Measurement of Resistance - 1 - GATE EE Electrical and Electronic Measurements

Applying KVL in loop 1:

Applying KVL in loop 2:

Applying KVL in loop 3:

Solving equations (i), (ii), and (iii), we get:

I₁ = 0.276 A

I₂ = 0.184 A

I₃ = 0.189 A

The current through the galvanometer is: 

I = 0.189 - 0.184

I = 0.005 A = 5 mA

• Creeping in energy meters is a phenomenon due to which the Almunium disc rotates due to only voltage applied to the pressure coil and no current flows in the current coil
• The necessary condition for creeping is that the Pressure coil (PC) must be energized
• Creeping increases the speed of the disc under low load conditions which increases the meter reading
• The primary reason for creeping is over-compensation for friction
• Generally, the friction compensation is so adjusted to give a diving torque required to compensate for the starting friction of the disc
• Another cause of creeping is the excess voltage, vibrations, and stray magnetic fields across the PC.
• This develops the excessive friction compensating torque at low load and the disc rotates

Hence we could observe that 

Statement P is incorrect, and statement Q is correct 

Kelvin's Double bridge:

• Kelvin Bridge is a modified Wheatstone bridge and provides high accuracy especially in the measurement of low resistance below 1 Ω.
• The error introduced by the contact/lead resistance is quite large and hence, will give an inaccurate reading if measured through a Wheatstone bridge.
• In order to avoid this problem, Kelvin's double bridge is used. It is so constructed that the contact resistance will not come into the picture while measuring the value of low resistance.

Advantages:

• It can measure the resistance value in the range of 0.1 µA to 1.0 A.
• Power consumption is less.
• Simple in construction.
• Sensitivity is high.

Construction:

 

• The Kelvin double bridge uses a second set of ratio arms, which is why it is called a "double bridge" and has low resistance arms with four terminal resistors. The first ratio of the arm is P and Q, and the second ratio of the arm is p and q.
• The Galvanometer is connected between P and p at point d. The point d is placed in the middle of the lead resistance r which is between unknown resistance R and standard resistance S. Thus, the effect of lead resistance r gets eliminated.

• Megger is a portable instrument to measure high insulation resistances
• It basically works on the principle of electromagnetic induction
• The electrical power to a megger is provided by permanent magnet D.C. generator
• The test voltages are usually of order 500, 1000, or 2500 V are generated by hand driven generator (permanent magnet D.C. generator)

Kelvin's double bridge is used for measuring low values of resistance.

Note:

Megger:

• Megger is a portable instrument that is used to measure the insulation resistance of the electrical machinery or system.
• It can be battery operated or mechanically operated (hand crank dc generator) and gives a direct reading in ohms.
• It basically works on the principle of electromagnetic induction.
• The electrical power to a megger is provided by a permanent magnet D.C. generator.
• The operating voltage of a megger is about 50 to 100 V.

Points to remember:

• Megger is also called an insulation tester because it is used to measure the insulation resistance of underground cables, motor windings, etc.
• The loss of charge method also used to measure the insulation resistance in MΩ.

• Megger is a portable instrument to measure high insulation resistances
• It basically works on the principle of electromagnetic induction
• The electrical power to a megger is provided by permanent magnet D.C. generator
• The test voltages are usually of order 500, 1000, or 2500 V are generated by a hand-driven generator (permanent magnet D.C. generator)
• The operation of a megger is based on moving coil meter

Murray loop test is the most common method for locating high resistance faults in low resistance conductor circuit. It employs the principle of Wheatstone bridge to determine the fault location.

Important Points:

• Hopkinson’s Test is a method of determining the efficiency of a DC machine. It is a full load test and it requires two identical machines which are coupled together. One of the two machines is operated as a generator and the other is operated as a motor to drive the generator. It is also called a back-to-back test or regenerative test.
• Open circuit test is done on the transformer to determine core losses in transformer and the parameters of the shunt branch of the equivalent circuit of the transformer. This test is done on LV side while keeping HV side of the transformer open-circuited. 

A Deflecting and Control coil is connected parallel to the generator, mounted at right angle to each other and maintain polarities in such a way to produced torque in opposite direction. Hence in a megger, controlling torque is provided by coil.

The Measurement of Resistance according to classification is shown below: