Control of Model Electric Locomotives (Traction)

All models which use batteries or alternators as their power source and electric tracion motors need to be wired in a way which functions and to protect the various components. They also all need some form of speed controller, and a means to reverse the direction of travel. This article describes the various connection and methods of achieving those aims.

Basic Connections. Figure 01 below shows the basic connections required for all battery powered models.

Basic schematic
Fig. 01

There are several important points to note.

  • There should be a main fuse or circuit breaker (F1) right at the battery (or as close as practical) for protection and safety reasons, and to prevent damage to the battery or other wiring in the event of a fault.
  • There should be a Battery Isolator switch (S1).
  • There should be separate switches for Traction ON/OFF (S3) and Auxiliary ON/OFF (S2).
  • The main connection to the controller and traction motor(s) should be run separately with heavy duty wire to handle the high traction currents.

Suitable battery isolators and fuses/circuit breakers are readily available from motor accessory, caravaning and boating stores.

 

Forward/Reverse. All models require For/Rev and the simplest connection is to use a double pole double throw (DPDT) switch. See Figure 02 for details.

DPDT Reverser
Fig. 02

While this circuit works quite well, and has been successfully used, there are several problems:-

  • The reversing switch must carry the full traction current, and such heavy duty switches are not easy to obtain.
  • Due to the heavy traction currents and heavy wiring required, the switch must be on the model itself making a small hand-held controller impossible.
  • It is either forward or reverse, there is no neutral or off position, unless a suitable DP3T switch can be found.
  • Multiple unit operation is not possible.

 

Relay Reverser.
Fig. 03

A better alternative is to use relays to do the actual high current switching, and just use a normal low-power switch to control the relay coils, see Figure 03. Remote operation via an umbilical cord to a hand-held controller, and multiple unit operation are now possible. Also, it is good practice to use a separate relay for Forward and another relay for Reverse, with no traction power to the motor(s) possible unless one relay is ON. This makes it failsafe in the event of the model breaking away and severing the control connection - both relays simply drop out cutting traction power, and the model just coasts to a stop.

When using 2 relays like this, they MUST be interlocked so that only one relay is ever operated at any one time, never both, otherwise there will be short circuit across the battery, and you will be glad of the circuit breaker/fuse! Interlocking can be achieved by using spare contacts on the relays, or by using smaller relays with more contacts to control the larger traction relays.

 

Reversing for different types of motors. As there are different types of motors used, each has different connections, and requirements for reversing. See the discussion paper on motors for more details on suitable motor types.

Below are the typical circuit diagrams for reversing the different types of motors used for traction purposes. The reverse switching is as per Figures 02 or 03 above.

Reversing PM motors.
Fig. 04
Reversing series motors.
Fig. 05
Reversing shunt sepex motors.
Fig. 06
Permanent Magnet
motors
Series Wound
motors
Shunt Wound motors
(separately exited)

 

Controllers:- Every model must have some form of control. The very simplest is just an ON/OFF switch, like a light switch. This gives just 2 speeds Stop and Max Speed. It works, it is simple, but is not overly useful or convenient! Something a little more user friendly is required, and the complexity of control systems ranges from quite simple to very sophisticated, and the cost varies accordingly.

The fundamental requirement is to vary the amount of current flowing through the armature of the motor, in an acceptably smooth fashion from zero (OFF) to maximum (Full speed).

Resistor controller. This is the traditional method, and has been used for over 100 years in railway and tramway use, and for cranes and elevators etc. Refer to Figure 07. A resistor is placed in series with the motor, and as the speed increases, sections of this resistor are switched out to make a progressively lower resistance until it is zero value (equals Max speed). The different switch positions are commonly called 'notches' giving rise to the term 'go up a notch' referring to increasing speed. Changing from one resistor setting (notch) to the next causes the small jerk felt sometimes during acceleration of vehicles equipped with this type of controller.

Resistor controller
Fig. 07

Notes about resistor controllers

  • The number of notches is typically between 3 and 8 or 9, although any number can be used.
  • Either one tapped resistor, or individual resistors may be used.
  • As the large traction currents flow though the resistor(s), there may be significant heat generated in them, and thought should be put into their physical location in regards to cooling and proximity to adjacent equipment.
  • As resistors of this type and power have typically bare metal exposed, this is "live" at the full battery voltage. Care should be taken to avoid any metal objects accidentally touching it. A guard may be a useful addition.
  • As some power is used and dissipated as heat in the resistors as part of the control function, resistor controllers are not as efficient as modern semiconductor ones. However, they are time proven, cheap, very rugged against overloads and rough handling, simple to understand, build and operate.

