Thursday, 15 August 2013

Electrical Instruments



The instruments which directly indicate the value of the electrical quantity at the same time when it is being measured are called indicating instruments. In these instruments a pointer moves over a graduated scale and gives the value of the quantity being measured.

Parts of indicating instruments:

Methods of Supporting Moving Systems: There are two methods of supporting the moving system

(1)   by pivoting and
(2)   by thread suspension. 

In pivoted system the moving system is mounted on a spindle of hardened steel, having the ends conical and highly polished to form pivots. The ends of the spindle fit into conical holes in jewels which form the bearings on the fixed part of the instrument. Sapphire is commonly used for the manufacture of jewels. This type of support is robust; of low frictional resistance and is sensibly independent of whether the instrument is used in a vertical or horizontal position.

Suspension system is usually used for galvanometers, where leveling of instruments is essential before use. In this system the moving system is suspended by means of a fine ribbon shaped metal strip from the top. The suspension system is advantageous when the operating forces are very small as compared to the weight of the moving system. This system eliminates the friction at the bearings. Such suspensions are delicate and are required to be protected against mechanical vibrations and shocks etc. The suspension system has been recently improved by introducing a taut-ribbon. The taut ribbon is also used for carrying the current to the coil in moving coil instruments in addition to providing the controlling torque.  Instruments of this type do not require as much careful leveling as the former type but still need careful handling

Permanent Magnets: Special alloy steels are used in instrument magnet construction. Tungsten steel has been popular since the early days of electrical instruments. Cobalt-chrome steels have also been used because of their higher coercively and where economy of weight and space is important.  But in recent years, alnico magnet has been used to an increasing extent in instrument applications.  In most of the applications, the field strength may be expected to be between0.05 and 0.25 T in the air gap of 1.5 – 2.5 mm length; depending on the size and type of instrument.  For efficient design, the x-section of the magnet may be large and its length small for materials of high coercively and low permanence whereas the length of the magnet should be large and x-section small for materials of low coercivity and high permanence. 

Scales: The scale is usually printed on the enameled surface of the metal plate, or on paper or card board firmly cemented to a metal backing plate.  In switchboard and panel instruments, scale markings are usually broad and for easy reading, the scale is usually subdivided into 2.5 and 10 parts between major scale divisions.  In portable instruments, division lines are laid out individually for each instrument in contrast to scales of switch-board of panel instruments which are usually pre-printed.  This procedure is necessary because of higher accuracy expected in the indications of the portable instruments.

In most of the instruments, the moving system rotates through an angle of 90° for full-scale defection though some instruments have been designed to give full-scale deflection through an angle of 120° or even greater.  The greater the angle for full-scale deflection more clear is the scale of the instruments.  The length of scale in most of the instruments is about 15 cm.

Covers or Cases: Covers or cases of the instruments must be dust and moisture proof in construction. These are usually made of hard wood, brass, Bakelite, cast iron or pressed steel. The steel cases are preferred in the case of instruments affected by external magnetic fields due to generators, current carrying conductors and earth field. The steel case bypasses practically the entire flux and thus prevents it from affecting the sensitive portions of the instruments. The smaller the opening in the shell, the more effective the screening. When the steel covers are used, the moving system is mounted in a position as far away from the case as possible in order to avoid errors due to hysteresis and eddy currents effects in the cases.

Commonly used terms in electrical measurements and instrumentation are defined below:

Measured Variable: A variable is that quantity of characteristic which is the object of measurement in an instrumentation /control system.  Variable may be measurement variable, instrumentation variable or process variable. The physical quantity, property or condition which is to be measured is referred as the measured and common measured variables are displacement, force, speed, pressure, temperature, rate of flow, thickness etc.

Input Signal: It is a signal applied to a device, element or system such as pressure applied to the input connection of a pressure transmitter.

Output Signal: It is a signal delivered by a device, element or system.

Range: The region between the limits within which a quantity is measured, received or transmitted, expressed by stating the lower and upper limits, is called the range such as 0-50 A, 50-500 V, -20°C-80°C.

Span: The algebraic difference between the upper and lower range values is called the span.  For example for range 0 to 50 A spans is 50 A, for range of -20°C to 80° the span is 100°C. 

