Moving-Iron Instruments (Ammeters and Voltmeters)

Iron instruments are commonly available in laboratories and switch boards as they have the required accuracy and are available at a cheap price. There are two types of moving iron instruments, namely:

1. Attraction Type
2. Repulsion Type

Attraction type repulsion is in which the soft iron sheet gets attracted towards the solenoid and in the case of repulsion type the soft iron gets repelled away from the solenoid.

   4.1. Attraction type

The figure below represents the Attraction type iron instruments. The sectional front and the elevated iron type instrument is represented. It contains a coil or solenoid and a disc, which is oval. Attached to this the pointer of iron named as P. This is deflected easily with the help of a scale. The soft iron instrument is especially made of a sheet that allows it give a shape as uniform as possible.

When the solenoid receives the current, a magnetic field is created inside the coil. This in turn makes the iron a magnet. Resultantly, the soft iron gets attracted towards the coil making it rotate and spindle. The V vane provides damping that is attached with the spindle and moves in the air.

   4.2. Repulsion type

In the diagram below, two irons are attached, one is mounted and the other one is attached.  The irons are in between the magnetic field because of the presence of the coil. When no current is passed by the coil the two irons almost coincide with each other. However, when the current is passed, the iron coils gets magnetised in the same direction. This means that they are repelling against one another. These type of instruments generally come with air friction damping and spring control.

In a commercial setting, the first iron can be arranged in the form of a thin curved plate and the other iron can be arranged in the form of a curved sheet. This helps in giving longer and more stable set up.

Deflecting Torque:

In both the types of iron movements, it was found that the deflection of the irons was directly proportional to the current square.

Thus, the iron movements can be used to calculate both alternating current and direct current.

   4.3. Advantages and disadvantages of moving-iron instruments

• Advantages are as follows:
• Usable in both AC and DC currents
• Simple in design and construction
• Good operating torque
• Can bear overload for sometime
• They are cheapest as the parts used are simple
• Best suited for high power circuits
• Capable for proving precise results and proper grades

Disadvantages are as follows:

• The scales are not stable
• The power consumption is higher for low voltage range
• Errors are caused as a result of overstress in the operation
• Change in frequency, causes grave mistakes
• The stiffness increases with the rise in temperature.

  4.4. Sources of errors

Hysteresis: In the soft iron part, an error may occur due to hysteresis. This means that very high values are recorded because of increasing current and very low readings are recorded with the decrease in the current.

Stray Field Errors: Stray magnets affect the correct functioning of the instrument. Shielding of magnet can be done by covering the case of iron cast

AC errors: Due to change in frequency, many errors are produced such as:

Change in the coil impedance

Change in eddy current magnitude. In ammeters, this error is negligible as the external circuit helps in determining the coil current.

5. Moving-Coil Instruments

There are two types of coil instruments:

• Permanent Type of magnet that is useable in the DC only
• Dynamometer that is useable in both DC and AC.

  5.1. Permanent-magnet moving coil type (PMMC) instruments

A PMMC instruments works on the principle that, “when a current-carrying conductor is placed in a magnetic field, it is acted upon by a force which tends to move it to one side and out of the field.” The instrument comprises of the permanent magnet and an insulated copper that is fitted to the steels that are polished. The magnet is made of soft iron steel.  The main functions of the central core include:

• Intensifying the fields of magnet by decreasing the length of the gap of air
• To provide uniform density by allowing the scale to be divided equally

The springs of both the coil is spiralled in repelling directions. When current is passed via the coil a deflection torque is produced.

Advantages:

• Consumption of power is low
• Scales are stable
• No loss of hysteresis
• High Torque
• Current damping and effective damping
• With the help of shunts range can be extended
• There is absence of stray magnetic field

Disadvantages:

• Expensive as compared to moving iron
• Not applicable for AC measurements
• There are chances of increased errors

The following are the instruments that are used for increasing the range of instruments:

• Multipliers
• Shunts
• Transformers
• Potential Transformers

Ammeter Shunt: This shunt is a low resistance shunt that is placed with the coil circuit. The greater part of the current in the main region is spread around the coil. The shunt has four terminals that is connected across the terminals. If this was connected to the current of terminal, the chances of error might increase as the resistance of these terminals is lower as compared to the shunt.

