Getting power from the engine to the wheels in a controlled manner
It is a popular misconception that class 50s are complicated locomotives. It is perhaps true if compared to say a class 40, but more modern designs such as the unique 89 or the classes 92 and 67 are orders of magnitude more sophisticated.
The Electrical System
The Electrical System
Electricity can best be simply explained by analogy with water. Water flows in pipes as direct current (d.c.) electricity flows in cables. In electrical terms the voltage determines how much current flows, just as the pressure on water will dictate how much will flow through a pipe.
The power plant on a Fifty turns three generators each with its own particular role. The first generator (Main) supplies power to move the train, the second supplies the train heating, and the last supplies the auxiliary systems on the locomotive. An electrical generator can be considered to be a pump, with its output current (volume of electricity flowing) set by the strength of the magnetic field applied to it.
The output from the generator feeds three parallel circuits each containing two motors (see simplified circuit diagram). One motor in each arm of the circuit is mounted on each bogie. The volume of electricity, or current applied to the motor dictates how much work the motor can do, in the same way that the volume of water applied to a water wheel determines how much work it can do. The work done by the motors causes the train to accelerate.
When the driver wants to apply power to move the train, he moves the power handle on his desk from 0 to 1, this closes the contactors (M1-M6 in the diagram), and the control system applied a field to the main generator, this allows the main generator to "pump" electricity through the motors (1-6), the motors turn the wheels and the train starts moving.
The Control System
The Control System
The control system on the class fifty is unique, and it is this part of the locomotives that makes them more sophisticated than their predecessors. When the driver moves his power controller on a class fifty he is effectively setting the desired current he wants to draw from the generator. He can select any one of an almost infinite number of possible current settings between minimum and full current using the power handle between positions 1 and 7.
The current output from the generator is measured and compared with that requested by the driver in a current controller, this outputs an error signal which is a measure of the difference between the required current and the actual current.
The error signal is feed to the generator field thyristor controller, this supplies the power to the magnetic field on the main generator. If the current output from the generator is too low, the error signal will make the field controller increase its output and thereby strengthen the magnetic field so the generator will produce more current. If the generator current is higher than requested by the driver, the generator magnetic field is decreased, and hence less current flows. The control system also prevents the driver from overloading the main generator by preventing the generator field being strengthened any further once maximum current is reached.
This form of control gives the best possible utilisation of the diesel sets’ power output as well as coping automatically with varying gradients without overloading the generator, and with minimum driver intervention.
A locomotive needs to be able to run in both directions (forward and reverse), to achieve this on a class 50 the direction of the current following in the field compared to the rotating part of the motor needs to change. This is done by the Reverser (labelled R in the diagram above). In one Reverser position the follow of electricity in the field is +ve to -ve and so is the flow in the rotating part and the motor will turn clockwise, with the reverser switched over, the flow in the field is now -ve to +ve but the flow in the rotor is still +ve to -ve, so the rotor will turn anticlockwise.