Special-Purpose Meters

Frequency Meters

Most components in a power system are designed to run on a specific ac frequency or limited range of frequencies. The frequency meter is important to test these frequencies. The frequency meter works very much like the digital voltmeter. A digital meter can generate a pulse train to be counted. Since the test frequency already has waveforms that can be counted, the clock and decade divider circuits only have to supply the gate pulse. The test signal is amplified, then converted to square wave pulses, generally by a Schmill trigger stage. The pulses are gated, then further shaped, and then counted to determine their frequency for display.

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Digital Amplifiers

Electronic amplifiers are used with analog meters to increase their ranges, to make them more sensitive, to improve their accuracy, and to minimize their effects on the circuit under test. But, any analog meter movement can be used effectively without amplifiers; they need only multiplier resistors and/or shunt resistors to work on different ranges.

Digital meters differ from analog meters because they need electronic amplifiers if they are to work at all. The electrical characteristics that we work with are analog in nature, so some electronic circuits are necessary for the test reading to go through an analog-to-digital conversion to get a digital readout. Several different techniques were tried in the past, including electromechanical servo control, oscillator frequency control, and the one method that works best, pulse gate control.


Analog-to-Digital Servo

Servo systems are motor systems that can be used to position the component being driven by the motor. The servo motor does this by producing a feedback signal voltage of its own while being driven by the test voltage. When the motor reaches the position dictated by the value of the test voltage, the feedback voltage cancels, or nulls, the test voltage to stop the motor. Each different test voltage level will cause the motor to position the potentiometer at a different place to produce an equal but opposite null voltage. The shaft of the motor also drives a series of drum wheels, like a car’s odometer, with digital readouts to match the servo’s positions.

The digital servo was one of the earliest systems tried to produce a digital readout. It was expensive, a little clumsy, slow, required more power, and not accurate enough. It is not a purely digital device since the servo follows an analog positioning of the drum scale and the digits on the scale simulate digital positioning.


Analog-to-Digital Frequency Control

The first attempt to develop a digital meter that did not rely on electromechan­ical devices used an oscillator circuit to produce a string of pulses which could represent a string of digits. The frequency of oscillation of the circuit was designed to be dependent on the test signal voltage. The higher the test signal voltage, the higher would be the frequency produced, and the higher the frequency, the more pulses per second there would be. At the right frequency, each pulse could represent a digit, the higher the test voltage, the higher the frequency, the more the pulses, and the higher the pulse or digit count. For each test voltage level, the pulses can be counted during a fixed time interval. A gate circuit was used to go on and off at fixed intervals to let through a string of pulses to be counted, then to drive a digital readout.

The variable oscillator was a good start, but it had serious limitations. An oscillator circuit tends to function most efficiently at a given frequency, or a relatively narrow range of frequencies. Getting the oscillator to vary its frequency in a wide enough range, or sets of ranges requires numerous complex circuits that are difficult to get to function reliably and accurately.


Analog-to-Digital Pulse Gate Control

The pulse gate control method of analog-to-digital conversion solves the problem of an unstable oscillator that must have its frequency shifted because the gate control method uses a fixed frequency oscillator. Also, instead of having a fixed time interval gate, this method uses a variable interval gate, which is narrower for low test voltages and gets wider as the test voltages go higher. So, with a fixed frequency, the variable-width gate will let the number of sampled pulses go up as the test voltage goes up.

The design of this kind of digital meter circuit can establish a basic fixed oscillator frequency and a pulse count availability so that each pulse in a gate sampling will represent one digit, and the actual count of the number of pulses will be the actual digit value that is generated.

This type of analog-to-digital conversion does not merely produce a digit that simulates the analog value. It produces true digital values.






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Electromagnetic Meter Construction: Part 2


In both the moving-coil and moving-iron meter movements, the current being measured flows through the coil. Except for this similarity, the coils in each type of movement are different. When current flows through the coil of the moving-coil meter movement, a magnetic field is produced that causes the coil to rotate. For the coil to rotate easily, it must be as light as possible. To make the coil light, it is wound on an aluminum frame. Furthermore, the coil is made from very fine wire, and when compared with the coil in the other meter movements, it contains very few turns to keep it as light as possible.

The coil in this type of movement remains stationary, and the magnetic field about the coil moves an iron vane. Because this iron vane is relatively heavy, a strong magnetic field is required to move it. Therefore, the coil of a moving-iron meter movement contains many turns of wire to produce this strong magnetic field.

