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The electrical contractor is charged with a responsibility to carry out a number of tests on an electrical installation and electrical equipment. The individual tests are dealt with in the NEC Code / Regulations and described later in this section. Electrical measuring instruments are identified by the way in which the instrument movement is deflected in operation. Thus a moving coil instrument movement consists of a coil which is free to move in operation.
The connection of a test instrument into a circuit can change the circuit resistance and cause error readings. For example, some power is drawn from the test circuit to drive the instrument movement across a scale. Fortunately for electrical installation students, this is not a common problem found in power engineering, but does need consideration when testing electronic circuits. Electronic test equipment is discussed in Section 4. Analogue and digital meters, their advantages and disadvantages.
Moving coil instruments
A delicate coil is suspended on jeweled bearings between the pole pieces of a strong permanent mag net. The test current is fed to the moving coil through the spiral control springs which also return the pointer to zero after each test reading. The construction is shown in Ill. 28.
Ill. 28 Moving coil instrument.
When the test current flows in the moving coil, a magnetic flux is established.
This flux interacts with the magnetic flux from the permanent magnet and since lines of magnetic flux never cross, the magnetic flux is distorted and a force F is exerted on the coil, rotating it and moving the pointer across the scale.
The purpose of the soft iron armature A is to establish a uniform magnetic field of equal flux density for the coil's rotation. This gives the moving coil instrument the qualities of sensitivity and a uniform scale.
The basic moving coil movement will only respond satisfactorily to a d.c. supply since an a.c. supply would reverse the current and magnetic flux in the coil at the supply frequency, resulting in the pointer trembling at some useless mid-point on the scale. To overcome this disadvantage, a.c. test circuits are connected to the moving coil meter movement through a rectifier circuit, as shown in Ill. 30.
Ill. 29 Operation of a moving coil instrument.
Ill. 30 Rectified input to moving coil meter movement.
The moving coil instrument presents a high impedance to the test circuit and draws very little current from it. When used with the rectifier circuit, it will give accurate readings over a frequency range from 50 to 50 kHz. This makes it suitable for power or electronic circuit measurements. Many commercial multirange analogue instruments such as the AVO use the moving coil movement because of these advantages, together with a high sensitivity and linear scale.
The wide range of scales achieved by the same meter movement in a commercial instrument is discussed in this section under the heading 'Range extension'.
Moving iron instruments
Moving iron instruments operate on basic magnetic principles, either attracting or repelling a piece of soft iron attached to a pointer which moves across a scale.
These instruments can be used to test a.c. or d.c. supplies without the addition of a rectifier circuit.
ATTRACTION-TYPE MOVING IRON INSTRUMENTS
The construction of an attraction-type moving iron instrument is shown in Ill. 31(a). The test current passing through the instrument solenoid establishes a magnetic flux which attracts the soft iron towards the solenoid, moving the pointer across the scale.
The flux density of the magnetic field in which the iron disc moves varies. This creates a non-linear force on the iron as it moves through the magnetic field and therefore the scale is non-linear. This instrument is usually arranged to have gravity control, by which the force of gravity returns the pointer to zero, and it can only be operated vertically.
REPULSION-TYPE MOVING IRON INSTRUMENTS
The repulsion-type moving iron instrument is the moving iron instrument most often found today. The construction is shown.
Ill. 31 Moving iron instruments: (a) attraction type; (b) repulsion type.
Ill. 32 Repulsion-type moving iron instrument
Current passing through the solenoid coil establishes a magnetic field inside the solenoid. This magnetic field magnetizes two pieces of soft iron, one fixed and the other moving, which are close to each other.
Since like magnetic poles repel, the moving iron is repelled away from the fixed iron and the pointer is deflected across the scale. The forces exerted between the fixed and moving iron are proportional to the square of the current flowing in the solenoid, which results in a non-linear scale.
A linear scale is highly desirable, and to achieve an almost linear scale the manufacturers of commercial moving iron repulsion-type instruments shape the iron pieces. Rectangular moving iron pieces and fixed iron pieces in the shape of a tapered scroll have been found to give good results.
