I studied electromagnetics and with that understanding when I recall the Working of DC Series motor I stumbled upon an inherent negative feedback action in the DC series motor during starting as follows:
Considering the DC Series Motor is started with No Load:
1) At the instant of powering up the motor armature current is low (Transient)
2) Implies the current in the field coil is low
3) So the flux due to field coil in the air gap is low
4) The armature starts rotating because of Lorentz force
5) By faraday's law- the Back Emf is low as the air gap flux is low
6) So the armature current increases as per KVL
7) This, in turn, increases the torque on the armature.
8) However simultaneously the current through field coil also increase(since Ia =If )
9)The FEEDBACK action ----Because of Step 8 the flux increases and thus back EMF induced increases which in turn reduces the armature current. Thus the Torque on armature also reduces and so the speed also inturn reduces!!
Isn't this a negative feedback action? Won't this stabilise the DC series motor at some operating speed?
Why it is said that motor speed increases drastically and reaches un-stability when started with no load?