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]]>Hydraulics/Pneumatics
Solenoid-operated valves typically control hydraulic and pneumatic equipment. A solenoid is used to move the valve spool that controls the flow of fluid (air or oil) in a directional control valve.
A directional control valve is a valve that is used to direct the flow of fluid throughout a fluid power system. Directional control valves are identified by the number of positions, number of ways, and type of actuator.
Positions.
A manual directional control valve is placed in different positions to start, stop, or change the direction of fluid flow. See Figure 1. A position is the number of locations within the valve in which the spool can be placed to direct fluid through the valve. A directional control valve normally has two or three positions.
Figure 1. Positions are the number of locations within the valve in which the spool can be placed to direct fluid through the valve.
Ways.
A way is a flow path through a valve. Most directional control valves are either two-way or three-way valves. The number of ways required depends on the application.
Two-way directional control valves have two main ports that allow or stop the flow of fluid. Two- way valves are used as shutoff, check, and quick-exhaust valves. See Figure 2.
Figure 2. A way is a flow path through a valve.
Valve Actuators.
A manual directional control valve uses a handle to change the valve spool position. An electrical control valve uses an actuator to change the position of a valve spool. In an electrical control valve, the solenoid acts as the actuator. See Figure 3.
Figure 3. In an electrical control valve, the solenoid acts as the actuator.
Refrigeration
Direct-acting, two-way valves are commonly used in a refrigeration system. Two-way (shutoff) valves have one inlet and one outlet pipe connection. These units may be constructed as normally open (NO), where the valve is open when de-energized and closed when energized; or they may be constructed as normally closed (NC), where the valve is closed when de-energized and open when energized. See Figure 4.
Figure 4. In a refrigeration system, direct-acting two-way valves may be constructed as normally open (NO), where the valve is open when de-energized and closed when energized; or they may be constructed as normally closed (NC), where the valve is closed when de-energized and open when energized.
A number of different solenoids may be used in a typical refrigeration system. The liquid line solenoid valves could be operated by two-wire or three-wire thermostats. The hot gas solenoid valve remains closed until the defrost cycle and then feeds the evaporator with hot gas for the defrost operation. See Figure 5.
Figure 5. Refrigeration systems may use different solenoids, such as liquid line solenoids and hot gas solenoids.
Combustion
Solenoids may also be used in an oil-fired single- burner system. See Figure 6. The solenoids are crucial in the startup and normal operating functions of the system.
Figure 6. Different solenoids are used for the safe operation of an oil-fired single-burner system.
General Purpose
In addition to commercial and industrial use, solenoids are used for general-purpose applications. Typical general- purpose applications include products such as printing calculators, cameras, and airplanes. See Figure 7.
Figure 7. Solenoids are used for general-purpose applications, such as those in airplanes
Pneumatic Robotics
Industrial robots are used in all kinds of applications from welding, painting, sorting, and assembling extremely small to extremely large parts. They can replicate human movement with the added advantage of being able to lift objects of almost any size or weight repeatedly in almost any type of environment.
Industrial robots use fluid power (hydraulic and/ or pneumatic) cylinders (linear motion), actuators (rotary motion), and grippers to provide the required power and movement. The cylinders, actuators, and grippers are controlled by solenoid-operated valves. See Figure 8.
In the robotic assembly example, cylinder 1 advances to move the arm out when the system starts. Rotary actuator 1 closes to grasp the part in the part feeder.
Cylinder 2 advances to move the part up and out of the part feeder. Cylinder 1 retracts to move the part away from the part feeder. Rotary actuator 2 rotates counter- clockwise to turn the part over. Cylinder 2 retracts to move the part over the sub-assembly. Rotary actuator 1 opens to release the part that drops into the sub-assembly. Rotary actuator 2 rotates clockwise to return the arm to the start position.
Figure 8. Pneumatic robots can be used to replicate human movement with the added advantage of being able to lift objects of almost any size and weight repeatedly.
