We can say that among the main categories of electric motors of alternating current, we must emphasize the synchronous motor, since it is perfectly capable of operating with a fixed speed, having its use only for large powers, since they present a very high cost for sizes, or if you need to achieve an invariable speed.
Nowadays it is plausible to control the speed of the induction motors with the help of frequency inverters. DC motors cost more and necessarily require a specific direct current source or an element that is capable of converting ordinary alternating current to continuous.
This type of engine operates with speed expressively adjustable between wide limits, having controls of great precision and flexibility. In this way, its application is only indicated in specific cases in which some factors can compensate the higher cost of the installation.
Induction motors can run at a steady speed, which can change with the mechanical load applied to the shaft. Because of its great simplicity, strength and low cost are considered as the best selling motors of all, making it suitable for almost all types of driven engines.