This instrument can usually be operated horizon tally since the movement is supported by jeweled bearings and a spiral spring provides the control torque which returns the pointers to zero after each test reading.
Moving iron instruments present a low impedance to the test circuit and will therefore draw current from the test circuit to provide the power necessary for the deflection of the pointer. The frequency range of a moving iron instrument is restricted to between about 50 and 400Hz, and so the applications of moving iron instruments are to be found in power circuits and electrical installations. They are not suitable for electronic circuit measurements.
If an analogue electrical measuring instrument is correctly connected to a live test circuit, the pointer will rise quite quickly and then settle on the true circuit reading, maintaining a steady value throughout the connection. This effect we take for granted, but it can only be achieved if the deflection system is critically damped.
In an undamped system the pointer would rise quickly to the true circuit reading but would then overshoot because of the inertia of the moving system.
The forces exerted on the moving system would be insufficient to maintain this higher value and the pointer would fall back towards the true circuit reading, gaining momentum which would take the pointer below the true reading. The forces exerted on the moving system would force the pointer towards the true value, again gaining momentum, resulting in the pointer oscillating about the true value for some time before finally coming to rest. This effect is shown in Ill. 33, where A represents the true reading of the circuit, curve B the undamped system and curve C the correctly or critically damped system. Note that curve C reaches its final steady value much more quickly than the undamped system of curve B.
To achieve damping in an electrical instrument a force is provided which opposes the rise of the moving system towards the final true value. This is achieved in one of three ways: eddy current damping; air vane damping; and air piston damping.
Ill. 33 Damping curves.
The electrical contractor is charged by the NEC Regulations for Electrical Installations to test all new installations and major extensions before connection to the mains supply. The contractor may also be called upon to test installations and equipment in order to identify and remove faults. These requirements imply the use of appropriate test instruments, and in order to take accurate readings consideration should be given to the following points:
++ Is the instrument suitable for this test?
++ Have the correct scales been selected?
++ Is the test instrument correctly connected to the circuit?
Test instruments normally consume a small amount of power in order to provide the torque required to move the pointer across a scale. Most instruments are constructed in a way which makes them highly sensitive, giving full scale deflection with only very small currents, but these currents are drawn from the circuit being tested. When testing power and electrical installation circuits and equipment, the power consumed by the test instrument can often be neglected, but electronic circuits use very little power and therefore when testing electronic circuits a high impedance test instrument must be used.
A 50_ load is connected across a 100 V supply. A multirange meter is correctly connected to read first the current and then the voltage. As an ammeter, the instrument has a resistance of 0.1-ohm& as a voltmeter a resistance of 20 k-ohm. The power loss in the instrument for each connection is as follows:
This instrument consumes approximately half a watt when taking the readings, which in a power or electrical installation circuit is negligible, but might destroy an electronic circuit.
Ill. 34 Eddy current damping.
EDDY CURRENT DAMPING
When a copper or aluminum disc is rotated between the poles of a permanent magnet, a current is induced into the disc, as shown in Ill. 34. The magnetic field due to this current sets up a force opposing the motion (Lenz's law) which produces a braking effect.
The braking effect only occurs when the disc is in motion, and this type of damping is used in moving coil instruments. The delicate moving coil is wound on an aluminum former and as the moving coil moves in the magnetic field so does the aluminum former.
Eddy currents are induced in the former, setting up a force to oppose the motion, and damping is achieved for the moving coil system.
AIR VANE DAMPING
This is usually achieved by attaching a thin rectangle of aluminum to the spindle of the instrument movement. When the instrument movement deflects, the aluminum rectangle or vane moves in a segment-shaped box, compressing the air, which exerts a damping force on the vane and on the instrument movement. Air vane damping is used on most moving iron instruments.
Ill. 35 Air vane damping.
AIR PISTON DAMPING
This is an alternative method of air damping. A piston is attached to the moving system of the instrument, as shown in Ill. 36. When the moving system deflects, the piston is pushed into an air chamber which compresses the air and applies a force which opposes the motion and causes damping of the meter movement.
