Download Laboratory Exercise 4 The Solenoid (two weeks – 200 pts)

Laboratory Exercise 4
The Solenoid
(two weeks – 200 pts)
Procedure – Week 1
Objective: The objective of this lab is to plot force-strokes and force-amp-turn graphs in order to
determine the capability of given solenoids to pull or push weights far out of its position. These
graphs shows that how much force we can expect from the solenoid. The knowledge of these
plots facilitate the customers to decide best suitable solenoid for their purpose.
Solenoids
A solenoid typically converts electrical power into linear mechanical power. One can think of
solenoid as a motor but the difference is that a motor converts electrical power into a rotational
mechanical power.
Two solenoids, SMO-0837L (turn ratio = 2000) and SMO-0420L (turns ration = 2190) are used
in this lab. Both are open frame and pull type (although one was labeled push type in one
literature cut:
Fig.1 Screenshot describing an open Frame Solenoid.
Although they are supplied from Tai Shing Electronics Components Corp, they were purchased
from Jameco for this class. In the Jameco catalog, one was described as a push and one as a pull
type solenoid. When bought, both turned out to be pull-type.
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Caution: Do not pull on the hot-glue gun glue dabbed on the end of the shaft!
Fig. 2 Tai Shing Electronics Components Corp Solenoids Used
The two solenoids have stroke vs force curves and ampere-turn vs force curves for various
percent on of the solenoid. We are assuming 100% for both solenoids since the solenoids are to
be turned on 100% for this test. Devise a test to substantiate these curves for various values of
length of stroke. Use the weights in the lab and remember that 1 gram = 0.035274 oz.
Procedure
Use the circuits given in Fig. 3 to test the two solenoids and collect data for the curves
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solenoid
Plunger:
attached to fixed
weights on string
12 VDC
-
Fig. 3 Circuit diagram
For the force, use hexagon nuts and tie them with plunger of the solenoid with the help of thread.
Before connecting the nuts, weight (in gm) the nuts the help of digital weigh scale. Fig. 4 is
showing images of experimental set-up.
A linear scale (or ruler) would be used to measure the distance (stroke) in mm. Tape the scale
with solenoid and a pencil as seen in figure 4.
The number of turns of the smaller solenoid (SMO-0420L-12AA) is 2190. Resistance = 120 Ω.
The number of turns of the larger solenoid (SMO-0837L-12AA) is 2000. Resistance = 36 Ω.
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Solenoid
Scale
Threads
Weights
Fig. 4 Set-up of the experiment
Use the following tables to record your values of force vs stroke and force vs ampere-turns for
the two solenoids. Use the following table to record your values.
Force(gf)
Stroke (mm)
Ampere-Turns
Force (gf)
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Figures as shown below are showing typical Stroke Vs Force and Ampere-Turns Vs Force
characteristics of the solenoid.
Fig. 5 Typical plots of Force Vs Strokes and Force Vs Ampere-turns
Conclusion
Once you get all the data filled in above tables, plot the graphs as shown in Fig. 5. Also measure
and record the solenoid resistance. Comment on the value compared to the published data.
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Procedure – Week 2
Devise a circuit to demonstrate the duty cycle approach to turning on a solenoid. Why is a diode
used in these circuits? Describe the circuit and success with turning on the solenoid. Describe
the circuit and record your results. Verify your results with the original solenoid graphs.
See the circuit at the bottom of page 17 for a suggested circuit.
To verify your design, show your results to the instructor and get initials: __________________
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Supplement to Lab 4 - Introduction to Solenoids / Basics of a Solenoid
Two basic laws govern solenoids:
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Faraday's Law
Ampere's Law
Faraday's Law
The voltage induced in a coil is proportional to the number of turns and rate of change of flux.
The induced current flows in the direction that opposes the changing flux. Flux has no source or
sink (What goes in comes out)
Ampere's Law
The magneto-motive force (mmf) around a closed loop is equal to net current enclosed by the
loop. The objective of solenoid design is to transfer the maximum amount of NI (energy) from
the coil to the working air gap.
