The higher the ohms/volt rating of a voltmeter, the less the voltmeter will upset circuit conditions. And the less circuit conditions are upset, the more accurate the reading will be. Most of the higher-end voltmeters and multimeters available now are rated at about 20,000 ohms/volt; more accurate voltmeters are rated at 100,000 ohms/volt. In some of the high-resistance circuits found in some present-day equipment, however, even a meter rated at 20,000 ohms /volt will greatly upset circuit conditions, and result in an incorrect reading. While a 100,000 ohms/voltmeter will give more accurate readings, even more accuracy is needed with some circuits. To overcome this problem, a device with a high ohms/volt rating called an electronic voltmeter was developed.
Because many insulation breakdowns occur during operation under the stress of high voltages, more dependable tests are performed with a megger (megohm-meter) and with hi-pot (high potential or high-voltage) equipment. The megger is a tiny, portable instrument that can be battery operated or powered by a hand-cranked generator. Meggers supply from 500 to 5000 volts for the insulation test. They measure leakage current, but the readout is calibrated in insulation resistance (ohms), in ranges to 100 and often 2000 megohms (MQ). The megger can measure insulation resistance between wires, between wires and ground, and between wiring and casings. Normal insulation resistance depends on the wire, insulation, length, and rating by the manufacturer. The greater the leakage, the more defective is the insulation. The wire manufacturer’s rating should be the guide.
With voltage measurements, the total resistance of the voltmeter must be considered. Reading an analog voltmeter scale is like reading an analog ammeter scale. Some multirange voltmeters have only one range marked on the scale and the scale reading must be multiplied by the range switch setting to acquire the accurate voltage reading. Other voltmeters have individual ranges on the scale for every setting of the range switch. When utilizing these meters, ensure that you read the set of values that corresponds to the range switch setting. Many digital meters also have range switches, but they may additionally have a special feature known as autoranging. This means you can either utilize the range switch or let the digital meter set the proper range by itself.
Current-measuring meters are called ammeters. There are two ways of measuring the current flowing in a circuit. The first way is to open the circuit being tested and link the meter test leads into the circuit so the current flowing in the circuit flows in series through the meter as well. This is the in-circuit-type of ammeter sometimes referred to as an in-line meter. An external type of ammeter, known as a clamp-on ammeter, can be used without opening the circuit. The clamp-on ammeter has jaws that open and clamp around the sides of one wire in the circuit and measures the magnetic field that the current in the wire produces, to determine the current flow. The clamp-on ammeter is widely used and simplifies the measurement of current from one to hundreds of amperes. It is especially helpful for field testing and troubleshooting AC power lines.
The moving-coil meter movement uses a horseshoe-shaped permanent magnet. The moving coil is placed within the magnetic field between the magnet’s two poles. However, if a simple horseshoe magnet were used, many of the magnetic lines of force would not cut through the moving coil. Magnetic lines of force travel the path of least resistance. Soft iron offers less resistance to lines of force than air. Therefore, soft iron pole pieces are attached to the poles of the magnet to concentrate the lines of force between the magnetic poles.
Where there is an option for the types of tests or measurements that can be done, opt for the resistance check, which allows for all power to be “off. ” To play it safe in these instances, wherever possible, throw the mainline switch or associated circuit breaker off. Lock the switch off if possible or tag it. Wear clothing that covers as much of your body as possible to reduce the chances of inadvertent contact with “hot” points. Try to use one hand at a time to make connections. Wear rubber or plastic soled shoes that do not use nails and are not worn or frayed. Wear gloves for as many tests as possible and use tools that have insulated handles and test leads that are not frayed. When wiring must be disconnected for a voltage or current test, always disconnect the power before the wiring is disconnected or reconnected and discharge all capacitors.
How Current Affects a Magnetic Field
Current flowing through a coil produces the magnetic field that surrounds the coil. The strength of the magnetic field is proportional to the amount of current flowing through the coil. As the current increases, the strength of the magnetic field increases, and as the current decreases, the strength of the magnetic field decreases. For example, if the current through a certain coil is increased from 1 to 1.6 amperes, the magnetic field around the coil will be stronger for 1.6 amperes than it was for 1 ampere.
Keeping friction to a minimum permits measuring small currents, but creates a major problem when reading the meter.
When a meter measures current, the pointer should move across the scale and stop immediately at the correct reading. However, because of the very little friction of the rotating parts, the pointer does not come to rest immediately at the correct reading; it overshoots because of inertia, and then the spring pulls it back; it overshoots slightly again, and so on. As a result, the pointer tends to swing back and forth or vibrate, about the correct reading point many times before coming to rest.