The next question is what value (and power) resistor to use. A good guide it to have say 10%~15% of maximum current for the first notch. And some fine-tuning of the values after testing is often required. Some basic electrical calculations are required to calculate the required resistor value. A simple example may be of some help.

Assume you have one 24V 200W motor. The typical ratings of such a motor are 24V DC, 11A. Note that 11 amperes is the full-load rated current at 100% speed, but the stall current is say 18A. This is effectively the starting current and should be used for this calculation. 24V 18A represents a resistance of 1.33Ω (1.33 ohms) which equals the motor stall resistance. For 15% current (18A x 0.15 = 2.7A) we need a total resistance of approx 9Ω, or a resistor value of 8Ω when the motor resistance is subtracted (9Ω - 1.33Ω). So for a 5-notch controller, values of 8Ω, 6Ω, 4Ω, 2Ω and 0Ω are good starting points for the resistors. For the highest (max speed) notch, the resistor is switched out completely (zero ohms). Power in the 8Ω resistor at 2.7A is approx 60W, so the resistor must be rated at say 100~150W.

Switch-mode Controller (Basic type). Switch-mode controllers are electronic circuits using high power transistors (usually but not always, MOSFET type transistors), and work by rapidly switching the current on and off hundreds or thousands of time per second. See Figure 8a for the equivalent schematic.

Switchmode simplified schematic
Fig. 08a

It is an inherent property of motors that when rapidly varying power is applied, they tend to smooth out the current (i.e. the bit that does the useful work) to a steady average value with minimum ripple. The voltage may vary rapidly but the current won't.

Most switch-mode controllers switch the current on and off at 300 to several thousand times per second, and you may hear a high-pitched whine coming from the motors and/or wiring. This is normal as is the controller doing its job.

Another inherent property of motors is that when the current is switched off, it tends to keep flowing due to energy storage within the windings. A diode, connected anti-parallel with the motor, is required to keep the current flowing and help provide the smoothing, and to prevent voltage spikes damaging the transistors in the controller.

During the period when the switch is closed, the green arrow shows where the current flows. In the period when the switch is open, the blue arrow shows the current which continues to flow. This diode carries the full traction motor current and MUST have a current rating equal to or larger than the motor stall current. The inverse voltage rating of the diode should be at least 5x the battery voltage. Note that this diode may need to be mounted on a heatsink, as power dissipated by it is about 1V times the motor current, i.e. 18W at starting, 12W during running for a 24V 200W motor.

Switchmode controller
Fig. 08b

 

Switchmode controller
Fig. 08c

How do these controllers work?

They switch the current on and off at a relatively high frequency, and the motor receives a series of pulses [Figure 08b] that vary between 0% and 100% duty cycle.

 

The power to the motor is controlled by varying the width (duty cycle) of these pulses [Figure 08c].

 

 

At low speeds, these pulses are narrow [Figure 08d (1)] and the average power to the motor is low.

 

 

As the speed is increased, the pulse width is wider [Figure 08d (2)] and the corresponding average power to the motor is higher.

 

As the speed is increased even higher, the pulse width is much wider [Figure 08d (3)] and the corresponding average power to the motor is higher again.

 

Switchmode controller
Fig. 08d

Good design controllers can vary the pulse width from 0% to 100%.

Note that this controller does not provide any reversing capability, it simply controls the current in a more efficient way than the resistor controller above. Switched motor reversing is still required.

 

Switch-mode Controller (H-Bridge type). The next level of complexity in switch-mode controllers is the so-called H-bridge type, named after the basic shape of the schematic diagram. The ideas and principles are the same as for the standard switch-mode controller, except now the reversing function is added. See Figure 09 for the equivalent schematic.

H-bridge simplified schematic
Fig. 09

The H-bridge controller has 4 power switches, rather than just 1, and controls the motor by switching in a different pattern. Basic speed control is exactly the same as for the basic switch-mode controller above, with the pulse width being varied to vary the motor speed.

For forward operation, only switches 1 and 4 are used, and switches 2 and 3 remain open. The current flows through the motor in a L-R direction in this diagram (green arrows). For reverse operation, the situation is reversed and only switches 2 and 3 are used, and switches 1 and 4 remain open. The resultant motor current now flows R-L (blue arrows), and the motor turns in the opposite direction.

 

Further enhancements on the more expensive switch-mode controllers include, current limiting, acceleration control, regenerative braking, enhanced multiple unit control capability, wheel-slip control, ..... etc. But of course the complexity and cost is increasing. It is best to purchase a commercial ready-made controller unless you are sure of your electronic capabilities.

When purchasing, or designing/building ANY controller, it is the stall current of the motor(s) which should be used in the relevant calculations, not the running current which is usually marked on the label and spec sheets. The stall current may be 2 or 3, or even more, times the nominal running current. Failure to take the stall current into account may result in a dead controller.