VAC 350 VAC C RUN CAP 370 VAC Motor partially unloaded pulls 7A VOLTAGE INDUCED IN START WINDING S START WINDING

  6. Partially Unloaded… …motor under a partial load as if furnace panels and filter in place. RPM increases generating a higher Back EMF and decreased current.

  7. R RUN WINDING VOLTAGE INDUCED IN ROTOR 120 VAC 380 VAC 0 VAC C RUN CAP 370 VAC Motor mostly unloaded pulls 5A VOLTAGE INDUCED IN START WINDING S START WINDING

  8. Mostly unloaded… …low motor load is reduced due to very dirty filter, blocked Return Air, dirty Evaporator Coil, and/or crimped Supply Air Ducts. High RPM generates high Back EMF and low current.

  9. Mostly unloaded… …Back EMF generated may be higher than the Run Capacitor’s voltage rating leading to Capacitor failure or burning out the Start Winding.

  10. R RUN WINDING Back EMF used with Hard Start Kit VOLTAGE INDUCED IN ROTOR C START CAP RUN CAP VOLTAGE INDUCED IN START WINDING S START WINDING BACK EMF 1 5 2 POTENTIAL RELAY

  11. Pop up quiz… • The Run Capacitor is wired in series with the Run Winding in a PSC motor circuit. • A. True B. False

  12. Pop up quiz… • 2. Line Voltage is applied to the Run Capacitor when the PSC motor is operating. • A. True B. False

  13. Pop up quiz… • 3. When the Back EMF is the lowest, the motor’s current is high. • A. True B. False

  14. Pop up quiz… 4. Closing Supply Registers will… A. …load the Blower Motor increasing Back EMF. B. …unload the Blower Motor decreasing Back EMF. C. …unload the Blower Motor increasing Back EMF.

  15. Pop up quiz… 5. Could a loss of refrigerant charge cause the Run Capacitor on the Compressor circuit to fail? A. Yes B. No Loss of charge unloads the Compressor increasing Back EMF – possibly above the capacitor’s voltage rating.

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