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Tapiola Parish Model Railway Club


Turnout control -- CDU


Normally i.e. according to manufacturers guides -- turnout motors are operated with alternating current (ac). This ac is take from the aux. autput of a train contoller / transformer. Usually the transformers have separate windings for driving the train and for the aux. equipment.

Transformer [2K]

Turnout motors are usually of twin coil type. The two coils are wound next to each other on a common hollow core. The iron armature moves inside the hollow core. This article does not cover slow motion turnout motors such as Tortoise or Bemo

The principle of a point motor

Let's first take a look of the operating principle of a coil. If electric current goes through a wire a magnetic field is generated around it. If a loop is formed from the wire, the field is concentrated inside the loop. The higer the current, the stronger the field. adding revolutions in the loop to make it a coil also add to the field. The magnetic "power" of a coil depends on the amount of revolutions and the magnitude of the current.

As current goes through the coil, it attracts the ferrous armature inside the coil. As there are two coils on common core, driving current through either coil will cause the aramature to move iside this coil and via linkage move the turnout point blades. The coil is usually made of quite thin enameled copper wire. As the current available from the aux. output of power pack is limited, the resistance of the coil must be kept sufficiently high. By making the coil from thinner and longer wire the resistance is higer, hence the current is lower. To compensate magnetic power loss due to lower current, the length of wire enables to make more revolutions into the coil.

Internal cut-out switches

As current flows through the coil windings the coil warms up. This warming up is so fast that the coil may be energized for only a second or two. If the coil is energized longer the coil will overheat and the enamel will melt and cause shorts within the coil. This means that the coil shortens and current will rise causing even more rapid rising of the temperature. Finally the curent becomes so high that the coil wire will break. To prevent this from happening internal cut-out switches are encorporated within point motors. They will de-enrgize the coil as the armature hits the end position. These switches are of very light design and only tolerate alternating curent.

Self induction

If direct current goes through a coil, the cutting of the circuit causes an arch between the opening contacts of the switch, ruining the contacts of small inexpensive buttons in no time. Why? If direct current source is connected to a coil, the current through the windings will rise slowly as the coil resists changes of current flow. As the steady state is reached, a magnetic field is built around it. If the current is cut the collapsing magnetic field will generate a current that flows in the coil. To get current flowing the voltage across the coil terminals will rise until somewere a weak point is found to let the energy of the magnetic field to become converted into flow of current (and consequently into heat). The weak point will be the opening contacts of the switch. Air gap is arched and molten metal (plasma) will flow between contacts ruining them.

By placing a diode across the coil terminals into a normally non-conductive way, the voltage need not go sky high to get the current flowing. As the current flows in the same direction as the coil was energized, but the coil is now a power source the voltage across the terminals is reversed and diode will only coduct during disconnection phase.

Installing diodes inside small turnout motors may prove to be very difficult and will make the use of alternating current impossible.

With alternating current the direction of current flow is constantly chancing, thus, as contacts open, the current flow is bound to reverse soon, and the current will go momentarily into zero. This is suffecient to kill the arch and letting the air in the gap cool to prevent another arch being struck between departing contacts. Hence lightly built cut-out switches inside turnouts will last if operated with alternating current. This is also the reason for most switches being rated higer currents and voltages if ac is being used.

How to increase turnout motor power

If we are stuck with motors with internal cut-out switches, the only way to increase the power is to make sure there are no extra losses in the cabling. If 0.22 sq-mm (24 AWG) wire is used and the point motor is in five meters (15 ft) distance from the power source, the resistance of the cable will be 0.8 ohms. To use 0.5 sq-mm (20 AWG) cable the resistance will drop to 0.35 ohms. If the coil resistance is in the region of 30 ohms, the gain will be 3 percent. With Peco motors (normal type) having 3.3 ohms resistance the gain will be 26 percent (this is a simplified example).

