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
Famous quotes containing the words power, supply and/or modern:
“Now narrow minds can develop as well through persecution as through benevolence; they can assure themselves of their power by tyrannizing cruelly or beneficently over others; they go the way their nature guides them. Add to this the guidance of interest, and you will have the key to most social riddles.”
—Honoré De Balzac (1799–1850)
“There never has been a time in our history when work was so abundant or when wages were as high, whether measured by the currency in which they are paid or by their power to supply the necessaries and comforts of life.”
—Benjamin Harrison (1833–1901)
“The modern world needs people with a complex identity who are intellectually autonomous and prepared to cope with uncertainty; who are able to tolerate ambiguity and not be driven by fear into a rigid, single-solution approach to problems, who are rational, foresightful and who look for facts; who can draw inferences and can control their behavior in the light of foreseen consequences, who are altruistic and enjoy doing for others, and who understand social forces and trends.”
—Robert Havighurst (20th century)