Contenido

La parte fija del circuito magnético (estator) es un anillo cilíndrico de chapa magnética ajustado a la carcasa que lo envuelve. La carcasa tiene una función puramente protectora. En la parte interior del estator van dispuestos unas ranuras donde se coloca el bobinado(correspondiente).

En el interior del estator va colocado el rotor, que es un cilindro de chapa magnética fijado al eje. En su periferia van dispuestas unas ranuras en las que se coloca el bobinado correspondiente.

El entrehierro de estos motores es constante en toda su circunferencia y su valor debe ser el mínimo posible.

electrical circuits

The two electrical circuits are located, one in the slots of the stator (primary) and the other in the slots of the rotor (secondary), which is short-circuited.

The short-circuited rotor can be formed by coils that are short-circuited outside the machine directly or through rheostats; or, it can be formed by copper bars placed in the slots, which must be carefully soldered to two rings of the same material, called short-circuit rings. This set of bars and rings forms the squirrel cage motor.

There are also single-phase asynchronous motors, in which the stator has a single-phase winding and the rotor is a squirrel cage. They are small power motors and in them, by virtue of Leblanc's Theorem, the magnetic field is equal to the sum of two equal rotating fields that rotate in opposite directions. These single-phase motors do not start on their own, so some auxiliary means must be provided for starting (split phase: resistor or capacitor, shielded pole).

The rotation speed of the magnetic field or synchronism speed is given by:.

Where is the system frequency, in Hz, and is the number of pairs of poles in the machine. The speed being thus given in revolutions per minute (rpm).

What produces the induced voltage in the rotor bar is the relative motion of the rotor compared to the magnetic field of the stator, this can be seen in the following equation:

Where:.

: Bar speed in relation to the magnetic field.

: Magnetic flux density vector.

: Length of the conductor in the magnetic field.

: Represents the "vector product" operation.

The squirrel cage motor consists of a rotor "Rotor (electrical machine)") made up of a series of metal conductors (usually aluminum) arranged parallel to each other, and short-circuited at their ends by metal rings, this is what forms the so-called squirrel cage due to its graphic similarity to a squirrel cage. This 'cage' is filled with material, usually stacked sheet metal or aluminum. In this way, a -phase system of conductors is achieved (the number of conductors being, commonly 3) located inside the rotating magnetic field created by the stator, which results in a very effective, simple, and very robust physical system (basically, it does not require maintenance as it lacks brushes).

The wound rotor motor "Rotor (electrical machine)") has a rotor made up, instead of a cage, of a series of conductors wound on it in a series of slots located on its surface. In this way we have a winding inside the magnetic field of the stator, with the same number of poles, and in motion. This rotor is much more complicated to manufacture and maintain than the squirrel cage one, but it allows access to it from the outside through rings that short-circuit the windings. This has advantages, usually such as the possibility of using a starting rheostat that allows the starting speed and torque to be modified, as well as reducing the starting current.

In either case, the rotating magnetic field produced by the stator inductor coils generates induced currents in the rotor, which are what produce the movement.

The asynchronous motor works on the principle of Faraday mutual induction. By applying three-phase alternating current to the inductor coils, a rotating magnetic field is produced, known as a rotating field, whose frequency will be equal to that of the alternating current that is fed to the motor. This field, when rotating around the rotor in a state of rest, will induce electrical tensions that will generate currents in it. These will in turn produce a magnetic field that will follow the movement of the stator field, producing a couple or motor torque that makes the rotor rotate (principle of mutual induction). However, since induction in the rotor only occurs if there is a difference in the relative speeds of the stator and rotor fields, the speed of the rotor never reaches that of the rotating field. Otherwise, if both speeds were equal, there would be no induction and the rotor would not produce torque. This difference in speed is called "slip" and is measured in percentage terms, so this is the reason why induction motors are called asynchronous, since the rotor speed differs slightly from that of the rotating field.

The slip differs with the mechanical load applied to the rotor, being maximum with the maximum load applied to it. However, despite this, the motor varies its speed little, but the motor torque or coupling increases (and with it the intensity of current consumed) so it can be deduced that they are constant speed motors.

Electrically speaking, the asynchronous motor can be defined as an electrical transformer whose stator windings represent the primary, and the rotor windings are equivalent to the secondary of a short-circuited transformer.

