Measurement of High Resistance MCQ Quiz in मल्याळम - Objective Question with Answer for Measurement of High Resistance - സൗജന്യ PDF ഡൗൺലോഡ് ചെയ്യുക

The correct answer is option 1):(Rotating current reverser and rectifier )

Concept:

• The instrument or device used to find the resistance of the earth is known as Earth Tester or Earth Resistance Tester.
• The Earth Resistance Tester uses a hand-driven generator. The rectifier and the rotational current reverser are the two principal components of the Earth Tester. The rectifier converts AC (Alternating Current) into DC. The Earth Tester works only on DC (Direct Current)
• The construction of the earth tester is similar to the Megger with small modifications adding additional features. It consists of a dc generator, a rotating current reverser, a rectifier, and a PMMC instrument to measure resistance. The schematic diagram of the earth tester for measuring the resistance of earth soil is shown below

• The current reverser is used to convert dc to ac so that the earth resistance test can be done with alternating current. The ac supply from the current reverser is given to the earth electrode that is under test. Here the use of ac supply for measuring the earth resistance is, the undesirable effects due to back emf produced in the soil are eliminated.
• The rectifier is used because the tester is equipped with a moving coil instrument to measure resistance. Thus the pressure and the current coil of the instrument need DC supply. The rectifier converts alternating current coming from the electrode to direct current and supplies it to the current coil.
• The pressure or potential coil is connected across the generator and it measures the voltage applied. The current coil is connected in series with the dc  generator and it measures current through the electrode. Both the current reverser and rectifier are connected with the help of two commutators mounted on the shaft of the generator. Two pairs of fixed brushes are provided with each commutator to transfer power from the rotating shaft to the stationary parts.

Spectrum analyser:

• It measures the magnitude of an input signal versus frequency within the full frequency range of the instrument.
• The primary use is to measure the power of the spectrum of known and unknown signals.
• Spectrum analyzer is used to measure harmonics.

Digital counter measures the frequency by counting no of clock cycles.

Schering Bridge:

Schering bridge is used to measure the dissipation factor (dielectric loss) and capacitance.

The circuit of the Schering bridge as shown below:

When the bridge is in the balanced condition, zero current passes through the detector, which shows that the potential across the detector is zero.

At balance condition:

\(\frac{{{Z_1}}}{{{Z_2}}} = \frac{{{Z_3}}}{{{Z_4}}}\)

Z1 Z4 = Z2 Z3

Megger:

• An instrument that is used to measure insulation resistance is a Megger. It is also known as mega-ohm-meter.
• It is used in several areas like multi-meters, transformers, electrical wiring, Etc.
•  Megger device is for testing various electrical devices that can measure greater than 1000 meg-ohms.
• It consists of both a generator and an ohmmeter.
• The principle of Megger is based on moving coil in the instrument. When current is flowing in a conductor, which is placed in a magnetic field, it experiences a torque.

The correct answer will be option (4)  A - 3, B - 4, C - 1, D - 2

Explanation:

• The insulation resistance is measured by the megger circuit.
• The value of the insulation resistance is very high.

Classification of resistance:

The resistance is classified as:

Low Resistance - Resistance of the order of 1Ω and below are classified as low resistance.

Medium Resistance - Resistance ranging from 1Ω to 100kΩ is classified as medium resistance.

High Resistance - Resistance of the order of 100kΩ and above is classified as high resistance.

Based on their ohmic values, the resistance is measured by the following:

• 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 permanent magnet D.C. generator
• The operating voltage of a megger is about 50 to 100 V

Notes:

• A multimeter is used to measure current, voltage, and resistance.
• ​A voltmeter is an instrument that is used to measure the potential difference. The voltmeter is always connected in parallel across the points between which the potential difference is to be measured.
• An ammeter is an instrument that is used to measure electric current in a circuit. It is always connected in series in a circuit through which the current is to be measured. 
• An ohmmeter is an instrument for measuring electrical resistance, which is expressed in ohms.
• A Wattmeter is an instrument for measuring the electric power (or the supply rate of electrical energy) in watts of any given circuit.