To overcome this problem, the meter movement must be damped. Damping can be thought of as a braking action on the rotating parts. It almost completely eliminates the vibrating action of the pointer, resulting in quick, correct pointer indication.
Damping also eliminates another problem. When a meter is removed from an external circuit or when the circuit is de-energized, the pointer returns to zero. Because of the very low friction of the rotating parts, the springs tend to pull the parts back to zero very quickly; so quickly in fact, that the pointer could bend as it overshoots and strikes the left retaining pin. This is particularly true when the meter returns to zero from near full -scale deflection.
Damping a Moving-Coil Meter
Moving-coil meter movements make use of the aluminum frame on which the coil is wound to provide damping. Since aluminum is a conductor, the frame acts as a one-turn coil. When the coil assembly and pointer rotate to register current, the aluminum frame cuts through the field flux lines of the permanent magnet. Small currents, called eddy currents, are induced in the frame, which set up a magnetic field about the frame.
The polarity of this magnetic field is opposite to that of the magnetic field about the coil. Therefore, the magnetic field about the frame opposes the magnetic field about the coil. This action reduces the overall field of the moving coil so that it swings more slowly. In effect, the faster the coil swings, the more the aluminum frame’s field slows it down. This causes the coil and pointer to rotate relatively slowly and smoothly to the correct reading without vibrating. As soon as the coil assembly and pointer come to rest, no further eddy currents are induced in the frame and its magnetic field disappears.
When the meter is disconnected from the circuit, or when the circuit is de-energized, essentially the same action occurs. The coil assembly and pointer start to rotate very rapidly toward zero, but now, since it is rotating in the opposite direction, the eddy currents produced in the frame flow in the opposite direction. This results in a magnetic field with a polarity opposite to the one previously. As the springs pull the coil assembly and pointer closer and closer to zero, the like poles of the permanent magnet and the frame repel each other more and more so that the coil assembly is again braked or slowed down, and slowly comes to rest at zero. Thus, the pointer is prevented from striking the left retaining pin and, perhaps, bending around it.
Alternating current flows periodically first in one direction and then in the opposite direction. One direction is called a positive alternation and the other direction is called a negative alternation. A complete positive and negative alternation is called one cycle. The number of complete cycles that occur each second is the frequency and is designated in hertz, abbreviated Hz. Therefore, if one complete cycle occurs per second, the frequency is 1 Hz; if 5 cycles are completed per second, the frequency is 5 Hz, and so on.
The moving-coil meter movement can only be used to measure dc, however; there is no way that it can be used to measure ac directly. If ac was applied directly to the meter, one half of the cycle would try to make the meter pointer move in one direction and the other half would try to make the pointer move in the opposite direction. Even at very low frequencies, the pointer would not be able to move fast enough to follow the positive and negative alternations of the ac wave. Therefore, instead of moving across the scale, the pointer would simply vibrate about zero. But, if the ac is first changed to DC before applying it to the meter movement then the moving-coil meter can be used in AC applications as well as DC.
Alternating current can be converted to DC by special devices called rectifiers. These devices offer a high opposition to current flow through them in one direction and a low opposition to current flow in the other direction. Therefore, when a sine wave is applied to a rectifier, it will pass either the positive alternation or the negative alternation, depending upon how the rectifier is connected into the meter circuit. In no case, however, will it pass both alternations. Therefore, a rectifier changes a sine wave to a pulsating dc wave. In a rectifier symbol, the arrow points in the direction of high resistance, so current flow through it is in the opposite direction. Rectifier meters use semiconductor rectifiers.
The Half-Wave Rectifier Meter
There are two basic types of rectifier circuits: the half-wave type and the full-wave type. In the half-wave type, one alternation of current passes through the meter movement and the opposite alternation is bypassed by the rectifier. Even though current through the meter is pulsating, the meter pointer, because of its inertia, will not have sufficient time to follow these fluctuations. Therefore, the meter pointer will rest at the average value of the current flowing through it.
The average current for one alternation is 0.637 of peak value, but, for the next alternation, it is zero and that alternation is bypassed by the meter. Therefore, the average current for a complete cycle is the sum of both alternations divided by 2, or 0.637/2 = 0.313 of the peak value. The meter pointer then deflects to the position on the scale that represents 0.318 of the peak value of the current flowing through the meter. But, for the reading to be meaningful the scale is usually calibrated to show the equivalent effective or RMS value. Therefore, the points on the scale are calibrated at 0.707 of the equivalent peak values.