Motor design optimization by using copper rotors

13/12/2014 13
By Nuno Fernando

Electric motors are widely used in many industrial applications, and the use of high efficiency motors allows significant energy savings to be made. This was the thinking that led to the three-phase induction motors classification scheme EC/640/2009. It introduced two efficiency levels: “high efficiency” IE2 and “premium efficiency” IE3 (the latter corresponding to the US “NEMA Premium”).

What EC/640/2009 means in practice is that from 2015, induction motors in the range 7.5-375 kW must have the minimum efficiency level IE3 or the efficiency level IE2 if fed by inverter. From 2017, all induction motors must have the minimum efficiency level IE3 or the efficiency level IE2 if fed by inverter.

A consequence is that electric motor manufacturers will have to adapt their production processes and invest in development strategies for innovative and high efficiency motors. Looking even further ahead, the standard also mentions a future level above IE3 to be called IE4 super premium efficiency. Even though the standard does not yet provide the full technical specifications, a few manufacturers are already introducing IE4 motors.

 

Global optimization algorithm

With these more demanding requirements in mind, research being carried out at the Department of Industrial and Information Engineering and Economics, University of L’Aquila, Italy is focusing on optimizing the design of induction motors. In particular, they have worked with the Department of Computer and System Sciences at the University of Rome to develop a global optimization algorithm to attempt to reach this goal.

“Software was developed to calculate motor performance at different loads, taking into account magnetic saturation, the skin effect on rotor parameters, and thermal analysis,” says Marco Villani, Associate Professor in Design of Electrical Machines at the University of L’Aquila. “This software was linked with an optimization algorithm to optimize the induction motor design with reference to several objectives such as rated efficiency, starting torque, power density, and manufacturing cost.”

The overall aim was to design three-phase induction motors with aluminium and copper cages to fulfil the IE3 efficiency level according to typical performance and standard constraints. Recent work focused on TEFC (Totally Enclosed Fan-Cooled) 400 V, 50 Hz, S1 duty three-phase squirrel-cage induction motors, in the range 0.75-22 kW. Five motor sizes were selected: 1.5 kW-6 pole, 3 kW-4 pole, 7.5 kW-4 pole, 15 kW-4 pole and 22 kW-2 pole.

The motors’ designs, with aluminium and copper cages, were optimized to reach the minimum efficiency level IE3 at lowest active material costs and satisfy the physical and performance constraints of the designs.

 

Three investment strategies

A suitable optimization procedure was used which enabled the “best design” to be found. Three strategies with different investment costs were defined:

  • Low cost: premium steel, longer stack if possible, new stator winding, unchanged stator/rotor lamination.
  • Medium cost: All the above, plus a die-cast copper rotor.
  • High cost: All the above, plus changing the stator/rotor lamination, changing the outer stator diameter, and introducing a new housing.

To guarantee the feasibility of the optimized designs, several constraints were imposed, such as temperature, power factor, starting performance, breakdown torque, flux densities (yoke and teeth), and slot fill factor.

 

Copper rotor offers efficiency advantages

One of the team’s conclusions is that the performance of the IE3 motors with aluminium and copper cages are quite similar and consistent with typical performance of commercial aluminium motors of the same size. However, the copper motors always present an advantage in size (diameter/stack length) and total weight.

The total copper weight in the copper motors (stator winding and rotor cage) is higher than the copper weight (stator winding) in the aluminium motors, the difference diminishing from small to large sizes (in the case of the 22 kW motor, they are similar).

It also became clear that it is difficult to go beyond IE3 with aluminium technology because of the limitations in the housing and the inability to fit with the standard dimensions.

For the smaller sizes (1.5 kW and 3 kW), the copper cage motors are slightly more expensive than the aluminium motors, while for the 7.5 kW size the difference on the active material cost is very small; this difference could be reduced if the aluminium motor needs a new (out of line) housing.

For the larger sizes (15 kW and 22 kW), the copper cage motors present active material costs which are lower than the IE3 aluminium motors for all scenarios (excluded the cost of die-casting).

“The overall conclusion is that copper rotor motors can be a cost-effective way of meeting the new high efficiency IE3 standards,” remarks Villani. “In addition, copper rotors will be the way ahead to meet the requirements of the IE4 Super Premium efficiency motors.”