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]]>Coils
Magnetic coils are normally constructed of many turns of insulated copper wire wound on a spool. The mechanical life of most coils is extended by encapsulating the coil in an epoxy resin or glass-reinforced alkyd material. See Figure 1. In addition to increasing mechanical strength, these materials greatly increase the moisture resistance of the magnetic coil.
Because magnetic coils are encapsulated and cannot be repaired, they must be replaced when they fail.
Figure 1. The mechanical life of most coils is extended by encapsulating the coil in an epoxy resin or glass-reinforced alkyd material.
Solenoid coils draw more current when first energized than the amount that is required to keep them running.
In a solenoid coil, the inrush current is approximately 6 to 10 times the sealed current. See Figure 2. After the solenoid has been energized for some time, the coil becomes hot, causing the coil current to fall and stabilize at approximately 80% of its value when cold. The reason for such a high inrush current is that the basic opposition to current flow when a solenoid is energized is only the resistance of the copper coil. Upon energizing, however, the armature begins to move iron into the core of the coil. The large amount of iron in the magnetic circuit increases the magnetic opposition of the coil and decreases the current through the coil. This magnetic opposition is referred to as inductive reactance or total impedance. The heat produced by the coil further reduces current flow because the resistance of copper wire increases when hot, which limits some current flow.
Figure 2. Solenoid inrush current is approximately 6 to 10 times the sealed current.
Magnetic coil data is normally given in volt amperes (VA). For example, a solenoid with a 120 V coil rated at 600 VA inrush and 60 VA sealed has an inrush current of 5 A (600/120= 5 A) and a sealed current of 0.5 A (60/120 = 0.5 A). The same solenoid with a 480 V coil draws only 1.25 A (600/480= 1.25 A) inrush current and 0.125 A (60/480= 0.125 A) sealed current. The VA rating helps determine the starting and energized current load drawn from the supply line.
Tech Fact
Solenoids are rated for intermittent or continuous duty. An intermittent-duty solenoid is designed to produce a strong force in a small package but will overheat if current is continuously applied to the coil. A continuous-duty solenoid is designed to handle a continuous current but is larger to help dissipate the heat produced.
All solenoids develop a magnetic field in their coil when voltage is applied. This magnetic field produces a force on the armature and tries to move it. The applied voltage determines the amount of force produced on the armature.
The voltage applied to a solenoid should be ±10% of the rated solenoid value. A solenoid overheats when the voltage is excessive. The heat destroys the insulation on the coil wire and burns out the solenoid. The solenoid armature may have difficulty moving the load connected to it when the voltage is too low.
Pick-up voltage is the minimum voltage that causes the armature to start to move.
Seal-in voltage is the minimum control voltage required to cause the armature to seal against the pole faces of the magnet.
Drop-out voltage is the voltage that exists when voltage is reduced sufficiently to allow the solenoid to open.
Seal-in voltage can be higher than pick-up voltage because a higher force may be required to seal in the armature than to just move the armature.
Drop-out voltage is lower than pick-up voltage or seal-in voltage because it takes more force to hold the armature in place than to release the armature.
For most solenoids, the minimum pick-up voltage is about 80% to 85% of the solenoid rated voltage.
The seal-in voltage is somewhat higher than the pick-up voltage and should be no less than 90% of the solenoid rated voltage.
Drop-out voltage can be as low as 70% of the solenoid rated voltage.
The exact pick-up, seal- in, and drop-out voltages depend on the load connected to the solenoid armature and the mounting position of the solenoid. The greater the applied armature load, the higher the required voltage values.
Voltage Variation Effects
Voltage variations are one of the most common causes of solenoid failure. Precautions must be taken to select the proper coil for a solenoid. Excessive or low voltage must not be applied to a solenoid coil.
Excessive Voltage
A coil draws more than its rated current if the voltage applied to the coil is too high. Excessive heat is produced, which causes early failure of the coil insulation. The magnetic pull is also too high and causes the armature to slam in with excessive force. This causes the magnetic faces to wear rapidly, reducing the expected life of the solenoid.
