Michelle Bash with adviser Steve Pekarek, Purdue University
Single-phase induction motors are traditionally used to drive a reciprocating or scroll compressor in air conditioning systems used in residential applications. The motor is designed to minimize cost subject to a specified phase voltage, starting torque, and peak torque required at the desired compressor speed. In addition, an overall system efficiency referred to as a seasonal energy efficiency ratio (SEER) at a single thermal operating point is used to specify motor efficiency at the desired compressor speed. In testing a 35 000 BTU/hr compressor (single-phase, 2-pole, induction motor), at the single thermal operating point at which the SEER is measured, motor efficiency was found to be about 80%. However, as seasonal conditions vary (corresponding to different load torque applied to the motor), motor efficiency varies significantly, and can be as low as 50%.
As an alternative, a 3-phase motor was chosen to reduce torque ripple and potentially improve efficiency for a given motor mass. Although the 3-phase machine comes at the price of adding power electronics (residential power in the US is single-phase), it is speculated that the additional cost of a drive may be offset by reduced copper and steel in the 3-phase machine. In addition, rather than specifying a single-operating-point efficiency, the design constraint is to minimize the energy consumption over an entire seasonal cycle of a compressor.
An MEC model of a synchronous reluctance machine was configured for multi-objective optimization. Performance objectives were to minimize mass and minimize energy consumed over a seasonal cooling cycle, subject to constraints of voltage (input to the drive is 220 Vrms) and load torque. The design space spans 22 degrees of freedom, corresponding to geometry, winding layout, conductor/magnetic material, and stator input current. The load torque is modeled using a family of profiles obtained by modeling compressor dynamics over various speeds and pressure ranges. It was assumed that the machine is connected to a 3-phase inverter.
Using the MEC-based design code within a multi-objective optimization, a tradeoff between mass and efficiency for the HVAC application was established. As shown in Figure 7, there is an exponential increase in mass as efficiency is increased beyond 80%. If a synchronous reluctance is used in the compressor, it is clear that any mandates to increase motor efficiency would definitely lead to increased equipment cost. Studies are underway now to establish the mass versus efficiency tradeoffs for other machines (switched-reluctance, permanent magnet synchronous) which would enable comparison of alternative technologies applied to compressor applications.
This work is supported by the Grainger Center for Electric Machinery and Electromechanics.