Several electrical instruments are used by the air- conditioning serviceperson to see if the equipment is working properly. Studies show that the most trouble calls on heating and cooling equipment are electrical in nature.
The most frequently measured quantities are volts, amperes, and ohms. In some cases, wattage is measured to check for shorts and other malfunctions. A wattage meter is available. However, it must be used to measure volt-amperes (VA) instead of watts. To measure watts, it is necessary to use DC only or convert the VA to watts by using the power factor. The power factor times the volt-amperes produces the actual power consumed in watts. Since most cooling equipment use AC, it is necessary to convert to watts by this method.
A number of factors can be checked with electrical instruments. For example, electrical instruments can be used to check the flow rate from a centrifugal water pump, the condition of a capacitor, or the character of a start or run winding of an electric motor.
Ammeter
The ammeter is used to measure current. It can measure the amount of current flowing in a circuit. It may use one of a number of different basic meter movements to accomplish this. The most frequently used of the basic meter movements is the D’Arsonval type. See Fig. 1-40. It uses a permanent magnet and an electromagnet to determine circuit current. The permanent magnet is used as a standard basic source of magnet- ism. As the current flows through the coil of wire, it creates a magnetic field around it. This magnetic field is strong or weak, depending upon the amount of cur- rent flowing through it. The stronger the magnetic field created by the moving coil, the more it is repelled by the permanent magnet. This repelling motion is calibrated to read amperes, milliamperes (0.001 A), or microamperes (0.000001 A).
The D’Arsonval meter movement may also be used on AC when a diode is placed in series with the moving coil winding. The diode changes the AC to DC and the meter works as on DC. See Fig. 1-41. The dial or face of the instrument is calibrated to indicate the AC readings.
There are other types of AC ammeters. They are not always as accurate as the D’Arsonval, but they are effective. In some moving magnet meters, the coil is stationary and the magnet moves. Although rugged, this type is not as accurate as the D’Arsonval type meter.
Fig. 1-40 Moving coil (D’Arsonval) meter movement.
Fig. 1-41 Diode inserted in the circuit with a D’Arsonval movement to produce an AC ammeter.
The moving vane meter is useful in measuring current when AC is used. See Fig. 1-42.
The clamp-on ammeter has already been dis- cussed. It has some limitations. However, it does have one advantage in that it can be used without having to
break the line to insert it. Most ammeters must be connected in series with the consuming device. That means one line has to be broken or disconnected to insert the meter into the circuit.
The ampere reading can be used to determine if the unit is drawing too much current or insufficient current. The correct current amount is usually stamped on the nameplate of the motor or the compressor.
Starting and running amperes may be checked to see if the motor is operating with too much load or it is shorted. The flow rate of some pumps can be deter- mined by reading the current the motor pulls. The load on the entire line can be checked by inserting the ammeter in the line. This is done by taking out the fuse and completing the circuit with the meter. Be careful.
If the ammeter has more than one range, it is best to start on the highest range and work down. The reading should be in or near the center of the meter scale for a more accurate reading. Make sure you have some idea what the current in the circuit should be before inserting the meter. Thus, the correct range—or, in some instances, the correct meter—can be selected.
Fig. 1-42 Air-damping system used in the moving-vane meter.
Voltmeter
The voltmeter is used to measure voltage. Voltage is the electrical pressure needed to cause current to flow. The voltmeter is used across the line or across a motor or whatever is being used as a consuming device.
Voltmeters are nothing more than ammeters that are calibrated to read volts. There is, however, an important difference. The voltmeter has a very high internal resistance. That means very small amounts of current flow through its coil. See Fig. 1-43. This high resistance is produced by multipliers. Each range on the voltmeter has a different resistor to increase the resistance so the line current will not be diverted through it. See Fig. 1-44. The voltmeter is placed across the line, whereas the ammeter is placed in series. You do not have to break the line to use the voltmeter. The voltmeter has two leads. If you are measuring DC, you have to observe polarity. The red lead is the positive (+)
Fig. 1-43 An ammeter with high resistance in series with the meter movement allow it to measure voltage.
Fig. 1-44 Different types of multirange voltmeters. This view shows the interior of the meter box or unit.
Fig. 1-45 Internal circuit of an ohmmeter.
and the black lead is the negative (-). However, when AC is used, it does not matter which lead is placed on which terminal. Using a D’Arsonval meter movement, voltmeters can be made with the proper diode to change AC to DC. Voltmeters can be made with a stationary coil and a moving magnet. Others types of volt- meters are available. They use various means of registering voltage.
