Power Supply in Modern Trams
On modern trams, power supply is controlled with thyristors (which can switch power supply on and off much faster than a motorman could), freewheel diodes which supply power when it is not supplied via the thyristor, and lowpass filters, known as line filters, which attempt to remove any remaining AC "ripple". Sometimes, when more than one traction motor is used, an individual chopper is provided for each traction motor, operating out of phase, to cancel out as much interference as possible.
In simple terms the thyristors supply power to the motors by the very rapid switching on and off of the power supply. As more power is required, the "on" period is increased and the "off" period is decreased. This is handled automatically by the thyristors and the driver makes no conscious decision to switch the power on and off.
Because of the rapid on/off switching, in any setting between neutral and full power, undesirable "AC ripple" in the current can cause the motor housings to vibrate, increase the risk of motor bearing failure through shaft induced voltage and waste energy through eddy currents and switching losses. Ripple can also cause electrical interference (this is why modern trams tend to generate electrical "hum" at low speed). Although lowpass filters attempt to overcome this problem, modern trams are best run in full series or full parallel, as is the case with trams with older technology.
Read more about this topic: Tram Controls
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