SOLENOID TECHNICAL IMFORMATION
1. Introduction
A large number of application possibilities, simple design and long
service life make Mechetronics solenoids cost-effective solutions
for the most selected problems.
Applications range from general machine construction
through plant engineering, vehicle construction, robotics, precision
mechanics, household appliances and medical technology to hydraulic
and pneumatic controls.
High reliability, long service life and high efficiency
are all requirements that are met by Mechetronics solenoids through
precision manufacture, tight tolerances and suitable surface treatment. Customer-related solutions are developed in conjunction with the
customer.
2. Operation
Solenoids transform electrical energy into mechanical movement.
3. Direct Current (DC) Solenoids
Contrary to an alternating current (AC) solenoid, the power consumption
of a direct current (DC) solenoid is independent of the position
of the plunger. DC solenoids are also characterised by soft and
hard operation.
Inherently longer cycling times can be reduced
by special circuitry. It is also possible to modify the stroke-force
characteristics. High switching frequencies do not cause dangerously
high thermal loads in DC solenoids; the maximum switching frequency
is only limited by the pull-in and drop-out times.
4. Alternating Current (AC) Solenoids
Unlike DC solenoids, AC solenoids provide relatively high operating/cycling
frequencies and low cycling times; this results in hard operation
which influences service life. Power consumption depends on the
position of the plunger.
High cycling frequencies can cause dangerously
high thermal loading in AC solenoids and the maximum permissible
temperature is therefore the limiting factor for cycling frequency.
5. Force
Usually, the correct solenoid for a given application is the smallest
one that has adequate magnetic force.
5.1. Magnetic force
The magnetic force in Newtons is the usable portion (i.e. that portion
which is reduced by friction) of the mechanical force that is produced
by the solenoid. It is measured at 90% of the rated voltage at normal
operating temperature.
5.2. Holding force
The holding force of a solenoid is the force that is effective at
the end of the stroke.
5.3. Residual force
The residual force, generated by any remaining (residual) magnetism,
is the holding force that still applies after the electrical power
is stopped. This force can be influenced by design features.
5.4. Return force
The return force is the force required to return the plunger from
the end of stroke to the start of stroke.
5.5. Magnetic force / stroke characteristic
Traditional curves indicate the plunger movement toward the final
(energised) position.
6. Stroke
The stroke is the usable distance travelled by the plunger from
its initial position to the end of travel. As the stroke is increased
the force is reduced and vice versa.
6.1. Start of Stroke
This is the position of the plunger before it starts its travel. It is also the position when it returns upon conclusion of the complete
cycle.
6.2. End of Stroke
This is the designed final position of the plunger upon conclusion
of the work portion of the complete cycle.
6.3. Stroke Work
For the linear solenoid, the stroke work (in Newtons) is the magnetic
force over the magnetic stroke.
A solenoid is the correct size if the magnetic
force exceeds the opposing force at all times with only a slight
amount of excess force to assure long service life.
A solenoid is too small if the magnetic force
is less than the opposing force over a certain range.
7. Time terms
The use of solenoids necessitates a certain chronological sequence
best clarified with the following terms:
7.1. Power-off pause
The power-off pause (in seconds) is the time between switching off
the current and switching it on again.
7.2. ON period
This is the period (in seconds) between switching the current on
and off again.
7.3. Cycle period
This is the sum of the ON period and the Power-off pause.
7.4. Duty cycle
The ratio between the ON and the cycle period is the relative ON
period in %.
7.5. Cycling sequence
The cycling sequence (in seconds) is the single or periodically
repeated joining of cycle period values of very different durations.
7.6. Response time
The response delay (in seconds) is the time between application
of the current and initial movement of the plunger.
7.7. Stroke time
This time (in seconds) starts when the plunger begins to move from
its initial position and ends when it reaches its limit of travel.
7.8. Pull-in time
The sum of Response time and Stroke time is the time required by
the plunger to perform its work. Special measures in the circuit
can shorten the pull-in time.
7.9. Drop-out delay
Drop-out delay (in seconds) is the time from current cut-out until
the plunger starts to return to its initial position.
7.10. Return time
The return time (in seconds) is the time from the beginning of plunger
return motion until it has reached its initial position.
7.11. Drop-out time
The sum of Drop-out delay and Return time is the drop-out time (in
seconds).
8. Temperature terms and classes of insulating
material
When selecting a suitable solenoid, temperature must be considered.
8.1. Ambient temperature
The ambient temperature is the temperature (°C) surrounding
the solenoid when it is operating. If the range is outside +40°C
to -50°C design changes may be required.
8.2. Permanent operating temperature
The permanent operating temperature (in °C) is equilibrium reached
between the heat generated by the solenoid and that escapes. Equilibrium
has been reached when the temperature changes by no more than 1°C
in an operation period of 60 minutes. It is determined on a thermally
non-conductive support in still air at the rated voltage.
8.3. Reference temperature
This temperature (in °C) is the constant temperature of the
solenoid. This temperature may differ from the ambient temperature
if, for example, the solenoid is mounted on a hydraulic valve which
has warm hydraulic oil flowing through it.
