Electrical Measurements, types of Electrical Instruments & Different Types of Voltmeter and Ameter

Basic Electical Technology Assignment, Given By Sir Muhammad Rashid Memon

What is Electrical Measurements?

Introduction

Electrical measurements often come down to either measuring current or measuring voltage. Even if you are measuring frequency, you will be measuring the frequency of a current signal or a voltage signal and you will need to know how to measure either voltage or current. In this short lesson, we will examine those two measurements - starting with measuring voltage. However, first we should note a few common characteristics of the meters you use for those measurements. Many times you will use a digital multimeter - a DMM - to measure either voltage or current. Actually, a DMM will also usually measure frequency (of a voltage signal) and resistance. You should note the following about typical DMMs. Polarity is important. Usually the terminals of the DMM will be coded to indicate

polarity. Often that polarity is indicated by a red terminal (positive) and a black terminal (negative). In other cases, the polarity could be indicated by printed notes on the terminals.

Often one of the terminals on the DMM may be connected to the ground. That would normally be the black terminal, or it may be indicated with a ground symbol.

Measuring Voltage

Voltage is one of the most common quantities measured. That's because many other variables - like temperature, for example - are measured by generating a voltage with a sensor. So, even if you want to measure temperature you might end up having to measure a voltage and convert that reading into the temperature reading you wanted. Voltage is measured with a voltmeter. However, digital multimeters (DMMs) - which can function as voltmeters - often have considerably more capability and can measure current, resistance and frequency. And, there are other instruments - like oscilloscopes - that measure voltage and should be thought of as voltmeters. No matter what the instrument is, if it measures voltage you have to treat the instrument as a voltmeter. When you measure voltage you have to remember that voltage is an across variable. When you measure voltage you have to connect the voltmeter to the two points in a circuit where you want to measure voltage. Here is a circuit with a voltmeter connected to measure the voltage across element #4.

Note the following about this measurement. Notice that the voltmeter measures the voltage across element #4, +V4. (And, the plus

sign is important. Remember the polarity issue.) Notice the polarity definitions for V4, and notice how the red terminal is connected to the

"+" end of element #4. If you reversed the leads, by connecting the red lead to the "-"

Submitted By: Izhar Mithal Jiskani – 09 MN 98


terminal on element #4 and the black lead to the "+" end of element #4, you would be measuring -V4.

And, remember this as well.

When you measure voltage, the voltmeter should not disturb the circuit where you are attempting to measure the voltage. In the circuit above, that disturbance is the current drawn by the voltmeter. You want that current to be as close to zero as it can possibly be. That means that you need to have the resistance of the voltmeter as large as possible. There's more discussion of that effect in the lesson on measuring voltage. Ideally, the resistance of a voltmeter would be infinite.

There are numerous different instruments, and we have a separate lesson just on measuring voltage that discusses some of those instrument, and you can get to that lesson by clicking here.

Measuring Current Current is measured with an ammeter. While voltage is a more common measurement, it is often necessary to measure current. When measuring current, it is important to remember that current is a flow variable. Current flows through electrical elements, and if you want to measure current you have to get it to flow through the ammeter. Here's the same circuit we used in the example above. Consider what we would have to do to measure the current flowing through element #4.

If we want to measure the current through element #4, we have to get that current to flow through the ammeter. Here's a way to insert an ammeter into the circuit to measure that current. However, this doesn't give the whole picture. Remember that polarity is important. In the circuit the polarity for the voltage across element #4 is defined, but the current polarity is not defined. In the diagram below, we have defined the direction of that current, and given it an algebraic name, Im.

As with the voltmeter, you need to pay attention to the polarity, and you also want to remember this. When you measure current, the ammeter should not disturb the circuit where you are

attempting to measure the current. In the circuit above, that disturbance is the voltage across the ammeter. You want that voltage to be as close to zero as it can possibly be. That means that you need to have the resistance of the voltmeter as small as possible. Ideally, the resistance of an ammeter would be zero.


http://www.facstaff.bucknell.edu/mastascu/eLessonsHtml/Measurements/MeasVolt.html

http://www.facstaff.bucknell.edu/mastascu/eLessonsHtml/Measurements/MeasVolt.html


Different types of Electrical Instruments

Measuring Instruments:

Measuring instruments are classified according to both the quantity measured by the instrument and the principle of operation. Three general principles of operation are available:

(i) electromagnetic, which utilizes the magnetic effects of electric currents;

(ii) electrostatic, which utilizes the forces between electrically-charged conductors;

(iii) electro-thermic, which utilizes the heating effect.

