Solenoid Operation

Single Coil Solenoid

A solenoid is a device that converts electrical energy into mechanical force. The basic single coil solenoid is made of a free moving steel plunger that sits within a wound coil of copper wire. When electric current is introduced, a magnetic field forms which draws the plunger in. The exposed end of the plunger can be attached to equipment, and when the solenoid is activated, the plunger will move to open, close, turn on or turn off that equipment.

Solenoids are configured in many ways for different applications and operating characteristics. In most automotive applications, solenoids are used for operating engine run/stop levers, throttle control levers, chokes, valves, and clutches and to protect expensive diesel equipment from overspeed, low lube pressure, and high temperature.

Figure 1 shows the various parts of a basic single coil solenoid.

Dual Coil Solenoid

The dual coil solenoid offers high actuation force in a small package compared to a single coil solenoid. This type of solenoid uses two separate coil windings to allow the solenoid to be held energized for longer periods. The first wound coil (pull coil) operates at a high current level to provide maximum pull or push. The second wound coil (hold coil) simply holds the plunger in after it has completed its stroke and “bottomed out.”

Since the current required to hold the plunger is low (typically 1.2 amps), the hold coil can be energized continuously without overheating. This unique design results in a highly efficient and compact solenoid approximately one-half the size of a comparable single coil unit.

Three Methods for Turning Off the Pull Coil

After energizing and pulling in the plunger, the pull coil in a dual coil solenoid must be turned off as soon as possible to prevent overheating. The three basic methods for switching off the pull coil are discussed below.

EXTERNAL SWITCHING

The externally switched (3-wire) solenoid is used in applications where an operator/driver manually turns a key switch that temporarily energizes the pull coil to pull in the plunger. The most popular application is for start-stop control of engines in trucks and mobile equipment where moisture, dirt, dust, and high vibration are present. The sealed 3-wire solenoid is well suited for these harsh conditions.  

EXTERNAL SWITCHING WITH TIMER MODULE

With the addition of a Woodward pull coil timer module, the externally switched (3-wire) solenoid can be used not only in operator/driver controlled vehicles, but also in unattended equipment, throttle, and choke controls. The timer ensures that the pull coil is turned off within approximately 1-1/2 seconds after energizing, which prevents overheating of the coil in situations such as abusive overcranking of an engine.  

INTERNAL SWITCHING

The internally switched solenoid utilizes a mechanical double contact switch, mounted on the rear of the solenoid, to turn off the pull coil. Best suited for applications such as standby generator sets or other applications where vibration, dirt, moisture, and excessive cycling are not present.  

Externally Switched Solenoid Terminations

Externally switched solenoids are available with the standard leads termination. They are also available with switch caps and either screw or blade type terminals. See Figure 3 

Pigtail (leaded) termination: designed for leads to be fitted to a connector 2. Switch cap with spade type terminals: designed for harness leads to be connected without soldering 3. Switch cap with screw type terminals: designed for harness leads to be be screwed on.

Solenoids can be pull type or push type. In a pull type solenoid, which is the more common type, when the solenoid is energized the plunger pulls in towards the solenoid. Pull type solenoids can be internally or externally switched.

In a push type solenoid, when the solenoid is energized the plunger pushes out away from the solenoid. All push type solenoids are externally switched.

Both pull and push type solenoids are constructed in a similar manner and operate on the same principle. It is usually the application, mounting considerations, and direction of plunger motion needed that determine what type of solenoid will be used.

Energized-to-Run vs. Energized-to-Stop Applications

Solenoids can be used in both Energized-to-Run (ETR) and Energized-to-Stop (ETS) Applications.

In Energized-to-Run applications, which are more common, the equipment is turned on when the solenoid is energized.

In Energized-to-Stop applications the equipment is turned off when the solenoid is energized. For example, in marine applications, it is desirable to keep the engine running at all times while at sea. Therefore, the engine fuel control lever is held in the run position. To stop the engine, a solenoid is energized long enough to stop fuel completely and shut the engine off. When de-energized, the solenoid returns the lever to its minimum fuel lever for the next engine start.

In ETR applications, the solenoid is on continuously and therefore the ES solenoid requires some form of coil protection such as a Coil Commander unit to protect from engine over cranking. In ETS operation, since the application is being turned off, the need for protection is minimum.

Shutdown vs. Throttle (Choke) Applications

Shutdown applications control via the fuel shutoff lever. The lever controls fuel flow to the engine or application. Usually a relay in the engine start system switches from the pull coil to the hold coil. However, over cranking by the operator can burn out the solenoid and therefore including a timing device is preferred as a safety measure.

Throttle (Choke) applications use a solenoid to increase the amount of fuel going to the engine. It pushes or pulls the throttle control lever to a preset position. With an externally switched solenoid, a timing device (such as a Coil Commander) to switch from the pull coil to the hold coil is needed to limit the pull coil on time. As an alternative, an internally switched solenoid can also be used.

Shutdown solenoids are prone to failure because of over cranking by the operator, resulting in overheating, while throttle solenoids are prone to failure because of likely problems in the system.

Coil Protection

In an internally switched dual coil solenoid, the pull coil is energized at the same time as the hold coil, with the key switch in the “Run” position. The plunger switches off the pull coil when it bottoms out, provided there is enough voltage and the linkage is adjusted correctly.

In an externally switched dual coil solenoid, the hold coil is designed to be energized continuously with the key switch in the “Run” position. There is little or no danger of the hold coil failing.

The pull coil is designed to be energized for a short time during engine cranking with the key switch in the “Start” position. Thus, the pull coil current is considerably higher (30-50 amps depending on the model) than that of the hold coil.

Energizing the pull coil causes a significant rise in the internal temperature of the solenoid. Burnout of the coil can occur with excessive engine cranking. Therefore, the pull coil must be turned off, as soon as possible, after pulling the plunger to the “bottomed out” position.

For both internally switched and externally switched solenoids, it is recommended that engine cranking be limited to three 30-second attempts with a cool-down period of two minutes (120 seconds) between each cranking attempt. After three cranking attempts (total elapsed time of 5½ minutes), the solenoid must be allowed to cool down to ambient temperature before making further cranking attempts.

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