Megger:

• Megger is a portable instrument to measure high insulation resistances.
• It works on the principle of electromagnetic induction.
• The electrical power to a megger is provided by a 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 a moving coil meter.
• Earth pit resistance is measured with an earth tester, also called earth megger which can test the resistance across a range of currents and distances.
• Insulation megger is used to measure the insulation resistance of the cables.

Important Points

Examples of High Resistance Materials:

• Cable Insulation Resistance.
• All Motors, generators, and transformers winding's insulation resistance.
• All semiconductor devices when reverse biased.

Methods to measure High Resistance:

• ​Loss of charge method.
• Direct-Deflection method.
• Mega ohm Bridge.
• Megger.

Megger:

• Megger is a portable instrument to measure high insulation resistances.
• It works on the principle of electromagnetic induction.
• The electrical power to a megger is provided by a 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 a moving coil meter.
• Earth pit resistance is measured with an earth tester, also called earth megger which can test the resistance across a range of currents and distances.
• Insulation megger is used to measure the insulation resistance of the cables.

Important Points

Examples of High Resistance Materials:

• Cable Insulation Resistance.
• All Motors, generators, and transformers winding's insulation resistance.
• All semiconductor devices when reverse biased.

Methods to measure High Resistance:

• ​Loss of charge method.
• Direct-Deflection method.
• Mega ohm Bridge.
• Megger.

Concept:

Controlling Torque in megger is provided by the coil.

Torques in measuring instruments

1.) Deflecting torque

• The deflecting torque causes the moving system to move from its zero position.
• The deflecting torque is produced by utilizing one or more of the following effects of current or voltage.

2.) Controlling Torque

• The controlling torque (Tc) opposes the deflecting torque and increases with the deflection of the moving system.
• The pointer comes to rest at a position where the two opposing torques are equal (Tc = Td).
• The controlling torque brings the pointer back to zero when the deflecting torque is removed.
• The controlling torque in indicating instruments is provided either by spring control or gravity control.

3.) Damping Torque

• A damping torque is produced by a damping or stopping force which acts on the moving system only when it is moving and always opposes its motion.
• Such torque is necessary to bring the pointer to rest quickly.
• If there is no damping torque, then the pointer will keep moving to and fro about its final deflected position for some time before coming to rest, due to the inertia of the moving system.
• The controlling torque in indicating instruments is provided either by eddy current damping or air friction damping.

Megger:

• Megger is a portable instrument to measure high insulation resistances.
• It works on the principle of electromagnetic induction.
• The electrical power to a megger is provided by a 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 a moving coil meter.
• Earth pit resistance is measured with an earth tester, also called earth megger which can test the resistance across a range of currents and distances.
• Insulation megger is used to measure the insulation resistance of the cables.

Important Points

Examples of High Resistance Materials:

• Cable Insulation Resistance.
• All Motors, generators, and transformers winding's insulation resistance.
• All semiconductor devices when reverse biased.

Methods to measure High Resistance:

• ​Loss of charge method.
• Direct-Deflection method.
• Mega ohm Bridge.
• Megger.

• Megger is a portable instrument which 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 permanent magnet D.C. generator
• The operating voltage of a megger is about 50 to 100 V

Notes:

• A multimeter is used to measure current, voltage and resistance.
• ​A voltmeter is an instrument which is used to measure the potential difference. The voltmeter is always connected in parallel across the points between which the potential difference is to be measured.
• An ammeter is an instrument which is used to measure electric current in a circuit. It is always connected in series in a circuit through which the current is to be measured. 
• An ohmmeter is an instrument for measuring electrical resistance, which is expressed in ohms.
• A Wattmeter is an instrument for measuring the electric power (or the supply rate of electrical energy) in watts of any given circuit.

Megger:

• It is a measuring instrument used for the measurement of the insulation resistance of an electrical system.
• As the open-circuit has very high resistance, so the megger will so infinite resistance.
• Meggar does not need any controlling torque.
• An electrical system degrades its quality of insulation resistance with time and various environmental conditions including temperature, moisture, dust particles & humidity
• Even mechanical and electrical stress affects the insulation resistance which adds to the necessity of checking insulation resistance at regular intervals so as to avoid fatal errors or electrical shocks
• Megger is used for measuring the electrical leakage in wires, electrical insulation levels in generators, motors, etc