If the voltage is not known, use the highest scale on the meter. Turn the range switch to appoint where the reading is in the midrange of the meter movement.
Normal line voltage in most locations is 120 V. When the line voltage is lower than normal, it is possible for the equipment to draw excessive current. This will cause overheating and eventual failure and/or burnout. The correct voltage is needed for the equipment to operate according to its designed specifications. The voltage range is usually stamped on the nameplate of the device. Some will state 208 V. This voltage is obtained from a three-phase connection. Most home or residential volt- age is supplied at 120 V or 230 V. The range is 220 to 240 V for normal residential service. The size of the wire used to connect the equipment to the line is important. If the wire is too small, voltage will drop. There will be low voltage at the consuming device. For this reason a certified electrician with knowledge of the NEC should wire a new installation.
Ohmmeter
The ohmmeter measures resistance. The basic unit of resistance is the ohm (W). Every device has resistance. That is why it is necessary to know the proper resistance before trying to troubleshoot a device by using an ohmmeter. The ohmmeter has its own power supply. See Fig. 1-45. Do not use an ohmmeter on a line that is energized or connected to a power source of any voltage.
An ohmmeter can read the resistance of the windings of a motor. If the correct reading has been given by the manufacturer, it is then possible to see if the reading has changed. If the reading is much lower, it may indicate a shorted winding. If the reading is infinite (w), it may mean there is a loose connection or an open circuit.
Ohmmeters have ranges. Figure 1-46 shows a meter scale. The R ´ 1 range means the scale is read as is. If the R ´ 10 range is used, it means that the scale reading must be multiplied by 10. If the R ´ 1000 range is selected, then the scale reading must be multiplied by 1000. If the meter has a R ´ 1 meg range, the scale reading must be multiplied by one million. A meg is one million.
Fig. 1-46 A multimeter scale. Note the ohms and volt scales.
Multimeter
The multimeter is a combination of meters. See Fig. 1-47. It may have a voltmeter, an ammeter, and an ohmmeter
Fig. 1-47 Two types of multimeters.
in the same case. This is the usual arrangement for fieldwork. This way it is possible to have all three meters in one portable combination. It should be checked for each of the functions.
The snap-around meter uses its scale for a number of applications. It can be read current by snapping around the current carrying wire. If the leads are used, it can be used as a voltmeter or an ohmmeter. Remember that the power must be off to use the ohmmeter. This meter is mounted in its own case. It should be protected from shock and vibration just as any other sensitive instrument.
Wattmeter
The wattmeter is used to measure watts. However, when used on an AC line it measures volt-amperes. If watts are to be measured, the reading must be con- verted to watts mathematically. Multiply the reading on the wattmeter by the power factor (usually avail- able on the nameplate) to obtain the reading in watts. Wattmeters use the current and the voltage connections as with individual meters. See Fig. 1-48. One coil is heavy wire and is connected in series. It measures the current. The other connection is made in the same way as with the voltmeter and connected across the line. This coil is made of many turns of fine wire. It measures the voltage. By the action of the two magnetic fields, the current is multiplied by the voltage. Wattage is read on the meter scale.
The volt-ampere is the unit used to measure volts time amperes in an AC circuit. If a device has inductance (as in a motor) or capacitance (some motors have run-capacitors), the true wattage is not given on a wattmeter. The reading is in volt-amperes instead of watts. It is converted to watts by multiplying the reading by the power factor. A wattmeter reads watts only when it is connected to a DC circuit or to an AC circuit with resistance only.
The power factor is the ratio of true power to apparent power. Apparent power is what is read on a wattmeter on an AC line. True power is the wattage reading of DC. The two can be used to find the power factor. The power factor is the cosine of the phase angle. The power factor can be found by using a mathematical computation or a very delicate meter designed for the purpose. However, the power factor of equipment using alternating current is usually stamped on the nameplate of the compressor, the motor, or the unit itself.
Wattmeters are also used to test capacitors. Some companies provide charts to convert wattage ratings to microfarad ratings. The wattmeter can test the actual connection of the capacitor. The ohmmeter tells if the capacitor is good or bad. However, it is hard to indicate how a capacitor will function in a circuit with the voltage applied. This is why testing with the wattmeter is preferred.
Fig. 1-48 (A) Wattmeter connection for measuring input power. (B) Alternate wattmeter connection. (C) With load disconnected, uncompensated wattmeter measures its own power loss.
