2. 2
Objectives
Define the unit of electricity and current
flow
Define the three electrical qualities
present in electrical circuits.
State and apply Ohm’s Law
Measurement basics
3. 3
Definitions
Current (I): flow of electric charges per unit time,
measured in amperes or amps (A)
Electromotive Force (emf) (E): a potential difference or
“electric pressure” which drives the flow of charges,
measured in volts (V)
Resistance (R): an electrical circuit’s opposition to
current flow, measured in ohms (Ω)
Conductor: a material which offers little resistance to
current flow, e.g. silver, copper, iron, etc…
Insulator: a material which offers high resistance to
current flow, e.g. wood, paper, plastic, etc...
4. 4
Types of Electricity
Static Electricity - no motion of free
charges
Current Electricity - motion of free charges
Direct Current (DC)
Alternating Current (AC)
6. 6
Direct Current (DC)
Current flow is unidirectional and of constant
magnitude.
Conventional Current Flow
Electron Flow
+ -
-+
E
7. 7
Alternating Current (AC)
Current is constantly changing in
magnitude and direction at regular
intervals.
Current is a function of time and usually
varies as a sine function.
I
t
8. 8
Voltage – emf (electromotive force)
Voltage (E) – electrical pressure or force with which electrons
move
Measured in volts
9. 9
Electromotive force - Emf
Water
Pump
Shutoff
Valve
High
Pressure
Low
Pressure
Switch
Battery
R -
resistor
Low
Potential
High
Potential
E
10. 10
Resistance/Impedance
Resistance / Impedance (R / Z or Ω) - opposition that an element or
material has to the flow of electrons
Ohm’s Law states that one volt (E) will push one amp
of current (I) through one ohm (Ω) of resistance (R).
Resistance (DC circuit); Impedance (AC circuit)
Ohm’s Law formula: E=IR
11. 11
Ohm’s Law
The amount of current flowing in an
electrical circuit
(I - Measured in amperage) is dependent
upon the value of electrical pressure (E -
measured in volts) and the amount of
opposition to the flow of current (R -
measured in ohms).
R
E
I =
R
E
I =
12. 12
The Ohms Law Triangle
RIE ×=
R
E
I =
I
E
R =
E
I R
I
E
R =
13. 13
Ohm’s Law
I = E/R 20 amps = 120 volts / ? ohms
In simpler terms; One volt (E) will push one amp of
current (I) through one ohm (Ω) of resistance (R)
E = IR 120 volts = 15 amps x ? ohms
R = E/I 6 ohms = 120 volts / ? amps
P = IE 60 watts = 20 amps / ? volts
Ohm’s Law – In an electrical circuit, the current
passing through a conductor between two points is
proportional to the potential difference (i.e. voltage
drop or voltage) across the two points, and inversely
proportional to the resistance between them.
14. 14
Power
Electric power (P) is defined as the
amount of work done by an electric current.
Measured in watts
P = I x V
P is the power (watt or W)
I is the current (ampere or A)
V is the potential difference (volt or V)
15. 15
Fundamental Concepts & Terms
Kirchoff’s Voltage
and Current Laws
Kirchoff’s Voltage Law:
The algebraic sum of the
voltage (potential)
differences in any loop
must equal zero.
Kirchoff’s Current Law:
The sum of current into a
junction equals the sum of
current out of the junction.
R1
R2
R3
R5
v2
v1
v4
v5
v3
a b
d c
+
+
vg
i3
i4
i1
R1
i2
20. 20
Digital Multimeters
Measurement Device Circuit Symbol
Voltage Voltmeter
Current Ammeter
Resistance Ohmmeter
V
A
Ω
“Through”
“Across”
“Across”
(and Not in circuit)
30. 30
Testing Generators/Motors
When testing
generators, motors, or
transformers each
winding/phase should
be tested in sequence
and separately while all
the other windings are
grounded. Testing this
way, the insulation
between phases is also
tested.
31. 31
Power
Electrical power is defined as the rate at
which electrical energy is supplied to a circuit
or consumed by a load.
The watt (w) is the unit of power
IE
time
work
P ×==
Where E = volts and I = current
P
I E
Static electricity is a property in some bodies that cause them to exert forces on each other. Two bodies both with positive or both with negative charges repel each other, whereas bodies with opposite or ‘unlike’ charges attract each other. Electrons possess a negative charge, and protons an equal positive charge. The basic unit of electric charge is the coulomb (symbol C).
