MEASURING INSTRUMENTS - AMMETER AND VOLTMETER

Measuring instruments are an important part of electrical engineering and they are essential for analyzing the electrical circuits. Some of the important instruments which are used occasionally are ammeter, voltmeter, wattmeter, energy meter etc. But in this article we are about to discuss the two most fundamental of all - Ammeter and Voltmeter. Ammeters and voltmeters are generally classed together because of the similarities in their operating principles, with some exceptions of course!

WHAT IS AN AMMETER?

An ammeter is an electrical measurement instrument which is used to measure current through a point or wire in a circuit. It is connected in series with the circuit whose current is to be measured. Therefore, they should have a low electrical resistance. This is essential in order that they cause a small voltage drop and consequently absorb small power.

Ammeter can also be made from a galvanometer by placing a shunt resistance in parallel with the galvanometer.

WHAT IS A VOLTMETER?

A voltmeter is an electrical measurement instrument which is used to measure voltage across two given points in a circuit. It is connected in parallel with the circuit or the part of the circuit whose voltage is to be measured. They should have high electrical resistance so that the current drawn by them is small and ultimately the power absorbed is small.

voltmeter can also be made from a galvanometer by placing a high resistance in series with the galvanometer.

Ammeters and voltmeters can be of different types based on their construction and working.

TYPES OF AMMETERS AND VOLTMETERS –

  1. Moving iron type (both for AC/DC)

      (a)  Attraction type

      (b) Repulsion type

  2. Moving coil type

      (a)  Permanent magnet type (for DC only)

      (b) Electrodynamic or dynamometer type (for DC/AC)

  3. Hot wire type (both for DC/AC)

  4. Induction type (for AC only)

      (a)  Split phase type

      (b) Shaded pole type

  5. Electrostatic type (for voltmeters only) – Both DC/AC

MOVING IRON TYPE –

In moving iron type, there are two basic types, which are:

(a)  Attraction type

(b) Repulsion type

For both type of these instruments, the necessary magnetic field is produced by the ampere turns of a current carrying coil. As we are talking about ammeter; the coil has comparatively fewer turns of thick wire so that the ammeter has low resistance because it is to be connected in series with the circuit.

ATTRACTION TYPE INSTRUMENT –

The figure below shows the constructional details of an attraction type moving iron instrument.

ATTRACTION TYPE MOVING IRON INSTRUMENT

The coil is flat disc or a sector eccentrically mounted. When the current flows through the coil, a magnetic field is set up and the moving iron moves from a region of weaker field to region of stronger magnetic field, thereby deflecting the pointer over a calibrated scale.

The controlling torque is provided by springs but can also be gravity controlled. Damping is provided by air friction usually by a vane moving in a sector shaped chamber.

REPULSION TYPE INSTRUMENT –

REPULSION TYPE MOVING IRON ONSTRUMENT

In repulsion type, there are two iron vanes inside the coil; one fixed and other movable. When the current flows through the coil, these vanes get similarly magnetized and there is force of repulsion between the two vanes resulting in the movement of moving vane, and therefore the pointer.

Why moving iron can be used in both AC and DC?

The function of moving iron instruments depends on the attraction or repulsion of iron vanes. Thus, they are unpolarised i.e. they are independent of the direction in which the current flows. Therefore, they can be used both in AC as well as DC.

DEFLECTION PRODUCED –

The deflection in a moving iron instrument is given by

                        \[\theta =\frac{1}{2}\frac{{{I}^{2}}}{K}\frac{dl}{d\theta }\]

Thus, $\theta \propto {{I}^{2}}$

As, the deflection is proportional to the square of the current, it is evident that the scale of such an instrument is non-uniform.

If there is no saturation, the change of inductance with the angle of deflection is uniform i.e. $\frac{dl}{d\theta }$ is constant. Thus, the scale can be easily laid as the measured quantity is proportional to the square root of deflection.