Moving iron instruments may be constructed to read 10, 20 or 50A by increasing the thickness and number of solenoid conductors. However, moving coil instruments can only be constructed using a delicate lightweight coil whose maximum current carrying capacity is no more than about 75mA. To extend the range of a moving coil instrument, shunt or series resistors are connected to it.
To extend the range of an ammeter a low-resistance shunt resistor is connected across the meter movement. This allows the majority of the circuit current to pass through the shunt and only a very small part of the current to pass through the meter movement.
Ill. 36 Air piston damping.
Ill. 37 Range extension of moving coil movement.
Ill. 38 The resistor and coil arrangements in a multirange instrument
To extend the range of a voltmeter a high-resistance series resistor is connected to the meter movement.
This limits the current flowing through the meter movement to an acceptable low value.
A multirange instrument contains a number of shunt or series resistors which are connected by a range selector switch to form the various scales of the instrument, as shown in Ill. 38.
EXAMPLE A moving coil movement has a resistance of 5-ohm and gives full scale deflection when 15mA flows in the coil. Calculate the value of the resistor which must be connected to the movement in order that the instrument may be used (a) as a 5 A ammeter and (b) as a 100 V voltmeter.
For (a), […]
The voltage dropped across the moving coil is given by […]
Many commercial instruments are capable of making more than one test or have a range of scales to choose from. A range selector switch is usually used to choose the appropriate scale, as shown in Ill. 38. A scale range should be chosen which suits the range of the current, voltage or resistance being measured. For example, when taking a reading in the 8 or 9V range the obvious scale choice would be one giving 10V full scale deflection. To make this reading on an instrument with 100V full scale deflection would lead to errors, because the deflection is too small.
Ammeters must be connected in series with the load, and voltmeters in parallel across the load as shown in Ill. 39. The power in a resistive load may be calculated from the readings of voltage and current since P=VI. This will give accurate calculations on both a.c. and d.c. supplies, but when measuring the power of an a.c. circuit which contains inductance or capacitance a wattmeter must be used because the voltage and current will be out of phase.
Ill. 39 Wattmeter, ammeter and voltmeter correctly connected to a load.
A correctly connected wattmeter will give an accurate measure of the power in any a.c. or d.c. circuit. It is essentially a moving coil instrument in which the main magnetic field is produced by two fixed current coils. The moving coil is the voltage coil and rotates within the fixed coils, being pivoted centrally between them and controlled by spiral hair springs as shown in Ill. 40.
Ill. 40 A dynamometer wattmeter
The main magnetic field is produced by the current in the fixed coil and is proportional to it. The force rotating the moving coil is proportional to its current and the magnetic field strength produced by the fixed coils. The deflection is proportional to the product of the currents in the fixed and moving coils. Since the moving coil current depends upon the voltage and the
fixed coils depend upon the current, the meter deflection is proportional to V _ I _ power in watts.
Any change in the direction of the current in the circuit affects both coils and the direction of deflection remains unchanged, allowing the instrument to be used on both a.c. and d.c. circuits. On a.c. circuits the deflection will be the average value of the product of the instantaneous values of current and voltage, meaning that the wattmeter will measure the true power or active power in the circuit, in which the deflection is proportional to VI cos _ (watts). Damping is achieved by an air vane moving in a dashpot.
Measurement of power in a three-phase circuit
When three-phase loads are balanced, for example in motor circuits, one wattmeter may be connected into any phase, as shown in Ill. 41. This wattmeter will indicate the power in that phase and since the load is balanced the total power in the three-phase circuit will be given by:
Total power _ 3 _ Wattmeter reading
This is the most commonly used method for measuring power in a three-phase, three-wire system since it can be used for both balanced and unbalanced loads connected in either star or delta. The current coils are connected to any two of the lines, and the voltage coils are connected to the other line, the one without a current coil connection, as shown in Ill. 42. Then Total power _ W1 _ W2 This equation is true for any three-phase load, balanced or unbalanced, star or delta connection, provided there is no fourth wire in the system.
Ill. 41 One-wattmeter measurement of power.
Ill. 42 Two-wattmeter measurement of power.
Ill. 43 Three-wattmeter measurement of power.
