Direct Current Motor Control

Contents

DC Motors - DC Resistance Control - Motor Control and Protection - DC Power CircuitSeries-Parallel ControlRegenerative Braking - Rheostatic Braking

This page describes the development of electric train power equipment which uses resistance controlled Direct Current (DC) motors.  This was the most common form of electric train control for almost 100 years until the advent of power electronics.  On other pages you will find Electric Traction Drives, Multiple Unit Operation and Electronic Power Traction described.  There is also an Electric Traction Glossary.

DC Motors

DC Motor & Commutator - tractn02.gif (4353 bytes) The DC motor was the mainstay of electric traction drives on both electric and diesel-electric for many years.  It consists of two parts, a rotating armature and a fixed field.  The fixed field consists of tightly wound coils of wire fitted inside the motor case.  The armature is another set of coils wound round a central shaft.  It is connected  to the field through "brushes" which are spring loaded contacts pressing against an extension of the armature called the commutator.  The commutator collects all the terminations of the armature coils and distributes them in a circular pattern to allow the correct sequence of current flow. 

The motor works because, simply put, when a current is passed through the motor circuit, there is a reaction between the current in the field and the current in the armature which causes the armature to turn.  The armature and the field are connected in series and the whole motor is referred to as "series wound".

A series wound DC motor has a low resistance field and armature circuit.  Because of this, when voltage is applied to it, the current is high.  (Ohms Law: current = voltage/resistance).  The advantage of high current is that the magnetic fields inside the motor are strong, producing high torque (turning force), so it is ideal for starting a train.  The disadvantage is that the current flowing into the motor has to be limited somehow, otherwise the supply could be overloaded and/or the motor and its cabling could be damaged.  At best, the torque would exceed the adhesion and the driving wheels would slip.  Traditionally, resistors were used to limit the initial current.

DC Resistance Control

As the DC motor starts to turn, the interaction of the magnetic fields inside it causes it to generate a voltage internally.  This "back voltage" opposes the applied voltage and the current that flows is governed by the difference between the two.  So, as the motor speeds up, the internally generated voltage rises, the effective voltage falls, less current is forced through the motor and thus the torque falls.  The motor naturally stops accelerating when the drag of the train matches the torque produced by the motors. To continue accelerating the train, resistors are switched out in steps, each step increasing the effective voltage and thus the current and torque for a little bit longer until the motor catches up. This can be heard and felt in older DC trains as a series of clunks under the floor, each accompanied by a jerk of acceleration as the torque suddenly increases in response to the new surge of current. When no resistor is left in the circuit, the full line voltage is applied directly to the motor. The train's speed remains constant at the point where the torque of the motor, governed by the effective voltage, equals the drag - sometimes referred to as balancing speed.  If the train starts to climb a grade, the speed reduces because drag is greater than torque.  But the reduction in speed causes the back voltage to decline and thus the effective voltage rises - until the current forced through the motor produces enough torque to match the new drag.

Simple DC Circuit - dc-cir01.gif (5032 bytes) On an electric train, the driver originally had to control the cutting out of resistance manually but, by the beginning of the First World War in 1914, automatic acceleration was being used in the UK on multiple-unit trains.  This was achieved by an accelerating relay (often called a "notching relay") in the motor circuit (see next diagram below) which monitored the fall of current as each step of resistance was cut out.  All the driver had to do was select low, medium or full speed (called "shunt", "series" and "parallel" from the way the motors were connected in the resistance circuit) and the equipment would do the rest.

Motor Control and Protection

DC Circuit with relays - dc-cir02.gif (5809 bytes) As we have seen, DC motors are controlled by a "notching relay" set into the power circuit.  But there are other relays provided for motor protection.  Sharp spikes of current will quickly damage a DC motor so protective equipment is provided in the form of an "overload relay", which detects excessive current in the circuit and, when it occurs, switches off the power to avoid damage to the motors.  Power is switched off by means of Line Breakers, one or two heavy-duty switches similar to circuit breakers which are remotely controlled. They would normally be opened or closed by the action of the driver's controller but they can also be opened automatically by the action of the overload relay.

