In practice, the measurement of electrical quantities for commercial purposes, for example to measure consumption of electrical energy that has been used by the consumer (kWh-meter or kVAr-meter) can be done in two ways, namely direct measurement and indirect measurement. Measurement is directly applied to the installation of small-scale low-voltage electricity. In this case the measuring devices used directly connected to the load directly.
Measurements are not directly applicable to the employment-listrikan installation of low voltage / medium-large scale. In this case the measuring device used is connected to the load but rather indirectly through measuring transformers consists of a current transformer and voltage transformer. For the installation of low voltage electrification of large-scale typically only use current transformer, was for the installation of electricity medium voltage / high use current transformer and voltage transformer.
Measure transformer
Measuring transformer is a transformer that is designed specifically for the purposes of electrical measurement. There are two kinds of measuring transformers, which tarfo current (CT) and voltage transformer (PT). Due to function only as a tool in the measurement of the measuring tarfo designed with low power, for example for special use current transformer 30 VA.
In accordance SPLN 76-87, current transformer (CT) should have primary current from 10 A, 12.5 A, 15 A, 20 A, 25 A, 30 A, 40 A, 50 A, 60 A, 75 A and multiples thereof , while the CT secondary current is 1A, 2 A and 5 A. (mostly 5 A).
In accordance SPLN 77-87, voltage transformer (PT) must have a secondary voltage of 100 V with a power of 10, 15, 25, 30, 50, 75, 100, 150, 200, 300, 400 and 500 VA.
Current and voltage measurements in analog, uses the pointer to indicate the value of the measure. Basic movement of the pointer system is applied to the D’Arsonval voltmeter ampermeter and direct current and alternating current and multimeter. Basic movement of the needle on the meter D’Arsonval like permanent magnet direct current motor, as shown in Figure 6.6.
Figure 6.6 Principle of Power Torque In DC Motor
Interesting natural phenomenon discovered by Lorentz is if there are wires bearus are within the influence of a magnetic field as shown in Figure 6.6A, then on both sides of the wires under the influence of magnetic field will get him repulsive force that is dependent on the current direction and the direction of flux magnetic influence. Furthermore, the repulsive force called the Lorentz force.
Side wires that are on the side of the north pole (N) will get the upward force, while wires that are at the south pole (S) got a repulsive force that leads downwards, so that both the repulsive force pose clockwise torque. The relationship between the direction of electricity in the wires, the direction of magnetic flux and direction of a repulsive force proposed by Flemming. The relationship known as the Left Hand huku Flemmig as shown in Figure 6.6 B.
If the direction of the magnetic flux in the direction of the left hand index finger, and the direction of flow of the wires in line with the middle finger, then the direction of repulsion will be in line with the thumb.