Low Voltage
Low voltage on the coil produces low coil current and reduced magnetic pull. The solenoid may pick up but does not seal in when the applied voltage is greater than the pick-up voltage but less than the seal-in voltage.
The greater pick-up current (6 to 10 times sealed current) quickly heats up and burns out the coil because it is not designed to carry a high continuous current. The armature also chatters, which creates noise and increases the wear of the magnetic faces.
Solenoids are selected based on the outcome required. It is important to select the correct solenoid to achieve the desired outcome. Solenoid selection methods include push or pull, length of stroke, required force, duty cycle, mounting, and voltage rating.
Push or Pull.
A solenoid may push or pull, depending on the application. In the case of a door latch, the unit must pull. In a clamping jig, the unit must push.
Length of Stroke
The length of the stroke is calculated after determining whether the solenoid must push or pull. For example, a door latch requires a Z\x′′ maximum stroke length.
Required Force
Manufacturer specification sheets are used to determine the correct solenoid based on the required force. A solenoid is selected from the manufacturer specification sheets based on required solenoid function.
Duty Cycle
Solenoid characteristic tables are also used to check the duty cycle requirements of the application against the duty cycle information given for the solenoid. For example, an A 101 solenoid is required for an application requiring 190 operations per minute.
Mounting
Manufacturers provide letter or number codes to indicate the solenoid mount. See Figure 3. For example, an A solenoid is selected for a door latch application because the door latch application requires an end-mounting solenoid.
Figure 3. Manufacturers provide letter or number codes to indicate the solenoid mount.
Voltage Rating
Manufacturers provide letter or number codes to indicate the voltages that are available for a given solenoid. See Figure 4. For example, a 2 A solenoid may be used for an application that requires a 115 V coil.
Figure 4. Manufacturers provide letter or number codes to indicate the voltages that are available for a given solenoid.
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]]>Solenoids are configured in various ways for different applications and operating characteristics. The five solenoid types are clapper, bell-crank, horizontal- action, vertical-action, and plunger. See Figure 1.
Figure 1. The five solenoid configurations are clapper, bell- crank, horizontal-action, vertical-action, and plunger solenoids.
A clapper solenoid has the armature hinged on a pivot point. As voltage is applied to the coil, the magnetic effect produced pulls the armature to a closed position so that it is picked up (sealed in).
A bell-crank solenoid uses a lever attached to the armature to transform the vertical action of the armature into a horizontal motion. The use of the lever allows the shock of the armature to be absorbed by the lever and not transmitted to the end of the lever. This is beneficial when a soft but firm motion is required in the controls.
A horizontal-action solenoid is a direct-action device. The movement of the armature moves the resultant force in a straight line. Horizontal-action solenoids are one of the most common solenoid configurations.
A vertical-action solenoid also uses a mechanical assembly but transmits the vertical action of the armature in a straight-line motion as the armature is picked up.
A plunger solenoid contains only a movable iron cylinder, or rod. A movable iron rod placed within the electrical coil tends to equalize or align itself within the coil when current passes through the coil. The current causes the rod to center itself so that the rod ends line up with the ends of the solenoid if the rod and solenoid are of equal length.
In a plunger solenoid, a spring is used to move the rod a short distance from its center in the coil. The rod moves against the spring tension to re-center itself in the coil when the current is turned on. The spring returns the rod to its off-center position when the current is turned off. The motion of the rod is used to operate any number of mechanical devices. See Figure 2.
Figure 2. In a plunger solenoid, a spring is used to move the rod a short distance from its center in the coil. The rod moves against the spring tension to re-center itself when the current is turned on.
Solenoids are constructed of many turns of wire wrapped around a magnetic laminate assembly. Passing electric current through the coil causes the armature to be pulled toward the coil. Devices may be attached to the solenoid to accomplish tasks like opening and closing contacts.
Eddy Current
Eddy current is unwanted current induced in the metal structure of a device due to the rate of change in the induced magnetic field. Strong eddy currents are generated in solid metal when used with alternating current.
