Line-start synchronous motors are a good alternative to asynchronous motors used in fluid machinery, due to their greater energy efficiency, but have a significant disadvantage of limiting the maximum moment of inertia of the load mechanism to ensure successful and reliable start. In particular, this disadvantage is critical in the drive of centrifugal fans with a massive steel impeller. In this study, a mathematical model has been developed that allows us to examine the starting characteristics of line-start synchronous motors with or without permanent magnets as part of a fan unit. This mathematical model allows us to estimate the critical moment of inertia and the static torque of the load, at which the motor cannot successfully synchronize during start-up. Long-term operation in asynchronous mode for this type of electric motors leads to overheating of the windings and the motor failure. The simulation has been carried out using the parameters of a 0.55 kW, 1500 rpm line-start permanent magnet synchronous motors and a mass-produced centrifugal fan. A comparison of the results of simulation of dynamic and steady processes of the motor start-up in the fan drive with and without air flow control has been carried out. The simulation results show the possibility of increasing the maximum moment of inertia of the load upon successful start-up of the line-stat synchronous motor by adjusting the hydraulic part of the fan unit. The findings of this study can be used when choosing an electric motor to drive centrifugal fans and can contribute to the wider use of energy-efficient line-start synchronous motors.