Measurement of Power | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE) PDF Download

Measurement Of Power

  • Power Measurement in A.C. Circuit by Wattmeter Method : 
    • 3 general types of wattmeters are :
      • Dynamometer type
      • Induction type
      • Electrostatic type
  • Average power over a cycle in an AC circuits in given by,

Measurement of Power | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

where, Measurement of Power | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)  = RMS value of voltage

Measurement of Power | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)   = RMS value of current

and cosø = Power factor of the load

Elctrodynamometer Type Wattmeter : 

  • It consists of two coils, the “Fixed coil” or “Field coils” or “Current coils” (C.C) are connected in series with the load.

And the other coils is “Moving coils” or “Pressure coil” or “Voltage coil” or “Pontential coil (PC)“.

  • The CC is made up of thick conductor because it has to carry the higher load current.
  • The PC is made up thin conductor because it is designed to carry low load current.
  • Spring control is used in this instrumment.
  • Air friction damping is used.

Let rp and Lp are the resistance and inductance of the pressure coil or moving coil respectively and Rp = (rp + R) = total resistance of PC circuit

l = current in CC circuit = lCC
Since,

Measurement of Power | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

Measurement of Power | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)  = impedance of PC circuit.

Let load p.f. cosø lagging i.e. lags behind V by an angle ø .

 Measurement of Power | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

  • Deflecting torque  Measurement of Power | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)  

Measurement of Power | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

and controlling torque = Tc = K θ

At null deflection, Tc = Td
Measurement of Power | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

since, θ ∝ Vl cos ø i.e.

θ ∝ Power (P)

  • It indicates the uniform scale.

Measurement of 3-Phase Power with two wattmeter method : 

  • In a three phase, three wire system , we require 3-elements, but if we make the common points of the pressure coils coincide with one of the lines, then we will require only, n – 1 = 3 – 1 = 2, elements.
  • Instantaneous power consumed by load is given by, p

Measurement of Power | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

  • Let us consider two wattmeters connected to measure power in three phases circuit as shown in figure (star connection). Instantaneous reading of W1 wattmeter

P1 = i1(v1 – v3)

Instantaneous reading of W2 wattmeter

P2 = i2(v2 – v3)

Sum of instantaneous reading of two wattmeter is, P = P1 + P2

P = i1(v1 – v3) + i2(v2 – v3)

From KCL at node ‘O’, i1 + i2 + i3 = O, or (i1 + i2) = – i3

∴ sum of instantaneous readings of two wattmeters P = v1i1 + v2i2 + v3i3

  • Therefore, the sum of the two wattmeter reading is equal to the power consumed by the load.
  • This method is irrespective of whether the load is balanced or unbalanced.
  • Observation : 
    • The phasor diagram for a balanced star connected load as shown in figure

  Measurement of Power | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

Let V1, V2, V3, be the rms values of phase voltage and I1, I2, I3 be the rms values of phase currents.

Since the load is balanced, therefore,

Phase voltages, V1 = V2 = V3 = V(say)

Line voltages, Measurement of Power | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE) 

 Phase currents, I1 = I2 = I3 = I(say)

and, Power factor = cos ø

  •  Actually phase currents and line currents both are same in star-connection. then reading of wattmeter

Measurement of Power | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)
and reading wattmeter W2 is
Measurement of Power | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)
Sum of reading of two wattmeter is, P = P1 + P2

  Measurement of Power | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

P = 3 VI cosø = 3 - ø power
This shows the total power consumed by the load.

Since,

Measurement of Power | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

Measurement of Power | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

Measurement of Power | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

Measurement of Power | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

Measurement of Power | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

Measurement of Power | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

Measurement of Power | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)  

From this equation (4.38) we have also find pf = cosø.

The document Measurement of Power | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE) is a part of the Electrical Engineering (EE) Course Electrical Engineering SSC JE (Technical).
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FAQs on Measurement of Power - Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

1. What is power measurement in electrical engineering?
Ans. Power measurement in electrical engineering refers to the process of determining the amount of electrical power consumed or generated by a device, circuit, or system. It involves measuring the voltage and current in the circuit and calculating the power using mathematical formulas such as P = VI or P = I^2R, where P represents power, V represents voltage, I represents current, and R represents resistance.
2. Why is power measurement important in electrical engineering?
Ans. Power measurement is crucial in electrical engineering for several reasons. Firstly, it helps in assessing the performance and efficiency of electrical devices and systems. By measuring power, engineers can identify any inefficiencies, losses, or faults in the system and take appropriate measures for improvement. Secondly, power measurement is essential for billing purposes, as it helps in accurately calculating the energy consumption of consumers. Additionally, power measurement is vital for maintaining the stability and reliability of power grids, as it allows engineers to monitor and balance the supply and demand of electricity.
3. What are the different methods of power measurement in electrical engineering?
Ans. There are various methods of power measurement in electrical engineering. Some commonly used methods include: - Direct measurement using power meters: This involves using specialized instruments called power meters to directly measure the power consumption or generation of a device or system. - Wattmeter method: A wattmeter is used to measure the power in AC circuits by measuring both the voltage and current simultaneously. - Power analyzer method: Power analyzers are advanced instruments that can measure various parameters such as voltage, current, power factor, and harmonic distortions to provide comprehensive power measurement. - Energy meter method: Energy meters are used for measuring the energy consumption of consumers over a period of time, typically in kilowatt-hours (kWh). - Current clamp method: This method involves using a current clamp meter to measure the current flowing through a conductor, which can then be used to calculate power.
4. Can power measurement be done in DC circuits?
Ans. Yes, power measurement can be done in DC circuits as well. In DC circuits, the power can be calculated using the formula P = VI, where V is the voltage and I is the current. However, it is important to note that in DC circuits, the power is constant and does not fluctuate like in AC circuits. Therefore, power measurement in DC circuits is relatively simpler compared to AC circuits.
5. How accurate are power measurements in electrical engineering?
Ans. The accuracy of power measurements in electrical engineering depends on various factors such as the type of measurement instrument used, calibration of the instrument, and the precision of the measurement technique employed. High-quality power meters and analyzers can provide accurate measurements with a typical accuracy of around 0.2% to 0.5%. However, it is important to ensure proper calibration and regular maintenance of the measurement instruments to maintain accuracy. Additionally, factors such as electrical noise, harmonics, and power factor can also affect the accuracy of power measurements and need to be considered during the measurement process.
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