I've considered using three relays to implement a wye-delta transformer, so I can turn the motor to wye or delta to change its Kv at any time, high Kv for high power and low Kv for high efficiency. But I need the transformer not to bring in much resistance to the system, while most relays have a contact resistance of >50mΩ. 50mΩ relays will bring in 100mΩ resistance at the wye state, creating more energy loss compared to the energy loss of diodes in the ESC when using a high Kv motor and low PWM duty cycle.
Does low resistance relay resistance with contact resistance less than 10mΩ ever existed? Maybe relay is not a good component to achieve that, what is the common solution to build a low resistance wye-delta transformer? -----update-----
I want to use wye-delta transformation to increase efficiency at low throttle. To get same power output, high duty cycle with low Kv has higher efficiency than low duty cycle with high Kv. A test in an anwser of this question has showed that.
High Kv motors have less inductance, creating more ripple current in the coils, causing higher crest factor thus more heat generation. Higher PWM frequency is required to decrease the coil current crest factor, putting more streess on the ESC.
At low duty cycle, freewheeling current will take a larger partion of total current in coils. More freewhelling current means more current pass through diodes with higher voltage drop.
An answer said that this can be done with MOSFET, I come up with an image of using 12 MOSFETs, or 6 pairs to build 6 relays. 3 connect the coil ends to a COM point, and 3 connect each coil end to the next coil head.
In this image, there will be 7 individual points with sources connected, each having a different changing voltage to GND. So we need 7 some kind of isolated drivers to apply voltages between gates and sources. Is this image a proper solution?