EKT112 Principles of Measurement and Instrumentation Power Measurement 1
Outline Power? Power in DC and AC Circuits Power Measurements Power Instrumentation (Wattmeter) 2
Concept of Electric POWER Power can be defined as the time rate of energy transfer or energy dissipation in a load. Power is the rate of using or supplying energy. The rate at which work is done to maintain an electric current in a circuit is termed ELECTRIC POWER. Electric power is measured in watts (W). The SI unit of power is the watt (W), where W = 1 J/s. The kilowatt is a commonly used unit where I kilowatt = 1000 watts. 3
Electric POWER Formula Normally electric power is useful, making a lamp light or a motor turn. ELECTRIC POWER equals the product of the current I and the potential difference V. Power = Current x VoltageDifference P( Watts) = I( Amperes) x V ( volts) 4
If P is positive, the component absorb power. If P is negative, the component produces power. 5
POWER in DC Circuits The POWER in DC circuit is equal to the product of voltage and current. [Power = Current Voltage] When the system voltage is constant, ammeter readings are almost a sufficient indication of the POWER taken. The POWER is calculated by using voltmeter and ammeter or wattmeter. P = I V = I² R = V² / R where: P = power in watts (W) I = current in amps (A) R = resistance in ohms ( Ω) V = voltage in volts (V) 6
Example How much power is used in a circuit which is 110 volts and has a current of 1.36 amps? P = IV = 1.36 A x 110 V = 150 W 7
Examples Label on TV state 720W/120V. Find the current supply to this TV. A heater has 30Ω resistor connected to voltage source 120V. Find the total power which changed to heat. 8
POWER in AC Circuits In AC circuits the voltage and current are changing their magnitude and polarities with reference to time. The electric power in the circuit at any instant is equal to the product of the current and the voltage across its terminals at that instant.[p = vi] The instantaneous power, 9 p = vi = V = V max max sinθ x I max I max sin( θ φ) sinθ sin( θ φ)
The mean power 2π 1 p = V 2π 0 max I max VmaxImax cosφ = 2 P = VI cosφ sinθ sin ( θ φ) dφ V and I are rms value of voltage and current and to calculate power, we should know the power factor of the load in AC circuit. Power factors are usually stated as "leading" or "lagging" to show the sign of the phase angle of current with respect to voltage. 10
Example The real power is 700 W and the phase angle (φ) between voltage and current is 45.6. The power factor = cos(45.6 ) = 0.700. The apparent power = 700 W / cos(45.6 ) = 1000 VA 11
The power output rating of motors is usually expressed in a power unit call the horsepower (hp) The relation between horsepower and watts is 1hp = 746W Electric motors and other systems have an efficiency (η) of operation η = P out P in x100% 12
Examples 1. Calculate the power for a 12V battery supply 250A to a starter motor. 2. Find the current drawn from a 115V line by a DC electric motor that delivers 1hp by assuming 100% efficiency of operation. 3. What is the operating efficiency of a fully loaded 2hp DC electric motor that drawn 19A at 100V? 13
Quiz The d Arsonval meter movement known as Permanent Magnet Moving Coil (PMMC) used to be as voltmeter and ammeter. For both instruments, what we uses to increase the voltage range and the current range? Answer in 5 minutes and submit. 14
Why Wattmeter Needed? Ammeter measures load current IL and there is voltage drop VA across ammeter. VL=V VA Pdc= VL IL = (V VA)IL = VIL VAIL [Power measured by meters] = [Power consumed by load] + [Power loss in ammeter] The product of ammeter and voltmeter does not give correct power consumed by load 15
If voltmeter shifted across the load to measure the load voltage, it measures VL correctly but ammeter measures current I. I = IL + IV Pdc= VL IL = VL (I - IV)= VL I VL IV [Power measured by meters] = [Power consumed by load] + [Power loss in voltmeter] Power measured is higher than power actually consumed by load. 16
* The power loss in the instrument (ammeter or voltmeter) near the load cause an error. To avoid that errors in power measurement, we need a device called wattmeter which gives direct reading of power. Ammeter and Voltmeter is not sufficient to measure power in AC power because the measurement of power consumption in circuit considering the effect of cos φ. 17
POWER Measurement A wide variety of instrumentation and transducers for the measurement of POWER in AC and DC circuit. Important primarily for the testing, monitoring and maintenance of the energy supply network and electrical equipment. Required in high frequency and low power circuits. A wattmeter suitable used for power meter measurement in DC and AC systems, which will give the same angle of deflection for a given power. 18
Power Factor Meter Single Phase and Three Phase This instruments indicate the power factor of a circuit directly instead of obtaining the watts applied to a circuit dividing by the volt amperes in the circuit Class of power factor meter 1. Dynamometer type power factor meter 2. Moving iron type power factor meter 19
Wattmeters Three types of wattmeters 1. Dynamometer 2. Induction - can be used on AC circuit only when frequency and supply voltage are constant. 3. Electrostatic Class of Dynamometer Suspended coil torsion head Pivoted coil direct indicating 20
21 Dynamometer Wattmeter