Calibration: It is to ascertain output of a device corresponding to a series of the quantity the device is to measure, receive or transmit.  Data so obtained are used for (a) determination of location at which scale graduations are to be placed (b) adjustment of output to bring it to the desired value within a specified tolerance and (c) ascertaining the error by comparing the device output reading against a standard.

Accuracy: It is a degree of conformity of an indicated value to a recognized accepted standard value or ideal value.  Conformity may be conceived as the maximum difference, over the range of the instrument, between indicated value and the true value under measurement.

Error: The algebraic difference between the indicated value and the true value of the measured signal is called the error i.e. Error = Indicated value – true value. +ve error denotes that the indicated value is higher than true value.

It is helpful to use the word error in this concept only and to describe it as an instrumental error meaning the difference between the average of a series of up and down readings, as indicated by the instrument output, and the corresponding true values of input.

Zero Error: It is an error of a device operation under the specified conditions of use when the input is at the lower range-value. The term zero-shift is often used to represent a change or drift in zero error with time.

Span Error: The difference between the actual span and the ideal span is called the span error and it is usually expressed as a percentage of ideal span.

Correction: The algebraic difference between the true value and the indicated value of the measured signal is called the correction i.e. correction = true value – indicated value.  Correction is a quantity which is added algebraically to the indicated value so as to have true value.

Hysteresis: It is that property of an element evidenced by the dependence of the value of the output, for a given excursion of the input, on the history of prior excursions and the direction of the current traversed.

It is usually determined by subtracting the value of the dead band from the maximum measured separation between up-scale going and down-scale going indications of the measured variable (during a full range traverse, unless otherwise specified) after transients have decayed. This measurement is sometimes called hysteresis error.

Dead Band: It is the range through which an input can be varied without initiating observable response and is usually expressed in percentage of span.

Repeatability: The closeness of agreement among a number of consecutive measurements of the output for the same value of the input under the same operating conditions, approaching from the same direction for full range traverses is called the repeatability.

Deviation: It is a departure from a desired or expected value or pattern and may also be described as the difference between measured value and true value for a particular input value. The deviation is given a plus or minus sign, depending on whether the measured values are above or below the true value.

Linearity: It is the closeness to which a curve approximates a straight line.  It is usually measured as a non-linearity and expressed as linearity e.g. a maximum deviation between an average curve and a straight line. The average curve is determined after making two or more full range traverses in each direction.  The value of linearity is referred to the output unless otherwise specified.

Damping: The progressive reduction or suppression of oscillations in a device or system is called the damping.

Noise: It is an unwanted component of a signal or variable which obscures the information content.

Signal to Noise Ratio: Ratio of signal amplitude may be peak or rms.  For non-sinusoidal signals, peak values should be used.

Dynamic Response: The behavior of the output device as a function of the input, both with respect to time is called the dynamic response.

Accuracy is a closeness with which the instrument reading approaches the true value of the variable under measurement while precision is a measure of the reproducibility of the measurements i.e. precision is a measure of the degree to which successive measurements differ from one another. IN brief, accuracy can be defined as conforming to truth and precision can be defined as sharply or closely defined.

The proper operation of the indicating instruments depends on the three torques listed below:

Deflecting (Operating) torque.
Controlling (Restoring) torque.
Damping torque.

Deflecting Torque: 

It utilises the effects of electric current or voltage and causes the movement of the moving system and thereby the pointer from the zero/initial position. For producing deflecting torque, Td it is necessary to connect the instrument in the circuit to measure the given electrical quantity. This is achieved by magnetic effect in moving iron instruments and by electro-dynamic effect in moving coil instruments. The deflecting torque causes the moving system and hence the pointer from zero position over a graduated scale to indicate the value of the electrical quantity being measured.

Controlling Torque:

With deflecting torque the pointer would swing indefinitely and go over the maximum deflected position irrespective of the magnitude of the electrical quantity being measured. To ensure that the pointer does not go beyond the range of the scale, a controlling or opposing torque, Tc is required which opposes the deflecting torque. The pointer comes to a rest or standstill when the deflecting and controlling torques are equal to each other.

Tc increases with the deflection of the moving system so that the pointer on the scale will be according to magnitude of the quantity being measured.

It restores the pointer to zero position when Td is removed.

Methods:-
 Spring control.

Gravity control.