Measurement of Voltage and Current

Measurement of voltage and current ( Moving coil and moving iron instrument)

Generally, voltage and current are measured using voltmeter and ammeter. There are two classes of these meters. They are:

1.Analog meter (Voltmeters and ammeters)

2.Digital meter (Voltmeters and ammeters)

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Ammeters and voltmeters, except electrostatic voltmeters, operate on the same principle. In case of analog or pointer indicating ammeter and voltmeter, there are two classes of instruments depending upon the operations:

1.Moving coil Instrument(voltmeter/ammeter)
2.Moving iron Instrument(voltmeter/ammeter)

Moving coil Instrument(voltmeter/ammeter):

Permanent Magnet Moving Coil Instrument:

The permanent magnet moving coil Instrument or PMMC uses two permanent magnets in order to create the stationary magnetic field. These types of instruments are only used for measuring the dc quantities as if we apply ac current to these type of instruments the direction of current will be reversed during the negative half cycle and hence the direction of torque will also be reversed which gives an average value of torque zero. The pointer will not deflect due to high frequency from its mean position showing zero reading. However, it can measure the direct current very accurately.

let us move towards the constructions of permanent magnet moving coil Instrument. We will see the construction of these types of instruments in five parts and they are described below:

Stationary part or magnet system: In the present time we use magnets of high field intensities, high coercive force instead of using a U-shaped permanent magnet having soft iron pole pieces. The magnets which we are using nowadays are made up of materials like alcomax and alnico which provide high field strength.

  • Moving coil: The moving coil can freely move between the two permanent magnets as shown in the figure given below. The coil is wound with many turns of copper wire and is placed on rectangular aluminum which is pivoted on jeweled bearings.
  • Control system: The spring generally acts as a control system for PMMC instruments. The spring also serves another important function by providing the path to lead currentin and out of the coil.
  • Damping system: The damping force hence torque is provided by movement of aluminum formed in the magnetic field created by the permanent magnets.
  • Meter: Meter of these instruments consists of a lightweight pointer to have free movement and scale which is linear or uniform and varies with angle.

Errors in Permanent Magnet Moving Coil Instruments

There are three main types of errors:

  1. Errors due to permanent magnets: Due to temperature effects and aging of the magnets the magnet may lose their magnetism to some extent. The magnets are generally aged by the heat and vibration treatment.
  2. The error may appear in PMMC Instrument due to the aging of the spring. However the error caused by the aging of the spring and the errors caused due to the permanent magnet are opposite to each other, hence, both the errors are compensated with each other.
  3. Change in the resistance of the moving coil with the temperature: Generally, the temperature coefficients of the value of the coefficient of copper wire in moving coil is 0.04 per degree celsius rise in temperature. Due to the lower value of temperature coefficient the temperature rises at a faster rate and hence the resistance.Due to this significant amount of error is caused.

Advantages of Permanent Magnet Moving Coil Instruments

  1. The scale is uniformly divided as the current is directly proportional to the deflection of the pointer. Hence, it is very easy to measure quantities from these instruments.
  2. Power consumption is also very low in these types of instruments.
  3. A Higher value of torque is to weight ratio.
  4. These are having multiple advantages, a single instrument can be used for measuring various quantities by using different values of shunts and multipliers.

Instead of various advantages, the permanent magnet moving coil instruments or PMMC Instrument possess few disadvantages.

Disadvantages of Permanent Magnet Moving Coil Instruments

  1. These instruments cannot measure ac quantities.
  2. The cost of these instruments is high as compared to moving iron instruments.

Moving-iron instruments

Construction and basic principle operation:

Moving-iron instruments are generally used to measure alternating voltages and currents. In moving iron instruments, the movable system consists of one or more pieces of specially-shaped soft iron, which are so pivoted as to be acted upon by the magnetic field produced by the current in the coil.

