Electric motor

An electric motor is an electromechanical energy conversion machine (i.e., capable of converting electrical energy into mechanical energy), consisting of a fixed part (stator) and a moving part (rotor or slider, depending on whether the machine is rotating or linear) separated by a small air gap of uniform thickness, called the air gap.

Classification of electric motors

They can be classified in different ways, for example:

  • DC, asynchronous, synchronous, brushless, stepper, SR;
  • alternating current;
  • with brushes, brushless;
  • rotary (ideal for most applications), linear, alternating;
  • low dynamic (for fans, pumps, etc.), high dynamic (for machine tools, robots, etc.), incremental motion (for printers, etc.);
  • with constant or slightly decreasing speed as the load increases (advantageous in many applications, for example the rotation speed of a lathe spindle must not depend on the size of the chip removed);
  • with speed strongly decreasing as the load increases (convenient in applications where the torque variation range is very wide, for example in electric traction, where the use of motors with constant or slightly decreasing speed as the load increases is uneconomical since the dimensioning is determined by the maximum torque).

Regarding the choice of motor type for a given application, this is influenced by different factors, such as:

  • purchase costs;
  • operating costs (efficiency, periodic maintenance);
  • torque/inertia ratio (for high performance systems);
  • power-to-weight ratio (for airborne systems, robot arms);
  • complexity of motion control;
  • reliability;
  • torque ripple;
  • range of speed variability;
  • starting and braking characteristics;
  • environment in which they must operate;
  • type of operation (at constant speed and load or selected from a small set of values, intermittent, cyclic, varied);
  • size to be regulated (speed, position, torque), type of regulation (coarse, very precise).

Materials used in electric motors

The materials used to make electric motors are essentially magnetic, conductive and insulating. Among the conducting materials, copper is by far the most widely used conductor due to its low resistivity, excellent technological properties (drawability, ease of rolling, weldability, etc.) and high mechanical properties. Aluminum is another conductor material used but, compared to copper, although less expensive and having lower values of specific gravity and melting temperature, has lower electrical and mechanical characteristics.

Ferromagnetic materials are divided into soft and hard or permanent magnets, depending on whether their magnetization processes are practically reversible or have a considerable hysteresis.

Finally, we have insulating materials, on which the operation and durability of electrical machines depend, since they are the elements most sensitive to thermal, dielectric and mechanical stresses. The main properties of insulating materials are: dielectric strength (the highest value of the voltage gradient that the material can withstand without discharge), dielectric constant and thermal conductivity.

Piloting

Depending on their structure, electric motors can be controlled in different ways, more or less simply and to a greater or lesser extent.

In the past, DC motors were used for traction, driven by a rheostat. Later DC motors were used with separate excitation and then characterized by rotor and stator wound (also known as universal motor), these motors while being characterized by a higher consumption than the DC motor without brushes (permanent magnet rotor) allow a greater and easy regulation of the motor, as well as a more constant performance at all speeds, allowing the modification of the armature currents (rotor) and excitation (stator) through the use of vector inverters.

The knowledge derived from these applications have allowed with the relative adjustments to be able to drive then asynchronous motors with the same effectiveness, but in this case we drive the two components of the stator current, one of which generates the part of stator flux that interacts with the rotor flux, while the other compensates the demagnetizing action of the rotor current.

To do this and then drive the asynchronous motor it is necessary to know for each instant the axis of the rotor flux in order to determine the two components of the statoric current. This is possible through a huge computing power in real time, thanks to the use of computers and an algorithm that interprets the operation of the motor through a mathematical model, which requires the knowledge of inductances and resistances, which vary according to temperature and saturation of the magnetic circuit.

To drive the brushless motor (brushless DC), the vector inverter is required, which must know the correct position of the rotor with respect to the stator (absolute position sensor), while the driving of induction motors (but also for universal motors) is through a motor speed sensor.

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