19 Analog Ammeters and Voltmeters

Vinay Gupta

    Introduction

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 current to be measured or by a definite fraction of it. In a voltmeter this torque is produced by a current which is proportional to the voltage to be measured. Thus, all 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.

Ammeters are connected in series in 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.

Voltmeters are connected in parallel with the circuit whose voltage is to be measured. They should have a high electrical resistance. This is essential in order that the current drawn by them is small and consequently the power absorbed is small.

Power Loss

Let Ra be the resistance of the ammeter and I be the current being measured.

Power loss in ammeter Pa = I2 Ra watt

If Rv be the resistance of the voltmeter and V be the voltage being measured,

Power loss in voltmeter= V2/Rv  watt

Thus in order that the power loss in instruments is small, resistance of ammeters should be small and that of voltmeters should be large.

Types of instruments.

The main types of instruments used as ammeters and voltmeters are:

(I)  Permanent magnet moving coil (PMMC) (ii) Moving iron (iii) Electrodynamometer (iv) Hot wire (v) Thermocouple (vi) Induction (vii) Electrostatic (viii) Rectifier.

Of these the permanent-magnet moving-coil type can be used for direct-current measurements only, and the induction type for alternating-current measurement only. The other types can be used with either direct or alternating current.

The moving iron and moving coil types both depend for their action upon the magnetic effect of current. The former is the most generally used form of indicating instrument, as well as the cheapest. It can be used for either direct alternating-current measurements and, if properly designed is very accurate. The permanent-magnet moving-coil instrument is the most accurate type for direct-current measurements, and instruments of this type are frequently constructed to have substandard accuracy.

Electrodynamometer type of instruments are used both on a.c. as well as on d.c. Their calibration for both d.c. and a.c. is the same and hence they are very useful as “transfer instruments”.

Thermal instruments have the advantage that their calibration is the same for both d.c. and a.c. They are particularly suited for alternating-current measurements since their deflection depends directly upon the heating effect of the alternating current, i.e. upon the rms value of the current. Their readings are thus independent of the frequency or wave-form of the current, and of any stray magnetic fields which may exist in their vicinity.

As voltmeters, electrostatic instruments have the advantage that their power consumption is exceedingly small. They can be made to cover a large range of voltage and can be constructed to have substandard accuracy. Their main disadvantage is that the electrostatic principle is only directly applicable to voltage measurements.

The induction principle is more generally used for watt-hour meters than for ammeters and voltmeters owing to the comparatively high cost, and inaccuracy of induction instruments of the latter types.

Errors in Ammeters and Voltmeters

There are certain errors which occur in most types of instruments, while other errors occur only in those of particular type.

Of the errors common to most types of instruments, friction and temperature errors are perhaps the most important. To reduce the effect of friction torque, and consequently the error produced by it the weight of the moving system must be made as small as possible compared with the operating force, i.e. the ratio of torque to Weight must be large (about 1/10 for full deflection).

A vertical spindle is generally preferred to a horizontal one from the point of view of a small friction torque.

The most serious error is produced by the heat generated in the instrument, or by changes in ambient (room) temperature, is that due to a change in the resistance of the working coil. Such a change or resistance is of little importance in ammeters, but in voltmeters, in which the working current should be directly proportional to the applied voltage, it is essential that the resistance of the instrument remains as nearly constant as possible.

Thus, the power loss in the instrument should be small, and resistance coils which are likely to produce appreciable heating should be mounted, if possible, in such a position that they are well ventilated. In order to eliminate temperature errors, the working coil is wound with copper wire and is of comparatively low resistance. A high ”swamping” resistance of material whose ‘resistance temperature’ coefficient is small, is connected in series with the coil, so that, although the resistance of the coil may change considerably, the change in total resistance is small.

Other errors resulting from heating may be caused by expansion of the Control spring, or of other parts of the instrument, although such errors are usually small. Lack of balance in the moving system and changes in the strength of permanent magnets (if used) are other possible sources of error which are common to several types of instruments.

Permanent Magnet Moving Coil Instruments (PMMC)

The permanent magnet moving coil instrument is the most accurate type for d.c measurements. The working principle of these instruments is the same as that the d’ Arsonval type of galvanometer the difference being that a direct reading Instrument is provided with a pointer and a scale.

Construction

The general constructional features of this instrument are shown in Figure.

Moving Coil: The moving coil is wound with many turns of enameled or silk covered copper wire. The coil is mounted on a rectangular aluminium former which is pivoted on jewelled bearings. The coils move freely in the field of a permanent magnet. Most voltmeter coils are wound on metal frames to provide the required electro-magnetic damping. Most ammeter coils, however, are wound on nonmagnetic formers, because coil turns are effectively shorted by the ammeter shunt. The coil itself, therefore, provides electro-magnetic damping.

