तमन्ना जहाज के सफर की... क्यों करूं मैं, दिन भर की खुशियाँ लम्हों मे बर्बाद क्यों करूं मैं। - Amir

yep, totally related to electric machines and electric drives ;)
when you go for v/f control, your motor has 3 'zones' of operation: Constant Torque region, Constant Power region and High speed region.
in Both P7 and P5's graph you'll see a flat region in the beginning. this is constant torque region, where we maintain v/f ratio constant, which means torque is also constant. As you must have guesses, as speed increases, frequency of rotation also increases and hence to keep v/f ration constant you have to increase input voltage to motor. However...
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you can't keep on doing this for a long time as every motor has winding with insulation, which can tolerate an electric potential up to certain level only. once that is reached (2180 V, for both locos ) you can only increase frequency but not Voltage. this is the non linear dropping region you see, where torque drops as v/f ratio falls. Eventually though you also reach a point where you can't even increase frequency and you go for field weakening. that is the last, sharp drop region where troque eventually drops to zero. for P5, the point of 3rd region arrives later than that of P7, as its RPM is higher, hence you get to play in the higher torque region for longer time than P7 (160.3 Hz for P5, compared to 132 Hz for P7)
PS: I assume you know about v/f, without which my answer will read like Greek to you. The assumption is based on solid grounds though, i know you are well read, curious person so its unlikely you aren't aware of v/f control :P
Image source: click here