My book says in Star the line current is equal to line voltage divided by impedance times square root 3; in Delta equal to line voltage times square root 3 divided by impedance, giving a 1:3 ratio for respective star and delta line currents.

I see Ohms Law, but not how star impedance is times square root 3, delta voltage times square root 3

Also I have two books each with a diagram of the star delta circuit: one has overload relays before the motor and one after the motor.

These statements and diagrams seem variously contradictory and nonsensical. They don't seem to bear any relation to the actual mechanics of three phase generated electricity and power plant, which may be because I don't understand the actual mechanics of three phase and why we're even talking about square roots and not whole numbers.

For instance: how can line current and phase current be equal in Star, since by Ohms Law line and phase voltage are square root 3 times different? Only if it's the impedance which is respectively determined by square root 3, not the voltage..and isn't the impedance in windings negligible anyway?

Many thanks for any assistance