The devices which measure potential differences in a circuit are called voltmeters. A voltmeter is connected in parallel with the circuit element across which the potential differences are to be measured. The basic principle of an ammeter and voltmeter is the same. Both are current-operated devices i.e. deflecting torque is produced when current flows through their operating coils. In a voltmeter, the deflecting torque is produced by a current that is proportional to the potential difference we wish to measure. Thus the same instrument can be used as an ammeter or voltmeter with proper design.
Symbolic Representation and circuit Diagram
A voltmeter is used to measure the potential differences between two points of a circuit. It is thus connected in parallel with the circuit or some part of the circuit .A voltmeter must have high resistance so that:
- It is not injured by the current that flows through it.
- Power wasted is small as the same is given by
- It will not materially affect the current in the circuit to which it is connected.
Types of Voltmeter
- Permanent-Magnet Moving coil(PMMC)
- Dynamometer Type Instruments
- Moving Iron Voltmeters
- Electrostatic voltmeters
- Induction Type voltmeter
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 Voltmeter
In order to measure large voltages, a suitable high resistance R(called multiplier) is used. A moving coil meter in series with a high resistance is called a voltmeter. The value of series resistance is chosen according to the maximum voltage to be measured.
If we want to read voltage V volts at full-scale using a moving coil meter having full-scale deflection current Ig and resistance G. For this purpose we connect a suitable high resistance R in series with the meter so that current through the meter is Ig.
Form Ohm’s Law,
Since the value of R is large ,the resistance of the voltmeter will be very high. The series resistance has extended the voltage range and also increased the resistance of the voltmeter.
- 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.
Dynamometer Type Instruments
These instruments can be used as ammeters or voltmeters 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 using two spiral hair springs which also provide the controlling torque. Air friction damping is provided using the aluminum vanes that move in sector shaped chamber at the bottom of the instrument.
Working Principle of Dynamometer Type Instruments
For the use as an ammeter or voltmeter ,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 Voltmeter
when the instrument is used as voltmeter, both fixed coils and the moving coils are connected in series together with a high resistance Rs(multiplier) having a negligible temperature coefficient. Therefore current in both the coils is the same and is proportional to the voltage V being measured.
when the instrument is used as an ammeter or voltmeter the deflection proportional to the square of current or voltage being measured. The scale of dynamometer ammeter and voltmeter is not uniform being crowded at the beginning and open at the upper end of the scale.
Moving Iron Voltmeters
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 Voltmeters
The range of a moving iron ac voltmeter is extended by connecting a high resistance(multiplier) in series with it. For ranges higher than 0-750V where power wasted in the multiplier would be excessive, a 0-110V a.c voltmeter is used in conjunction with a potential transformer.
The potential transformer is a stepdown transformer i.e. number of primary turns is more than the secondary turns. The primary of the transformer is connected across the load across which voltage is to be measured. The a.c voltmeter is connected across the secondary. The potential transformer ratio is 20:1,the load voltage is equal to 2o times the reading on the a.c voltmeter.
Load voltage, VL=100*20=2000V
An electrostatic voltmeter is essentially an air condenser, one plate is fixed while the other ,which is coupled to the pointer ,is free to rotate on jeweled bearings. When a p.d to be measured is applied across the plates, the electric force between the plates gives rise to a deflecting torque. Under the action of the deflecting torque ,the movable plate moves and causes the deflection of the pointer to indicate the voltage being measured. This type of instruments is used to measure direct as well as alternating voltages.
There are mainly three types of electrostatic voltmeters:
- Attracted disc type -500V TO 500kv
- Quadrant type 250v to 10KV
- multicellular type 30V to 300V
Attracted disc type voltmeter
Attracted disc type voltmeter consists of two mushroom-shaped plates A and B each mounted on insulated pedestal. The plate B is fixed while the plate A has a movable central portion-the attracted disc. The movable plate A is attached to a horizontal rod which is suspended by two phosphor bronze strips. when p.d to be measured is applied across the plates, the plate A moves towards the fixed plate B and actuates the pointer through a pulley or link mechanism. The control force is provided by gravity and damping force by air dash pot. If the plates are too close together or if the applied voltage is too high ,a spark discharge may occur.so to prevent spark a ballast resistor is included in the circuit. The function of ballast resistor is to limit the current if any sparking-over occurs. If the applied voltage reverses in polarity, there is a simultaneous change in the sign of charge on the plates so that the direction of deflecting force remains unchanged.
