Sunday, January 11, 2015

AC Instruments and Meters : Thermal converters , The watt-hour meter and Digital multimeters .

THERMAL CONVERTERS

Thermal converters change ac voltage and current signals into a dc signal in millivolts. This signal is proportional to the product of VI cos 8. AC watts can be measured using a thermal converter with a dc recording or indicating device.

Figure 11–32 is a schematic diagram of a single-phase thermal converter. The currents in the resistors depend on the values of the voltage and current inputs and the phase angle 8 between the inputs. Thermocouples are attached to each resistor. These thermocouples are connected in series so that their thermal electromotive forces (emfs) subtract. The total thermocouple circuit output is proportional to the temperature difference between the two resistors.

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This thermocouple output is also proportional to the difference in power dissipation in the resistors. Thus, the output is equal to a constant times VI cos 8. This means that the dc millivolt signal output of the converter indicates the ac power in the circuit to which the V and I inputs are attached.

The outputs of two of these converters can be connected in series. As a result, a dc voltage is developed that is proportional to the sum of two ac power inputs. In summary, three-phase ac power can be measured in two ways: (1) using the two-wattmeter method or (2) using one dc millivoltmeter or recorder.

External phase-shifting autotransformers can be added so that the same circuit will measure three-phase VARs.

THE WATT-HOUR METER

Direct Current Fundamentals explained that electrical work is the use of electrical energy over a period of time. Electric power is the rate at which electrical energy is used. The basic unit of measurement for electric power is the watt. In ac circuits, power in watts is the product of the potential in volts, the current in amperes, and the power factor. The basic unit of measurement for electric energy is the watt-hour. This value is found by multiplying the power (in watts) of a circuit by the total time in hours during which the power is used in the circuit. The watt-hour meter measures the electrical energy consumed in the circuit.

The connections for a typical single-phase watt-hour meter are shown in Figure 11–33. The watt-hour meter is similar to the wattmeter because it also has current coils connected in series with the load and potential coils connected across the line voltage. The interacting magnetic field of the current and voltage coils produces a torque in an armature. This torque is always proportional to the power in the circuit. In the watt-hour meter, the armature is a disc that rotates at a speed proportional to the power in the circuit. The rate at which the disc rotates corresponds to the power. The number of revolutions of the disc corresponds to the total energy used.

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Components of the Watt-hour Meter

A single-phase watt-hour meter consists of the following parts: an electromagnetic element, the magnetic braking system, the register, the frame, the base (including terminal connections), and the cover.

An induction-type motor is used in a watt-hour meter. The rotor is an aluminum disc mounted on a shaft. This shaft is free to turn in bearings held in the metal frame. A worm gear drives the gear register. Generally, this gear is cut directly into the shaft. In some instances, the rotor may be suspended magnetically. Guide pins are then used to maintain vertical alignment of the shaft. In all cases, the disc is mounted so that a portion of it rotates in the airgap of the stator assembly or electromagnet.

Operation

The electromagnet has two sets of windings assembled on a laminated soft iron core. The potential coil winding has many turns of fine wire and a high impedance. This winding is connected across the source voltage. The current coil winding consists of a few turns of heavy wire. This winding is connected in series with the metered circuit. The core laminations are riveted together to form a rigid mechanical structure. The permanent alignment that results maintains the correct magnetic flux distribution and so ensures a consistent performance.

A torque results from the interaction between the flux (produced by the current in one of the coils) and the eddy currents (induced in the disc by the flux created by the other coil). This torque causes the disc to turn at any given instant. The current coil conducts the load current. Because this coil has only a few turns of large-size wire, its inductance is very small. This means that the current coil flux is nearly in phase with the load current. The current coil

flux produces eddy currents in the disc. These eddy currents lag the current coil flux by 90°. Recall that an induced emf always lags 90° behind the flux producing it.

The potential coil is highly inductive. Therefore, the current and flux of the potential coil lag the source voltage by nearly 90°. If a load has a unity power factor, then the potential coil flux is in phase with the eddy currents produced by the current coil. The potential coil poles are above that part of the disc where the eddy currents flow. These eddy currents react with the potential coil flux to develop a torque that is proportional to the line voltage and the load current.

The poles on which the current coil is wound are located beneath the part of the disc where the eddy currents from the potential coil flux flow. These eddy currents react with the flux of the current coil. This reaction produces an additional torque that is also proportional to the line voltage and the load current.