SOURCES OF ERROR –                                                                      

     (a)  Errors with both AC and DC work –

          (i)  Errors due to hysteresis -  

             Because of hysteresis in the iron parts of the moving system, readings are higher            for descending values and lower for ascending values. This can be completely              eliminated by using Mu metal or Perm-alloy, which has negligible hysteresis loss. 

         (ii)  Errors due to stray fields –

               Unless shielding is not done from external fields, the results obtained may be                  wrong. Thus, the instrument is shielded with cast iron.                              

     (b) Errors with AC work –

     Changes of frequency produce change in the impedance of the coil and change in          the magnitude of AC currents.

ADVANTAGES - 

• Cheap and robust.
• Can be used both in AC and DC.

DISADVANTAGE –

• They cannot be calibrated with DC because of the effect of hysteresis in the iron      vanes. Hence, they are usually calibrated by comparison with AC standard.

MOVING COIL INSTRUMENTS

Moving coil instruments can be classified into two types:

       1. Permanent magnet moving coil instrument (for DC)

       2. Dynamometer type (for both AC and DC)

PERMANENT MAGNET MOVING COIL –

PRINCIPLE OF OPERATION –

The operation of a permanent magnet moving coil instrument is based upon the principle that when a current carrying conductor is placed in a magnetic field, it experiences a force or torque which moves it.

CONSTRUCTION –

The instrument consists of a permanent magnet and a rectangular coil of many turns wound on a light aluminium frame inside which is an iron core. A soft iron cylinder is fixed between the magnetic poles which help to

(i) Make the field radial.

(ii) Decrease the reluctance of air path between the poles.

The rectangular coil on the iron core is attached to a light pointer. The sides of the coil are free to move in the air gaps between the poles.

CONTROL –

The controlling torque is provided by two phosphor bronze hair springs. These springs also lead the current in and out of the coil.

The control torque can also be provided by using a ribbon suspension and it is considered to be an advantageous method as it eliminates bearing friction.

DAMPING –

Damping torque is produced by the movement of the aluminium frame moving in magnetic field of the magnet.

DEFLECTING TORQUE AND DEFLECTION –

The deflecting torque produced in the coil is given by

${{T}_{d}}$ = Force * perpendicular distance

     $=NBIl\times b$$=NBIA$ N-m.

Where, N = no. of turns in coil

            B = flux density in Wb/m²

            I = current in the coil

            A = area of the coil

$\Rightarrow {{T}_{d}}\propto I$

Also, controlling torque ${{T}_{c}}\propto \theta $ (Because such instruments are spring controlled)

As T_d = T_c  $\Rightarrow \theta \propto I$

Therefore, such instruments have uniform scales.

Damping is electromagnetic i.e. by eddy currents induced in metal frame over which coil is wound.

ADVANTAGES –

• They have low power consumption.
• Their scales are uniform.
• They have high torque-to-weight ratio.
• No hysteresis loss.
• Effective and efficient eddy current damping.
• They are not much affected by stray magnetic fields, due to the presence of strong permanent magnets.

DISADVANTAGES –

•      Costlier as compared to moving iron instruments.
•      Errors occur with the ageing of instrument.

ELECTRODYNAMOMETER TYPE INSTRUMENTS

An electrodynamometer type instrument is a moving coil instrument in which the operating field is produced by another coil which is fixed. This type of instrument can be used either as an ammeter or as a voltmeter, but is generally used as a wattmeter.

ELECTRODYNAMIC INSTRUMENT AS AN AMMETER

ELECTRODYNAMIC INSTRUMENT AS A VOLTMETER

Electrodynamic instruments are also capable of functioning as transfer instruments. Besides, their use as an ammeter, voltmeter, and wattmeter; they are also used to transfer calibration of working instruments.

CONSTRUCTION –

The instrument consists of a fixed coil and a moving coil. The fixed coil is usually air-cored to avoid hysteresis effects when used on AC circuits. Fixed coils are wound with fine wire for use as a voltmeter.

But, if the instrument is to be used as an ammeter or wattmeter, then the fixed coils are wound with heavy wire carrying the main current.

 The moving coil is mounted on an aluminium spindle. It is wound either as self sustaining coil or else on a non-metallic former so as to prevent eddy currents. Moving coils are also air-cored.