For example, the excess electrons evenly distributed over an insulated spherical surface will remain at rest, static. If a wire connects the sphere to ground , the electrons will flow through the wire to the ground. The flow of charge constitutes an electric current. DC current if it only flows in one direction and AC current if it flows in both directions alternately.
DC current is the constant flow of an electrical charge, typically in an conductor like a wire, where the electrons (negative charge) flow in one direction, and that does not reverse its flow. The electricity produced by a battery is direct current.
AC current is current that flows for an interval of time in one direction and then in the opposite direction; that is, a current that flows in alternately-reversed directions through or around a circuit. Electric energy is usually generated as alternating current in a power station, and alternating currents may be used for both power and lighting.
If half an amp moves in one direction past a point in half a second and in the next half a second, half an amp reverses and moves past the same point in the opposite direction, then in total one coulomb of electrons or one amp has passed the point in one second. The current flow is 1 amp AC.
The advantage of alternating current over direct current (DC) as from a battery is that its voltage can be raised or lowered economically by a transformer; high voltage for generation and transmission, and low voltage for safe utilization.
The unit of electric current is the ampere. One ampere (A) represents a flow of charge at the rate of one coulomb per second past any point:
The direction of conventional current is always the same as the direction in which positive charges would move, even if the actual current consists of a flow of electrons
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Electrical Maintenance Fundamentals
A device with the ability to maintain a potential difference between two points is called a source of electromotive force (emf).
The most familiar sources of emf are batteries and generators. Batteries convert chemical energy to electric energy and generators transform mechanical energy into electric energy.
Voltage is a relative quantity { it only exists between two points.
Unlike current, which may be measured at a single point in a circuit, voltage is fundamentally relative: it only exists as a difference between two points. In other words, there is no such thing as voltage existing at a single location. Therefore, while we speak of current going through a component in a circuit, we speak of voltage being across a component, measured between two different points on that component. A good way to understand voltage is to experiment with a voltmeter, measuring voltage between different pairs of points in safe, low-voltage circuits. Another good way to gain proficiency is to practice on conceptual problems relating to the measurement of voltage in circuits.
In an electric circuit, the source of emf is usually represented by the symbol “E”. The function of a source of emf in an electric circuit is similar to the function of a water pump in maintaining the continuous flow of water through a system of pipes. In the diagram, the water pump must perform the work on each unit volume of water necessary to replace the energy lost by each unit volume flowing through the pipes. In the circuit the source of emf must do work on each unit of charge which passes through it in order to raise it to a higher potential. This work must be supplied at a rate equal to the rate at which energy is lost in flowing through the circuit.
By convention, we have assumed that the current consists of a flow of positive charge even though in most cases it is negative electrons. Therefore, the charge loses energy in passing through the resistor from a high potential to a low potential. In the hydraulic analogy, water passes from high pressure to low pressure. When the shutoff valve is closed, pressure exists but there is no water flow. Similarly, when the electric switch is open, there is voltage but no current.
Page <number>
Electrical Maintenance Fundamentals
Resistance R is defined as opposition to the flow of electric charge. Although most metals are good conductors of electricity, all offer some opposition to the surge of electric charge through them. This electrical resistance is fixed for many specific materials of known size, shape, and temperature. It is independent of the applied emf and the current passing through it.
The effects of resistance in limiting the flow of charge were first studied quantitatively by Georg Simon Ohm in 1826. He discovered that, for a given resistor at a particular temperature, the current is directly proportional to the applied voltage. Just as the rate of flow of water between two points depends on the difference in height between them, the rate of flow of electric charge between two points depends on the difference in potential between them. This proportionality is usually stated as Ohm’s law : The current produced in a given conductor is directly proportional to the difference of potential between its endpoints.
The amount of current flowing in an electrical circuit (I - Measured in amperage) is dependent upon the value of electrical pressure (E - measured in volts) and the amount of opposition to the flow of current (R - measured in ohms). The mathematical formula representing this relationship is:
I=E/R
Note: This formula is known as Ohm’s Law and serves as the basic formula for determining the behavior of an electrical circuit.
Calculate the current flowing in a circuit having 12 ohms resistance and 120 Volts of pressure.
Use the magic triangle to find the combinations of Ohm’s Law.