Electric measuring instruments and meters are used to indicate directly the value of current, voltage, power or energy. In this lesson, we will consider an electromechanical meter (input is as an electrical signal results mechanical force or torque as an output) that can be connected with additional suitable components in order to act as an ammeters and a voltmeter. The most common analogue instrument or meter is the permanent magnet moving coil instrument and it is used for measuring a dc current or voltage of a electric circuit. On the other hand, the indications of alternating current ammeters and voltmeters must represent the RMS values of the current, or voltage, respectively, applied to the instrument.

Types

Analogue instrumentsAll analogue electrical indicating instruments require three essential devices:

(a) A deflecting or operating device. A mechanical force is produced by the current or voltage which causes the pointer to deflect from its zero position.

(b) A controlling device. The controlling force acts in opposition to the deflecting force and ensures that the deflection shown on the meter is always the same for a given measured quantity. It also prevents the pointer always going to the maximum deflection. There are two main types of controlling device—spring control and gravity control.

(c) A damping device. The damping force ensures that the pointer comes to rest in its final position quickly and without undue oscillation. There are three main types of damping used—eddy-current damping, air-friction damping and fluid-friction damping.

Moving-iron instrumentAn attraction type of moving-iron instrument is shown diagrammatically (a). When current flows in the solenoid, a pivoted soft-iron disc is attracted towards the solenoid and the movement causes a pointer to move across a scale.



(b) In the repulsion type moving-iron instrument shown diagrammatically, two pieces of iron are placed inside the solenoid, one being fixed, and the other attached to the spindle carrying the pointer. When current passes through the solenoid, the two pieces of iron are magnetized in the same direction and therefore repel each other. The pointer thus moves across the scale. The force moving the pointer is, in each type, proportional to I2. because of this the direction of current does not matter and the moving-iron instrument can be used on d.c. or a.c. The scale, however, is nonlinear.

The moving-coil rectifier instrumentA moving-coil instrument, which measures only d.c., may be used in conjunction with a bridge rectifier circuit to provide an indication of alternating currents and voltages. The average value of the full wave rectified current is 0.637 Im. However, a meter being used to measure a.c. is usually calibrated in r.m.s. values. For sinusoidal quantities the indication is _0.707 Im_/_0.637 Im_ i.e. 1.11 times the mean value. Rectifier instruments have scales calibrated in r.m.s. quantities and it is assumed by the manufacturer that the a.c. is sinusoidal.

Shunts and multipliersAn ammeter, which measures current, has a low resistance (ideally zero) and must be connected in series with the circuit. A voltmeter, which measures p.d., has a high resistance (ideally infinite) and must be connected in parallel with the part of the circuit whose p.d. is required. There is no difference between the basic instrument used to measure current and voltage since both uses a milliammeter as their basic part. This is a sensitive instrument which gives f.s.d. for currents of only a few milliamperes. When an ammeter is required to measure currents of larger magnitude, a proportion of the current is diverted through a low-value resistance connected in parallel with the meter. Such a diverting resistor is called a shunt.

Electronic instrumentsElectronic measuring instruments have advantages over instruments such as the moving-iron or moving-coil meters, in that they have a much higher input resistance (some as high as 1000 M_) and can handle a much wider range of frequency (from d.c. up to MHz).

The digital voltmeter (DVM) is one which provides a digital display of the voltage being measured. Advantages of a DVM over analogue instruments include higher accuracy and resolution, no observational or parallex errors (see Section 10.20) and a very high input resistance, constant on all ranges.