Dynamometer Construction The moving coil is placed between the two fixed coils. The fixed coils is used as a current coils which are made of a few turns of thick copper wire and are connected in series with the load to carry the load current. The moving coil is used as pressure coil which is made of a very light wire several turns of fine copper wire. A high non inductive resistance is connected in series with the moving coil to limit the current. The jewel supported spindle carrying the pressure coil, a pointer damping vane and control springs. The flat aluminium thin pointer is fixed to the pressure coil spindle and moves over a suitably calibrated scale. A cast iron cylinder is placed around the coil to protect against stray magnetic fields. Air friction damping is used. 22
Dynamometer Operation The fixed coils carry load current and create a flux in the air gap between them. The pressure coil carries another small current proportional to load voltage and hence produces a flux in the air gap. The circuit coil produces a flux in phase with the load current. The flux produced by the pressure coil is very nearly inphase with the applied voltage. The magnetic fields of the fixed and moving coils react on one another causing the moving coil to turn its axis and a torque produced on the moving system. 23
The torque developed on moving system will be proportional to product of the two fields and in turn the voltage and current. Deflecting Torque, Control Torque, T c Under steady state condition, T d VI cosθ = W (power) θ T d = T c θ W power The angular displacement of the pointer is directly proportional to average power in circuits. 24
Dynamometer In dynamometer type instrument Deflecting torque is produced by magnetic effect of electric current. Control torque is provided by control springs. Damping torque is provided by Air Friction damping. 25
Advantages Can be used to measure power in DC and AC circuits. Gives fairly accurate readings. The scale is uniform. Free from hysteresis and eddy current losses 26
Disadvantages The torque-weight ratio is small (even when the current and pressure coils are fully exicited) High cost Affected by stray magnetic field The inductance of pressure coil tend to be large at low power factors introduces serious error. 27
Wattmeter's are designed for lower power factor rating. This improves precision & reduces error. As two different ranges of voltage and current are available for wattmeter to calculate actual power, multiplying factor should be used. Multiplyin g Factor = voltage range x current range x rated Full Scale Deplection power factor Measured Power = Wattmeter Reading x Multiplyin g Factor 28
Examples Let FSD of wattmeter be 1250W. If potential coil is connected across 250V and the current coil connected for a current range 5A, What is the multiplying factor? In a circuit power is measured with a wattmeter with 13A, 240V, 1500Watts F.S.D. The measurement reading was 700Watts. What is the power consumed by load? 29
Error in wattmeter Connection error can be reduced by used for low current, high voltage circuit. Wattmeter Reading = (power consumed by load) + (Power consumed by current coil) used for high current, low voltage circuit, by selecting a proper connection the Wattmeter Reading = (power consumed by load) + (power consumed by pressure coil) 30
Example A circuit takes 10A at 200V and the power absorbed is 1000W. If a current coil has a resistance of 0.15Ω resistance and a pressure coil has a resistance of 5000Ω, Inductance of 0.3H. Find The error due to resistance for each of two possible method of connection. The error due to the inductance if the frequency of 50Hz. The total error in each case. 31
32 Method of Connections
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Compensated Wattmeter While measuring power in low power factor circuits, error due to connection is not negligible. So compensating winding is used. Compensating winding is a winding connected in series with pressure coil. It has same number of turns as that of current coil, but has small cross section. 34
35 Low Power Factor Wattmeter
Modifications on LPF Wattmeter Pressure coil circuit designed to have low resistance to make more current can flow through for develop more torque. (10xUFP). The large current causes higher voltage drop and power loss in current coil which is introduce an errors. The provision of a compensating is a must to reduce flux created by current coil. Error caused by pressure coil inductance is proportional to sin θ (power factor angle) which is large at low power factor. Compensation is carried out by putting a capacitor across a part of the series resistance To have small control torque relative to smaller deflection torque which able to give higher angular displacement and suitable for circuits with power factors as low as 0.1. 36
A dynamometer type three-phase wattmeter consists of two separate wattmeter movements mounted together in one case with the two moving coils mounted on the same spindle. 37
There are two current coils and two pressure coils. A current coil together with its pressure coil is known as an element. Therefore, a three phase wattmeter has two elements. The connections of two elements of a 3 phase wattmeter are the same as that for two wattmeter method using two single phase wattmeter. The torque on each element is proportional to the power being measured by it. 38
The total torque deflecting the moving system is the sum of the deflecting torque of the two elements. Hence the total deflecting torque on the moving system is proportional to the total Power. In order that a 3 phase wattmeter read correctly, there should not be any mutual interference between the two elements. A laminated iron shield may be placed between the two elements to eliminate the mutual effects. 39
End of POWER Measurements 40
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