In AC solenoids, the magnetic assembly and armature consist of a number of thin pieces of metal laminated together. The thin pieces of metal reduce the eddy current produced in the metal. See Figure 3. Eddy current is confined to each lamination, thus reducing the intensity of the magnetic effect and subsequent heat buildup.
For DC solenoids, a solid core is acceptable because the current is in one direction and continuous.
Figure 3. In AC solenoids, the magnetic assembly and armature consist of a number of thin pieces of metal laminated together.
Armature Air Gap
To prevent chattering, solenoids are designed so that the armature is attracted to its sealed-in position so that it completes the magnetic circuit as completely as possible. To ensure this, both the faces on the magnetic laminate assembly and those on the armature are machined to a very close tolerance.
As the coil is de-energized, some magnetic lines of force (residual magnetism) are always retained and could be enough to hold the armature in the sealed position. To eliminate this possibility, a small air gap is always left between the armature and the magnetic laminate assembly to break the magnetic field and allow the armature to drop away freely when de-energized. See Figure 4.
Figure 4. A small air gap is left in the magnetic laminate assembly to break the magnetic field and allow the armature to drop away freely after being de-energized.
Shading Coil
A shading coil is a single turn of conducting material (normally copper or aluminum) mounted on the face of the magnetic laminate assembly or armature. See Figure 5. A shading coil sets up an auxiliary magnetic field that helps hold in the armature as the main coil magnetic field drops to zero in an AC circuit.
Figure 5. A shading coil sets up an auxiliary magnetic field that helps hold in the armature as the main coil magnetic field drops to zero in an AC circuit.
The magnetic field generated by alternating current periodically drops to zero. This makes the armature drop out or chatter. The attraction of the shading coil adds enough pull to the unit to keep the armature firmly seated. Without the shading coil, excessive noise, wear, and heat builds up on the armature faces, reducing the armature life expectancy.
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]]>A clamp-on ammeter is normally used to measure current in a circuit with over 1 A of current and in applications in which current can be measured by easily placing the jaws of the ammeter around one of the conductors.
Most clamp-on ammeters can also measure voltage and resistance. To measure voltage and resistance, the clamp-on ammeter must include test leads and voltage and resistance modes. See Figure 1.
Figure 1. A clamp-on ammeter includes test leads and voltage and resistance modes.
Electricians must ensure that clamp-on ammeters do not pick up stray magnetic fields by separating conductors being tested as much as possible from other conductors during testing. If stray magnetic fields are possibly affecting a measurement, several measurements at different locations along the same conductor must be taken.
AC or DC measurements using a clamp-on ammeter or a digital multimeter (DMM) with a clamp-on current probe accessory follow standard procedures. See Figure 2.
Figure 2. A clamp-on ammeter measures the current on a circuit by measuring the strength of the magnetic field around a single conductor.
To measure current using a clamp-on ammeter, the following procedure is applied:
Locating Branch Circuits Using Clamp-On Am- meters.
A technician must often locate one circuit in a switchboard, panelboard, or load center to turn off the power before troubleshooting or working on a circuit.
Switchboards, panelboards, and load centers are often crowded with wires that are not marked or that are mismarked. A technician cannot start turning off each circuit until the correct circuit is found because this disconnects all loads connected to that circuit.
Timers, counters, clocks, starters, and other control devices must be reset, otherwise critical equipment such as alarms and safety circuits may be stopped. A flashing lamp and a clamp-on ammeter may be used to isolate a particular circuit. See Figure 3.
Figure 3. A flashing lamp and a clamp-on ammeter may be used to isolate a particular circuit
Tech Fact
Today, clamp-on ammeters are available that are capable of measuring currents as low as 4 mA and as high as thousands of amps. In addition, clamp-on ammeters are available with removable display heads for remote monitoring, flexible cables that open to enable connection around large conductors and bus bars, and small jaws with an attached lead that can be detached from the meter body and connected easily in tight areas.
