PMMI type instruments

Permanent magnet moving iron type instruments

Analog and Digital type instruments    principal, Construction,  working  of  PMMI type instruments, Their

                   Advantages,

                   Disadvantages,

Source of errors and much more about 

PMMI   type instruments

PMMI  type  instruments  has  many importance in panel making In laborites , In industries ,in various fields for measuring the Various quantizes as shown in the slides .

These instruments are more accurate ,east to use in every field of measuring the quantity

An instrument for direct measurement of the strength of electric current, voltage, or the quantity of electricity in AC/DC circuits. The movable part of a permanent-magnet instrument is displaced by the interaction between the magnetic field of the permanent magnet and the conductor carrying the current.

The most widespread type of permanent-magnet instrument has a movable frame

located in the field of a permanent magnet. When a current flows through the windings of the frame, forces are generated that produce torque. Current is supplied to the frame through springs, or tension members, which generate a counteracting mechanical torque. The action of both tensional moments causes deflection of the frame through an angle proportional to the strength of the current in the frame

Extension of the measurement range for current and voltage is achieved by connecting shunts and series resistors to the frame; they may be external or built-in.

                                            Permanent-magnet instruments exist in which the permanent magnet is located in the interior of a moving coil, as well as with moving magnets mounted on an axle inside a moving coil. Permanent-magnet quotient meters are also used. 

Permanent-magnet instruments with moving magnets are simpler, smaller, and lighter, but they are less precise and sensitive than instruments with moving frames. Readings are shown by pointers or luminous indicators: a ray of light from a source is directed at a mirror mounted on the moving part of the instrument and is reflected from the mirror, generating a bright spot with a dark line in the center on the scale of the instrument.

The distinguishing features of permanent-magnet moving iron type instruments are

  Uniform scales,

  Good damping,

   High precision

And high  Sensitivity,

and low current consumption.

 Such instruments are sensitive to overloads

and mechanical shock and impact but are relatively insensitive to the effect of external magnetic fields and ambient temperatures

Basically an electrical indicating instrument is divided into two types. They are 

           i) Analog instruments   

           ii) Digital Instruments.

    Analog instruments are nothing but its output is the deflection of pointer, which is proportional to its input

Digital Instruments are its output is in decimal form.

    Analog ammeters and voltmeters are classed together as there are no fundamental differences in their operating principles.

    The action of all ammeters and voltmeters, with the exception of electrostatic type of instruments, depends upon a deflecting torque produced by an electric current.

In an ammeter this torque is produced by a current to be measured or by a definite fraction of it.

    In a voltmeter a current produces this torque, which is proportional to the voltage to be measured.

    Thus all analog voltmeters and ammeters are essentially current measuring devices.

The essential requirements of a measuring instrument are 

(i) That its introduction into the circuit,where measurements are to be made,

does not alter the circuit conditions;

(ii) The power consumed by them for their operation is small.

(iii) Their connection should be simple

(Iv) The measuring device should be easy to handle

(v) Device should be portable in nature Because such instruments are used in Labs and industries etc

Ammeters are connected in series 

           in the circuit whose current is to be measured.  The  power  loss  in  an ammeter is  I 2R  a where I is the current to be measured and R is the resistance of ammeter.  Therefore,ammeters should have a low electrical   resistance 

so that they cause a small  voltage  drop and  consequently  absorb  small   power.

Voltmeters are connected in parallel 

                       with the circuit whose voltage is to be measured.    There is also a power loss in voltmeters

The voltmeters should have a high internal electrical resistance,  in  order  that  the current drawn by them is small  and consequently the power consumed is small.

permanent magnet moving iron instruments are widely used in laboratories and switch boards at commercial frequencies because these are cheaper in cost , robust  in construction and can be manufactured with required accuracy.

   there are two general types of such       instrument  i.e

(i)   the attraction type

(ii)   the repulsion type .  

the  repulsion type instrument operate on the principle of repulsion of two adjacent iron piece magnetized by the same magnetic field. repulsion type instrument are more sensitive as in these instrument large operating torque is developed by having two iron element positioned close together inside the field coil

Where the magnetizing effect is maximum. in both type of these instrument the current under measurement is passed through a coil of wire. this current carrying coil sets up the necessary field. depending on the magnitude of the current to be measured the coil may be of a few turns of very heavy conductor or of many turns of fine wire. 