A digital multimeter is a DVM with additional circuitry which makes it capable of measuring a.c. voltage, d.c. and a.c. current and resistance. Instruments for a.c. measurements are generally calibrated with a sinusoidal alternating waveform to indicate R.M.S. values when a sinusoidal signal is applied to the instrument. Some instruments, such as the movingiron and electro-dynamic instruments, give a true r.m.s. indication. With other instruments the indication is either scaled up from the mean value (such as with the rectifier moving-coil instrument) or scaled down fromthe peak value.



Sometimes quantities to be measured have complex waveforms (see Section 10.13), and whenever a quantity is non-sinusoidal, errors in instrument readings can occur if the instrument has been calibrated for sine waves only. Such waveform errors can be largely eliminated by using electronic instruments.

The ohmmeterAn ohmmeter is an instrument for measuring electrical resistance. Unlike the ammeter or voltmeter, the ohmmeter circuit does not receive the energy necessary for its operation from the circuit under test. In the ohmmeter this energy is supplied by a self-contained source of voltage, such as a battery. Initially, terminals XX are short-circuited and R adjusted to give f.s.d. on the milliammeter. If current I is at a maximum value and voltageE is constant, then resistance R D E/I is at a minimum value. Thus f.s.d. on the milliammeter is made zero on the resistance scale. When terminals XX are open circuited no current flows and R_D E/O_ is infinity, 1 The milliammeter can thus be calibrated directly in ohms. A cramped (non-linear) scale results and is ‘back to front’. When calibrated, an unknown resistance is placed between terminals XX and its value determined from the position of the pointer on the scale. An ohmmeter designed for measuring low values of resistance is called a continuity tester. An ohmmeter designed for measuring high values of resistance (i.e. megohms) is called an insulation resistance tester (e.g. ‘Megger’).

MultimetersInstruments are manufactured that combine a moving-coil meter with a number of shunts and series multipliers, to provide a range of readings on a single scale graduated to read current and voltage. If a battery is incorporated then resistance can also be measured. Such instruments are called multimeters or universal instruments or multirange instruments. An ‘Avometer’ is a typical example. A particular range may be selected either by the use of separate terminals or by a selector switch. Only one measurement can be performed at a time. Often such instruments can be used in a.c. as well as d.c. circuits when a rectifier is incorporated in the instrument.

WattmetersA wattmeter is an instrument for measuring electrical power in a circuit. The instrument has two coils:(i) a current coil, which is connected in series with the load, like an ammeter, and(ii) a voltage coil, which is connected in parallel with the load, like a voltmeter.

Electro dynamic (or Dynamometer) type Instruments

Fixed coil: The magnetic field is produced by the fixed coil which is divided into two sections to give more uniform field near the centre and to allow passage of the instrument shaft.

Moving coil: The moving coil is wound either as a self-sustaining coil or else on a nonmagnetic former. A metallic former cannot be used, as eddy currents would be induced in it by alternating field. Light but rigid construction is used for the moving coil. It should be noted that both fixed and moving coils are air cored.



Springs: The controlling torque is provided by two control springs. These hairsprings also act as leads of current to the moving coil.

Dampers: Air friction damping is employed for these instruments and is provided by a pair of Aluminum-vanes attached to the spindle at the bottom. These vanes move in a sector shaped chamber.

Shielding: Since the magnetic field produced by fixed coils is weaker than that in other types of instruments, these meters need a special magnetic shielding. Electro-dynamic instruments are effectively shielded from the effects of external magnetic fields by enclosing the mechanism in a laminated iron hollow cylinder with closed ends.



Different Types of Voltmeter and Ameter

1. Voltmeter Voltmeter is a kind of Galvanometer that is used for measuring voltage potential of an electric circuit or the potential difference between two points. Technically speaking, all Voltmeters are basically Ammeters, as they measure current rather than the voltage. Voltage gets measured only when current is transmitted in a circuit through the resistance. No doubt Voltmeters are often termed as high resistance Ammeters.

A Voltmeter is a versatile instrument as they are adept in measuring not only voltage but also current and resistance. This is the reason why Voltmeter is also known as Multimeter. A Voltmeter can measure voltages of both direct current (DC) or alternating electric current (AC) . Typical a Voltmeter scale is graduated in volts, millivolts (1/1000 volt), or kilovolts (1,000 volts) A laboratory standard Voltmeter employs electromechanical mechanism for functioning and typically comes with ranges in between 1000 - 3000 volts (V).

