Variable-frequency drives (VFDs) are widely applied on induction motors that drive fans, pumps, and compressors. Under partial loads, VFDs not only adjust frequency to reduce motor speed and mechanical output power (load) but also adjust voltage to reduce motor electrical input power. Traditionally, VFD manufacturers recommend controlling the voltage so that it is proportional to the square of the frequency for variable torque motor loads on fans and pumps and proportional to the frequency for constant torque motor loads on compressors. The purpose of this paper is to investigate energy-efficient voltage frequency ratios of VFDs using the motor equivalent circuit method. First, the motor load and speed correlation are derived for different applications. Then, VFD voltage is optimized for a given VFD frequency to maximize motor efficiency. Finally, the motor efficiency is simulated and compared under the optimal voltage and different preset voltages. The simulation results show that the motor efficiency with the ratio of voltage to frequency to the power of 1.5 is mostly close to the optimal efficiency for variable torque motor loads, and the motor efficiency with the ratio of voltage to frequency to the power of O. 5 is mostly close to the optimal efficiency for constant torque motor loads with efficiency improvement by up to 3% over traditional ratios.