Types of Solenoids
There are two main categories of solenoids:
Rotary
Linear
Linear solenoids have applications in appliances, vending machines, door locks, coin changers,
circuit breakers, pumps, medical apparatus, automotive transmissions and postal machines to
name just a few.
Rotary solenoids have applications in machine tools, lasers, photo processing, media storage,
medical apparatus, sorters, fire door closures, and postal machines, also just to name a few.
Solenoids are used in almost every conceivable industry in the world and are well known as an
efficient, affordable and reliable actuation alternative.
Eight Essential Application Considerations when designing a solenoid into your assembly
 Stroke
 Force or Torque
 Voltage
 Current / Power
 Duty Cycle
 Temperature
 Operating Time / Speed
 Environmental
 AC / DC
 Life
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Stroke – when applying solenoids, keep the stroke as short as possible to keep the size, weight
and power consumption to a minimum.
Force – applies to linear products. Starting force is typically more important than ending force.
A safety factor of 1.5 is suggested. For example, an application requiring 3 pounds of force
should employ a solenoid that provides at least 4.5 pounds of force. Force is inversely
proportional to the square of the air gap with flat face plunger designs. The air gap is the space
in the magnetic circuit allowing the armature to move without interference, and the magnetic
flux to circulate with minimum resistance (reluctance).
To determine your requirements for force or torque, you need to consider the following:
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The actual load you are moving
Return spring force or torque
Frictional loads
Temperature rise
Duty cycle
Orientation of the solenoid vs. gravity (the weight of the plunger is added or subtracted
depending on how the solenoid is mounted.
In linear solenoids, force can be modified by the shape of the plunger used. A conical face
plunger is used for medium to long stroke applications. The effective air gap changes to become
a fraction of actual stroke. Flat face plungers are used for short stroke applications. Stepped
conical face plungers can provide various stroke (medium to long) dependent on the angle of the
step. These are advantageous for high holding force requirements.
Torque – applies to rotary products. Starting torque is typically more important than ending
torque. A safety factor of 1.5 is suggested. For example, an application requiring 3 pound of
torque should employ a solenoid that provides at least 4.5 pounds of torque. Torque produced by
Ledex™ Rotary Solenoids is inversely proportional to the total length of the stroke. The longer
the stroke, the lower the torque output. The shorter the stroke, the higher the torque output.
Voltage – the voltage source determines the coil winding to be used in the appropriate solenoid.
Common DC power supply ratings are 6,12,24,36, and 48 VDC. AC vs. DC solenoids – AC
solenoids are most commonly used in household appliances. Generally AC solenoids have been
specified when there was a high cost to rectify to DC. AC solenoids typically require twice the
in rush power of an equivalent DC solenoid. As a result, many more DC solenoids are chosen
for today's applications.
Current / Power – Force produced by a DC solenoid is proportional to the square of the number
of turns (N) in the coil winding and current flow (I). This determines the ampere turns or NI.
Solenoid coil requirements must match the power source.
Duty Cycle – The duty cycle of your application is the ratio of the "on-time" divided by the total
time for one complete cycle (on + off). Duty cycle is usually expressed as a percentage or a
fraction (50%, 100%). A more simplistic representation of duty cycle is to call < 100% duty
solenoids "Intermittent" and 100% duty cycle solenoids "Continuous". All intermittent duty
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solenoids (< 100% duty cycle) also must have a maximum "on-time" allowed to avoid
overheating that can eventually lead to a burned out coil. The "on-time" must not exceed the
power dissipation limits of the coil. Proper heat sinking and/or additional cooling improves heat
dissipation which allows a broader duty cycle range. Very close attention must be paid to the
maximum "on-time" data provided in conjunction with the duty cycle calculation to avoid
damaging your solenoids. For example, although an application with a one hour cycle time and a
3 hour off-time might calculate to a 25% duty cycle, this is not realistic in practice. A more
realistic solenoid application might be an on-time of one second and an off-time of 3 seconds for
the same 25% duty cycle.