The flashing lamp is plugged into any receptacle on the circuit that is to be disconnected. As the lamp flashes, a clamp-on ammeter is used to check each circuit. Each circuit displays a constant current reading except the one with the flashing lamp. The circuit with the flashing lamp displays a varying value on the ammeter equal to the flashing time of the lamp. This circuit may then be turned off for troubleshooting.
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]]>Objective
A learner will be able to:
Orienting Questions
Introduction
When dealing with extremely large or small quantities, scientists and engineers use scientific notation as a form of representation. In electronics, scientific notation is an important tool for representing electrical values. Important skills include being able to perform arithmetic (adding, subtracting, multiplying, and dividing) using scientific notation, and being able to convert between units of metric measures.
Very large and very small quantities are encountered a great deal in electronics. Scientific notation is used in the place of huge numbers of digits.
Scientific notation is a convenient way to express large or small numbers in order to perform arithmetic and other functions. Scientific notation uses a base number between 1 and 10, and a power of ten. A power of ten is a representation of a base number of ten and an exponent indicating how many times the base number is raised. The power of ten is represented by a symbol written above and to the right of a digit, or an exponent.
For example, if we were to represent 230,000 in scientific notation, we would move the decimal point left until we have a number between 1 and 10 on the left side of the decimal.
In this case, we would move the decimal point between the 2 and 3.
Next, we would count the number of digits to the right of the decimal. In our example, there are 5. So 230,000 would be represented as 2.3 X 10^{5}.
Scientific notation can only have a number less than 10 to the left of the decimal. Any numbers to the right of the decimal, greater than zero, must remain in the base number. As in the above example, we left the 3 in the base number since it is greater than zero.
To convert a number represented by scientific notation to a decimal number, we would simply move the decimal to the right the number of places indicated by the exponent.
Example
Let’s take the following number and convert it to scientific notation:
2,500,000 Our number
2.5 We place our decimal between the 2 and 5 which gives us our base number between 1 and 10.
2.5 X 10^{6} We moved the decimal 6 places to the left.
Small Numbers
When dealing with small numbers, the decimal is moved to the right. Instead of a positive exponent (power of ten), it is negative. This does not mean the number is negative.
For example, if we wanted to represent the quantity 0.00000362, we would move the decimal to the right until we have a number between 1 and 10. In this case, our decimal would be between the 3 and 6.
Next, we would count how many digits are on the left side of the decimal. In our example, we moved the decimal 6 places. Our example would be 3.62 X 10^{-6}.
Notice, we left the 2 because it is a number greater than zero.
To convert a small number represented by scientific notation to a decimal number, we move the decimal to the left the number of places indicated by the exponent.
Example
Let’s represent the following decimal number in scientific notation:
0.000 000 025 our number.
2.5 We moved the decimal point to the right to give us our base of 2.5 which is between 1 and 10.
2.5 X 10^{-8} We moved the decimal 8 places to the right, giving us our exponent of (-8).
More examples
516,570,000,000,000 = 5.1657 X 10^{14}
0.000100972 = 1.00972 X 10^{-4}
4,683.8 = 4.6838 X 10^{3}
0.05871 = 5.871 X 10^{-2}
7.55 X 10^{2} = 755
190 X 10^{6} = 190,000,000
1.23 X 10^{-6} = 0.00000123
9 X 10^{-3} = 0.009
View the video below before proceeding to the next section.
Scientific notation makes performing arithmetic simpler when dealing with very large and very small numbers. This leaves less room for errors.
Addition
We add numbers in scientific notation using the following method:
Example
How would we add 3 X 10^{5 }plus 6 X 10^{4}?
