In my time on the tube, electrical control on trains was dominated by resistors, contactors, tap changers and wotnot with conventional DC motors. Nowadays I get confused by all the modern guff with bi-polar and all that jargon. Not to mention AC traction motors. How exactly please [in simple terms] does this modern traction control system work?
The basic chain of events 'as the power sees it' is:
1 - If the supply is AC (ie overhead) it's transformed and rectified to a lower DC voltage.
2 - The DC is fed into an inverter which converts it to three phase AC current
3 - The AC current is fed to an induction motor which turns the wheels - this equipment may be duplicated
The traction motors are induction motors, also known as asynchronous motors. Induction motors are very common in household applications such as fans and tumble dryers, right up to the heaviest of industrial motors. The principle of operation is that windings are mounted in the case of the motor, which when fed with the appropriate current create a rotating magnetic field, which magnetises the rotor and causes it to spin. There is no electrical connection to the rotor, meaning there is no need for brushes. There is always a degree of 'slip', meaning that the rotor never rotates quite as quickly as the field (the 'synchronous speed') when the motor is producing torque. The current consumed, and the torque generated, vary in proportion with the amount of slip. So by varying the frequency of the AC current you can get very fine control over the torque. If you drop the frequency of the current below the synchronous speed, the motor becomes a generator and you have regenerative braking.
The inverter controls the frequency and voltage of the current fed to the traction motors. It works by switching on and off the DC current at high frequency under computer control. By varying the width of the switch pulses ('pulse width modulation') the average voltage produced can be chosen. The high frequency pulses are smoothed out by electronics, leaving a reconstructed AC voltage that gets sent to the motor. Three inverters work in tandem to give three phase current. The actual switches are either insulated gate bipolar transistors (IGBT) or gate turn-off thyristors (GTO Thyristor) which have the function of solid state relays, able to turn on and off many thousands of times per second.