The force of attraction F between the charged plates is given by:
x=distance between the plates
C=capacitance between the plates
Since x is always small, dC/dX is practically constant.
Quadrant type voltmeter
Quadrant type voltmeter consists of a light aluminum vane A suspended by a phosphor-bronze string mid-way between two inter-connected quadrant shaped brass plates BB. One terminal is joined to fixed plates BB and the other to the movable plate A. The controlling torque is provided by the torsion of the suspension string. Damping is provided by air friction due to the motion of another vane in a partially closed box.
Working principle Quadrant type voltmeter
When the instrument is connected in the circuit to measure the potential difference an electric force exists between the plates. The movable vane A move in between the fixed plates and causes the deflection of the pointer. The pointer comes to rest at a position where deflecting torque is equal to the controlling torque. Since the force of attraction between the movable plate A and the fixed plates BB is directly proportional to (p.d) square ,the instrument can be used to measure either direct or alternating voltages. When used in an a.c circuit it reads the r.m.s values.
The capacitance C between the plates depends upon deflection θ i.e. upon the position of the movable plate A. The applied alternating voltage is v.
If dC/dθ were constant ,then,
Multicellular electrostatic voltmeter
Quadrant type voltmeter has deflecting torque very small for low voltage. This type of instruments cannot be used to measure voltages below 250V. Multicellular voltmeter overcomes the difficulty of Quadrant type voltmeter.
Quadrant type voltmeter has ten moving vanes instead of one and eleven fixed plates forming “cells” in and out of which the vanes move. The moving vanes are fixed to a vertical spindle and suspended by a phosphor-bronze wire so that the vanes are free to move, each between a pair of fixed plates .At the lower end of the spindle ,an aluminum disc hangs horizontally in an oil bath and provides damping torque due to fluid friction. The controlling torque is provided by the torsion of the suspension wire as the moving system rotates. The upper end of the suspension wire is attached through a coach spring S to a torsion head H. The torsion head is provided with a tangent screw for Zero adjustment. The function of the coach spring is to prevent the suspension wire from breaking when accidently jerked.
The moving vanes be jerked downward, then the coach spring yields sufficiently to allow the safety sleeve E to come into contact with the guide stop G before the suspension wire is over strained. The scale is horizontal if the pointer is straight but the indications can be given on a vertical scale by bending the pointer at right angles.
Range Extension of Electrostatic Voltmeters
Range extension of voltmeter can be increased by the use of multipliers. There are mainly two types of multipliers employed for this:
- Resistance potential divider -for ranges up to 40 kV
- Capacitance potential divider-for ranges up to 1000kV
Resistance potential divider -for ranges up to 40 kV:
It consists of a high resistance with tapping’s taken off at intermediate points. The voltage V to be measured is applied across the whole of the potential divider and the electrostatic voltmeter connected across part of it. Since the voltmeter practically carries no current ,the p.d v across it is in the same fraction of the applied voltage V as the resistance across it is of the whole resistance (R) i.e
Thus if the voltmeter is connected across 1/5 of the whole resistance then voltage V to be measured is 5 times the reading of the voltmeter. The advantage of this method is that there is no shunting effect of the voltmeter. The drawback is that there is power loss in the resistance divider.
Capacitance potential divider-for ranges up to 1000kV:
It consists of a single capacitor of capacitance C connected in series with the voltmeter and the whole circuit is connected across the voltage V to be measured. Let v volts be the reading of the voltmeter. Since the voltage across a capacitor is inversely proportional to its capacitance,
Using the different capacitances different voltages ranges can be obtained. This method has advantage that the circuit consumes no power. The drawbacks is that capacitance current taken is greately increased.
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 voltmeter current proportional to the voltage to be measured is passed through the operating coil,
The instrument is spring controlled,
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.