MULTIPLE UNIT OPERATION

Introduction

Originally derived from lift operation over a hundred years ago, multiple unit (MU) control has become the most common form of train control in use around the world today.  This page describes how it started and its development in the century to date.

See also Electric Traction,  Electric Traction Drives,  Electric Traction Glossary and Electronic Power.

Contents

Origins - The Relay - The Contactor - Multiple Unit Control - Forward and Reverse

Origins

Electric locomotives were originally designed so that the motors were controlled directly by the driver.  The traction power circuits passed through a large controller mounted in the driving cab.  A handle was rotated by the driver as necessary to change the switches in the circuit to increase or reduce power as required.  This arrangement meant that the driver had to remain close to the motors if long and heavy, power-carrying cables were to be avoided. 

While this arrangement worked well enough, the desire to get rapid turnrounds on city streetcar railways led to the adoption of remote control.  The idea was that, if the motors could be remotely controlled, a set of driver's controls could be placed at each end of the train.  It would not be necessary to have a locomotive added at the rear of an arriving train to allow it to make the return journey.  A cab would be installed at each end of the train and the driver just had to change ends to change direction.  Once this idea was established, it was realised that the motors could be placed anywhere along the train, with as many or as few as required to provide the performance desired.  With this development, more but smaller motors were scattered along the train instead of building a few large motors in a locomotive.   This is how the concept of motor cars and trailer cars evolved.  Trailer cars are just passenger carrying vehicles but motor cars are passenger carrying vehicles which have motors and their associated control equipment. 

Multiple unit trains, as these trains became known, were equipped with control cables called train lines, which connected the driver's controls with the motor controls and power switches on each motor car.  The opening and closing of the power switches was achieved by electro-magnetic relays, using principles originally designed for lifts.  While the idea was being established on passenger trains, it was also adopted on locomotives.  It quickly became the standard method of control.

Full article: http://www.railway-technical.com/muops.shtml

Basic Model of a Wheelset, Degrees of Freedom

For more than 150 years, the wheel – rail system has provided a relatively safe system of transport. This safety level is so high that the mechanism is generally neglected and considered as a simple slider by most people.
However, the engineer’s point of view can be different, especially when taking into account responsibilities in a railway network. The wheel – rail contact is actually a complex and imperfect link. Firstly, it is a place of highly concentrated stresses. The conical wheel shape makes the wheelset a mechanical amplifier, limited by the transverse play, with partially sliding surfaces. The contact surfaces are similar to those in a roller bearing but without protection against dust, rain, sand, or even ballast stones.

If the track is considered to be rigid, then the railway wheelset has two main degrees of freedom:

• The lateral displacement, or shift, y
• The yaw angle, a

When the behaviour of a wheelset is unstable, the dynamic combination of these two degrees of
freedom is called “hunting.”
The lateral displacement and the yaw angle must be considered as two small displacements relative to the track. The play will be the limit of the lateral displacement between the two flange contacts. It is generally approximately +-8 mm.
The other degrees of freedom are constrained: the displacement along Ox and the axle rotation speed v around Oy are determined by the longitudinal speed Vx and the rolling radius of the wheel r0 with: Vx ¼ vr0: The wheelset centre of gravity height z and the roll angle around Ox are linked to the rails when there is contact on both rails.
The railway wheelset is basically described by two conical, nearly cylindrical wheels (Figure 1 and Figure 2), linked together with a rigid axle.

Figure 1 Wheelset degrees of freedom.

Figure 2 Rail, wheel and contact frames.

Each wheel is equipped with a flange, the role of which is to prevent derailment. In a straight line the flanges are not in contact, but the rigid link between the two wheels suggest that the railway wheelset is designed to go straight ahead, and will go to flange contact only in curves. This is the railway dicone or wheelset.
The interface between the wheel and the rail is a small horizontal contact patch. The contact pressure on this small surface is closer to a stress concentration than in the rest of the bodies. The centre of this surface is also the application point of tangential forces (traction and braking Fx, guiding or parasite forces Fy, see Figure 1). The knowledge of these forces is necessary to determine the general wheelset equilibrium and its dynamic behaviour.
In order to determine this behaviour and these forces, the first thing to do is to determine some contact parameters: the contact surface, the pressure and the tangential forces. This determination is generally separated into two steps:

1. The normal problem (Hertz theory)
2. The tangential problem (Kalker’s theory)