The iron vanes of the moving-iron meter movement are placed inside the coil. However, the shape of the coil is different for the concentric-vane meter movement and the radial-vane movement. The coil for the concentric­vane movement is constructed so that it can accept semicircular vanes and the coil for the radial-vane movement is constructed so that it can accept rectangular plates.



When a pointer is attached to the moving element of the meter movement, and a calibrated scale is put behind the pointer tip, the pointer will swing with the meter movement according to the amount of current flowing and stop at the proper place on the scale to show the current flow.

Because of the delicate nature of the meter movement, and the need to have it respond to current flow without having to overcome external forces, the pointer is generally made of thin aluminum to keep it light.

The tips of the pointer usually come in three shapes: spade, knife, and lance. The spade pointer is used when high visibility is needed, particularly at a distance. However, the broad width of the tip makes it difficult to differentiate scale index marks that are close together on high-accuracy scales. The thin, sliver-like knife pointer is best for high accuracy readings, but it is difficult to see at a distance.  The lance pointer is a compromise shape that gives good accuracy with acceptable visibility.

When knife-edge pointers are used for high accuracy, parallax error must be considered. This is the error in reading when the pointer appears to be pointing off the true reading because it is not being read exactly head-on. Good instruments contain a parallax mirror to show the error. The reading should be made when the mirrored reflection of the pointer is directly in line with the pointer. In this way, the meter will not be read at an angle. These are sometimes referred to as mirrored-scale meters.


Counterweights and Retaining Pins

Although the pointer in a meter is very light, the extremely delicate sensitivity of the meter movement is such that the pointer must be properly balanced so that it does not interfere with the accuracy of the movement.

When the meter is manufactured, counterweights are attached to the pointer assembly, and meticulously adjusted to balance the pointer on the pivot so that the weight of the pointer will neither aid nor oppose the motion of the meter movement.

To limit the range over which the meter movement can travel, and to protect it from being overdriven, retaining pins are placed on both sides of the pointer to keep the pointer from going too far off the bottom or top ends of the scale. The pins thus limit any undue pressure being applied to the delicate meter movement spring.







Current Tests: Part 2

Air Conditioner Current Test

Some motors are designed to reach rated rpm faster than other motors by using a special starting winding or capacitor circuit on startup, then switch over to the normal running circuit once normal rpm is reached. Motors used in air conditioners are of this type. Certain types of test jigs are available commercially.

The initial surge current of an air conditioner that requires a 15-ampere circuit is over 15 amperes, and this high current flows momentarily every time the motor starts up, when you first turn the equipment on, and whenever the compressor comes on. Therefore, it is recommended that a time-delay circuit breaker be used, and the air conditioner be on a separate, dedicated circuit. Set the ammeter range higher than 15 amps to allow for the surge; the normal run reading while the compressor is on will likely be less than 10 amps, and while the compressor is off, around 3 amps if the circulator fan is running, and zero if it is not.


Usage Tests

Usage testing refers to power usage more than it does to current usage, and it is the measure of the power used over a period. Essentially, a wattmeter is connected into the circuit, either permanently or for prolonged periods, to keep track of power consumed over specific lengths of time.

The most common type of usage tester, or wattmeter, is the utility’s monitor that is connected across the main input service line before it enters the building or, inside the building, before it enters the main service box. It monitors the line voltage and all current flows to the building and gives usage readings in kilowatt hours (kWh).

In other applications, separate circuits or equipment can be monitored to compare the power usage of the different systems. In a shopping mall, for example, the heating and/or cooling systems could be monitored separately, as well as the lighting systems, and people-moving systems, such as escalators and elevators. In industrial applications, individual production systems could be monitored. All of these usage monitoring systems give readings in kilowatt hours.


Load Tests

Typically, when a circuit is rated to carry a certain maximum current, say 20 amperes, there is little or no difference when a low current flows or when a higher current flows- as long as the current stays within limits. However, when certain problems exist, such as a high-resistance connection, the higher current that flows through such a connection will cause a voltage drop that will result in the line voltage in that circuit being reduced. How much the line voltage drops depends on how bad the connection is. In a case such as this, if the line voltage was tested when there was little current flowing, a normal reading might be obtained, because the line voltage drops under heavier load currents.

When circuits are suspected of this problem, a load tester can be used to simulate load conditions and read out the percentage that the line voltage drops as loads are simulated. A typical example of a load current simulator is one that can be set to draw 0, 10, 15, or 20 amperes in sequential steps. Do not set the simulator for a 20-amp load on a 15-amp circuit.