The devices which measures current in a circuit are called ammeters. An ammeter is connected in series with the circuit element whose current is to be measured. The basic principle of the ammeter is that the deflecting torque is produced when current flows through their operating coils. In ammeter the deflecting torque is produced by the current to measured or a certain fraction of that current.
Representation and connection of Ammeter
Basically ammeter is connected in series with the circuit element whose current we wish to measure.
An ammeter is used to measure current in a circuit. It is thus connected in series with the circuit under test so that current to be measured or a fraction of it passes through the instruments itself. Its resistance must therefore be as small as possible:
- Since power wasted in the instruments is given by
where Im is the meter current and Rm is the meter resistance.
- so that it does not increase the resistance of the circuit into which it is inserted.
Types of Ammeter
The following are the types of ammeters:
- Permanent-magnet moving coil type
- Dynamometer type
- Moving-iron type
- Induction Type
1.Permanent-magnet moving coil(PMMC)
It is only suitable for d.c work only. It is based on the principle that when a current carrying coil is placed in a magnetic field, torque acts on the coil. Permanent magnet moving coil consists of a light rectangular coil of many turns of fine wire wound on an aluminum former inside which is an iron core. The coil is delicately pivoted upon jewel bearings and is mounted between the poles of a permanent horse-shoe magnet .Attached to these poles are two soft-iron pole pieces which concentrate the magnetic field. The current is led into and out of the coil by means of two control hair-springs, one above and the other below the coil. These springs also provide the controlling torque. The damping torque is provided by eddy currents induced in the aluminum former as the coil moves from one position to another.
Working of Permanent-magnet moving coil
When the instruments is connected in the circuit to measure current or voltage the operating current flows through the coil. since the coil is carrying current and is placed in the magnetic field of the permanent magnet, a mechanical force acts on it. As a result, the pointer attached to the moving system moves in a clockwise direction over the graduated scale to indicate the value of current or voltage being measured .If the current in the coil is reversed, the deflecting torque will also be reversed since the direction of the field of the permanent magnet is the same. As a result the pointer will try to deflect below zero. Deflection in this direction is prevented by a spring. since the deflecting torque reverses with the reversal of current in the coil it is used to measure direct currents.
The magnetic field in the air gap is radial due to the presence of soft-iron core. Therefore, when operating current flows through the coil, a constant torque Td acts on the coil and given by:
Since the values of B,N and A are fixed,
Td ∝ I
The instrument is spring controlled so that Tc ∝ θ.Pointer comes to rest when Td=Tc.
θ ∝ I
Since the deflection is directly proportional to operating current. Hence such instruments have uniform scale.
Range extension of PMMC ammeter
To measure a large currents a suitable low resistance S called shunt is connected in parallel with the instrument. The shunted moving coil instrument is called an ammeter. The value of shunt is chosen according to the maximum current to be measured.
To measure I amperes at full-scale using a moving coil instrument having full-scale deflection current Ig and resistance G. when the circuit current is I ,current Ig flows through meter. The current I-Ig flows through shunt S.
The potential difference across the shunt is the same as across the meter,
The value of Rm will be less than S. Since the value of S is very small, the ammeter resistance will be very low. Shunt has extended the current range and also lowered the resistance of the ammeter.
- Used for the measurement of direct currents and voltages.
- D.C galvanometer is used to measure small currents.
- Ballistic galvanometers are used mainly for measuring changes of magnetic flux linkages.
2.Dynamometer Type Instruments
These instruments can be used as ammeter or voltmeter but are generally used as wattmeter. These are suitable for d.c as well as a.c work. The operating principle of such instruments is that mechanical force exists between the current carrying conductors.
Dynamometer consists of a fixed coil and a moving coil. The fixed coil is split into two equal parts(F,F)which are placed close together and parallel to each other. The moving coil(M) is pivoted in between the two fixed coils and carries a pointer. The current is led into and out of the moving coil by means of two spiral hair-springs which also provide the controlling torque. Air friction damping is provided by means of the aluminum vanes that move in sector shaped chamber at the bottom of the instrument.
Working Principle of Dynamometer Type Instruments
The fixed coils(FF) and moving coil M are connected together so that the same current flows through the two coils. Mechanical force exists between the coils due to the flow of current. Hence moving coil M moves the pointer over the scale and pointer comes to the rest at a position where deflecting torque is equal to the controlling torque. since the polarity of the fields produced by both fixed and moving coils is reversed by the reversal of current, the deflection of moving system is always in the same direction regardless of the direction of current through the coils.
Since the instruments is spring-controlled, the controlling torque is proportional to the angular deflection,θ i.e
Tc ∝ θ
In the steady state position of deflection,
Thus the deflection(θ) is directly proportional to the product of currents in the fixed coils and the moving coils.
Range extension of Dynamometer ammeter
when the instrument is used as an ammeter ,the fixed coils and the moving coils are connected in series so that the same current flows through the two coils.