CONTROLLING TORQUE –

Controlling torque is provided by two control springs. These springs act as leads to the moving coil.

DAMPING –

Air friction damping is provided by aluminium vanes attached to the spindle at the bottom.

DEFLECTING TORQUE –

Let the currents passing through fixed and moving coils be I₁ and I₂ respectively. Since, there is no iron, field strength and hence flux density is proportional to I₁.

                \[\therefore B=K{{I}_{1}}\] (K is constant)

Force on each side of the moving coil$=NB{{I}_{2}}l$

Torque produced on whole of the coil$=NB{{I}_{2}}l\times b=NB{{I}_{2}}A$

Which can be written as, ${{T}_{d}}=NK{{I}_{1}}{{I}_{2}}A$ N-m                                 (By putting B=KI₁ )

$\Rightarrow {{T}_{d}}\propto {{I}_{1}}{{I}_{2}}$

It shows that the deflecting torque is proportional to the product of currents flowing in the fixed and the moving coil.

Since, the instrument is spring controlled; the restoring or controlling torque is proportional to angular deflection$\theta $.

                                                                              

                                            \[{{T}_{c}}\propto {{K}_{2}}\theta \]

As, the deflecting torque is equal to the controlling torque, $\Rightarrow \theta \propto {{I}_{1}}{{I}_{2}}$ . The deflection produced in the instrument is proportional to the product of currents flowing in the fixed and the moving coil.

ADVANTAGES –

• These instruments are free from hysteresis and eddy current losses.
• They can be used on both AC and DC.
• They are used as transfer instruments.

DISADVANTAGES –

• Low torque/weight ratio and hence low sensitivity.
• Costlier than PMMC and moving iron type.
• Non-uniform scale.

INDUCTION TYPE INSTRUMENTS

The operation of induction type instruments depends on the production of torque due to reaction between two magnetic fluxes having some phase difference OR reaction between flux of an AC magnet and the eddy current induced by this flux. These types of instruments are used only foe AC measurements.

Before getting into the constructional and working details of these instruments; let us first have a general look on the torque produced in these instruments.

The torque produced in induction type instruments depends on the two fluxes; the net torque acting on the disc is

                          \[T=K\omega {{\phi }_{1m}}{{\phi }_{2m}}\sin \alpha \]

Where, ${{\phi }_{1m}},{{\phi }_{2m}}$ = maximum fluxes produced by the currents

               $\alpha $ = phase difference between the two fluxes

And if both the fluxes are produced by the same alternating current, then

                                     \[T={{K}_{1}}\omega I_{m}^{2}\sin \alpha \]

Where, I_m= maximum value of current. Therefore, torque is proportional to the square of current for a given value of frequency  and angle . If the disc has spring control, then at some point the controlling torque will be equal to the deflecting torque which will help the disc to attain a steady deflected position. And if the disc is attached to a pointer, then this arrangement can be used for measurement of current.

This was a general discussion about induction type instruments. For the sake of a healthy discussion we will take on these instruments by their types.

Induction type instruments are of two types:

1. Split-phase type
2. Shaded pole type

SPLIT-PHASE TYPE –

The diagram for the split phase type induction instrument is shown here.

SPLIT PHASE TYPE INDUCTION INSTRUMENT

CONSTRUCTION –

In this arrangement, there are two AC magnets M₁ and M₂, which are connected in series. The winding in M₂ is shunted by a resistance R.  The current in the M₂ winding lags with respect to the total line current. This helps to develop the necessary phase angle $\alpha $ between the two fluxes.

DEFLECTION –

If the hysteresis effects are neglected, then deflecting torque is

                      \[{{T}_{d}}\propto {{\phi }_{1m}}{{\phi }_{2m}}\sin \alpha \]

(Where all the signs have their usual meanings as stated before)

Or, ${{T}_{d}}\propto {{I}^{2}}$ (if fluxes are produced by the same current)

Note that, here I is the r.m.s value of current.

