Introduction to Industrial Automation

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  Introduction to Industrial Automation

2.1.1   Electric  Motors

Furthermore, electric actuators include the category of electric motors of all kinds, such as the 

stepper motors, servomotors, linear motors, and solenoids. However, in the industrial world, the 

term “actuator” is usually connected to low-power actuating devices and not to high-power elec-

tric motors. For an industrial engineer, the motor is a separate category itself and is not always 

straightforwardly connected to the actuators, although its definition includes them. Thus, the rest 

of this book will adopt the following approach: separate the electric motors for high mechanical 

power production from other types of actuators. In general, electric motors have the capability to 

convert the electrical energy into mechanical or kinetic energy. All electric motors*, AC (alternate 

current) or DC (direct current), use the principle of electromagnetic induction and the subsequent 

interaction of two magnetic fields to generate torque on a rotational element called a “rotor” inside 

a stationary housing called a “stator”. In Figure 2.2, an indicative internal construction for the case 

of AC and step motors is provided, where the existence of the coils gives a first impression of the 

generated magnetic forces that are responsible for achieving motor rotation.

More analytically, most DC motors operate by electric current flowing through a number of 

coils at the rotor (depending on the number of poles in the motor), which are positioned between 

the poles of a permanent magnet or electromagnet of the stator. The interaction of the two mag-

netic fields, one created by the rotor and the second one due to the stator, causes the rotor-shaft 

to rotate. To reverse the motor, it is needed to change the polarity of the supply voltage to either 

field winding or armature winding, but not both, since this will cause no change in the direction 

of rotation.

In the case of AC induction motors, only the stator has coils by means of a three-phase wind-

ing circuit, which produce a rotating magnetic field. This field induces an alternating current in 

the rotor, which consists of a cylindrical laminated core with slots that can carry conductors from 

*

 

P. Vas, Electrical Machines and Drives: Space Vector Theory Approach, Oxford Science Publications.

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