For the measurement of large currents, the moving coil is shunted being in series with the fixed coils .The fixed coils carry the main current while the moving coil carries a current proportional to the main current.
3.Moving Iron Ammeters
It is used for measurement of alternating currents and voltages though it can be used for d.c measurements. Mainly there two types of moving-iron instruments i.e
- Attraction type
- Repulsion type
Attraction type moving iron instrument consists of a cylindrical coil or solenoid which is kept fixed. An oval-shaped soft-iron is attached to the spindle in such a way that it can move in and out of the coil. A pointer is attached to the spindle so that it is deflected with the motion of the soft-iron piece. The controlling torque is provided by one spiral spring arranged at the top of the moving element. The damping torque is provided by an aluminum vane, attached to the spindle ,which moves in a closed air chamber.
Working principle of Attraction type
When the instrument is connected in the circuit to measure current or voltage, the operating current flowing through the coil sets up a magnetic field. The result is that the pointer attached to the moving system moves from Zero position. The pointer will come to rest at a position where deflecting torque is equal to the controlling torque. If the current in the coil is reversed, the direction of magnetic field also reverses and hence the magnetism is produced in the soft-iron core.
The force F pulling the soft-iron piece towards the coil is directly proportional to
- field strength H produced by the coil
- Pole strength m developed in the iron piece
If the permeability of iron is assumed constant, then,
H ∝ i where i is the instantaneous coil current.
since the deflection is proportional to the square of coil current, the scale of such instruments is non-uniform; being crowded in the beginning and spread out near the finish end of the scale.
Repulsion type consists of two soft-iron pieces or vanes surrounded by a fixed cylindrical hollow coil which carries the operating current. One of these vanes is fixed and the other is free to move. The movable vane is of cylindrical shape and is mounted axially on a spindle to which a pointer is attached. The fixed vanes, which is wedge-shaped and has a larger radius, is attached to the stationary coil. The controlling torque is provided by one spiral spring at the top of the instrument. Damping is provided by air friction due to the motion of a piston in an air chamber.
Working principle of Repulsion Type
When the current to be measured flows through the coil, a magnetic field is set up by the coil. This magnetic field magnetizes the two vanes in the same direction i.e similar polarities are developed at the same ends of the vanes. since the adjacent edges of the vanes are of same polarity, the two vanes repel each other. As the fixed vanes cannot move, the movable vane deflects and causes the pointer to move from Zero position. The pointer will come to rest at a position where deflecting torque is equal to the controlling torque provided by the spring.
If the current in the coil is reversed, the direction of deflecting torque remains unchanged. The reversal of the field of coil reverses the magnetization of both iron vanes so that they repel each other regardless of which way the current flows through the coil .In such, instruments can be used for both d.c and a.c applications.
If the permeability of iron is assumed constant, then,
H ∝ i where i is the coil current.
since the deflection is proportional to the square of coil current as in the case with attraction type moving-iron instrument.
Range Extension of Moving-Iron Instruments
Here shunts are not used to extend the range of moving-iron a.c ammeters. It is because the division of current between the operating coil and the shunt varies with frequency. The range is extended by the following methods:
- By changing the number of turns of the operating coil. For example ,suppose that full-sacle deflection is obtained with 400 ampere-turns. For full-scale reading with 100A,the number of turns required would be 400/100=4.Similarly,for full-scale reading with 50A,the number of turns required is 400/50=8.Thus the ammeter can be arranged to have different ranges by having different number of turns on the coil. The usual ranges by this method are 0-250A.
- For ranges above 0-250A a current transformer is used in conjunction with 0-5 A a.c ammeter. The current transformer is step-up transformer i.e number of secondary turns is more than the primary turns. The primary of this transformer is connected in series with the load and carries the load current. The a.c ammeter is connected across the secondary of the transformer. The current transformer ratio is 10:1 ,it means that the line current is equal to 10 times on the a.c meter.
Load current, IL=3*10=30A
Induction type can be used for a.c measurement only. It can be of shaded-pole type or split-phase type.
- Shaded-pole Type
- split-phase Type
Shaded-pole type consists of a specially shaped aluminum disc coupled to a pointer and suspended in jeweled bearings. Disc passes through two air-gaps. The first located in an electromagnet having a shading coil and second in a permanent magnet(provides the necessary damping torque).Controlling torque is provided by a spiral spring attached to the moving system.
when shaded-pole type is used as an ammeter the current to be measured or a part of it passed through the operating coil of the instrument. since both the fluxes are produced by the same alternating current I(rms value).
In split-phase the winding of the two electromagnet A and B are connected in parallel across a single phase supply an inductive coil L in series with one and a resistance R in series with the other. The values of R and L are selected that the currents through the two windings have a phase difference of nearly 90º.This produces the deflecting torque on the aluminum disc. The permanent magnet provides the necessary damping torque. The controlling torque is provided by a spiral spring attached to the moving system.
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