 5.2. Electrodynamic or dynamometer instruments

In this instrument, the operating field is created not by permanent magnet but fixed coil. This has usage as a voltmeter or ammeter but is mostly used as wattmeter. They mostly consist of wire coil that carries currents. The air coated wire prevents error of hysteresis. The fixed coil is divided into two parts one is parallel to the other and the other is close.

Deflecting Torque:  This is created due to involvement of the magnetic currents produced with the help of currents.

Advantages are as follows:

• Is useable both as AC and DC
• Free from error of hysteresis
• It is easy to construct ammeters that range up to 10A

Disadvantages are as follows:

• Sensitivity of the instrument is low
• The scale of the instrument of not uniform
• The cost of the related instruments are higher as compared to the moving iron instruments
• Higher losses because of friction

6. Rectifier Instruments

using of D Arsonval along with the rectifier to change AC to DC are called rectifier instruments. Thus, a direct current can be flown with the help of alternating current. Using rectifier type instruments, means providing low frequencies giving good indication of 20 Khz.

Features:

1. The AC measurements is provided in economical and easy means in communication and radio circuits.
2. A rectifier instrument is very sensitive even more than an electrodynamometer or a moving iron
3. They are measured in linear scales
4. They have sensitivity of more than 1000 to 2000 ohms per volt
5. The rate of power consumed is mostly high as compared to other instruments
6. The instruments are named as micro ammeters and milli ammeters.
7. Shunting in these instruments is not practical as there is a change in both the amount and temperature of current.

Advantages of Rectifier Instruments:

• The main advantage of rectifier instrument is that it is more sensitive as compared to other voltmeters. Thus, it is suitable for measuring AC voltages
• These instruments can be included in any kind of ammeter in combination with the coil milli ammeter.

7. Wattmeters

A wattmeter consists of two coils that is a combination of a voltmeter and an ammeter. The two coils are pressure coil and current coil. Due to the interaction of fluxes, the operating torque is produced. There are three types of wattmeter:

• Dynamometer
• Induction
• Electrostatic

7.1. Dynamometer wattmeter

A dynamometer is combined to a wattmeter and can be used as a wattmeter. If the coils are combined then the current proportional flows to one coil and the value proportional flows to the other. The indication relies on the two magnetic fields and the strength of the magnetic field is relied on the value of the current flowing. The calibration of the meter can be done in watts. For circuits flowing with fluctuating torque, the instant torque is directly related to the power.

Errors:

Due to the inductance of the voltage coil, the errors may creep in. The error in the indicated power may be because of drop in the voltage current and the voltage coil taken by the current.

However, the errors can be overcome easily by having a winding connected in series and placed in opposite position of the coils.

Ranges:

For transformers the circuit current range is 0.25 to 100

Potential circuit range is 5 to 750 v.

Advantages:

• The scale is stable as the true power is proportional to the deflecting torque.
• The accuracy from the instrument is very high and it can be used for the purpose of calibration

Disadvantages:

• The error that may result due to the inductance of pressure coil can be very serious in nature.
• The reading of the instrument may affect stray field. Magnetic shielding can be provided in this case.

7.2. Induction wattmeters (IW)

This instrument can be used only in the AC circuits and are best working when the frequency and supply of the voltage are constant.

The operation of the IW relies on the formation of torque and the reaction that takes place between the flux and the eddy currents. Knowing that the magnitude of the eddy currents depends on the flux, the constant value of the torque is directly related to the square of the voltage. The value of the deflecting torque is directly related to the square of the current.

Induction wattmeter contains two electromagnets. The first one is excited and the other is connected to the main circuit when connected to a series of magnet. The exciting coil is parrelly arranged with the circuit. The eddy currents are produced on the disk and the copper rings are fixed on the limb of the magnet that is shunt. The spring of this instrument is connected to a spindle.

