Analog Electronic Wattmetr 424r44

Development of an Analog Electronic Wattmeter Mahfoozur Rehman and M. R. Saad

Shamsuddin Ahmad and M.S. Ansari

School of Electrical and Electronics Engg. University Science Malaysia Penang, Malaysia [email protected]

Department of Electrical Engineering ZHCET, AMU Aligarh, India [email protected]

Abstract—The paper deals with the development of an analog Electronic wattmeter which is simple in design and can measure power, from very low values to very high values, with simple modifications, at different frequencies and with good order of accuracy. It can also be used in the measurement of power of distorted signals and can be interfaced with out any loading error. Basically, it measures I cosΦ with good order of accuracy and effect of variation of voltage is taken care off by introducing a voltage sensitive resistance. By introducing electronic circuits at different stages, phase angle errors due to imperfections can be compensated. Output voltage, at the final stage, will be proportional to average power of the system.

I. INTRODUCTION Wattmeter is one of the most important instruments of any laboratory, industry, power distribution system etc. Mostly, Dynamometer type watt meters are used in the measurement of power. However, this meter has a number of limitations, when used for accurate measurement of power, due to current based deflecting system. The finite impedances of pressure and current coils limit the accuracy of measurement [1]. However some electronic circuits have been suggested to improve their performance at low power factors [2]. Wattmeters based upon analog electronic devices, like Hall Effect sensors and Electronic Multipliers have specific applications and are not used as general purpose instrument. Recently, digital watt meters have become popular but have number of limitations due to cost and method of getting final results [3]. In this paper we have presented the details of an analog wattmeter which can be used in a wide rage of power measurement and can be prepared in any laboratory at a low cost. It is composed of a current to voltage converter, analog switches and voltage comparator. Output of the current to voltage converter is applied to the input of the analog switch and its controlling signal is obtained from the main supply, with the help of a voltage divider. The average value of out put voltage of the analog switch will be proportional to I.cosΦ. The averaging can be done with the help of electronic circuit as well as with the help of permanent magnet moving coil instrument. Hence deflection of the PMMC instrument can be calibrated in of power. II.

THEORETICAL BACKGROUND

The complete circuit diagram is shown in Fig.1. It is designed to measure power of a load of 1Amp at a voltage of 220 volt. Current to voltage conversion is obtained with the

Figure 1. Schematic Diagram of the proposed electronic wattmeter

help of a current transformer and opamp based current to voltage converter. Current transformer will accurately bring down the current level and a simple opamp based current to voltage converter will convert current to voltage. It will provide isolation between supply and measuring circuit and opamp will manage the required value of the output voltage. Current to voltage converter can also manage the phase error of the current transformer by simply varying the ive components used in the feed back path of the current to voltage converter. When load is connected across supply, a current will flow in the circuit which may be given by following expression, IL =

V ZL

(1)

This current will through the primary winding of the current transformer and a current Is will flow in the secondary winding as well as through the current to voltage converter. The output voltage of the current to voltage converter may be given by following expression, Vo =

IL RC NC

(2)

This voltage is connected to the input terminal-1 of analog switch. Control signal of this control switch is obtained from the supply voltage by using a potential divider and a comparator. The output signal, as shown in the Fig.2, will be

III.

Figure 2. Different waveforms: (a) waveform of the applied voltage; (b) waveform of the load current; (c) waveform of the output of the comparator; (d) waveform of the output of analog switch

proportional to the cosine of the angle between supply voltage and load current and its average value may be given by the following expression, R V o' = k .I L . C .cos φ (3) NC or (4) Vo' = k1 I L cos φ If permanent magnet moving coil instrument is used for averaging and display purposes, a current I m, will flow through the meter. Magnitude of this current can be given by following expression (excluding the resistance of the dotted area), V' V0' k1 I L cos φ (5) = = k 2 I L cos φ Im = o = RT

Rm + Rop + Ras

Rm + Rop + Ras

Where, Rm, Rop, Ras are the resistances of permanent magnet moving coil instrument, output resistance of operational amplifier, on resistance of analog switch respectively. From (5) it is clear that current of the instrument is directly proportional to IL .cos Φ. However, voltage variations will not be taken care off. To make the system sensitive to voltage variation (upto10%) a voltage sensitive resistance made of FET is used. It is given in Fig.1 in the dotted area. The resistance offered by FET may be represented as follows, k (6) R fet = 3 Vs Where, R fet is the resistance offered by the FET when a portion of Vs is applied across its gate. In this case, current through meter, will be modified as follows, I m' =

Vo' Rm + Rop + Ras + R fet

(7)

V o' V o' V ' .V = = o S 2R fet 2 ( k 3 V S ) 2k 3

Experiments were conducted to show the successful working of the proto-type model. However it was tested in the Laboratory in the power range of 1kW and lagging power factor. Applied voltage was varied from 180 to 230V at 50Hz. Circuit was arranged as shown in Fig.1. Resistance of analog switch under ‘ON’ condition and resistance of FET by varying the gate voltage of the FET were measured to analyze the effect of approximations made in (9). Actual power was measured with the help of sub-standard wattmeter. Power factor was also varied to assess the working at different power factors.. Results obtained are given in Table-1. Only one table is given to the working of the wattmeter. Relationship between power and deflection is appreciably linear. For every reading, proportionality constant K is determined which comes out to be nearly constant. TABLE I. S.No

THE EXPERIMENTAL RESULTS WITH VOLTAGE VARYING FROM 180-230 VOLTS

1. 2.

Voltage V(Volts) 180 190

Current I(Amp) 0.68 0.71

Power (Watts) 102 123

Deflection PMMC 2.5 3.0

K 40.800 40.666

3. 4. 5.

200 210 220

0.72 0.74 0.76

143 163 184

3.5 4.0 4.5

40.857 40.750 40.888

6.

230

0.84

224

5.5

40.727

IV. CONCLUSION A simple, low cost design of an analog type Electronic Wattmeter is presented. Experimental results conform the theory and design has enough potential to be developed as an accurate wattmeter. PMMC instrument can be replaced by electronic averaging circuit, analog to digital converter and digital display. FET based voltage dependent resistance can easily be replaced by other more accurate designs. However present design can also be extended in the measurement of Iron and dielectric losses with proper phase adjustments if needed. Unlike multiplier type wattmeter, spikes will not increase the power indication because averaging of complete cycle is made. With further development work, circuit can be developed as a general purpose wattmeter. REFERENCES

If, R fet =Rm +Rop +Ras , then Eq.7 may be written as, I m' =

EXPERIMENTAL METHOD AND RESULTS

(8)

Substituting the value of Vo in Eq.8, we get, I m' = ( k 1 .V S .I L .cos φ ) 2k 3 = K .V S .I L .cos φ (9) It is clear from (9) that Im is directly proportional to power in the load. Hence reading of the Permanent magnet moving coil instrument can be calibrated in of power.

[1]. E.W.Golding and F.C.Widdis, Electrical Measurements and Measuring Instruments, 5th Edition,1963 [2]. M.Rehman, ,M.T.Ahmad, and M.Q. Rafiq , “ Realization of electronically compensated precision type dynameter wattmeter,” Proc. IEEE Conference on Consumer and Industrial Electronics and Applications, Singapore, pp 1-3, April, 1984 [3]. K.A.Hammad,M.Y. Jabouri and Al-Khateeb, “ Digital active and reactive power measurement using sample/hold circuit” International Journal of Electronics, vol. 65, no.2, pp263-267.