3. P7 and P7i :-
'i' stands for IGBT (Insulated Gate Bipolar Transistor). Earlier, GTO (Gate turn-off) thyristors were used as inverters and auxiliary converters which had the following issues :-
(I) Higher switching losses.
(II) Obsolescence
(III)...
more... Group drive resulting in poor slip/slide control
(IV) More noise
(V) Larger size and weight
(VI) Lower switching frequency
IGBT is much smaller and lighter. Hence, it was possible to add more components without exceeding weight and space limitations. Older locos featured one inverter per bogie (set of two or three axles), newer locos have one per every traction motor. As a coolant, a mixture of water (70%) and ethylene glycol (30%) is used which is largely inorganic unlike the oil that is used in GTO. Switching frequency is 3-4 times higher than that of GTO and the absence of complex snubber circuits reduced the switching losses by ~50%. Noise could also be reduced by ~10%.
Newer P7's have IGBT based traction and auxiliary converters. Some sheds include the suffix 'i' (like ELS/BIA) while most of them don't.
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4. P5 and P5i :-
Same as P7 and P7i.
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5. Power conversion in 3-phase locomotives :-
This happens in 3 stages :-
(i) Input Converter : This rectifies the AC from the OHE to a specified DC voltage using GTO (gate turn-off) thyristors. A transformer section steps down the voltage from the 25kV input. It has filters and circuitry to provide a fairly smooth (ripple-free) and stable DC output, at the same time attempting to ensure that a good power factor presented to the electric supply. The power factor is around 0.9.
(ii) DC Link : This is essentially a bank of capacitors and inductors, or active filter circuitry, to further smooth the DC from the previous stage, and also to trap harmonics generated by the drive converter and traction motors.
(iii) Drive Converter : This is basically an inverter which consists of three components (GTO/IGBT) that switch on and off at precise times under the control of a microprocessor (pulse-width modulation). The three components produce 3 phases of AC (120 degrees out of phase with one another). Additional circuitry shapes the waveforms so that they are suitable for feeding to the traction motors. The microprocessor controller can vary the switching of the thyristors and thereby produce AC of a wide range of frequencies and voltages and at any phase relationship with respect to the traction motors.