Temperature – Both the ambient temperature of the solenoid environment and the self -heating of
the solenoid at work must be considered. The resistance of the coil varies with temperature
which affects force output. The self-heating temperature is dictated by the duty cycle. Each 1
increase above 20º C equates to an increase of 0.39% of rated resistance; thereby reducing force
or torque output. There are various ways to compensate for temperature restrictions:
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Specify a Class C Coil
Specify an over-molded coil
Use a E Model Rotary solenoid vs. the S Model
Actuate at one power level and cut back to a reduced power level for holding (pick and
hold)
Use a latching solenoid
Use a multiple winding solenoid
Operate intermittently, not at continuous duty
Use a larger solenoid
Use a heat sink
Add a cooling fan
The limiting factor of operating temperature of a solenoid is the insulation material of the magnet
wire used. Insulation classes:
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Class B- 130º C
Class F- 155º C
Class H- 180º C
Class C- 220º C
A typical solenoid requires 10% of the normal current to remain energized. To accomplish this,
you may choose to use one of the following:
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Mechanical hold in resistor
Capacitor discharge and hold in resistor
Transistorized hold in circuit
Pulse-width modulation
Pick and Hold
Dual voltage
Multiple coils
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Operating Time / Speed – Factors affecting time and speed include the mass of the load,
available power / watts and stroke. De-energizing also plays an important role and is affected by
the air gap, coil suppression, the plunger or armature return mechanism, and residual magnetism.
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The air gap is the space in the magnetic circuit allowing the armature to move without
interference, and the magnetic flux to flow with minimum resistance (reluctance). The
smaller the air gap, the longer it takes for the magnetic field resulting from the excited
coil to diminish. This causes a longer de-energizing time.
The application of electronic protection devices to reduce spikes caused by interrupting
the current in the coil is necessary to ensure protection of your switching device. Coil
suppression tends to increase the de-energizing time of the solenoid.

Since solenoids have force in one direction only, there must be some restoring force
(such as gravity or a spring) to take the solenoid back to the starting or de-energized
position. This positions the solenoid for the next operation.

Air gap surfaces of a solenoid become the north pole and south pole of a magnet when
energized. When the solenoid is off, a small but measurable magnetic attraction between
the poles still exists called residual magnetism. Residual magnetism can be reduced by
hyper-annealing the solenoid parts of by increasing the size of the air gap.
Environmental – Many environmental factors must be noted when choosing a solenoid. These
include temperature, sand/ dust, humidity, shock, vibration, altitude, vacuum, chemicals and
paper dust.
Solenoid Life – Life is determined by / optimized by the:
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Bearing system and shaft surface finish
Side loading and load alignment
Preventing the pole pieces from slamming together
Reducing impact shock upon energizing
Solenoid life expectations range from 50 thousand cycles to over 100 million cycles.
Custom Solenoids - 80% of solenoids used are custom designs. Typical modifications include
termination, lead wires, plunger configurations, shaft extensions, mounting changes and
linkages.
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Application Hints 
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To achieve extended life, try the following options:
o Drive the load from the armature end of a rotary solenoid rather than the base end
o Use vespel or oilite bearings in a low profile solenoid design
o Use dual ring bearings or a groove in the shaft to act as a lube reservoir
o Use glass-filled or carbon-filled nylon couplings
To achieve increased holding torque / force performance try the following options:
o Use indented ball races in a rotary solenoid
o Use flat pole pieces
o Use latching solenoids
To determine the temperature at which a coil has stabilized follow this sequence of steps:
o Measure the coil resistance at room temperature
o Measure the current at the stabilized temperature and determine the coil resistance
using Ohm's Law
o Divide this resistance by the resistance at room temperature to obtain the
resistance factor
o Using the resistance factor chart, read the temperature at which the solenoid coil
has stabilized.
To compensate for temperature rise:
o Mount the solenoid on a metal surface (heat sink)
o Use a cooling fan
o Use a larger solenoid
o Operate at < 100% duty cycle
o Consider a higher insulation class
o Use a solenoid with multiple windings
o Use a pick and hold circuit such as PWM
The Linear Solenoid
Another type of electromagnetic actuator that converts an electrical signal into a magnetic field is
called a Solenoid. The linear solenoid works on the same basic principal as the
electromechanical relay (EMR) seen in the previous tutorial and like relays, they can also be
controlled by transistors or MOSFET. A Linear Solenoid is an electromagnetic device that
converts electrical energy into a mechanical pushing or pulling force or motion.