Page <number>
Electrical Maintenance Fundamentals
Using ohms law to solve we find:
R = E/I = 120 volts / 20 amps = 6 ohms
R = E/I = 120 volts / 15 amps = 8 ohms
E = P/I = 60 watts / 20 amps = 3 volts
Page <number>
Electrical Maintenance Fundamentals
Kirchoff’s Voltage Law as interpreted from the diagram shows that the voltages will add to zero as shown in the formula V1+V2+V3+V4=0 for the illustrated circuit.
For example, if V4 = 24 volts and V1 = -12 volts, V2 = -6 volts, V3 = -6 volts, using the voltage sum formula. the result will be zero.
This knowledge can be beneficial in troubleshooting circuits when one realizes that all the voltage drops (V1, V2, V3) in a circuit should equal the supplied voltage, which in this case, is V4.
Kirchoff’s Current Law as interpreted means that the current supplied to a point in the circuit equals the current that leaves or is drawn from that same point which is shown in the formula i1 + i4 = i2 + i3 for the illustrated circuit.
For example, if i2=3 amps, i3 = 1 amp, i4 = 2 amps, what should i1 equal? Remember i2 and i3 are supplying the current and therefore i2 + i3 = 4 amps which is the current supplied. If i4 is 2 amps, then i3 must be 2 amps to satisfy the current law.
This is also helpful when troubleshooting for circuit problems. If a low resistance (for example, a relay has a bad coil) draws more current than normal, then the other circuit may not have its desired current.
The current that passes through resistor R1 must also pass through R2. Thus the total of the Voltage drops v1 plus v2 must equal v4.
Following Ohm’s law of E = IR in a series resistive circuit the total resistance is therefore the sum of the resistances in series.
Based on Kirchoff’s law the total current entering point “a” is equal to the current exiting point “b”. Again Ohm’s I=E/R law works. Since the voltages are equal V4=V1=V2 and since I1= I2 +I3 then substitution gives us the basic formula shown in the slide.
Series/Parallel resistance combinations follow these basic rules shown.
Knowledge of this basic function of Ohm’s law and series/parallel affects of current, voltage and resistance can aid in basic troubleshooting of a circuit to help evaluate the proper function of devices if we just perceive that basically all loads in a circuit are resistors.
The ability to make safe and accurate electrical circuit measurements is an integral part of measuring performance and diagnosing faults of electric drives common to most rotating equipment.
Everyone involved must exercise caution in order to remain insulated from low and high voltage circuits being worked on. Before attempting to measure any electrical circuit for current, voltage or resistance, be aware of the various Federal, Provincial, State or any other regulatory body which may have rules and regulations identifying what is allowed in the trade area when it comes to performing electrical work.
Voltage, current, resistance and power measurements are routinely made in electrical circuits of those electric drives and switching components for rotating equipment.
Electrical instruments are used to measure and monitor these circuit values.
The most common field instruments used by tradesmen and technicians to test and/or troubleshoot electrical circuits are voltmeters, ammeters, ohmmeters, megohmmeters, and occasionally wattmeters.
An electrical measuring device that combines an ammeter, an ohmmeter, a voltmeter, and occasionally other measurement or testing devices into one unit. Multimeters are designed to allow the user to choose the type of electrical unit (current, voltage, resistance) to be measured by means of a selector switch. Multimeters are available in analog type (needle movement), or in digital type (DMM - digital multimeter).
Basic meter operation is based on magnetic principles or motor action in the galvanometer.
Any device used to detect an electric current is called a galvanometer. The operating principle for the majority of such instruments is based on the torque exerted on a coil in a magnetic field. The essential parts are shown in the illustration. A coil, wrapped around a soft-iron core, is pivoted on jeweled bearings between the poles of a permanent magnet. Its rotational motion is restrained by a pair of spiral springs which also serve as current leads to the coil. Depending upon the direction of the current being measured, the coil and pointer will rotate in either a clockwise or a counterclockwise direction.
The coil and pointer of the laboratory galvanometer are shown in the equilibrium position. The permanent magnets are shaped to provide a uniform radial magnetic field. This ensures that the pointer deflection will be directly proportional to the current in the coil. Thus the position of the pointer on a marked scale is a measure of the current magnitude. Reversing the current direction will cause an equal pointer deflection in a counterclockwise direction.
The sensitivity of a galvanometer of the type shown in is determined by the spring torque, the friction in the bearings, and the strength of the magnetic field.
