Copyright Goodheart-Willcox Co., Inc. Chapter 17 Electrical Troubleshooting Fundamentals 381 6 Frequency Check Frequency is a measure of the number of cycles that occur per second in an ac circuit. Most power in North America is sup- plied at 60 Hz. However, some electronic devices, such as vari- able speed motors, may require special frequencies supplied by VFDs (variable frequency drives). As a result, it may be nec- essary to use a multimeter’s frequency function to verify that an electrical load is receiving power at the correct frequency. It is also a good practice to confirm that the incoming line volt- age is in fact operating at 60 Hz. If a motor or other device is not receiving the proper frequency, it may cause the motor or device to draw too much current and overheat. 17.1.5 Power Factor Meters A circuit’s power factor is the relationship between the circuit’s true power (in watts) and apparent power (in volt-amperes). Power factor is the ratio of a wattmeter reading (true power) to calculated power (apparent power). A power factor meter is an instrument used to provide a direct reading of the power factor in an electrical circuit. A power factor meter saves time because it prevents the technician from having to take several measurements at different settings and then perform calcula- tions to determine the power factor. A power factor meter first determines the apparent power in a circuit. It finds the product of the measured voltage multi- plied by the measured current. It then determines the circuit’s true power by measuring its wattage. The ratio of the true power to the apparent power is determined and displayed. This value is the power factor. It is always a number between 0.0 and 1.0 (or, a percentage between 0% and 100%). Power factor meters can be used to assist in increas- ing the effectiveness of a circuit. For example, assume a circuit has a measured power factor of 0.8, meaning that 80% of the total possible power is being used. This value is quite common in an inductive circuit, such as one with a motor or transformer. The proper addition of a capaci- tive load would result in an increase in true power. This increase would then bring the power factor closer to 1.0. Electric utility companies usually limit the power factor allowable in industrial and commercial loads. Normally, a power factor of at least 0.85 or 85% is required. 17.1.6 Wattmeters A wattmeter is an instrument used to measure the true power, or wattage, used by a circuit or an electrical load. Like an ammeter, a wattmeter is connected in series with the cir- cuit being measured. Its reading automatically adjusts for a circuit’s power factor. To help in checking electrical circuits, many companies list the volt-ampere (VA) values for their equipment. A wattmeter is often used to confirm that a load is operating at the correct power level. For example, a manu- facturer may list a motor as a 120 V single-phase motor with 1.1 VA under a full load. The technician can then check the motor with a voltmeter and clamp-on ammeter to find out if the motor is operating correctly, or simply use a wattmeter to compare to the rated value. 17.1.7 Electrical Insulation Testers Extreme temperatures, dirt, oil, and mechanical stress can break down insulation on conductors over time. For instance, when a motor overheats, the insulation on the motor windings often breaks down. The insulation on windings keeps current in the motor circuit, but burnt or heat-damaged insulation may allow current to seek ground out of the circuit. This can create a dangerous shock hazard and also cause an equip- ment breakdown due to a short circuit in the windings or a short to ground (ground fault). An electrical insulation tester is an instrument used to detect current leaks or possible areas of insulation failure along conductors, Figure 17-9. Wattage Readings HVACR technicians use wattage readings to determine if an electrical load is operating efficiently. For example, if a motor is consuming more than its rated watts, it is consuming more electricity, making its operating costs higher. Thinking Green Scale in megohms (MΩ) Lights indicate insulation condition Sealed Unit Parts Co., Inc. Figure 17-9. An insulation tester is used for testing the resistance of electrical insulation and can measure electrical resistance up to 1000 MΩ.