Performance - Traction Motor Controls
The primary objective of the Traction Control System is to provide tractive effort from the traction motors corresponding to the demands of the driver as set by the master controller. The Class 60 traction system is based around the 6 Brush TM2161A Traction Motors which are axle hung machines and drive the wheel-sets through single reduction gearing. This type of traction motor is separately excited ( Sepex ) and requires a separate power supply for its traction motor field windings. This gives major advantages in the control of wheel slip compared to the conventional series field motor where the armature current is fed through the field as well. Although the series system is simpler, ( as used on the Class 59, 66 & 67 ) the Sepex motor is much more capable of being controlled precisely when a wheel slip commences and hence rail burns are avoided because wheel slips are much less severe. Most important of all, the Sepex motors allows the Class 60 control system to deliver high tractive efforts by making full use of the available wheel/rail adhesion.
The basic advantage of the Sepex motor is that by using a separate supply to magnetise the motor field poles, the machine is much less subject to influences from the armature current. On a series motor, a wheel slip commencing can cause a runaway effect as the accelerating armature causes its current to drop, which in turn causes the motor field to weaken, hence encouraging the motor to increase its speed further. This effect, combined with a simple wheel slip system can lead to rail burns ( especially when starting off ) and failure of trains because adequate T.E. cannot be sustained for long enough periods. ( Many of the older loco's suffered these problems )
The Sepex motor is inherently easier to manage when adhesion is low and much closer control of motor speed and T.E. can be maintained. The separately excited DC traction motor was used on many other Brush vehicles and is in many ways superior to the later AC drive systems used by other manufacturers. It is much more tolerant of wheel diameter variations between the axles and saves on wheel turning. It takes the advantages of the fully controlled DC motor but does not require the complexities of inverter drives to power it. each motor has a speed probe mounted in it to give indication of axle speed. The traction motor control system uses six independent T.M. field converters to excite the individual motor fields. These are fully variable, and reversible, and are electronically controlled. Armature current is provided by the Main Alternator via the Main Rectifier, the alternator output and hence D.C. output from the rectifier being controlled by the main alternator field. This is energised by the fully variable Main Alternator Excitation Converter which is operated from the Control Electronics System. These systems provide all the necessary traction motor control features required to control locomotive T.E.
OUTLINE OF TRACTION MOTOR CONTROL SYSTEM
At standstill, when power is demanded by the driver, and, when all input signals are seen to be correct and the system is fault free, the Control System closes the appropriate contactors, enables the T.M. field and Main Alternator Excitation Converters, and increases engine speed to an appropriate setting. Each traction motor is set to full field and the armature currents are increased smoothly to the required value. At all times, the engine loading is monitored to ensure that the engine power requirement is built up smoothly and that the engine is operating within its limits. The " Load Control " feature involves the Engine Governor closely. A separate " Power Control " scheme operates within the Control System and acts as a backstop to Load Control to ensure that the driver cannot demand more power that the locomotive can produce.
All these functions within the Control System will combine to provide T.E. as demanded by the driver and within the power constraints of the locomotive, unless a loss of rail adhesion occurs, leading to wheel slip.
All wheel speeds are continuously monitored and any loss of adhesion giving rise to wheel speeds above the true locomotive rail speed is corrected by the intervention of the wheel creep and wheel slip system; this takes corrective action to optimise the locomotive performance for the prevailing rail conditions. Wheel Creep control operates from standstill, and gives the loco its unique all weather hill start performance.
As locomotive speed increases, field weakening takes place to allow the locomotive speed to increase further by ensuring that the power unit output power can always be delivered to the traction motors and that no off loading occurs. The weakening of each motor field is achieved by gradual reduction of each Field Converter output. It is step-less and requires no contactors or divert resistors.