Also, ${{T}_{c}}\propto \theta $ (Because spring control is used)

In the final deflected position, T_c=T_d.

 Therefore, deflection ($\theta $)$\propto {{I}^{2}}$

DAMPING –

Eddy current damping is used in this instrument.

SHADED POLE TYPE –

Shaded pole type induction instrument uses a single winding to produce flux. The flux produced by this winding is split up into two fluxes , having phase difference  with respect to each other. The phase difference is usually 40 to 50 degrees and can be varied by varying the size of shading band. This is done by making a narrow slot in the poles of the electromagnet. A copper strip is placed around the smaller of the two areas formed by the slot. This copper shading band acts as a short circuited secondary winding.

CONSTRUCTION –

The constructional features of a shaded pole type induction instrument are shown in the figure below.

The exciting coil is placed on the poles and a current proportional to current or voltage being measured is passed through it. An aluminium disc which is mounted on a spindle is inserted in the air gap of the electromagnet. The spindle carries a pointer and has a control spring attached to it. The controlling torque is provided by this spring only.

DEFLECTION –

As the net driving torque is due to the fluxes and the difference in the phase angle of these fluxes, then deflecting torque can be written as

                         \[{{T}_{d}}\propto {{\phi }_{1}}{{\phi }_{2}}\sin \alpha \]

If are produced by same current I, then ${{T}_{d}}\propto {{I}^{2}}$

As the instrument is spring controlled, ${{T}_{c}}\propto \theta $

For steady deflection, ${{T}_{c}}={{T}_{d}}\Rightarrow \theta \propto {{I}^{2}}$

DAMPING –

Damping is provided by a permanent magnet placed at the opposite side of the electromagnet, so that the disc can be used for production of both deflecting and damping torque.

ADVANTAGES –

• A full scale deflection of over 300 degrees can be obtained.
• Good damping.
• Less effect of stray magnetic fields as the operating fields are large.

DISADVANTAGES –

• Errors are caused due to changes in frequency and temperature.
• Non-uniform scale.
• Large power consumption and high cost.
• Can be used for AC only.

HOT WIRE TYPE INSTRUMENT

The working of a hot wire type measuring instrument is based on the heating effect of the current. These type of instruments are not much in use nowadays because if their instability in working. Nevertheless, they paved the way for further advancements of their own type.

CONSTRUCTION –

HOT WIRE TYPE INSTRUMENT

The adjacent figure shows the constructional features of a hot wire type instrument. It consists of a platinum-iridium wire AB stretched between a fixed end B and tension adjusting screw at A. The platinum-iridium wire used here can withstand oxidation at high temperatures. Now, a phosphor bronze wire CD is attached or hooked up to the centre of AB. This wire CD is further attached to a silk fiber which after passing round the pulley is attached to a spring S.

HOW DOES IT WORK?

When the current is passed through the fine wire AB, it gets heated up and expands. The sag of the wire is magnified and the expansion is taken up by the spring. This causes the pulley to rotate and the pointer to deflect, indicating the value of the current.

The expansion is proportional to the heating effect of the current and hence to the square of the rms value of the current.

DEFLECTION –

The deflection of the pointer is proportional to the expansion of AB which in turn is proportional to I².

Hence, $deflection\propto {{I}^{2}}$

As, spring control is used, ${{T}_{c}}\propto \theta $

                                          \[\therefore \theta \propto {{I}^{2}}\]

These instruments read the rms value of current and their readings are independent of its form and frequency.

DAMPING –

Damping is provided by eddy currents produced in the aluminium disc. This thin disc is attached to the pulley such that its edge moves between the poles of the permanent magnet M.

ADVANTAGES –

• Suited for both AC and DC work.
• Readings are independent of waveform and frequency.
• Unaffected by stray fields.

DISADVANTAGES –

• Sluggish response due to temperature dependent working.
• High power consumption.
• Mechanical shocks.
• They are fragile.
• Inability to withstand overloads.

These disadvantages have made this instrument commercially unsatisfactory. As such, they are now obsolete and have been replaced by thermo-electric instruments.