Help me understand the operation of this DC Contactor coil

I came across one Contactor rated 400A in which the coil is fed through a electronic circuit in which pulsating DC (waveform of full wave rectifier) & each pulse having PWM pulses inside it ( as seen on Oscilloscope) is given to the coil. I can see that the E shaped steel stampings riveted together to form a solid block in which the coil sits, doesnt have a shading ring. So definitely its meant to be used for DC. On analysing the ckt. I found out that the duration of the PWM pulses is- active high: 94 uS & low 6 uS( micro seconds), which keep going on to keep the contactor coil energized. The coil is designed to work on 220Volts & the contactor is supposed to pick up at 140 volts & drop out at 350 volts. The ckt. also takes care of the surges & makes sure using a microcontroller that the contactor only remains energized with in 140V to 350 Volts. It is a kind of safety for the coil because below 140V the coil will not magnetise the core up to the level that it attracts the other end & above 350V the coil may burn & also the ckt has been so designed.

Can some one please tell me:-
1) Why PWM is used in this case & why a a particular this particular duration on Active high & low. What is the relation?
2) When I give AC instead of DC the contactor makes a lot of vibrations. I understand that is because of shading rings not used.
3) What are the things to be considered when designing such a ckt for a coil of contactor.

Thanks

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