Q: I heard that if we have a 3 phase induction motor constantly operating under 40% of rated power, we can reduce its losses using a Wye connection. But I can't find any reference of recommendation about that. So is it a practical solution to maintain energy efficiency? And is there any practical intelligent Wye-Delta switch?
The voltage across each winding drops by sqrt(3). So the torque-speed curve is reduced (by a factor of 3). The equilibrium speed is only slightly changed, but the torque and hence real power reduces to 1/3 (The magnetizing/reactive power is also reduced due to the lower voltage). This reduces the winding and rotor currents, and so reduces losses in the windings and the rotor bars. The delivered power reduces by slightly more than the losses (because of the equilibrium speed change), so i think the efficiency will worse.
Peak and starting torques drop by the factor of 3 to (typically) 66.7% and 46.7% of (original) rated output. So a load of 40% of rated output may not start successfully (depending on its torque-speed relationship) and has little stall margin. For a high-inertia load the motor safe start time may be exceeded.
Most loads are variable torque and reducing the voltage does reduce the equilibrium torque as well as speed.
A fraction of loads are constant torque when at steady state: conveyors, materials handling, lifts (elevators) and escalators, some machine tools. Load torque is by definition given by t=r x F where F=m.a is the opposing force and in each of these cases the mass, the opposing acceleration (mostly gravity), and the effective radius are all constant, hence so is the load torque.
If conveyors do not have net elevation change, they are constant torque only if their bearing (and other frictional) losses are independent of speed. This is often true, but my understanding is, not always.
Reducing voltage to reduce energy is the wrong fix to the wrong problem (The system should have been engineered correctly in the first place), and often ineffective in any case as loads are often dominated by HVAC.