Linear Solenoid
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Solenoids basically consist of an electrical coil wound around a cylindrical tube with a ferromagnetic actuator or "plunger" that is free to move or slide "IN" and "OUT" of the coils body.
Solenoids are available in a variety of formats with the more common types being the linear
solenoid also known as the linear electromechanical actuator (LEMA) and the rotary solenoid.
Both types, linear and rotational are available as either a holding (continuously energized) or as a
latching type (ON-OFF pulse) with the latching types being used in either energized or power-off
applications. Linear solenoids can also be designed for proportional motion control were the
plunger position is proportional to the power input.
When electrical current flows through a conductor it generates a magnetic field, and the direction
of this magnetic field with regards to its North and South Poles is determined by the direction of
the current flow within the wire. This coil of wire becomes an "Electromagnet" with its own
north and south poles exactly the same as that for a permanent type magnet. The strength of this
magnetic field can be increased or decreased by either controlling the amount of current flowing
through the coil or by changing the number of turns or loops that the coil has. An example of an
"Electromagnet" is given below.
Magnetic Field produced by a Coil
When an electrical current is passed through the coils windings, it behaves like an electromagnet
and the plunger, which is located inside the coil, is attracted towards the center of the coil by the
magnetic flux setup within the coils body, which in turn compresses a small spring attached to
one end of the plunger. The force and speed of the plungers movement is determined by the
strength of the magnetic flux generated within the coil.
When the supply current is turned "OFF" (de-energized) the electromagnetic field generated
previously by the coil collapses and the energy stored in the compressed spring forces the
plunger back out to its original rest position. This back and forth movement of the plunger is
known as the solenoids "Stroke", in other words the maximum distance the plunger can travel in
either an "IN" or an "OUT" direction, for example, 0 - 30mm.
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Linear Solenoids
This type of solenoid is generally called a Linear Solenoid due to the linear directional
movement of the plunger. Linear solenoids are available in two basic configurations called a
"Pull-type" as it pulls the connected load towards itself when energized, and the "Push-type" that
act in the opposite direction pushing it away from itself when energized. Both push and pull
types are generally constructed the same with the difference being in the location of the return
spring and design of the plunger.
Pull-type Linear Solenoid Construction
Linear solenoids are useful in many applications that require an open or closed (in or out) type
motion such as electronically activated door locks, pneumatic or hydraulic control valves,
robotics, automotive engine management, irrigation valves to water the garden and even the
"Ding-Dong" door bell has one. They are available as open frame, closed frame or sealed tubular
types.
Rotary Solenoids
Most electromagnetic solenoids are linear devices producing a linear back and forth force or
motion. However, rotational solenoids are also available which produce an angular or rotary
motion from a neutral position in either clockwise, anti-clockwise or in both directions (bidirectional).
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Rotary solenoids can be used to replace small DC motors or stepper motors were the angular
movement is very small with the angle of rotation being the angle moved from the start to the
end position. Commonly available rotary solenoids have movements of 25, 35, 45, 60 and 90o as
well as multiple movements to and from a certain angle such as a 2-position self-restoring or
return to zero rotation, for example 0-to-90-to-0o, 3-position self-restoring, for example 0o to
+45o or 0o to -45o as well as 2-position latching.
Rotary solenoids produce a rotational movement when either energized, de-energized, or a
change in the polarity of an electromagnetic field alters the position of a permanent magnet rotor.
Their construction consists of an electrical coil wound around a steel frame with a magnetic disk
connected to an output shaft positioned above the coil. When the coil is energized the
electromagnetic field generates multiple north and south poles which repel the adjacent
permanent magnetic poles of the disk causing it to rotate at an angle determined by the
mechanical construction of the rotary solenoid.
Rotary solenoids are used in vending or gaming machines, valve control, camera shutter with
special high speed, low power or variable positioning solenoids with high force or torque are
available such as those used in dot matrix printers, typewriters, automatic machines or
automotive applications etc.
