Electric Traction Drives

Introduction

This page describes the way electric motors on locomotives and multiple units drive the axles and wheels. See also the Electronic Power, Multiple Unit OperationDC Traction Motor Systems and Electric Traction Glossary pages.


Contents

The DC Traction Motor: How it Drives the Axle - AC and DC Motors - Nose Suspended Motor - Quill Drive - Monomotor Bogie - Linear Motor

The DC Traction Motor: How it Drives the Axle

tractn01.gif (7282 bytes) The traditional DC (Direct Current) electric motor driving a train or locomotive is a simple machine consisting of a case containing a fixed electrical part, the stator (called the stator because it is static and comprising what is called the field coils) and a moving electrical part, the rotor (because it rotates) or armature as it is often called.  As the rotor turns, it turns a pinion which drives a gearwheel.  The gearwheel is shrunk onto the axle and thus drives the wheels as shown in the diagram above.

The motion of the motor is created by the interaction of the magnetism caused by the currents flowing the the stator and the rotor.  This interaction causes the rotor to turn and provide the drive. 

The stator and the rotor of the DC motor are connected electrically.  The connection consists of fixed, carbon brushes which are spring loaded so that they remain in contact with an extension of the armature called the commutator.  In this way, the field coils (the stator) are kept in the circuit with the rotor (the armature and commutator).

AC and DC Motors

Both AC (Alternating Current) and DC motors have the same basic structure but there are differences and, for various reasons, the DC motor was originally the preferred form of motor for railway applications and most systems used it.  Nowadays, modern power electronics has allowed the use of AC motors and, for most new equipments built today, the AC motor is the type used. 

Often, people ask about the differences between AC and DC motors as used in locomotives and multiple-units.  In the early days of electric traction at the beginning of this century both types were tried. The limits of the technology at the time favoured the DC motor. It provided the right torque characteristic for railway operation and was reasonably simple to control.

By the early 1980s, power electronics had progressed to the stage where the 3-phase AC motor became a serious and more efficient alternative to the DC motor because:

1. They are simpler to construct, they require no mechanical contacts to work (such as brushes) and they are lighter than DC motors for equivalent power.

2. Modern electronics allow AC motors to be controlled effectively to improve both adhesion and traction.

3. AC motors can be microprocessor controlled to a fine degree and can regenerate current down to almost a stop whereas DC regeneration fades quickly at low speeds.

4. They are more robust and easier to maintain than DC motors.

This type of motor is commonly called the Asynchronous Motor and was often referred to as the squirrel cage motor on account of its early design form.  The photos below show a DC and an AC motor.

DC motor brushes.jpg (47568 bytes)
DC traction motor brushes

Click for full size
Modern AC traction motor

The DC motor is similar to look at externally but there are differences in construction, particularly because the DC motor has a commutator and brushes which the AC motor does not.

Nose Suspended Motor

The following diagram shows the layout of the traditional DC motor mounted in a bogie as a "nose suspended motor".

whl008.gif (11693 bytes) In electric trains or locomotives, the DC motor was traditionally mounted in the bogie frame supported partially by the axle which it drove and partially by the bogie frame.  The motor case was provided with a "nose" which rested on a bracket fixed to the transom of the bogie.  It was called a "nose suspended motor" (see diagram above) and is still common around the world.  Its main disadvantage is that part of the weight rests on the axle and is therefore unsprung.  This leads to greater wear on bogie and track.  Nowadays, designers try to ensure all the motor weight is sprung by ensuring it is carried entirely by the bogie frame - a frame mounted motor.

Quill Drive

whl010.gif (14932 bytes) This is a simplified diagram of a quill drive.  A quill is described in the dictionary as, "the hollow stem of a feather" and "a bobbin or spindle", as well as a "feather" and, alternatively, what a porcupine has on its back. 

In railway traction terms, a quill drive is where a hollow shaft is placed round the driving axle and the motor drives the quill rather than driving the axle as it does with a nose suspended drive. The quill itself is attached, at one end, to one of the wheels by means of rubber bushed links and, at the other end, to the gearwheel by similar links. The big advantage of such drives is that all the weight of the motor is carried in the bogie frame (so it is a frame mounted motor) instead of it being directly supported by the axle and therefore partially unsprung.

Click for full size An example of a traction motor with quill drive appears in the following photo.  Click on it for a full size view and the part names.  Various forms of quill drive have been used over the years.  Older versions used radially mounted coil steel springs instead of rubber to connect the links to the wheels.  Some, like the example shown here, have the motor mounted parallel with the axle.  Others have the motor at a right angle to the axle, as in the the UK Class 91 electric locomotives.

In German the quill is called "Hohlwelle" (hollow shaft) and used in the ICE1 and ICE2 as well as the electric locomotive Class 101. (Source Tobias Benjamin Koehler 19 Oct 98). 

Monomotor Bogie

Click for full size As its name implies, the monomotor bogie has a single motor which drives both axles.  Click on the thumbnail to see a photo with the part names.

The design is much favoured in France, where it was introduced in the 1950s for the rubber tyred train concept.  The motor is mounted longitudinally in the centre of the bogie and drives each axle through a differential gearbox, similar to a road vehicle.  The differential gears are required to compensate for the operation of the rubber tyres round curves.  It requires a special bogie frame construction to accommodate the motor.

Another version of the monomotor bogie has also been applied to a number of French locomotive designs but here the arrangement is more conventional.  Each bogie has a single motor mounted transversely over the centre as shown in the diagram left.

monomot.gif (9605 bytes) The motor is fully suspended in the bogie frame and drives both axles through the gear train, which is contained in a single, large, oil filled gearcase (not shown).  This type of drive is referred to in the locomotive wheel arrangement called a B-B, as opposed to a more conventional locomotive with four motors, each driving its own axle, which is called a Bo-Bo.


Linear Motor

Linear Motor - tractn03.gif (5144 bytes) A new form of traction which has appeared in recent years is the linear motor.  The principal, compared with a standard motor, is shown here.  This simple diagram shows the principal of the linear motor.  The conventional DC motor consists of a fixed part (the stator) and a moving part (the rotor).  Both parts are contained in a case on the train and the rotor is connected to the axle by a pinion/gear arrangement.  When the armature turns, the wheel turns.

The two parts of the linear motor are separated and one is placed on the train and the other on the the track.  Both parts are unwrapped and they are swapped so that the fixed part of the DC motor becomes the moving part of the linear motor mounted on the train while the former moving part of the DC motor is fixed to the track.  The electro-magnetic interaction between the current in the fixed part and that in the moving part causes the train to be drawn along the line.  There is a very small air gap (about 10 mm) between the two parts as shown in this photo.

LIM Air Gap.jpg (27584 bytes) The efficiency of the linear motor is about 60% of the conventional motor but it has the advantage of less moving parts and it does not have the reliance on adhesion of the conventional motor.



See also the Electronic Power, Multiple Unit OperationDC Traction Motor Systems and Electric Traction Glossary pages and Power Supplies pages.