Acoustics of air-water heat pumps as a function of electric control of compressors

To reduce CO2 emissions, many buildings should be heated by heat pumps. Installation in densely populated residential areas requires low noise emissions, not only to comply with legal requirements, but also to contribute to the well-being of the residents. One major noise source of a heat pump is the compressor. Due to recent advances in control technology, it seemed promising to investigate the noise reduction of heat pumps through optimized compressor control. The control strategy Current Shaping (CS) applies current harmonics to the compressor electric machine. CS is known to reduce electric machine vibration and noise. However, it has not yet been investigated if it is effective for a compressor, where noise and vibration are mainly excited by pressure and mass imbalances and not by the electric machine. Hence this thesis aims to evaluate if and how CS reduces compressor induced heat pump noise. To investigate the effects of CS inside the compressor and consequentially evaluate the potential of CS, compressor vibration simulations are performed. The excitation is derived from a state-of-the-art compressor excitation model, where the forces inside the compression chamber are calculated. These forces are applied to a Multi-Body (MB) compressor vibration model, which simulates the whole compressor structure and predicts the compressor vibrations. The MB model is improved by considering the resonance behavior of the compressor structure. To this end, a method is developed to perform a modal analysis in a MB simulation and the results are compared with a Finite Element (FE) simulation and an Experimental Modal Analysis (EMA). Experiments are performed on a compressor test bench and in a semi-anechoic sound chamber to analyze the whole path from CS to heat pump sound. This path is defined as follows. CS changes the currents from the inverter to the electric machine of the compressor. This changes the behavior of the electric machine, consequently the compressor vibrations and finally the heat pump sound power. On a compressor test bench, compressor vibrations are minimized by applying suitable current harmonics using CS. Currents, vibrations, and pressure pulsations are then measured to evaluate the effect of CS. Afterwards, the optimized currents are applied to a heat pump in a sound chamber, and it is proven that heat pump noise is reduced. From the investigations the following is derived. Compressor vibrations are dominating the heat pump sound in the investigated frequency range of 0 to 500 Hz. The compressor vibrations are reduced by CS, whereas pressure pulsations in the pipes are not affected. By applying current harmonics, CS generates a driving torque ripple, which reduces tangential vibrations. Tangential vibration means that the direction vector coincides with the tangent of a circle around the axis of rotation of the compressor. Thus, CS behaves like to well-known compressor control strategies Load Torque Compensation (LTC) and Speed Ripple Reduction (SRR) when the load torque ripple dominates the vibrations. CS is beneficial compared to LTC and SRR, as it can reduce resonances in tangential direction as well. The vibration reduction potential of CS depends on the compressor operating condition. It is increasing with higher temperature difference and decreasing with higher compressor rotation speed. A maximum vibration reduction of 23 dB for one harmonic is achieved. Thus, shaping the currents with CS is a suitable method to reduce heat pump noise. For some harmonics a sound pressure reduction of up to 12 dB is achieved.