Advantages:

• Long scale value
• Free from the effects of stray fields
• Good damping
• Free from errors

Disadvantages:

Subjected to serious temperature errors

8. INTEGRATING METERS (ENERGY METERS)

Integrating meters ate used to calculate the quantity of electric energy supplied to any circuit at a given time. There is no direct power indication as to which energy is supplied as the registrations are autonomous of the rate at which the energy is being used up.

The major difference between a wattmeter and an energy meter is that in an energy meter instant readings can be taken by connecting it with some kind of registration mechanism while in the case of wattmeter the value is read.

     8.1. Essential characteristics of energy meters

The important characteristics of integrating meters are as follows:

1. They are simple to use and understand and do not consist of any part that gets spoiled easily.
2. The readings can be set in dials and they do not take into consideration the multiplying factors
3. The meter’s casing is watered and dusty and free from insects
4. To attain permanent calibration, it is important that the friction and the torque friction brakes stay stable. The magnets should be placed in such a way so that the strength of the field is not affected by the coil.
5. The loss via friction should be very less and it should remain constant over long period of time
6. There should be less friction in the device. The torque’s meter should be high so that the errors gets rectified
7. The accuracy of the energy meter should be maintained
8. The loss of energy in the meter should be small

• Types of energy meters

The types of energy meters are as follows:

• Electrolytic Metres
• Motor Metres
• Clock Metres

    8.3. Motor meters

Motor metres can be used in both ac and dc. The following are regarded as essential parts in the motor meters:

1. Torque system as it helps in the production of a torque and causes the system to move continuously.
2. A braking device as this induces magnetic current that will in turn produce braking torque.
3. Revolution Registering Device as this helps in the driving of wheels and registers the same in ampere or watt.

Types of Motor Meters:

• Mercury Motor
• Commutator Motor
• Induction Motor

8.4. Motor-driven meter-watt-hour meter

The motor driven consists of a small motor and a disc made of aluminium. The winding is connected in serial order and the strength of the field is related to the load of current.

The armature is connected to the source and this is also directly proportional to the line voltage. The torque here is directly proportional to the consumption of power.

The armature speed is controlled by the aluminium speed. The disc creates a magnetic field and the force increases the rotational strength of the disc. The force is created by the conductor cutting through lines of fore. At very light loads, the meter should overcome the friction and the indicators. A segment of the field is created by the current. This coil is wrapped to help the field and overcome the friction of meter.

 8.5. Induction type watt-hour meter

The advantages of induction type meter are:

Easy to operate

High Torque

Inexpensive

Proper Registration at even low power

Unaffected by variations in the temperature

Provides accurate calibration

Single Phase Energy Meters: This are considered as the best of all the meters. The measure of this type of meter is done in KWH.

This metre works as the same principle of watt meters. In this case, the brake magnet is employed to the spindle.

The construction of this type of meter is shown in the diagram below it comprises of the following:

Series Magnet

Shunt Magnet

Brake Magnet

Rotating Disc

A pulsating character of magnetic field is produced in the electromagnet. There is production of eddy currents and there is lack of driving force. Phase displacement of 90 degrees is conducted to create a magnetic field. The reaction within the eddy currents and the magnetic fields built up a torque.

Sources of Errors:

The following are the sources of errors:

The fluxes do not have correct magnitude. This gives rise to unwanted load.

The phase relation of the fluxes are not correct. This may lead to abnormal frequencies.

Unsymmetrical structure of the disc. There may be continuous rotation in the disc.

Changes in the resistance power of the disc. This may happen due to change in temperature.

 

Links of Previous Main Topic:-

• Current Electricity Basic Concepts
• Introduction to Alternating Current
• Introduction Three Phase A C Circuits
• Magnetic Field
• General Aspects
• Elementary Theory of Ideal Transformer
• General Aspects Polyphase Induction Motors
• Single Phase Motors
• Characteristics of D C Generators
• Measuring Instruments
• Moving Iron Instruments Ammeters and Voltmeters

Links of Next Electrical Engineering Topics:-

• Measurement of Resistance
• Power Supply System