Types

Digital VoltmetersWith the advent of digital electronic devices, Digital Voltmeters are now in vogue in most of the physics laboratories. Digital Voltmeters can measure the current flow in amperes and then display the voltage digitally. The Digital Voltmeters provide accurate details about current draw and current continuity thus helping the users to troubleshoot erratic loads and trends. Modern Digital Voltmeters can They have both positive and negative leads and feature extremely low internal resistance A digital voltmeter shows voltage directly as numerals. Some of the new types of Digital Voltmeters can ascertain voltage readings to several significant figures.

Electrostatic VoltmetersElectrostatic Voltmeters are instruments that can effectively measure voltages without any charge transfer. Electrostatic voltmeters are particularly suitable in cases where voltage measurements cannot be performed using conventional contacting voltmeters. Conventional contacting Voltmeters need charge transfer to the voltmeter, thus leading to loading and adjustment of the source voltage.

An essential feature of an Electrostatic voltmeter is that it is able to precisely measure surface potential (voltage) on any type of material without the need for physical contact. So there is no occurrence of charge transfer as well as loading of the voltage source.

Solenoid VoltmeterA Solenoid voltmeter is a type of voltmeter that is in use mostly by electricians for testing and debugging of electrical power circuits. Solenoid voltmeters are known for extreme ruggedness and do not damaged easily because of rough handling or electrical overload.



Vacuum Tube VoltmeterA Vacuum tube Voltmeter is aso known as electronic voltmeter, or vacuum tube voltmeter (VTVM). Though not in much use a VTVM employs a tube or transistor circuit for amplifying the input voltage. This facilitates dual objectives. First it give rise to increased sensitivity, secondly i9t gives rise to increased input impedance. A Vacuum tube Voltmeter generally has an input resistance in the range of of 10 to 20 megohms.

Microvoltmeter Microvoltmeter is a multipurpose instrument for measuring low DC voltage. These days Digital Microvoltmeters can measure voltage down to even 1µV. Some of the exceptional features offered by the latest range of Microvoltmeters include.

» A low temperature drift » Automatic polarity indication » Recorder facility» 100% overranging.» Digital panel meter for better accuracy and convenience» IC regulated internal

Some voltmeters in use

Calibrating one of our voltmeters using

PCE-123 series voltmeters.

Testing the voltage in a cable at a distance using the PCE-DC3 series

voltmeters.

Analazing a electronic plate using PCE-OC 1 series voltmeters.

Testing the voltage of a station using PCE-PA6000 series voltmeters.

Comprobating current cablesusing PCE-EI-3000 series voltmeters.

Testing electrical ground of a vehicle

using the PCE-DM 22 series voltmeters.



2. An Ammeter

An ammeter is a device that detects an electric current. With some readily available materials, you can build your own ammeter and use it to measure current produced by batteries including homemade batteries and generators.

Types

GalvanometerThe first type of ammeter was a galvanometer, which reads direct current flow. The magnetic field created as current flows through a coil acts on a spring, which in turn moves an indicator needle.

Moving Iron AmmeterA moving iron ammeter replaces the spring and pointer system of the galvanometer with a piece of iron, which is acted on by the magnetic field created in the coil; the moving iron ammeter measures both alternating and direct currents.

ShuntShunts, which act as resistors, are inserted into ammeter systems to measure larger currents; the known quantity of resistance is used to obtain an accurate reading; digital ammeters use analog to digital converters to measure current across the shunt.

Zero-Center AmmeterZero-center ammeters are used in systems where voltage needs to be monitored in two directions, and are typically used along with a battery.

Clamp-on AmmeterA clamp-on ammeter is equipped with two jaws that can be fixed around the conductor; the magnetic field of the conductor moves an iron cylinder inside the ammeter to create a reading.


Documents

Electrical Measurements, types of Electrical Instruments & Different Types of Voltmeter and Ameter