The die-cast copper motor rotor – a new copper market opportunity
Induction motor manufacturers have long known that substitution of copper for diecast aluminum in the rotor squirrel cage would significantly reduce motor losses and improve the electrical energy efficiency. Aluminum is readily die-cast to produce the complex squirrel cage within and around the iron laminations of the rotor. Copper is much more difficult to die-cast due to its high melting temperature which leads to premature die failure. The work reported here addresses the die life problem in die casting electrical grade copper. A number of candidate high temperature die materials were surveyed. Computer simulation of the cyclic thermal and stress gradients in dies provided insight into the “heat checking” failure mechanism and how it could be minimized or avoided. A nickel-base alloy die system preheated to and operated at ~625°C has been developed and shown to greatly extend die life compared to die steels operated conventionally. A die insert heating and insulation system was developed to make extended runs possible. Copper motor rotors were die cast over a range of machine operating parameters in joint effort with several motor manufacturers. Motor test results showing the predicted 15 to 20% reduction in motor losses and reduced motor operating temperatures are presented.
A study of metal/die interfacial heat transfer behavior of vacuum die cast pure copper
High pressure die casting copper is used to produce rotors for induction motors to improve efficiency. Experiments were carried out for a special “step-shape” casting with different step thicknesses. Based on the measured temperature inside the die, the interfacial heat transfer coefficient (IHTC) at the metal/die interface during vacuum die casting was evaluated by solving the inverse problem. The IHTC peak value was 4.5×10 3 –11×10 3 W·m −2 ·K −1 under the basic operation condition. The influences of casting pressure, fast shot speed, pouring temperature and initial die surface temperature on the IHTC peak values were investigated. Results show that a greater casting pressure and faster shot speed could only increase the IHTC peak values at the location close to the ingate. An increase of pouring temperature and/or initial die surface temperature significantly increases the IHTC peak values.
Thermal Modelling of an Induction Motor
The comparison of motor losses is a significant item in the study of inverter control strategies for variable frequency, variable speed ac drives. For our study, since it is necessary to differentiate small changes in loss, we have decided to use steady state temperature rise as a direct indicator of loss rather than measurement as the small difference between input and output power. An essential preliminary, the determination of temperature rise with normal sinusoidal voltages, is described here. It has established that a simple thermal model could define the steady state temperature rise with acceptable repeatability and accuracy.