There are two general types of moving iron instruments namely:

  1. Repulsion(or double iron) type
  2. Attraction(or single-iron) type

The brief description of different components of a moving-iron instrument is given below:

  • Moving element:a small piece of soft iron in the form of a vane or rod.
  • Coil:to produce the magnetic field due to the current flowing through it and also to magnetize the iron pieces.
  • In repulsion type, a fixed vane or rod is also used and magnetized with the same polarity.
  • Control torque is provided by spring or weight (gravity).
  • Damping torque normally pneumatic, the damping device consisting of an air chamber and a moving vane attached to the instrument spindle.
  • Deflecting torque produces a movement of an aluminum pointer over a graduated scale.


Working principle:

The deflecting torque in any moving-iron instrument is due to forces on a small piece of magnetically ‘soft’ iron that are magnetized by a coil carrying the operating current.In repulsion type moving–iron instrument consists of two cylindrical soft iron vanes mounted within a fixed current-carrying coil.

One iron vane is held fixed to the coil frame and other is free to rotate, carrying with it the pointer shaft. Two irons lie in the magnetic field produced by the coil that consists of only a few turns if the instrument is an ammeter or of many turns if the instrument is a voltmeter.

Current in the coil induces both vanes to become magnetized and repulsion between the similarly magnetized vanes produces a proportional rotation. The deflecting torque is proportional to the square of the current in the coil, making the instrument reading is a true ‘RMS’ quantity Rotation is opposed by a hairspring that produces the restoring torque. Only the fixed coil carries load current, and it is constructed so as to withstand high transient current.

Moving iron instruments having scales that are nonlinear and somewhat crowded in the lower range of calibration.

Measurement of Electric Voltage and Current

  • Moving iron instruments are used as Voltmeter and Ammeter only.
  • Both can work on AC as well as on DC.

Ammeter

  • The instrument used to measure current in the circuit.
  • Always connected in series with the circuit and carries the current to be measured.
  • This current flowing through the coil produces the desired deflecting torque.
  • It should have low resistance as it is to be connected in series.

Voltmeter

  • The instrument used to measure the voltage between two points in a circuit.
  • Always connected in parallel.
  • Current flowing through the operating coil of the meter produces deflecting torque.
  • It should have high resistance. Thus, a high resistance of the order of kilo-ohms is connected in series with the coil of the instrument.

Ranges of Ammeter and Voltmeter

  • For a given moving-iron instrument the ampere-turns necessary to produce full-scale deflection are constant.
  • One can alter the range of ammeters by providing a shunt coil with the moving coil.
  • Voltmeter range may be altered connecting a resistance in series with the coil. Hence, the same coil winding specification may be employed for a number of ranges.

Advantages

  • The instruments are suitable for use in AC and DC circuits.
  • The instruments are robust, owing to the simple construction of the moving parts.
  • The stationary parts of the instruments are also simple.
  • The instrument is low cost compared to moving coil instrument.
  • Torque/weight ratio is high, thus less frictional error.

Errors

  • Error due to variation in temperature.
  • Error due to friction is quite small as the torque-weight ratio is high in moving coil instruments.
  • Stray fields cause relatively low values of magnetizing force produced by the coil. Efficient magnetic screening is essential to reduce this effect.
  • Error due to the variation of frequency causes a change of reactance of the coil and also changes the eddy currents induced in neighboring metal.
  • Deflecting torque is not exactly proportional to the square of the current due to non-linear characteristics of the iron material.

References-

1.Douglas Hubbard.(2007).How to Measure Anything.Wiley

2.http://www.electrical4u.com/

3.Stevens, S.S. (1946)On the theory of scales and measurement. Science

  • Generally, voltage and current are measured using voltmeter and ammeter. There are two classes of these meters. They are:

    a.Analog meter (Voltmeters and ammeters)

    b.Digital meter (Voltmeters and ammeters)

  • The permanent magnet moving coil Instrument or PMMC uses two permanent magnets in order to create a stationary magnetic field to measure the dc voltage and currents.

  • Moving-iron instruments are generally used to measure alternating voltages and currents.
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