Three-phase motor winding head and tail speed judgment circuit

Circuit working principle: aa', bb' and cc' in the figure are three-phase windings respectively. Double throw
switch K1-1 to K1-4. Both are turned to the left, that is, "l" and "2" of the switch are connected. At this time,
aa' and bb' are connected in series on the secondary side of transformer T, and the 6.3 V small bulb H is connected in parallel to both
ends of cc'. If a and b are connected to the heads or tails of windings aa' and 66', then when
button AN is pressed, an induced current will be generated in winding ce' and H will light up. On the contrary, if a and b are not
at the same end, the magnetic fields generated by aa' and bb' will cancel each other, there will be no induced current in cc', and
H will not light up. Based on this, we found out the names of aa' and bb'.