Many important dc measuring instruments utilize a galvanometer as an indicating element. Two of the most common are the voltmeter and the ammeter. The analog multi-meter illustrated previously uses the galvanometer principle for its operation.
To measure voltage, a voltmeter (either AC or DC) is connected across an electrical component.
Remember to observe polarity marks when measuring DC. This type of connection is known as a parallel connection and is shown in the illustration.
Note the polarity of the voltmeter connections: positive is connected to positive and negative to negative.
In this illustration, the voltage of the battery is being measured. Note that the leads of the voltmeter are directly connected across the battery terminals.
In the illustration, the voltage drop across the resistor is measured. The resistor is considered the ‘load” in this type of circuit.
The circuit is not broken, and the voltmeter leads are connected directly across the resistor leads.
In this example there is significant conductor resistance and the motor is drawing a substantial current. In this case, the voltage drop across the motor or load L will be less than that at the battery.
Note: Voltmeters are always connected across (parallel to) circuit components, never in series.
A device used to measure current in a circuit is called an ammeter.
A multimeter will also have the ability to measure current in amps as well.
Usually there are several ranges that can be selected for measuring current.
An ammeter is always connected into the electrical circuit (series connection) as shown. The polarity of the circuit must be considered depending on if the current is AC or DC.
Connecting an ammeter to a live high-energy circuit is very dangerous. Use safe work practices with all circuits, and dangerous habits will not develop.
Always refer to the meters operating manual when using to verify proper application of the meter.
Clamp-on current meters in a convenient method for checking current in conductor.
The user must be aware of the type of current being monitored: AC or DC. Some clamp-on meters can only measure AC and some can only measure DC.
Note: never clamp two wires when taking readings. Why? Depending on the direction of current flow the wires can either cancel the reading altogether or the reading can be double the value expected.
Resistance is an opposition to the flow of current. If the resistance of a circuit is doubled, the current is reduced to one-half. Resistance can be very useful in rotating equipment drives when the flow of current must be controlled.
For example, a component called a “rheostat” can be added to a motor circuit. A rheostat is an adjustable resistor. Current flow in a circuit must overcome the resistance in the circuit. As the rheostat is adjusted for more resistance, less current flows and the motor slows down. As the rheostat is adjusted for less resistance, the current flow increases and the motor speeds up. Resistance can also be used to control illumination, loudness, and many other useful circuit functions.
In some cases, resistance is undesirable where the conductors used to carry the motor current have excess resistance. If the motor is used where it must develop maximum output at all times, the conductor resistance is resistance that prevents the motor from developing its full output and some electrical energy is wasted since heat is produced by the current flowing in the wires. In severe cases, the wires could be damaged by the heat or a fire could result. Conductor resistance becomes a significant problem when the wires are very long and high currents must flow.
The instrument commonly used to measure resistance is an ohmmeter. An analog ohmmeter can consist of a battery and a variable resistor connected in series with a basic meter movement. The variable resistor is used to calibrate the meter to obtain a full scale deflection (zero ohms showing on the meter) when the two terminal leads are shorted together. (See example above)
Calibration is done to compensate for changes in the internal battery voltage due to aging (only needs to be done on analog type ohmmeters).
Note: Each time an analog ohmmeter’s scale is changed the meter must be recalibrated.
Note: Shorting leads in a digital ohmmeter will display the resistance of the meter leads. However, it can also quickly be used as an indicator that the meter is functioning correctly.
Resistance must never be measured in a circuit that is energized.
A megohmmeter, popularly known as a “megger”, is an instrument that is used for measuring very high resistance values.
The term megohmmeter is derived from the fact that the device measures resistance values in the megohm range.
The megohmmeter’s primary function is to test insulation resistance of power transmission systems, electrical machinery (motors, generators), transformers, and cables. A basic megohmmeter insulation tester consists of a hand-driven generator and a direct-reading true ohmmeter.
The megger is a portable instrument used to measure insulation resistance
and consists of a hand-driven DC generator and a direct reading ohm meter.
The megohmmeter is used to measure the unknown resistance in the cable. The unknown resistance is connected between the terminals marked line and earth.
The hand crank is turned at a moderate speed (approximately 120 RPM) and a DC voltage is generated. The scale is calibrated so that the pointer directly indicates the value of the resistance being measured (all values shown are in megohms).
The purpose of the G terminal (guard ring) is to eliminate surface current leakage across exposed conductors and or ground.