Fig. Permanent magnet moving coil instrument.

Magnet Systems: There has been considerable development in materials for permanents magnets and, therefore, magnet assemblies have undergone a lot or change in the recent past. Old styIe magnet systems consisted of a relatively long U shaped permanent magnet having soft iron pole pieces. Owing to development of material like Alcomax and Alvico, which have a high coercive force, it is possible to use smaller magnet lengths and high field intensities. Thus in small instruments it is possible to use a small coil having small number of turns and hence a reduction in volume is achieved. Alternatively in instruments having a large scale length it is possible to increase the air gap length to accommodate large number of turns.

Control: When the coil is supported between two jewel bearings the control torque is provided by two phosphor bronze hair springs. These springs also serve to lead current in and out of the coil. The control torque is provided by the ribbon suspension. This method is comparatively new and is claimed to be advantageous as it eliminates bearing friction.

Damping: Damping torque is produced by movement of the aluminium former moving in the magnetic field of the permanent magnet.

Pointer and Scale: The pointer is carried by the spindle and moves over a graduated scale. The pointer is of light-weight construction and, apart from those used in some inexpensive instruments, has the section over the scale twisted to form a fine blade. This helps to reduce parallax errors in the reading of the scale. In many instruments such errors may be reduced further by careful alignment of the pointer blade and its reflection in the mirror adjacent to scale. The weight of the instrument is normally counter balanced by weights situated diametrically opposite and rigidly connected to it.

Torque Equation:

The torque for a moving coil Instrument is given by

Deflection torque is Td = NB I dl = GI

where G = a constant = NB Id

The spring control provides a restoring (controlling) torque Tc=KӨ

where K=spring constant.

For final steady deflection Tc = Td or GI = KӨ

:.  Final steady deflection Ө=GI/K

or current l = KiӨ

where Ki = K/G a constant

Shape of Scale: As the deflection is directly proportional to the current passing through the meter, we get a uniform (linear) scale for the Instrument.

Non-linear Scales: The conventional permanent magnet moving coil mechanism uses a core and pole pieces having concentric faces. This means that the field is uniform and radial throughout the air gap. When the current being measured is directly passed through the meter, the instrument shows a uniform scale.

Range

D.C. Ammeter

(l) Instrument alone, 0-5 μA to 0-20 mA. (2) With internal shunts, upto 0-200 A. (3) With external shunts upto 0-5000 A.

D.C. Voltmeter

(I) Instrument alone, 0-50 or 0-100 mV. (2) With series resistance upto 20,000 to 30,000 V.

In micro-ammeters and low range milli-ammeters upto about 20 mA, the entire current to be measured is sent through the moving coil. This is because instrument springs serve as current leads to the moving coil. Their current carrying capacity limits the current which can be safely carried to about 20 mA. For higher currents (usually above 20 mA) the moving coil is shunted to bypass current around the coil and the spring.

It may be mentioned here that ammeters or range lower than 25 μA are not likely to be robust on account of extremely delicate construction required for proper sensitivity. Thus Instruments of range lower than 25 μA are normally not manufactured. Voltmeters or all ranges use a moving coil together with sufficient series resistance (known as multiplier) to limit the instrument current to the desired value.

D.C. ammeters are normally designed to have a voltage drop of nearly 50 mV to 100 mV for full scale deflection. Most d.c. voltmeters are designed to produce full scale deflection with a current of 10, 10, 5 or 1mA. Normally a value of 1 mA is used.

Thus, excluding low range current measuring instruments, most d.c. ammeters are actually 50 mV (or 100 mV) millivoltmeters operated with a suitable shunt, while voltmeters are low range milli-ammeters operated with a suitable series resistance.

Questionnaire

1. Ammeters are connected in series in the circuit whose current is to be measured. (True/False)
2. Voltmeters are connected in series in the circuit whose voltage is to be measured. (True/False)
3. Power loss in ammeter is given by ______.
4. Power loss in voltmeter is given by ______.
5. Describe the construction of Permanent Magnet Moving Coil Instruments (PMMC).

    References:

1. Electronic Measurements and Instrumentation by Bernard M. Oliver and John M. Cage.
2. Measurement and Instrumentation Principles by Alan S. Morris.
3. Instrumentation and Measurement in Electrical Engineering by Roman Malaric.
4. Measurement and Instrumentation Systems by William Bolton.
5. Engineering Measurements and Instrumentation by Leslie Frank Adams.
6. Electrical Measurements and Instrumentation by U. A. Bakshi.
7. Introduction to Measurements and Instrumentation by Arun K Ghosh.