Electrical Measurement Instrument

Instrument Transformer and Power Management (P1) Course
Chapter (4) : Power Measurement in 1-ph Circuits
4.2 Electrical Measurement Instrument ;
       The measuring procedure is performed in measuring circuit, this is joined to a circuit of the quantity to be measured, it is important that the procedure in the measuring circuit shall not disturb the process giving rise to the quantity to be measured.
        Now let's talking about the electric measuring instruments which shall use our lab, tests:

  1. Ammeter.
  2. Voltmeter.
  3. Wattmeter ( single phase and three phase)
  4. Varmeter (single phase and three phase)
  5. Megger.
  6. Power factor meter.
4.2.1) Ammeter :
       Ammeter is a basic device used to measure current in amperes, the symbols for the ammeter is a circle with enclosed letter "A' as shown in Fig. (1). 
       An ammeter is inserted into the path of current, that in series to carry the current to be measured, if the ammeter is d.c type it has polarity marks to ensure proper connection for an up scale indication.
       If the meter is a.c type, current direction is of no significance and no polarity marks appear on the instrument. 
      However, the meter is still connected in series with the part of circuit for which current is being determined.
Fig. (1) : Ammeter : its symbol and its circuit connection.

4.2.2) Voltmeter:
       Voltmeter is a basic device used to measure electrical potential or voltage in volts. The symbol for the voltmeter is a circle with enclosed letter "V' as shown in Fig. (2).
        A voltmeter is connected across in parallel with the element or circuit portion for which the voltage is being measured.
        As with d.c ammeter a d.c voltmeter has polarity marks, an a.c voltmeter doesn't have polarity marks.

Fig.  (2) : Voltmeter : its symbol and its circuit connection

4.2.3) Wattmeter :

A) Single phase wattmeter :
       Wattmeter is a basic device used to measure real power flow in a circuit, it has tow input circuits, one for the voltage and the other for the current both are marked at one of their terminals to show polarity.
       The symbols of wattmeter is a circle with the enclosed tow coil, a current coil and a potential coil, a current coil connected in series with the circuit and the voltage coil connected in parallel with the circuit as shown in Fig. (3).
       The correct connection can be obtained by thinking about a.c circuit as d.c circuit and connecting the positive voltage lead to the marked voltage terminal, and connecting the current flows into the marked terminal.
Fig. (3) : Wattmeter : its symbol and its circuit connection
B) Three phase wattmeter :
       Three phase wattmeter consists of tow separate wattmeter mounted together in one one case, there are tow current coils and tow potential coils used for three wire connection. For four wire connections there are three current coils and three potential coils.
       A current coil together with its potential coil are known as element, therefore three phase wattmeter has tow elements.
       The connection of tow elements of three phase wattmeter are the same as that for two wattmeter method using tow single phase wattmeter as shown Fig. (4).
 Fig. (4)

4.2.4) Varmeter :
        The reactive power in a circuit is Q = V x I x (sin the phase angle), the reactive power serves as check on power factor measurements, since ratio of reactive power and active power is (tan the phase angle = Q/P).
       Also the apparent power V x I which determine the line and generator capacity may be determined from measurement of active and reactive power 
V x I = ( P2 + Q2 )1/2

A) A single phase varmeter :
       In a single phase circuit reactive power can be measured by varmeter ( volt ampere reactive power). This is an wattmeter in whose potential coil circuit a large inductive reactance  is substituted for the series resistance so that the potential coil current is quadrature with the voltage.
       Under these condition the wattmeter reads :
        This equal to the reactive power. The phasor diagram of the varmeter is as shown the following  Fig.
B) Reactive power measurement in three phase circuit :
        In case of balanced three phase circuit. it is simple to use single phase wattmeter to read reactive power.
       The current coil of the wattmeter is connected in one line and the potential coil is connected across the other tow lines as shown in Fig. (5).
      Referring to the fig :
- Current through current coil = I2
   - Voltage through voltage coil = V13
       Then, reading of wattmeter =
V13  x I2 x cos( 90-Φ ) = √3 VI cos ( 90-Φ ) = -√3 VI sinΦ
Total reactive power Q = 3VI sinΦ = - √3 x Reading of wattmeter.
Fig. (5) : Reactive power measurement in three phase
4.2.5) Megger :
        Most electric break downs and accidents are due to insulation failure. The megger insulation tester, however provides a means  of detecting insulation at an early stage so that defective apparatus may be attended to in time to prevent a costly breakdown or a series accident.
       The instrument consisting of a hand driven generator and a direct reading ohmmeter mounted together in case.
       There are tow voltage coil, the combined action of them may by considered as though the coil constitute a spring of variable stiffness, being very stiff near the zero end of the scale where the current is very large, and very weak near the infinity end of the scale.
       Thus, this effect compress the low resistance portion of the scale and opens up the high resistance, since this instrument is meant to be used as "insulation tester' and the insulation resistance are equal high.

4.2.6) Power factor meter :
        Power factor meter like wattmeter have a current circuit and potential circuit.
       The current circuit carries the current in the circuit whose power factor to be measured, the potential circuit is connected across the circuit whose power factor is to be measured and is usually split up into tow parallel paths.
       The inductive and the other is one inductive, then the deflection of the instrument depends on the phase difference between the tow current and the tow currents of the tow paths of the potential circuit, i.e. up on the power factor, the deflection will be indicated by a pointer.

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