Multi-turn Energy Recovery Linacs (ERLs) powered by Superconducting Radio Frequency (SRF) cavities enable the production of continuous bright high-current beams while being very efficient. The low energy consumption can be attributed to diminishing refrigeration costs from increasingly advanced SRF materials and the efficient use of rf power in maintaining the accelerating gradient. Operating the main linac cavities of SRF ERLs with large QL reduces average rf power consumption but increases its sensitivity to resonance detuning. We discuss passive measures as well as develop a novel Active Noise Control algorithm to reduce microphonics detuning and ensure stable field. This dissertation describes the complete rf commissioning process of a SRF main linac in detail including the use of automation in various steps. These techniques were utilized in the Cornell BNL ERL Test Accelerator (CBETA), the first multi-turn SRF ERL, which achieved an energy recovery efficiency of 99.4 % during high-current operations in the single turn mode. We further analyze the effect of transient beam loading using a novel linear model which predicts the existence of reactive beam loading arising from relativistic effects in the presence of low energy beam. Finally, we discuss a novel thermometry system to image local heating and thermal runaway on the surface of SRF cavities, thus aiding the development of more efficient SRF materials for future ERLs.