Study of Instrument workshop-observe the construction of PMMC, Dynamometer, Electro thermal and rectifier type Instrument, Oscilloscope and digital multimeter

Objective:

To study the construction of PMMC, Dynamometer, Electro thermal and Rectifier type instrument, Oscilloscope and Digital multimeter.

Theory:

Electric measuring instruments and meters are used to indicate directly the value of current, voltage, power or energy. These measuring instruments are classified according to both the quantity measured by the instrument and the principal of operation. There are generally, three general principles of operation available: (i)electromagnetic, which utilizes the magnetic effects or electric currents; (ii) electrostatic, which utilizes the forces between electrically-charged conductors; (iii)electro-thermic, which utilizes the heating effect.

There are various forces required in measuring instruments:

Deflecting force: the deflection of any instrument is determined by the combined effect of the deflecting torque, control  torque and damping torque.

Controlling force: this force must act in the opposite sense to the deflecting force, and the movement will take up an equilibrium or definite position when the deflecting and controlling torque are equal in magnitude.

Damping force: A damping force is required to act in a direction opposite to the moving system. This brings the moving system to rest at the deflected position reasonable quickly without any oscillation or very small oscillation.

(i)Permanent magnet Moving coil(PMMC) Instrument:

The permanent magnet moving coil instruments are most accurate type of for direct current measurements. The action of these instruments is based on the motoring principle. When a current carrying coil is placed in the magnetic field produced by permanent magnet, the coil experiences a force and move. As the coil is moving and the magnet is permanent, the instrument is called permanent magnet moving coil instrument. This basic principle is called D’Arsonval principle. The amount of force experienced by the coil is proportional to the current passing through the coil.

The moving coil is either rectangular or circular in shape. It has number of turns fine wire. The coil is suspended so that it is free to turn about its vertical axis. The coil is placed in uniform, horizontal and radial magnetic field of a permanent magnet in the shape of a horse-shoe. The iron core is spherical if coil is circular and is cylindrical if the coil is rectangular. Due to iron crore, the deflecting torque increases, increasing the sensitivity of the instrument. The controlling torque is provided by two phosphor bronze hair springs.

The damping toque is provided by eddy current damping. It is obtained by movement of aluminum former, moving in the magnetic field of the permanent magnet.

The pointer is carried by the spindle and its moves over a graduated scale. The pointer has light weight so that it deflects rapidly.

The mirror is placed below the pointer to get the accurate reading by removing the parallax.

The weight of the instrument is normally counter balanced by the weights situated diametrically opposite and rapidly connected to it. The scale markings of the basic d.c. PMMC instruments are usually linearly spaced as the deflecting torque and hence the pointer deflections are directly proportional to the current passing through the coil.

Advantages of PMMC:

  • It is uniform scale.
  • With a powerful magnet, its torque to weight ration is very high. So operating of PMMC is small.
  • The sensitivity is high.
  • The eddy currents induced in the metallic former over which coil is wound, provide effective damping.
  • It consumes low power, of the order of 25W to 200Mw.
  • It has high accuracy.
  • Instrument is free from hysteresis error.
  • Extension of instrument range is possible. Not effected by external magnetic fields called stray magnetic fields.

Disadvantages of PMMC:

  • PMMC is suitable for the direct current measurement only.
  • Ageing of permanent magnet and the control springs introduces the errors.
  • Delicate construction and accurate machining.

(ii) Dynamometer type Instrument:

Electrodynamometer type instruments are similar to the PMMC instruments except the magnet is replaced by two serially connected fixed coils are spaced far enough apart to allow passage of the shaft of the movable coil. The movable coil carries a pointer, which is balanced by counter weights. Its rotation is controlled by springs. The motor torque is proportional to the product of the currents in the moving coil reverse at the same time, and the turning force continues in the original direction. Since reversing the current direction does not reverse turning force, this type of instruments can be used to measure AC or DC current, voltage or its major application as a watt-meter (in our case) for power measurement.

(iii) Rectifier type instrument:

Mainly there are two types of thermal instruments:

  • Hot-wire type
  • Thermocouple instrument

Hot-wire and thermocouple meter movements use the heating effect or current flowing through a resistance to cause meter deflection. Each uses this effect in a different manner. Since their operation depend only on the heating effect of current flow, they may be used to measure both direct and alternating currents of any frequency on a single scale.

