I read lot of articles praising copper rotors instead of conventional aluminum rotors.
It is claimed that as conductivity of copper is more it has less resistance and low slip,lower rotor losses ans improved efficiency.
My problem is that in order to have a good starting characteristics we need to have
a definite rotor resistance.The choice is a compromise between lower losses and starting torque. This choice is without regard to material copper or aluminum.
So How does copper rotor helps to reduce losses at the same time maintaining
the required starting torque?
subrao

Hello subrao

Welcome to the forum.

Yes, you have struck the real heart of a problem.
In the major push to improve the operating efficiencies of induction motors, the starting characteristics have been severely compromised.

Part of the efficiency push, is to reduce the slip losses. To do so, requires that the rotor has a very low resistance. As you increase the rotor resistance, you increase the slip and therefore the slip losses. (The slip loss is equal to the torque times the slip)

Copper bars exhibit a lower resistance than aluminium for the same cross sectional area and are therefore used on larger motors to improve the efficiency.
Small motors commonly use diecast aluminium rotors because of the cost of manufacture.

Start current is influenced by the rotor impedance which is a combination of both resistance and reactance. The position of the bars within the rotor alters the reactance of the bars, and the cross sectional area and material of the bars influences the resistance of the rotor.
Start torque. is the power dissipated in the rotor during start, so it is a function of rotor current squared times effective rotor resistance.
If a bar is positioned at the surface of the rotor, it exhibits a low reactance. If the bar is positioned deeper into the rotor, it exhibits a higher reactance.
By using multiple bars, or thin bars with deeper penetration into the rotor, the effective resistance at start can change from the effective resistance during run.
At start, the rotor is stationary and the frequency of the current in the rotor is equal to the line frequency. Reactance is very frequency dependent. At run, the frequency of the current in the rotor is very close to 0Hz and so the reactance drops to almost 0 also. If we have two equal bars in the rotor, one deep and one shallow, then most of the current at start, will be concentrated in the outer bar. This will almost double the effective resistance of the rotor and result in a higher starting torque, but at full speed, both bars are effectively in parallel, reducing the running resistance and slip losses.

Double cage motors used to be a relatively common means of developing a very high start torque with good running characteristics, but we rarely see them today. With the traditional double cage motors, the outer bars could be made of a high resistance material such as brass and the inner bars of copper.

see Induction_Motors#Rotor_Design

Best regards,