Two-speed stator with wound rotor

[EngRepair]

It sounds strange, but several times this type of crane motor arrived in our workshop (mostly just the stator).
The stator is a two-speed motor 8 and 4 poles, constant HP connection but the rotor has an 8-pole winding.
Costumer claims that the motor runs at low speed with rotor resistances connected in order to get three low speeds. At higher speed (4 poles) the resistors are switched off and the rotor rings are not short circuited (see the photos of external wiring attached).
Is there any explanation?

 

[zlatkodo]

There is such a series of Chinese motors spread all over the world with a very advanced, cunning 8/4 pole stator winding (constant HP) and a wound 8-pole rotor:
YZRDW200L-8/4, 15 / 15KW
YZRDW225M-8/4, 24 / 24KW
YZRDW250M-8/4, 30 / 30KW
YZRDW250L-8/4, 37 / 37KW
YZRDW280M-8/4, 37 / 37KW
Winding is very hard to properly record. We have managed to recreate and analyze few of them.
The low speed is working with an additional part of winding and resistors connected to rings unlike the high speed with open rotor rings.

[URL unfurl="true"]https://winding.wixsite.com/design[/url]