If the installation is four-wire, and the load on each phase is unbalanced, then three wattmeter readings are necessary, connected. Each h […]
wattmeter measures the power in one phase and the total power will be given by
Total power _ W1 _ W2 _ W3
Ill. 44 Construction of an energy meter.
The current and voltage coils are wound on the two magnets as shown in Ill. 44. The current coil establishes a flux _1 which is proportional to the current, and the voltage coil establishes a magnetic flux _v.
The rotation of the aluminum disc is due to the interaction of these magnetic fields. The magnetic flux establishes eddy currents in the disc which produce a turning force. The force exerted is proportional to the phase angle between the voltage and current coil fluxes;
maximum force occurs when they are 90° out of phase.
This force is proportional to the true power VI cos _, which is equal to the speed of rotation of the disc. The number of revolutions in a given time will give a measure of energy since energy _ power _ time.
The rotating disc spindle is attached through suit able gearing to a revolution counter which is calibrated to read kilowatt-hours, which is the Board of Trade unit of electric energy.
The tong tester or clip-on ammeter works on the same principle as the bar primary current transformer. The laminated core of the transformer can be opened and passed over the busbar or single core cable. In this way a measurement of the current being carried can be made without disconnection of the supply. The construction is shown.
Ill. 45 Tong tester or clip-on ammeter.
Phase sequence testers
Phase sequence is the order in which each phase of a three-phase supply reaches its maximum value. The normal phase sequence for a three-phase supply is R-Y-B, which means that first red, then yellow and finally the blue phase reaches its maximum value.
Phase sequence has an important application in the connection of three-phase transformers. The secondary terminals of a three-phase transformer must not be connected in parallel until the phase sequence is the same.
A phase sequence tester can be an indicator which is, in effect, a miniature induction motor, with three clearly color-coded connection leads. A rotating disc with a pointed arrow shows the normal rotation for phase sequence R-Y-B. If the sequence is reversed the disc rotates in the opposite direction to the arrow.
However, an on-site phase sequence tester can be made by connecting four 230V by 100W lamps and a p.f. correction capacitor from a fluorescent luminaire as shown in Ill. 46.
Ill. 46 Phase sequence test by the lamps bright, lamps dim method.
The capacitor takes a leading current which results in a phase displacement in the other two phases. The phasor addition of the voltage in the circuit results in one pair of lamps illuminating brightly while the other pair are illuminated dimly. Two lamps must be connected in series because most of the line voltage will be across them during the test.
Test equipment used by electricians
The NEC which advise electricians and other electrically competent people on the selection of suitable test probes, voltage indicating devices and measuring instruments. This is because they consider suitably constructed test equipment to be as vital for personal safety as the training and practical skills of the electrician. In the past, unsatisfactory test probes and voltage indicators have frequently been the cause of accidents, and therefore all test probes must now incorporate the following features:
1. The probes must have finger barriers or be shaped so that the hand or fingers can't make contact with the live conductors under test.
2. The probe tip must not protrude more than 2mm, and preferably only 1mm, be spring-loaded and screened.
3. The lead must be adequately insulated and colored so that one lead is readily distinguished from the other.
4. The lead must be flexible and sufficiently robust.
5. The lead must be long enough to serve its purpose but not too long.
6. The lead must not have accessible exposed conduct ors even if it becomes detached from the probe or from the instrument.
7. Where the leads are to be used in conjunction with a voltage detector they must be protected by a fuse.
A suitable probe and lead is shown in Ill. 47.
Ill. 48 Typical voltage indicator.
Ill. 47 Recommended type of test probe and leads.
NEC also tells us that where the test is being made simply to establish the presence or absence of a voltage, the preferred method is to use a proprietary test lamp or voltage indicator which is suitable for the working voltage, rather than a multimeter. Accident history has shown that incorrectly set multimeters or makeshift devices for voltage detection have frequently caused accidents. -- shows a suitable voltage indicator. Test lamps and voltage indicators are not fail-safe, and therefore NEC recommends that they should be regularly proved, preferably before and after use, as described in the flowchart for a safe isolation procedure.
Next: Test procedures- Exercises (cont. and conclusion)
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Updated: Sunday, 2016-06-12 22:50 PST