On a historical note, early equipment (pre-1905) had a huge fuse instead of an overload relay. Some of these lasted into the 1970s and I recall the complications of changing one, which involved inserting a wooden board (called a "paddle") between the shoes and the current rail. This was to isolate the current from the locomotive while you changed the fuse.

A further protective device is also provided in the classic DC motor control circuit.  This is the "no-volt" relay, which detects power lost for any reason and makes sure that the control sequence is returned to the starting point (i.e. all the resistances are restored to the power circuit) before power could be re-applied.  This is necessary to ensure that too much current is not applied to a motor which lost speed while current was off.

DC Power Circuit

DC control sequence - dc-cir03.gif (6423 bytes) This diagram shows a simple traction motor power control circuit.  Most DC motor circuits are arranged to control two or four motors.  The control range is enhanced by changing the connections to the motors as the train accelerates.  The system is known as "series-parallel control".





Series-Parallel Control

Transition sequence - dc-cir04.gif (4730 bytes) This diagram shows the principle of series-parallel control.  There are three stages, "series", "transition" and "parallel", which operate in that order.  The connections are changed automatically as the train accelerates.  Upon starting, the motors are in series with each other and with all the resistance.  The resistance is cut out in steps and the train accelerates to "full series" when all the resistance is out of circuit.  The train may be running at about 30 km/h now. 

If full speed has been selected, the transition circuit will provide a parallel connection between the two legs of the series circuit.  Immediately this is done, the two series connections will be opened and the resistances inserted back into each motor circuit.  The resistances are then cut out in steps again until all are out of circuit.  The motors are now running at "full parallel" and the train speed will rise to the design speed.  

During this whole process, the correct control sequence is maintained by the low voltage control circuit under the overall control of the driver selecting "shunt", "series" or "parallel" on his master controller.

Field Weakening

field.gif (8408 bytes) The DC motor can be made to run faster than the basic "balancing speed" achieved whilst in the full parallel configuration without any resistance in circuit.  This is done by "field shunting".  An additional circuit is provided in the motor field to weaken the current flowing through the field.  The weakening is achieved by placing a resistance in parallel with the field.  This has the effect of forcing the armature to speed up to restore the balance between its magnetic filed and that being produced in the field coils.   It makes the train go faster.

Various stages of field weakening can be employed, according to the design of the motor and the intended purpose.  Some locomotives used as many as six steps of field weakening.

Regenerative Braking

Since the DC motor and a DC generator are virtually the same machine mechanically, it was immediately realised that a train could use its motors to act as generators and that this would provide some braking effect if a suitable way could be found to dispose of the energy. The idea formed that if the power could be returned to the source, other trains could use it. Trains were designed therefore, which could return current, generated during braking, to the supply system for use by other trains. Various schemes were tried over many years with more or less success but it was not until the adoption of modern electronics that reliable schemes have been available.

Rheostatic Braking

The major drawback with the regenerative braking system is that the line is not always able to accept the regenerated current.  Some railways had substations fitted with giant resistors to absorb regenerated current not used by trains but this was a complex and not always reliable solution. As each train already had resistors, it was a logical step to use these to dispose of the generated current. The result was rheostatic braking.  When the driver calls for brake, the power circuit connections to the motors are changed from their power configuration to a brake configuration and the resistors inserted into the motor circuit.  As the motor generated energy is dispersed in the resistors and the train speed slows, the resistors are switched out in steps, just as they are during acceleration. Rheostatic braking on a DC motored train can be continued down to less than 20 mph when the friction brakes are used to bring the train to a stop.

Before the advent of power electronics, there were some attempts to combine the two forms of what we now call "dynamic braking" so that the generated current would go to the power supply overhead line or third rail if it could be absorbed by other trains but diverted to on-board resistors if not.

In the case of diesel-electric locomotives, dynamic braking is restricted to the rheostatic type. Racks of resistors can often be seen on the roofs of heavy-haul locomotives for which dynamic braking is a big advantage on long downhill grades where speed must be maintained at a restricted level for long periods.