Note: Because the amount of power that the megger can produce is small, the test is considered to be non-destructive, meaning permanent damage is not likely to be caused in the insulation system of the device being tested. However, the level of output voltage is high enough to present a personal safety hazard if incorrect testing procedures are used.
Never connect a megger insulation tester to energized lines or equipment.
Proof testing
Electricians and engineers perform proof tests to insure proper installation and integrity of conductors. The proof test is a simple, quick test used to indicate the instantaneous condition of insulation. It provides no diagnostic data and the test voltages used are much higher than the voltages used in predictive maintenance tests. The proof test is sometimes called GO/NO GO TEST because it tests cable systems for maintenance errors, incorrect installation, serious degradation, or contamination. The installation is declared acceptable if no breakdown occurs during testing.
Proof test procedure
To conduct an installation proof test, use the following procedure:
Use a multimeter or the voltage measurement function on the MegOhmMeter to make sure there is no power applied to the tested circuit.
Select the appropriate voltage level.
Plug one end of the black test lead to the common terminal on the meter and touch the test probe to a ground (earth) or another conductor. Sometimes it is helpful to ground all conductors that are not part of the test. Alligator clips can make measurements easier and more accurate.
Plug one end of the red test lead to the volt/ohm terminal on the meter and connect the test probe to the conductor to be tested.
Press the test button to apply the desired voltage and read the resistance displayed on the meter. It could take a few seconds for the reading to settle. The higher the resistance is the better.
Test each conductor against ground and against all the other conductors present in the conduit. Keep a dated record of the measured values in a safe place.
If some of the conductors fail the test, identify the problem or re-pull the conductors. Moisture, water, or dirt can create low resistance readings.
Testing connections in generators, transformers, motors, and wiring
To test the insulation resistance in generators, transformers, motors, and wiring installations, we can employ any of the previously mentioned predictive maintenance tests. Whether we choose the spot-reading, step voltage, or time-resistance tests depends on the reason for testing and the validity of the data obtained. When testing generators, motors, or transformers each winding/phase should be tested in sequence and separately while all the other windings are grounded. In this way, the insulation between phases is also tested.
Testing generators and motors
When testing the resistance of the stator coils make sure the stator winding and phases are disconnected. Measure the insulation resistance between windings and windings to ground. Also, when DC generators or motors are being tested, the brushes should be raised so the coils can be tested separately from the armature.
Testing transformers
When testing single-phase transformers, test winding to winding, winding to ground, or test one winding at a time with all others grounded.
Testing wiring and cable installations
When testing wires or cables, they should be disconnected from panels and machinery to keep them isolated. The wires and cables should be tested against each other and against ground.
As the current passes through the external circuit, work is done by the current on the components of the circuit. In the case of a pure resistor, the energy is dissipated in the form of heat. If a motor is attached to the circuit, the energy loss is divided between heat and useful work. In any case, the energy gained in the source of voltage must equal the energy lost in the entire circuit.
The rate at which heat is dissipated in an electric circuit is referred to as the power loss. When current is flowing continuously through a circuit, this power loss is given by the formula Power equals Voltage time Current.
Power is defined as the rate of expending energy or rate of doing work. The watt (w) is the unit of power. A wattmeter measures the true power of an electrical circuit and is effective in both AC and DC circuits.
A wattmeter consists of two electromagnetic coils with one coil connected across (parallel) the electrical circuit being measured, and the other coil being connected into (series) the electrical circuit. The interaction of the magnetic fields of these two coils will result in a net value of power (P = V x I).
The illustration shows a wattmeter measuring power in a single-phase circuit.
Wattmeters have polarity marks which must be observed to ensure connections are made correctly. Remember that the current should (normally) enter at the polarity mark and leave at the non-polarity mark for both the voltage and current coils.
Wattmeters work by finding the average of the product of voltage and current . This is done either electronically in newer meters or with the interaction of magnetic fields as in meters with coil movements. In either case, connections must be made for both the current and the voltage.
A typical wattmeter connection is shown above. It is always a good idea to put an ammeter in series with the wattmeter to make sure the current coil of the wattmeter is not being overloaded.
The power relationship of three-phase can be measured using two wattmeters as shown.
It is usually referred to as the two wattmeter method of measuring power.
Note that because of the phase relationships between current and voltage, one of these wattmeters could read a negative value but the sum of the wattmeter readings will be positive.