Hot-wire Instrument:

The hot-wire meter movement deflection depends on the expansion of a high resistance wire caused by the heating effect of the wire itself as current flows through it. A resistance wire is stretched between the two meter terminals, with a thread attached at a right angles to the center of the wire. A spring connected to the opposite end of the thread exerts a constant tension on the resistance wire. Current flow heats the wire, causing it to expand. This motion is transferred to the meter pointer through the thread and a pivot the basic arrangement of a hot wire type instrument.

Advantages of Hot-wire type Instruments:

  1. The deflection depends upon only the rms value of the current flowing through the wire, irrespective if its waveform and frequency. Hence, the instruments can used for ac as well as dc system.
  2. The calibration is same for ac as well as dc measurement. So it is a transfer-type instrument.
  3. They are free from stray magnetic fields because no magnetic field is used to cause their operation.
  4. It  is cheap in cost and simple in construction.

Thermocouple- type Instrument:

When two metals having different work functions are placed together, a voltage is generated at the junction which is nearly proportional to the temperature of the junction. This junction is called a thermocouple. This principle is used to convert heat energy to electrical energy at the junction of two conductors. The heat at the junction is produced by the electrical current flowing in the heater element while the thermocouple produces an emf at its output terminals,which can be measured with the help of a PMMC meter. The emf produced is proportional to the temperature and hence to the rms value of the current. Therefore, the scale of the PMMC instrument can calibrate to read the current passing through the heater. The thermocouple type of a thermocouple instrument is that they can be used for measurement of current and volatages at very high frequency. In fact, these instruments are very accurate well above a frequency of 50 MHz.

Advantages of thermocouple-type instruments:

  1. These are not affected by stray magnetic fields.
  2. They have very high sensitivity.
  3. The indication of these instruments are practically unaffected by the frequency and waveform of the measuring quantity. Hence these instruments can be used for measurement of currents up to frequencies of 50MHz and give accuracy as high as 1%.
  4. These instruments are very useful as transfer instruments for calibration of dc instruments by potentiometer and a standard cell.

(iv) Rectifier-type Instruments: 

The basic arrangements of a rectifier type of instrument using a full-wave rectifier circuit. If this instrument is used for measuring ac quantity then first the ac signal is converted to dc with help of the rectifier. Then this dc signal is measured by PMMC meter .These types of instruments are used for light current work where the voltage is low and resistances high.

(V)Oscillpscope:

The cathode Ray Oscilloscope(CRO) is a very useful and versatile laboratory instrument used for display, measurement and analysis of waveform and  other phenomena in a electrical and electronics circuits, CRO are, in fact, very fast X-Y plotters, displaying an input signal versus another signal or versus time.

The normal form of a CRO uses a horizontal input voltage which is an internally  generated ramp voltage called ‘time base’. The horizontal voltage moves the luminous spot periodically in a horizontal direction from left to right over the display area or screen. The vertical input to the CRO is the voltage under investigation. The vertical input voltage with respect to time. When the input voltage repeats itself at a fast rate, the display on the screen appears stationary on the screen. The CRO thus provides a means of viewing time-varying voltages. As such, the CRO has become a universal tool in all kinds of electrical and electronic investigation. A basic block diagram of a general-purpose oscilloscope in given figure and a schematic of internal parts of CRT is shown in given figure.

(vi)Multimeter:

A digital multimeter is an electronic instrument which can measure very precisely the dc and ac voltage, current(dc and ac) and resistance. All quantities other than dc voltage is first converted into an equivalent dc voltage is first converted into an equivalent dc voltage by some device and then measured with the help of digital voltmeter. The block diagram of a digital multimeter is shown in given figure.

The procedures of measurement of different quantities are described below.

For measurement of ac voltage, the input voltage, is fed through a calibrated compensated attenuator, to a precision full-wave rectifier circuit followed by a ripple reduction filter. The resulting dc is fed to an Analog Digital converter(ADC) and the subsequent display system.

In ac measurement the reading is often average or rms values of the unknow current. Sometimes for measurement of current, a current-to-voltage converter may also be used.

For resistance measurement the digital multi-meter operates by measuring the voltage across the externally connected resistance, resulting from a current forced through it from a calibrated internal current source. The accuracy of the resistance measurement is of the order of 0.1 to 0.5% depending on the accuracy and stability of the internal current sources.

Procedure:

  1. Observe the construction of the instruments.
  2. Switch on CRO and digital Multimeter to observe the output.
  3. Relate the practical meters with the theoretical working principles.

 

Conclusion:

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