Coil Switch Machines

TOPICS IN THIS SECTION

COIL MACHINE OPERATING BASICS

OPERATION OF TWIN COIL MACHINES

TYPES OF TWIN COIL MACHINES

TWIN COIL CURRENT CONSIDERATIONS

TABLE OF TWIN COIL DRIVE POWER REQUIREMENTS

COIL MACHINE OPERATING BASICS

Coil based switch machine are, as the name implies, based on a coil or more accurately a solenoid. Technically the word "solenoid" is another name for a coil. However in the industrial industry, solenoid also refers to a coil based machine that can create linear mechanical motion to do some kind of work. The motion is very limited but can be very powerful. The solenoid is the bases of making relays (contactors), electric valves or in our case a turnout switch machines.

To understand how the switch machine works, we need to understand how a solenoid works. A solenoid consist of Terminals (or wires to reach the coil), a Coil, an Armature (Plunger or slug) and a frame (open or closed) to hold it all together. A picture of a simple open frame solenoid is shown below:

Construction of the solenoid is a coil wrapped around a hollow tube. Inside the tube is the solid metal armature which is allowed to freely slide up and down the length of the tube. However to prevent the armature from sliding completely out of the tube, some form of mechanical stops are included.

A solenoid's key to operation is the coil. When we pass electricity or more accurately current threw the coil, the coil creates a magnetic field. The more current we pass through the coil, the stronger the magnetic field gets. The stronger the magnetic field, the more mechanical force is available to do work.

Iron based metal is attracted to magnetic fields. The Armature is made of steel (iron). When the coil is energized (turned on) with current, the armature is attracted to it and will slide within the tube up or down as required to reach the strongest concentration of the magnetic field within the tube. The strongest magnetic field is found in the very center of the coil in terms of the coils physical length. For most solenoids, that will also be the very center of the tube.

But what happens to the armature when the coil is de-energized (turned off)? The Armature will stay right where it is assuming gravity is not a factor. However, if the armature is left in the center of the tube, the armature will not move at all the next time the coil is energized! The solenoid will not work. What is required is some EXTERNAL force on the armature that will cause it to move out of the center of the tube. Often part of that external force is integrated into the solenoid's tube in the form of an internal spring. With the coil off, the spring will push the armature out one side of the tube until it hits a mechanic stop.

However, such a solenoid design is not useful for a turnout. Why?

1) The spring will alway push the points back to one direction of the turnout when you turn off power to the solenoid! That means to keep the given turnout thrown the other way, you will have to provide power to the solenoid constantly.

2) If you have to energize 50% of the solenoid on the layout to keep trains running, the amount of power required will force the use of a big power supply or powerpack. That translates into costing more money in terms of the size (high power rating) of the power pack and the use of lots of electricity.

3) The coils, when continuously energized, will get hot which means they must be designed to handle the heat. The will lead to a larger mechanical design to allow the heat to dissipate into the air. More material means more cost. In addition, scale modeling need small switch machines for both practical and cosmetic reasons and this will work against that goal.

How do you solve these problem?

Use a "Dual Solenoid" or "TWIN COIL" solenoid or switch machine.

OPERATION OF TWIN COIL MACHINES