As we all know the voltage across the transformer terminals will drop as load is increased. This droping is caused by internal resistance of the source. The real power source may well be described as having an ideal ("non-dropping") source and a resistance in series with it. This internal resistanse acts like the transmission losses described above. By using powerful enough transformer -- with output voltage within the limits of the recomendations of the maker of the point motor -- the internal resistance will be lower and thus more energy is supplied to the coil.

Capacitor discharge unit -- CDU

If we use point motors with no internal cut-out switches we may rise the power of the motors considerably and provide protection to the coils by using a device called capasitive discharge unit (CDU). A fully charged capacitor resembles ideal power source: the output current of a typical capacitor used in capacitor discharge units is considerably higher than a reasonably sized transformer will ever deliver. The electronic components of the unit will disconnect the capacitor from the power source as the turnout motor is operated, thus the motor will only receive the "portion" or amount of energy stored in the capacitor, and so effectively preventing the coil from overheating. As the resistance in the circuit is less, the self induction becomes an increasingly important matter to be taken account of. A diode must be placed next to each coil.

How does a capacitor discharge unit work?

How does a capacitor discharge unit work? Below is an example of a design used at Tapiola.

Capacitor discharge unit

Diode D1 rectifies alternate current into a half wave direct current. Full bridge rectifier would only drop the voltage more but provide no additional advantage. The capacitors C1 and C2 are connected via transistor T1. By connecting the base of the transistor to the supply, control current may flow through resistor R1 to the base of the transistor and on to the emitter of the transistor and onwards to the capacitors. The current flowing through the base-emitter joint causes high current to flow from the diode D1 to the collector of the transistor T1 and on to the capacitors. This heavy current is in porpotion of the control current. The ratio depends on the current gain (hfe) of the transistor. With heavy duty power transistors this ratio is usually below 100. The resistor R1 will be selected according to the ac source. The transistor is a power transistor which must tolerate roughly twice the supply voltage and be capable of withstanding the collector-emitter current adjusted by the resistor R1. As the capacitor is being charged the current must be limited and thus the transistor operates like a shunt resistor. It must therefore also withstand the power dissipated during this phase.

When the turnout is operated the coil is connected across the output terminals. The capacitors will be discharged to the coil. To switch of the supply to the capacitors, the base of the transistor must be grounded. By providing a path from the transistor base to the output terminal, the point motor will ground the base, preventing current flow through the transistor. As the button is released, the diode D2 prevents the discharged capacitors shorting the base of the transistor. Diode D3 is an added precaution aginst reverse voltage generated by the self induction of the point motor coil, in case no diode is connected across the point motor coil terminals. If the diodes near motor coils are left out, the buttons will be ruined in no time!

The Light emitting diodes -- LEDs -- L1 and L2 are from the "Bells and Whistles" Dept. Led L1 (green) will light as the capacitors is charged and L2 (red) when the unit is connected to load. The leds may well be replaced with wire if no fancy lights are needed. It may be more economical to purhase two smaller capacitors and connect them as shown above instead of one huge one. The capacitors are of electrolytic type and must have voltage rating of roughly twice the supply voltage. The capacitance should be in a region of 2200 mmF (microfarads) or more. The resistance of the resistor could be 2.2 kohms and 2N3055 will do for the transistor.

How to operate more motors at the same time

To operate more turnouts at a time a diode matrix is often used. This is not possible with alternating current (turnout motors with cut-out switches). In such cases the turnouts may be operated with triacs. If the triacs are to be fired with direct current they must be capable of operating in all four quadrants. The Finnish National Railway Museum at Hyvinkää has (or had?) a huge layout where a number of turnouts had to tun at once and be operated with a reed switch. The switches were rated 30 mA, but the old Fleishmann motors took nearly an ampere each, and there were four of them turning at one time! With triacs this was no longer a problem. Each motor was equipped with a pair of triacs and current limiting resistors (and should have been equipped with small capacitors to prevent misfiring!). This system worked for years!

This also brings us to the negative side of CDU: The buttons are going to have the heavy current passing through them. There is an excellent explanation of a distributed capacitor discharge system in TracTronics web site. Their SwitchWitch [TM] unit is operated with few milliamps.


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