Simple controllers for DC motors (inc PWM) inertia and braking (sim)

Simple Controllers a modular approach.

Firstly, I am not a qualified electronic technician. I am entirely self taught and simply research and design circuits for my individual needs.
These controllers were designed to drive model locomotives but the circuits should work with any single phase DC motor as long as the power requirements are taken into account when selecting components.

In order to control a model locomotive, we need to supply it with electrical power at varying levels for speed control, and we need to be able to reverse the polarity for directional control.

This article attempts to describe simple, effective ways that the home constructor can achieve this. The controllers are kept as simple as possible and are intended for DC (as opposed to DCC) use with N, OO or HO gauge, but should work with any nominally low voltage (12 - 18V) rated DC motors.

Before we begin to look at controllers a few words of warning. Firstly I am not an electrical engineer, most of these circuits have been adapted from well tried and tested ideas. Secondly MAINS VOLTAGES CAN KILL! Do not attempt to work on any type of mains voltage supply unless you are confident that you know what you are doing. For testing purposes I would recommend that you look at the section where power sources are discussed, or use an old 12V car battery as your power source.

I wanted a custom built control panel and therefore a custom made controller. As ever, being poor, it had to be cheap and easy to understand.
Here's my control panel. The motor controller is the lower right hand side of the panel.

Step 1 Explanation

In the early days of electric model railways the controllers most often found were of the ‘split rheostat’ type. This consisted of two semi-circular wire wound resistors with a wiper and an electrically dead centre. With the control knob at the centre, no power is supplied to the track. As the knob is advanced either to the left or the right, power is applied to the rails, but with the resister ahead of the wiper, both voltage and current are reduced. As the knob (and therefore the wiper) are advanced further, the amount of resistance is reduced and the voltage and current to the track is increased. In this way speed control is achieved. Reversing was simply accomplished by moving the control knob back passed the centre dead section and in the other direction.

The advantages of this arrangement were, no need for any directional switching, robustness (many of these controllers are still in use) and heat dissipation being achieved via the rheostat itself. The major disadvantages were cost of manufacture (rheostats are expensive), erratic behaviour as the wiper contacts begin to wear and get dirty, and finally the one that will haunt all further discussion. Poor starting and slow speed operations.

The problem of slow speed and starting control is a common one. Locomotive motors develop most power and torque at or near their main operating voltage (nominally 12Vdc). In order to start moving they have to overcome inertia, magnetic force and friction from bearings and gears. This means that you may have to move your speed control to 60 or 70 percent of full power, then the loco will race off at an unrealistically high speed. Once moving you find that you can back the control off for slow running, but at certain slow speeds performance can be erratic as the voltage and current drop to a level where the motor is struggling.

Using more modern (and cheaper) components we will attempt to address these problems one by one.

Below are three initial circuits so you can see where we are going. More detailed information in the next steps.

Comments

[park47]

Thanks...for your PWM circuit... :)

[gmoon]

Good article.

You, my friend, are a jack-of-all-trades...