8.4. Differential temperature
This is the number of degrees (°C) between the temperature of
the solenoid and that of the cooling medium designated for the solenoid.
8.5. Limiting temperature
The upper limiting temperature (in °C) is the highest temperature
permitted for the solenoid or any part thereof. The lower limiting
temperature (in °C) is the lowest temperature permitted for
the solenoid or any part thereof.
8.6. Maximum temperature above normal
This is the maximum permissible number of degrees (°C) of Differential
temperature.
8.7. Thermal insulation classes
Thermal insulating materials are divided into the following classes
based on their thermal resistance.
|
Thermal Insulation
Class |
Maximum Temperature
(°C) |
Maximum Temperature
Rise |
|
Y |
90 |
50 |
|
A |
105 |
65 |
|
E |
120 |
80 |
|
B |
130 |
90 |
|
F |
155 |
115 |
|
H |
180 |
140 |
9. Electrical terms
If solenoids are to operate reliably, they must be provided with
suitable power; the following are a few terms to aid in understanding:
9.1 Nominal voltage
The nominal voltage is that with which a solenoid is normally operated,
the tolerance is +5% to -9%.
9.2. Nominal current
The nominal current given in the data sheets is always referenced
to the rated voltage and a winding temperature of 20 °C.
9.3. Nominal power
The nominal power (in W) is calculated from the rated voltage and
current (at a winding temperature of 20 °C) given in the data
sheets.
9.4. Test voltage
Solenoids are tested for electrical insulation and dielectric strength
at a certain test voltage that lies above the rated voltage. The
test voltage is applied between the exciter winding and the metal
parts of the unit that can be touched.
10. Protection classes
Protection can be divided into three
classes:
- Class I – voltage-carrying parts have
only an operating insulation and a connection for the neutral
line
- Class II - operating and protective insulation
provided but no connection for the neutral line
- Class III operates at less than 42 Volts and
has no circuit designed for any higher voltage
10.1. Types of protection
The following types of protection are standardised in DIN
400501. They concern protection against touching, foreign bodies
and humidity.
IP 65 is a common example where IP is the code
for the standardised type of protection, the first digit relates
to touching or the entry of foreign bodies and the second digit
concerns protection against the penetration of water.
|
First Digit |
Protection against touching
and foreign bodies |
| 0 |
no protection |
| 1 |
protected against large foreign bodies |
| 2 |
protected against medium-sized foreign bodies |
| 3 |
protected against small foreign bodies |
| 4 |
protected against grain-sized foreign bodies |
| 5 |
protected against dust deposits |
| 6 |
protected against dust entry |
|
Second Digit |
Protection against water |
| 0 |
no protection |
| 1 |
protected against vertically falling water |
| 2 |
protected against water falling at an angle |
| 3 |
protected against sprayed water |
| 4 |
protected against splashing |
| 5 |
protected against water jets |
| 6 |
protected in case of flooding |
7 |
protected in case of immersion |
8 |
protected in case of submersion |
11. Sample circuits
Suitable circuitry will influence the operating times and service
life of the solenoid.
In an AC circuit the over-voltage at cut-off is fully damped however
this severely delays the drop-out time.
In a DC circuit the over-voltage at cut-off is not damped. This
circuit usually uses magnetic units of low electrical power in order
to shorten the drop-out time. There are ways of reducing the contact
wear of DC solenoids
12. Damping
There are three ways of damping solenoids:
12.1. Damping by Ohmic resistance
A parallel resistor can be used to limit the voltage surge that
occurs when the power to the solenoid is cut off. As a result, however,
the drop-out time increases as does the power requirement. Both
are reduced as the parallel resistance is reduced.
12.2. Damping by Varistor (voltage-dependent resistor)
A Varistor may be used to damp the voltage surge at cut-off. This
causes only a slight rise in power requirement.
12.3. Damping by diode
Diodes will completely damp the cut-off surge voltage, however,
the drop-out time will be greater.
13. Variable current (e.g. using a resistor)
Varying the current applied makes it possible to use a smaller solenoid. To prevent the winding from overheating the current is limited by
a resistor after the plunger reaches the end of the stroke. This
circuit cannot be used with high operating frequencies. The size
of the dropping resistor depends on the resistance of the winding.
14. Installation Guidelines
- DC solenoids may be installed in any position.
- AC solenoids are prone to buzzing if they
are not installed squarely in the application.
- The plunger should only be used in the axial
direction.
- In order to achieve maximum service life solenoids
should be loaded with at least 70% of the magnetic force.
- It is imperative that voltage, ON period,
temperature and protection be checked before a solenoid is operated.
- If a neutral lead is required it should be
fitted provided by the customer in compliance with VDE 0100.
15. Solenoid specification
To help us identify the correct solenoid for your application please
provide us with as much of the following information as possible:
- Model Number
- Voltage
- ON period
- Stroke (in mm)
- Magnetic force / stroke characteristics
- Ambient temperature
- Stroke force (in Newtons)
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