Assembly. An electromagnet assembly is shown in Figure 11–34 for a single-phase watt- hour meter. For a load with a power factor other than unity, the eddy currents lag or lead the fluxes with which they react. The amount of lag or lead corresponds to the phase difference between the line voltage and the load current. The torque developed in each of the eddy current and flux reactions is reduced by a proportional amount.

It is possible for the disc to turn at an excessive speed. To prevent this, a magnetic braking system is used. This system consists of two permanent magnets mounted so that the disc is located between the poles of the magnets. As the disc rotates, it cuts the flux of the two permanent magnets. Eddy currents are thus induced in the disc. The eddy currents react with the permanent-magnet flux to produce a damping torque. The torque opposes the meter torque, and the disc turns at the desired speed for a given load.

The gear register is located on the disc shaft and consists of a train of gears driven by a worm gear or a pinion gear. The gear register turns several dial pointers to show the number of times the disc has turned. This means that the watt-hour meter determines and adds together

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(integrates) all of the instantaneous power values. As a result, there is an indication of the total energy used over a period of time. Compare this action with that of a wattmeter, which indicates only the instantaneous power or rate of energy use in a circuit.

Interpreting the Dial Readings of the Watt-hour Meter

The watt-hour constant of the meter is the number of watt-hours represented by one revolution of the disc. The pointer of the right-hand dial of the gear register indicates one kilowatt-hour after the disc makes the required number of revolutions. The gearing is arranged so that each division on the right-hand dial is one kilowatt-hour (kWh). On the second dial from the right, each division represents 10 kWh. The third dial from the right has divisions representing 100 kWh. For the dial on the left, each division represents 1000 kWh.

The register ratio is the number of revolutions made by the first gear wheel as it meshes with the worm or pinion gear on the disc shaft for one revolution of the right-hand dial pointer. The gear ratio is the number of revolutions made by the meter disc in causing one revolution of the right-hand dial pointer.

DIGITAL MULTIMETERS

Digital multimeters have become increasingly popular. The most apparent difference between digital meters and analog meters is the fact that digital meters display their read- ings in discrete digits rather than with a pointer and scale. A digital multimeter is shown in Figure 11–35. Some digital meters have a range switch similar to the range switch used with analog meters. This switch sets the full range value of the meter. Many digital meters have volt- age range settings from 200 mV to 2000 V. The lower ranges are used for accuracy. For example, assume that it is necessary to measure a voltage of 16 V. The meter will be able to make a more accurate measurement when set on the 20-V range than it will when set on the 2000-V range.

Some digital meters do not contain a range setting control. These meters are known as autoranging meters. They contain a function control, which permits selection of the electrical quantity to be measured such as ac volts, dc volts, or ohms. When the meter probes are connected to the object to be tested, the meter automatically selects the proper range and displays the value. Appearance is not the only difference between digital meters and analog meters. Analog meters change scale value by inserting or removing resistance from the meter circuit (Figure 11–36). The typical resistance of an analog meter is 20,000 D/V for dc and 5000 D/V ac. This means that if the meter is set for a full-scale value of 60 V, there will be 1.2 MD of resistance connected in series with the meter if it is being used to measure dc (60 X 20,000 = 1,200,000) and 300 kD if it is being used to measure ac (60 X 5000 = 300,000). The impedance of the meter is of little concern if it is used to measure circuits that are connected to a high-current source. For example, assume that the voltage of a 480-V panel is to be measured with a multimeter having a resistance of 5000 D/V. If the meter is set on the 600-V range, the resistance connected in series with the meter is 3 MD (600 X 5000 = 3,000,000). This will permit a current of 160 µA to flow in the meter circuit (480/3,000,000 = 0.000160). This 160 µA of current would not be enough to affect the circuit being tested.

Now assume that this meter is to be used to test a 24-V circuit that has a current flow of 100 µA. If the 60-V range is used, the meter circuit contains a resistance of 300 kilohms (60 X 5000 = 300,000). This means that a current of 80 µA will flow when the meter is

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connected to the circuit (24/300,000 = 0.000080). The connection of the meter to the circuit has changed the entire circuit operation.

Digital meters do not have this problem. Most digital meters have an input impedance of about 10 MD on all ranges. This is accomplished by using field effect transistors (FETs) and a voltage divider circuit. A simple schematic for this circuit is shown in Figure 11–37. Notice inthis circuit that the meter input is connected across 10 MD of resistance regardless of the range setting of the meter. If this meter is used to measure the voltage of the 24-V circuit, a current of µA will flow through the meter. This is not enough current to upset the rest of the circuit, and voltage measurements can be made accurately.