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NORA Oilheat Technicians Manual

nect the two leads from the capacitor terminal. 3. Connect one ohmmeter lead to the L1 motor power lead and the other meter lead to each of the capacitor leads, one at a time. See Figure 10-15. A B 4. Record the two resistance values. 5. Repeat by measuring the other motor power lead (L2) and each of the capacitor leads, one at a time. 6. Check with the manufacturer’s instructions. From one of the power leads, you should have measured 3-6 ohms and 9- 18 ohms. From the other power lead, you should have measured a short (less than 1 ohm) and 15-25 ohms. If you do not observe these resistances, the motor windings are faulty, and the motor should be replaced. Miscellaneous things to look for: • Make sure bearings are in good condition and oiled. • Check and be sure there is no excessive end play on the shaft. This can cause noise and hard starting. • Is all the wiring in good condition? If not, replace and secure with appropriate connectors. • On blower motors check belt size, width, and tension. Remember, overtightening is as bad as a loose belt. Chapter 10 Motors 10-12 Motors Figure 10-14: Multimeter Figure 10-15: PSC Motor wiring 3. Using the multimeter on the ohms scale, observe the meter’s response when the leads of the meter are connected to the terminals. Note: Because the meter charges the capacitor slightly in order to make a resistance measurement, if you desire to repeat the measurement, discharge the capacitor first (step 2). 4. The ohmmeter reading should jump immediately to a non-infinite resistance value and then quickly increase again to infinity. This should happen in a fraction of a second since the capacitor will charge quickly and then resist any more charge. If the meter settles to zero ohms, the capacitor has short-circuited. If the meter resistance is infinite the entire time, the capacitor is open circuited. A failed capacitor (open or short circuited) should be replaced by a capacitor of the same capacitance (microfarads or μF) and a voltage rating at least as great as the original one. In most cases it is best to replace the entire motor. Test tip: The quick capacitor response is more easily observed with an analog “needle” meter (see Figure 10-13), than a digital meter. With a digital meter, the resistance reading should gradually increase to infinite resistance (either quickly or slowly, depending on the meter). 5. Using the capacitance function of the multimeter, see Figure 10-14, and after shorting the capacitor, determine the microfarad output of the capacitor. The reading should be between 5-10% of the rating. Checking the PSC motor windings: 1. Remove power from the burner and detach the motor power leads from the burner. 2. Discharge the capacitor and discon- Figure 10-13: Analog “needle” meter Main: 3 - 6 Ohms Auxiliary: 14 - 18 Ohms L1 L2 Capacitor


NORA Oilheat Technicians Manual
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