Linear Solenoids

Linear Solenoids

Another type of electromagnetic actuator that converts an electrical signal into a magnetic field is called a Solenoid. Linear Solenoids work 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 MOSFETs. A Linear Solenoid is an electromagnetic device that converts electrical energy into a mechanical pushing or pulling force or motion. They basically consist of an electrical coil wound around a cylindrical tube with a ferro-magnetic 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 being the Linear Solenoid, Rotary Solenoid both available as Holding or Latching types.

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 centre of the coil by the magnetic flux setup within the coils body, which inturn 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 "IN" or "OUT" direction, for example 0 - 100mm.

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.

Example of a Pull-type Linear Solenoid Structure and Connection

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 force either clockwise, anti-clockwise or in both directions (bi-directional). Rotary solenoids can be used to replace small d.c. motors where the angular movement is very small with the more common types being 2-position self restoring or return to zero, for example 0 to 90^o, 3-position self restoring, for example 0 to +45^o or 0 to -45^o and 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 energised 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 and can be either 10, 30, 45 or 90^o etc.

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 D.C. voltages but they can also be used with A.C. sinusoidal voltages by using full wave bridge rectifiers to rectify the supply which then can be used with D.C. solenoids. 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 a "Flywheel Diode" is used.

Switching Solenoids using a Transistor

Reducing Energy Consumption

One of the main disadvantages of solenoids and especially Linear Solenoids 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. 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 self generated 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 position. One way of achieving this is to connect a suitable "holding" resistor in series with the solenoids coil, for example:

Reducing Solenoid Energy Consumption

Here, the switch contacts are closed shorting out the resistance and passing full current to the coil windings. Once energized the contacts which are mechanically connected to the solenoids plunger action open connecting the holding resistor in series with the solenoids coil. 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 and 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.

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. Intermittent duty cycle switching is a very effective way to reduce the total power consumed by the coil.

The Duty Cycle 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 and is expressed as a percentage, for example:

Then if a solenoid is switched "ON" or energised 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%.

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 energised without overheating.