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Electrical Instrumentation and Measurements

1.      (a)  i. Briefly explain the three (3) main functions of instruments and give examples each.

      ii. Explain loading effects in a measurement system.

(b) A Maxwell bridge in Fig 1 below is used to measure an inductive impedance. The bridge constants at balance are:

R1 = 235 kΩ, C1 = 0.012 µF, R2 = 2.5 kΩ, R3 = 50 kΩ

Find the series equivalent of the unknown impedance.

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2.      (a) The power input to a 3-phase induction motor is read by two wattmeters. The readings are 920 W and 300 W. Calculate the power factor of the motor.

(b) A moving-coil milliammeter having a resistance of 10 ohms gives full scale deflection when a current of 5mA is passed through it. Explain how this instrument can be used for measurement of

                                i.            Current upto 1 A

                              ii.            Voltage upto 5 V

 

 

(b)  A current transformer with a bar primary has 250 turns in its secondary winding. The resistance and reactance of the secondary circuit are 1.4 Ω and 1.1 Ω respectively including the transformer winding. When 5 A currents flows in the secondary winding, the magnetizing mmf is 80 AT and iron loss is 1.1 W. Determine the following:

                                i.            Ratio Error

                              ii.            Phase angle error                     .

 

3.      (a) With the aid of a diagram, explain the construction and the working principles of the permanent magnet moving coil meter movement.

(b) Considering the characteristics of the Current Transformer, what is the effect of change of IP, IS and the frequency?


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Answer #1
1 (b) answer:

A maxwell bridge in big 1 below && used to measure an inductive impedance. The bridge constand at balance are RA = 935 k-2, C

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Answer #2
2. answer:

& power given that Wi= 920W factor angle W 3.00w We know that tane - Blewet bare tand B ( 920-300 tand 1900 3 x 620 1900 10.8Q = tani l 0-8918) 41189 NOW power factoren 0,745 3E Cas 41.89 Pf 1b) for enhancement of Range of ammeter we connect a shuntRm Rsh = m-) (200-1) a voltmeter WP connect a seriest 10 10 199 Rsh - 0.0503 R Vj) For enhancement of range of Resistance toRse (mt) Rom Rse (100-)) RM Rse gg x 10 ggor

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