How do the new 3-phase AC locos (WAP-5, etc.) work, and how do they compare with the earlier locos?

Three-phase AC locos such as WAP-5 use some fairly new technology as compared to the earlier generations of diesel-electrics and electrics. In most of the earlier locos, the traction motors driving the axles are DC motors. DC motors were used because they afforded (in those days) far superior speed and torque control compared to AC motors — the latter require variation of input frequency and voltage for effective control, which was not an easy matter earlier.

Modern microprocessor technology and the availability of efficient and compact power components have changed that picture. In 3-phase AC locos, the input (single-phase AC) from the OHE is rectified and then 3-phase AC is generated from it, whose voltage, phase, and frequency can be manipulated widely, without regard to the voltage, phase, frequency of the input power from the OHE. AC traction motors can thus be driven with a great degree of control over a wide range of speed and torque.

AC traction motors are also used on diesel- ... more.

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