Study of DC-AC Measuring Instruments - 2 | Basic Electrical Technology - Electrical Engineering (EE) PDF Download

 Advantages, Limitations and sources of errors 

Advantages: 

• The scale is uniformly divided (see at steady state ,
• The power consumption can be made very low ( 25 μ Wto 200 μ W ). 
• The torque-weight ratio can be made high with a view to achieve high accuracy.
• A single instrument can be used for multi range ammeters and voltmeters.
• Error due to stray magnetic field is very small. 

Limitations: 

• They are suitable for direct current only.
• The instrument cost is high.
• Variation of magnet strength with time.

The Errors are due to:

i) Frictional error, ii) Magnetic decay, iii) Thermo electric error, iv) Temperature error. 

Errors can be reduced by following the steps given below:  

• Proper pivoting and balancing weight may reduce the frictional error.
• Suitable aging can reduce the magnetic decay.
• Use of manganin resistance in series (swamping resistance) can nullify the effect of variation of resistance of the instrument circuit due to temperature variation.
• The stiffness of spring, permeability of magnetic core (Magnetic core is the core of electromagnet or inductor which is typically made by winding a coil of wire around a ferromagnetic material) decreases with increases in temperature.

Ammeter Sensitivity: Ammeter sensitivity is determined by the amount of current required by the meter coil to produce full-scale deflection of the pointer. The smaller the amount of current required producing this deflection, the greater the sensitivity of the meter. A meter movement that requires only 100 microamperes for full- scale deflection has a greater sensitivity than a meter movement that requires 1 mA for the same deflection.

Voltmeter Sensitivity: The sensitivity of a voltmeter is given in ohms per volt. It is determined by dividing the sum of the resistance of the meter (R_m), plus the series resistance (R_s), by the full-scale reading in volts. In equation form, sensitivity is expressed as follows: 

This is the same as saying the sensitivity is equal to the reciprocal of the full-scale deflection current. In equation form, this is expressed as follows: 

Therefore, the sensitivity of a 100-microampere movement is the reciprocal of 0.0001 ampere, or 10,000 ohms per volt.  

 Construction and Basic principle operation of Moving-iron Instruments 

 We have mentioned earlier that the instruments are classified according to the principles of operation. Furthermore, each class may be subdivided according to the nature of the movable system and method by which the operating torque is produced. Specifically, the electromagnetic instruments are sub-classes as (i) moving-iron instruments (ii) electro-dynamic or dynamometer instruments, (iii) induction instruments. In this section, we will discuss briefly the basic principle of moving-iron instruments that 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 coil. There are two general types of moving-iron instruments namely (i) Repulsion (or double iron) type (ii) 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 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 is normally pneumatic, the damping device consisting of an air chamber and a moving vane attached to the instrument spindle.
• Deflecting torque produces a movement on an aluminum pointer over a graduated scale.

Construction of Moving-iron Instruments 

The deflecting torque in any moving-iron instrument is due to forces on a small piece of magnetically ‘soft’ iron that is magnetized by a coil carrying the operating current. In repulsion (Fig.42.7) 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 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. Another type of instrument that is usually classed with the attractive types of instrument is shown in Fig.42.8. 

Fig. 42.7: Repulsion type.

This instrument consists of a few soft iron discs ( B ) that are fixed to the spindle (D ), pivoted in jeweled bearings.  The spindle (D ) also carries a pointer (P ), a balance weight ( W₁),  a controlling weight (W₂) and a damping piston (E) which moves in a curved fixed cylinder (F ). The special shape of the moving-iron discs is for obtaining a scale of suitable form.  

Remark: Moving-iron vanes instruments may be used for DC current and voltage measurements and they are subject to minor frequency errors only. The instruments may be effectively shielded from the influence of external magnetic fields by enclosing the working parts, except the pointer, in a laminated iron cylinder with laminated iron end covers. 

Torque Expressions: Torque expression may be obtained in terms of the inductance of the instrument. Suppose the initial current is I , the instrument inductance L and the deflection θ . Then let I change to I + dI , being a small change of current; as a result let  θ changes to (θ + dθ) ,and L to ( L + dL ) . In order to get an incremental change in current dI there must be an increase in the applied voltage across the coil.  

Applied voltage 

The electric energy supplied to the coil in dt is  v I dt = I ² dL+ IL dI  

Increase in energy stored in the magnetic field = 

(neglecting second and higher terms in small quantities)

If T is the value of the control torque corresponding to deflection θ , the extra energy stored in the control due to the change dθ is Tdθ . Then, the stored increase in stored 

From principle of the conservation of energy, one can write the following expression  Electric energy drawn from the supply = increase in stored energy + mechanical work done 

 Controlling torque:

i Spring control:  T_s =K_s θ where K_S is the spring constant. 

ii Gravity control:   T_g=K_g sin θ . Where K_g = mgl

At equilibrium i.e. for steady deflection, Deflecting torque = Controlling torque. If the instrument is gravity controlled 

 Ranges of Ammeters and Voltmeters 

 For a given moving-iron instrument the ampere-turns necessary to produce fullscale deflection are constant. One can alter the range of ammeters by providing a shunt coil with the moving coil. 

 Shunts and Multipliers for MI instruments 

For moving-iron ammeters: For the circuit shown in Fig.42.9, let R_m and L_m are respectively the resistance and inductance of the coil and R_sh and L_sh the corresponding values for shunt. 

Fig 42.9

The ratio of currents in two parallel branches is 

The above ratio will be independent of frequency ω provided that the time constants of the two parallel branches are same i.e

In other words,

Now, 

Multipliers for the shunt = It is difficult to design a shunt with the appropriate inductance, and shunts are rarely incorporated in moving iron ammeters. Thus the multiple ranges can effectively be obtained by winding the instrument coil in sections which may be connected in series, parallel or series-parallel combination which in turn changing the total ampere-turns in the magnetizing coil. 

For moving-iron voltmeters: 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. Let us consider a high resistance R_se is connected in series with the moving coil and it is shown below.

Note: An ordinary arrangement with a non-inductive resistance in series with the fixed coil – results in error that increases as the frequency increases. The change of impedance of the instrument with change of frequency introduces error in signal measurements. In order to compensate the frequency error, the multiplier may be easily shunted by the capacitor.

Flg.42.10t Connection for method of compensating frequency error in moving-iron voltmeter.

Advantages: 

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

Errors:

      i. Errors due to temperature variation.

      ii. Errors due to friction is quite small as torque-weight ratio is high in moving-iron instruments.

      iii. Stray fields cause relatively low values of magnetizing force produced by the coil. Efficient magnetic screening is essential to reduce this effect.

      iv. Error due to variation of frequency causes change of reactance of the coil and also changes the eddy currents induced in neighboring metal.

      v. Deflecting torque is not exactly proportional to the square of the current due to non-linear characteristics of iron material.