We need to first express the numbers using the same power of ten:
(3 X 10^{5}) + (60 X 10^{5})
Add the base numbers:
3 + 60 = 63
Bring the power of ten down:
63 X 10^{5}
Simplify so that the base is a number between 1 and 10:
6.3 X 10^{6}
Subtraction
The following method is used when subtracting powers of ten:
Example
Here is an example of subtracting numbers expressed in powers of ten:
Subtract 3.5 X 10^{-12 }from 9.5 X 10^{-11}
First we represent both numbers in the same power of ten:
(9.5 X 10^{-11}) – (.35 X 10^{-11})
Subtract the base numbers:
9.5 – .35 = 9.15
Bring down the power of ten:
9.15 X 10^{-11 }
Scientific Notation: Addition and Subtraction
Multiplication
To multiply numbers expressed in scientific notation use the following method:
Example
Multiply 6 X 10^{3} by 4 X 10^{-5}
Multiply the base numbers: (6)(4) = 24
Add the exponents: 3 + (-5) = -2
The product is: 24 X 10^{-2}
Simplified: 2.4 X 10^{-1}
Division
To divide numbers expressed in scientific notation use the following method:
Example
Divide 7 X10^{9 }by 3.5 X 10^{4}:
Represent problem as a fraction:
$\frac{7~X~{{10}^{9}}}{3.5~X~{{10}^{4}}}$
Divide the base numbers:
7 / 3.5 = 2
Subtract the exponents:
9 – 4 = 5
The quotient is: 2 X 10^{5}
Scientific Notation: Multiplication and Division
In the electronics field, you will deal with measureable quantities. You will measure voltage, current, and resistance as well as many other electrical quantities. All of these measurements have certain units and symbols that are used in combination with engineering notation.
Similar to scientific notation, engineering notation uses the same “power of ten” concept. A difference is that engineering notation can have up to three digits to the left of the decimal. Also, engineering notation can only have exponents that are multiples of three (3, 6, 9, etc.).
Example
Below are a few examples of numbers represented by both scientific and engineering notations:
Number Scientific notation engineering notation
23,000 2.3 X 10^{4 } 23 X 10^{3}
500 5 X 10^{2} 500 or .5 X 10^{3}
0.000052 5.2 X 10^{-5} 52 X 10^{-6}
Electrical units and quantities are represented by a letter symbol. Below is a table of some common electrical quantities, SI (International Standard), and symbols:
QUANTITY | SYMBOL | SI UNIT | SYMBOL |
Voltage | V | Volt | V |
Current | I | Ampere (Amp) | A |
Charge | Q | Coulomb | C |
Resistance | R | Ohm | Ω |
Capacitance | C | Farad | F |
Inductance | L | Henry | H |
Power | P | Watt | W |
Energy | W | Joule | J |
Time | T | Seconds | S |
Frequency | F | Hertz | Hz |
Primer on Electrical Units, Abbreviations and Symbols 1-2
Metric prefixes represent some of the most common powers of ten in engineering notation. Below is a table showing the most common metric prefixes:
Prefix | Prefix Symbol |
Value | |
Pico | P | 10^{-12} | = 0.000 000 000 001 |
nano | n | 10^{-9} | = 0.000 000 001 |
micro | µ | 10^{-6} | = 0.000 001 |
milli | m | 10^{-3} | = 0.001 |
kilo | k | 10^{3} | = 1000 |
Mega | M | 10^{6} | = 1000 000 |
Giga | G | 10^{9} | = 1000 000 000 |
Tera | T | 10^{12} | = 1000 000 000 000 |
Example
Show the following number with the prefix and the unit symbols:
0.005 Volts Our number
5 X 10^{-3} Volts in scientific notation
5 m Volts On our table we see 10^{-3} is represented by m
5 mV The symbol for volts is V
In order to do some calculations involving metric units, it is more convenient to convert the metric prefixes. There are some basic guidelines to follow when converting prefixes:
Example
Using the table above, we see that mF is millifarads (10^{-3}). Microfarads is 10^{-6}. Since microfarads are smaller than millifarads we would move the decimal three places to the right. This would give 300µF.
We would move the decimal three places to the left.
4000nA = 4000 X 10^{-9}A = 4 X 10^{-6 }= 4µA
We would move the decimal three places to the left.
1600kΩ = 1600 X 10^{3} = 1.6 X 10^{6 } = 1.6MΩ
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