Copper Rotor Motors

[subrao]

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

[marke]

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,

[edingwall]

Hi Marke, 

Is my understanding of double cage motors correct?

At the start, when the stator flux cuts through the rotor bars, as it has only penetrated mainly through the surface where the high resistance bars are fitted, current is induced in the outer bars and the rotor operates at a high resistance with good starting toque characteristics. As the rotor accelerates to full speed, the flux lines penetrate deeper within the rotor resulting in current flowing through the inner bars. At rated speed, both bars are effectively in parallel however the majority of the current flows through the lower resistance inner bars.

Best regards,

Ewan 

[marke]

Essentially yes, this is a valid description.

I always thing of the equivilent circuit of each bar, outer bars have higher resistance, (may be brass or similar) and lower inductive reactance. Innerbars have lower resistance and higher inductance. The higher frequency currents (high slip) flow through the outer bars generating high torque at high slip and the lower frequency currents (low slip) pass through the inner bars offering low slip, high efficiency operation.

[edingwall]

Thanks Marke for your response. 
 

 

[edingwall]

Hi again Marke,

What is the relationship between rotor resistance and starting current? is it proportional?

Alot of info is available for relationships regarding torque/applied stator voltage etc but not for rotor resistance and starting current 

Thanks again,

Ewan 

[marke]

Hello Ewan

The rotor current is dependent on the rotor impedance at the actual slip.

When the rotor speed is zero, then the slip is 100% and the rotor current frequency is equal to the line frequency.
As the rotor speeds up, the slip reduces as does the frequency of the current flowing in the rotor.
The rotor impedance is made up of both a resistive component and an inductive component. Hence the flux field is at an angle that is dependent on the effective power factor of the rotor current,and the torque is dependent on the "in phase" component of the field, so proportional to the resistive component of the rotor impedance.

The start current is determined by the rotor impedance,and the torque is dependent on the rotor resistance.

A rotor with a high resistance rotor will have a low start current and a high start torque.

A rotor with a low resistance rotor will have a high start current and a low start torque, and a low full load slip. (higher efficiency)

 

[edingwall]

Thanks Marke, you're the best.