Altern current motors are electric motors that are powered by alternating current. Electric motors convert electrical energy into mechanical rotation energy through the mutual action of magnetic fields.
There is a wide variety of altern current motors, among which the following basic types stand out:
- The universal motor that can also be direct current.
- The synchronous motor. In this type of electric motor the speed of rotation is constant and depends on the frequency.
- The asynchronous motor. It is a three-phase motor.
In some cases, such as boats, where the main source of energy is direct current, or where a large range of speeds is desired, direct currentelectric motors can be used. However, most modern motors work with alternating current sources
Its constitution is similar to that of a direct current series motor, although with some modifications:
- The polar nuclei, and the entire magnetic circuit, are built with silicon iron plates isolated and stacked to reduce energy losses by parasitic currents. These currents occur because of magnetic flux variations when connected to an alternating current network.
- Lower number of turns in the inductor in order not to magnetically saturate the core and thus reduce losses due to eddy currents and hysteresis, increase the current intensity and, therefore, the motor torque and improve the power factor.
- Increased number of turns in the armature to compensate for the decrease in flow due to the lower number of turns of the inductor.
The use of these motors in alternating current is very widespread due to the higher starting torque with respect to that of the induction motors and because of its high rotation speed, which allows to reduce its size and its price. Thus, it is used in portable machines of all types, small household appliances, etc.
A synchronous motor is a type of altern current motor. Its speed of rotation is constant and depends on the frequency of the voltage of the electrical network to which it is connected and the number of pairs of poles of the motor; this speed is known as "synchronism speed".
Synchronous motors work much like an alternator.
There are four types of different starts for this type of motor:
- As an asynchronous motor.
- As an asynchronous motor, but synchronized.
- Using a secondary or auxiliary motor for starting.
- As an asynchronous motor, using a different type of outrigger: it will have some slip rings that will connect the motor's polar wheel to the starter.
Asynchronous motor or induction motor
The asynchronous motor is also known as an induction motor.
The three-phase asynchronous motor consists of a rotor and a stator. The rotor can be of two types: squirrel cage or winding. In the stator are the inductor coils. These coils are three phase and are offset by 120 degrees in space. According to Ferraris' Theorem, when a balanced three-phase current system circulates through these coils, whose phase shift in time is also 120º, a rotating magnetic field is induced that surrounds the rotor. This variable magnetic field will induce a voltage in the rotor according to Faraday's Law of Induction.
Then the Laplace effect (or motor effect) occurs: every conductor through which an electric current flows, immersed in a magnetic field, experiences a force that tends to set it in motion. Simultaneously Faraday effect (or generator effect) occurs: in any conductor that moves within a magnetic field a voltage is induced. The rotating magnetic field, at synchronism speed, created by the stator winding, cuts the rotor conductors, thereby generating an electromotive induction force.
The mutual action of the rotating field and the existing currents in the rotor conductors, originate an electrodynamic force on said rotor conductors, which rotate the rotor of the motor. The difference between the speeds of the rotor and the magnetic field is called sliding.
The squirrel-cage engine is a type of asynchronous motor.
Most of the single-phase AC motors have the squirrel-cage type rotor. The actual squirrel-cage rotors are much more compact and have a laminated iron core.
A squirrel-cage rotor is the rotating part commonly used in an AC induction motor. Once installed, the motor is a cylinder mounted on a shaft. Internally it contains longitudinal aluminum or copper conductor bars with grooves. The bars are connected together at both ends by short circuiting the rings that form the cage. The name is derived from the similarity between this cage of rings, the bars and the wheel of a hamster.
The rotor base is built with stacked iron sheets.
Often, the drivers are inclined slightly along the length of the rotor to reduce noise and reduce the fluctuations of the torque that might result, at some speeds, and because of the interactions with the stator bars. The number of bars in the squirrel cage is determined according to the currents induced in the stator coils and therefore according to the current through them. Constructions that offer less regeneration problems use prime numbers of bars.
The inductor windings in the stator of an induction motor incite the magnetic field to rotate around the rotor. The relative motion between this field and the rotation of the rotor induces electric current, a flow in the conductor bars. In turn these currents that flow longitudinally in the conductors react with the magnetic field of the motor producing a force that acts tangent to the rotor, resulting in a torque to turn the shaft. In effect, the rotor is carried around the magnetic field, but at a slightly slower rate of rotation. The difference in speed is called sliding and increases with the load.
The iron core serves to carry the magnetic field through the motor. In structure and material it is designed to minimize losses. The thin sheets, separated by the varnish insulation, reduce the parasitic currents that flow resulting from eddy currents (in English, eddy current).
The material, a steel low in carbon but high in silicon (called silicon steel), with several times the resistance of pure iron, in the additional reducer. The low carbon content makes it a soft magnetic material with low loss due to hysteresis.
The same basic design is used for single-phase and three-phase motors over a wide range of sizes. Three-phase rotors have variations in the depth and shape of the bars to meet design requirements. This motor is very useful in variable speed drives.