. the instrument to be used as an ammeter is provided with a coil of thick wire in order to have low resistance and carry large current                      and to be used as a voltmeter is provided with a coil of large number of turns of fine wire in order to have high resistance and draws as small current as possible.

permanent magnet moving iron instruments let us discuss first Repulsion type moving iron instruments

Construction. The main components in Repulsion type moving iron instruments are

1 moving iron vane

2 fixed iron vane

3fixed cylindrical coil

4shaft

6hair spring

7 pointer

8 scale

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.

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.

In permanent moving iron type instruments

Weather they are attraction type or repulsion type, the deflection is directly proportional to the square of electrical current flowing ,when connected in the circuit , that is why the scale of these instruments are non linier or non uniform, so these instruments works on both type of supply AC 

as well DC. 

In the repulsion type, there are two vanes inside the coil one fixed and other         movable as discussed earlier

 These are similarly magnetized when the current flows through the coil and there is a force of repulsion between the two vanes resulting in the movement of the moving vane.

 Two different designs are in common use

(I)Radial Vane Type

 In this type, the vanes are radial strips of iron.

•

•   The strips are placed within the coil as shown

•   The fixed vane is attached to the coil and the movable one to the spindle of the instrument

•. (a)    Radial vane  

•  (b) Co-axial vane type

      

i) (b) Co-axial Vane Type

    In this type of instrument, the fixed and moving vanes are sections of co axial cylinders as shown previously The controlling torque is provided by springs. Gravity control can also he used in vertically mounted instruments. The damping torque is produced by air friction as in attraction type instruments.

•    The operating magnetic field in moving iron instruments is very weak and therefore eddy current damping is not used in them as introduction of a permanent magnet required for eddy current damping would destroy the operating magnetic field.

•    It is clear that whatever may be the direction of the current in the coil of the instrument, 

•     That there is always a force of attraction in the attraction type and repulsion in the repulsion type of instruments.

    Thus moving iron instruments are unpolarised instruments i.e., they are independent of the direction in which the current passes

that is why these instruments can be used on Ac as well as on dc both types of supply, no rear of reading to go in reverse direction.

Attraction type

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

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

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 of  low cost as compared to moving coil instrument.

• Torque/weight ratio 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. 

with both D.C. and A.C

i)          Hysteresis Error

ii)          Temperature error

iii)         Stray magnetic field

Errors with only A.C

 Frequency errors

This error is introduced only if there is a change in supply frequency ,due to change in frequency ,readings of such type of instruments may affects or alters.

Advantages & Disadvantages

 Universal use

 Less Friction Errors

Cheapness 

 Robustness 

Accuracy

    Scale 

    Errors

  Waveform errors

Non linear scales.

Is digital ammeter is a moving iron Instrument or moving coil instrument?

 Both moving iron and moving coil   instruments are analogue instruments, relying on mechanical movement to display a measurement. A moving iron instrument works on the principle of attraction or repulsion between an electromagnet and an iron armature to drive  its   pointer. 

. A moving coil /iron instrument works on the principle of the interaction of two magnetic fields to drive its pointer.

A digital instrument, on the other hand, is completely solid-state, using electronic circuits to digitally process the information supplied by the circuit under test, and then displays the result using a digital output

What is difference between Ammeter and voltmeter

Main difference is of internal resistance of their operating coils, in case of ammeter ,the internal resistance is low as length of coil is less, in case of voltmeter the internal resistance is high ,another difference is of their connection , Ammeter is connected in series of supply 

Question----- the scale of permanent magnet moving iron type instruments are non uniform or non linier why?

Answer--- ---- the scale of permanent magnet moving iron type instruments are non uniform or non linier because in such type of instruments the deflection or torque is proportional to the square of current ( not current)  that is why the scale is crowded near about zero so that interval is not uniform