It is certainly possible to use switches to switch in power factor
correction capacitors--but on a fractional horsepower motor? In fact it
seems strange to have such a three phase motor anyway.
Again, for large motors, it might make sense for wound rotors to have
series resistance in order to increase starting torque without crazily
low power factor (high current).
It's pretty obvious that it's to detect the motor has stopped rotating,
I've said that from the beginning.
Whether the original intent for this particular motor was to tell the
controller to reverse the rotation to back off whatever jammed the
equipment the motor was running, cut the power, or to sound an alarm to
alert somebody that the motor has jammed, failed, or otherwise stopped
turning we may never know. I can think of numerous uses for it, but have
never actually encountered any of them in practice. I haven't seen one
of these in the field and apparently nobody else here has either.
There are two explanations that I can see- James has indicated one - an
auxiliary shut of switch. Another could be that this motor was designed as a
one size fits all- if 3 phase supply available- run as a 3 phase machine-
otherwise an easy modification(with help of a capacitor) to single phase
operation. Mass manufacturing cost savings being the motive.