Solenoid Switching
Generally solenoids either linear or rotary operate with the application of a DC voltage, but they
can also be used with AC sinusoidal voltages by using full wave bridge rectifiers to rectify the
supply which then can be used to switch the DC solenoid. Small DC type solenoids can be easily
controlled using transistor or MOSFET switches and are ideal for use in robotic applications, but
again as we saw with relays, solenoids are "inductive" devices so some form of electrical
protection is required across the solenoid coil to prevent high back emf voltages from damaging
the semiconductor switching device. In this case the standard "Flywheel Diode" is used.
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Switching Solenoids using a Transistor
Reducing Energy Consumption
One of the main disadvantages of solenoids and especially the linear solenoid is that they are
"inductive devices" which convert some of the electrical current into "HEAT", in other words
they get hot!, and the longer the time that the power is applied to a solenoid coil, the hotter the
coil will become. Also as the coil heats up, its electrical resistance also changes allowing more
current to flow.
With a continuous voltage input applied to the coil, the solenoids coil does not have the
opportunity to cool down because the input power is always on. In order to reduce this selfgenerated heating effect it is necessary to reduce either the amount of time the coil is energized
or reduce the amount of current flowing through it.
One method of consuming less current is to apply a suitable high enough voltage to the solenoid
coil so as to provide the necessary electromagnetic field to operate and seat the plunger but then
once activated to reduce the coils supply voltage to a level sufficient to maintain the plunger in
its seated or latched position. One way of achieving this is to connect a suitable "holding"
resistor in series with the solenoids coil, for example:
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Reducing Solenoid Energy Consumption
Here, the switch contacts are closed shorting out the resistance and passing the full supply
current directly to the solenoid coils windings. Once energized the contacts which can be
mechanically connected to the solenoids plunger action open connecting the holding resistor, RH
in series with the solenoids coil. This effectively connects the resistor in series with the coil.
By using this method, the solenoid can be connected to its voltage supply indefinitely
(continuous duty cycle) as the power consumed by the coil and the heat generated is greatly
reduced, which can be up to 85 to 90% using a suitable power resistor. However, the power
consumed by the resistor will also generate a certain amount of heat, I2R (Ohm's Law) and this
also needs to be taken into account.
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Duty Cycle
Another more practical way of reducing the heat generated by the solenoids coil is to use an
"intermittent duty cycle". An intermittent duty cycle means that the coil is repeatedly switched
"ON" and "OFF" at a suitable frequency so as to activate the plunger mechanism but not allow it
to de-energize during the OFF period of the waveform. Intermittent duty cycle switching is a
very effective way to reduce the total power consumed by the coil.
The Duty Cycle (%ED) of a solenoid is the portion of the "ON" time that a solenoid is energized
and is the ratio of the "ON" time to the total "ON" and "OFF" time for one complete cycle of
operation. In other words, the cycle time equals the switched-ON time plus the switched-OFF
time. Duty cycle is expressed as a percentage, for example:
Then if a solenoid is switched "ON" or energized for 30 seconds and then switched "OFF" for 90
seconds before being re-energized again, one complete cycle, the total "ON/OFF" cycle time
would be 120 seconds, (30+90) so the solenoids duty cycle would be calculated as 30/120 secs or
25%. This means that you can determine the solenoids maximum switch-ON time if you know
the values of duty cycle and switch-OFF time.
For example, the switch-OFF time equals 15 secs, duty cycle equals 40%, therefore switch-ON
time equals 10 secs. A solenoid with a rated Duty Cycle of 100% means that it has a continuous
voltage rating and can therefore be left "ON" or continuously energized without overheating or
damage.
The TIP 115 Darlington and 1N4001 diode are shown connected to a power supply and a
frequency generator to demonstrate the duty cycle circuits above. Demonstrate it with a solenoid
and turn the duty cycle down until the solenoid’s magnetic force weakens sufficiently to allow
the solenoid to be pulled apart easily. Demonstrate this to your instructor.
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solenoid
1N4001
12 V DC
Oscilloscope
-.5 - 4.5V
from
Source
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