High-performance motors operate at lower temperatures than conventional motors, if they are of the same power, and allow greater overload capacity. Although higher cost due to better quality of their materials, if used mainly in continuous regimes, these engines recover the additional investment in a time much smaller than its useful life, due to its energy saving. https://www.mrosupply.com/automation/ac-variable-frequency-drive/2527061_17465400_leeson/
Due to the great need to control the speed and the automation of the processes it is necessary to use frequency inverters for this function in the three phase induction motor and so even though the prices of these electronic devices are diminished over time, they are more sophisticated and more expensive than an AC-DC converter, used to drive the DC machine, which makes it cheaper to analyze the overall cost. In particular, the three-phase squirrel-cage rotor induction motor is more economically feasible and the most commonly used in general industries because it is smaller in size and lighter than a DC motor of the same power and the need for maintenance of the machine alternating current is lower than that of the DC machine.