3-Voltmeter Method of Measuring Choke Coil Parameters [PDF]

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3-Voltmeter Method of Measuring Choke Coil Parameters 3-Voltmeter Method of Measuring Choke Coil Parameters:The choke coil parameters we are going to measure in this 3-voltmeter method are – the inductance, resistance as all choke coils have inherent resistance in addition to their inductance. We also measure the quality factor and power absorbed by the given choke coil. A given choke coil is usually represented by a pure inductance (L) in series with equivalent resistance (r). This equivalent resistance takes into effect the iron losses in the core of the choke coil and the inherent resistance of the choke coil. 3-Voltmeter method and 3-Ammeter method are two of the best ways to measure these two parameters. Thus the equivalent resistance accounts for the copper loses in the choke coil and the iron loses in the iron core. The following figures represent the circuit diagram of 3-voltmeter, equivalent circuit of choke coil.



Supply voltage ‘Vs‘ can be varied by means of the single phase variac. VR and VL are the voltmeter readings across the resistance and choke coil. The phasor diagram for the measurement of choke coil parameters by 3-voltmeter method is as shown below :



From the phasor diagram it is clear that,



Procedure for 3-voltmeter method of finding choke coil parameters :1. Give the connections as in the circuit diagram. 2. Apply 230V, 1-φ A.C voltage by means of 1-φ A.C variac to the series combination of choke coil and external resistance. 3. Note down the readings of the 3 voltmeters. 4. Apply different voltages in suitable steps and note down the readings of voltmeter. 5. Calculate coil resistance, inductance and Q-factor using the above mentioned formulae.



Three Voltmeter Method Three Voltmeter Method For Measurement Of Power: As we know, wattmeters are used for measurement of power in inductance AC circuits, but in some cases it is not possible to use wattmeters because of their incorrect readings or sometime wattmaters may not available. So in such cases three voltmeters or three ammeter method is used for measurement of power. Three Voltmeter Method: Following figure shows the circuit diagram for three voltmeter method.



Three voltmeter method



Three voltmeter method vector diagram V1, V2 and V3 are the three voltmeters and R is a non-inductive resistance connected in series with the load as shown in figure. From the phasor diagram, we have:



The assumptions are made that the current in the resistor R is same as the load current. Disadvantages:  Supply voltage higher than normal voltage is required because an additional resistance R is connected in series with the load Z (inductive circuit).  Even small errors in measurement of voltages may cause serious errors in the value of power determined by this method. Let us solve one simple numerical example based on three voltmeter method for clear understanding. Example 1: The following readings were obtained from three voltmeters used for a single phase power measurement: V2 = 180 volts across a non-inductive resistor; V3 = 200 volts across an inductive load; V1 = 300 volts across the two in series. Calculate the power factor of the inductive load. Solution: Given: V2 = 180 V; V3 = 200 V; V1 = 300 V Power factor, cos ф = (V1^2 – V2^2 – V3^3)/2V2V3 Or cos ф = [(300^2) – (180^2) – (200^2)]/(2*180*200) = 0.244 (Ans.)