At the moment of start-up, as a result of the rotor's resting state, the relative speed between the stator and rotor fields is very high. Therefore, the current induced in the rotor is very high and the rotor flux (which always opposes that of the stator) is maximum. As a consequence, the stator impedance is very low and the current absorbed from the network is very high, reaching values ​​of up to 7 times the nominal current. This value does not cause any damage to the motor since it is transient, and the strong starting torque makes the rotor turn immediately, but it causes abrupt and momentary drops in voltage that manifest themselves mainly as flickering of the lamps, which is annoying, and can cause damage to sensitive electronic equipment. Induction motors are all prepared to withstand this starting current, but repeated and very frequent starts without rest periods can progressively raise the stator temperature and compromise the useful life of the stator windings until causing failures due to melting of the insulation. That is why electronic "soft start" devices are used in medium and large powers, which minimize the motor starting current.

As the rotor gains speed, its current decreases, the rotor flux also decreases, and with it the impedance of the stator windings, remember that it is a phenomenon of mutual induction. The situation is the same as connecting a transformer with the secondary shorted to the AC network and then with a variable resistor inserted, progressively increasing the load resistance until reaching the nominal current of the secondary. Therefore, what happens in the stator circuit is a reflection of what happens in the rotor circuit.

Asynchronous motors are essential in various applications due to their robustness and efficiency. Next, we analyze the industrial applications of these motors:.

Las técnicas de control más comunes son las 4 siguientes:.

Control of line voltage applied to the stator

Line voltage control is carried out by varying the voltage supplied to the stator winding. This voltage can be regulated by varying the firing angle of the thyristors. This system introduces harmonics into the network and a good power factor is not achieved.

Line voltage-frequency control. Scalar control

A drop in frequency would produce a drop in the inductive impedance and if the voltage is maintained it causes a rise in current, which could cause the motor to burn out. This effect is avoided by modifying both parameters, but keeping the V/Hz ratio constant.

The technique used to vary the voltage applied on the stator and the applied frequency in proportion is pulse width modulation or PWM through DC/AC conversion. Using PWM modulation, a DC input voltage is converted to a symmetrical AC output voltage with the desired magnitude and frequency.

Depending on the supply network, the regulation is different:

ç.

Field Oriented Control (FOC)

The purpose of vector modulation or oriented field control applied to asynchronous machines is to achieve a type of linear control, making the current that produces the magnetic flux independent of the current that produces the motor torque. To achieve this, because asynchronous machines do not have two decoupled windings, a fictitious and equivalent circuit reference is created, with two windings arranged in quadrature (at 90° electrical angle) in the stator, replacing the three real windings.

In this way, the three-phase system of stator currents is transformed into a two-phase system of quadrature currents, non-stationary, that rotates synchronously with the rotor's magnetic field.

Consequently, these two currents represent the two decoupled windings and therefore can be controlled independently. In this new reference system the two stator currents are processed as rotating vectors, hence the name Vector Control or Spatial Vector Modulation (SVM) or Field Oriented Control (FOC).

There are two types of field-oriented control:

Direct Torque Control (DTC)

In direct torque control it offers a very fast torque response and good dynamic behavior. The instantaneous values ​​of torque and flux are calculated from the machine stator variables.

Torque and flux are directly and independently controlled by the optimal section of the inverter switching states. The values ​​are compared to the reference and generates an error.

The error enters a controller and generates a logic signal that modifies the space vector of the inverter voltage so that it adopts the most suitable value. Thus it forces the air gap flux vector to vary according to the reference value.

The motor torque is controlled by the rotation of the stator flux vector using appropriate switching states. At the same time, the magnitude of the stator flux vector is controlled in the same way, that is, with the use of the switching state of the inverter. This value can be changed according to the flow setpoint requirements. The calculated torque and flux values ​​are compared with their setpoints, the errors enter the hysteresis controllers. Its outputs are logic signals of discrete values ​​± 1.0 that are applied to the switching table that chooses one of the eight possible states of the inverter voltage space vector.

Its main difference from the other previous control methods is that in DTC there is no separate PWM modulator, but rather the position of the power converter switches is determined directly by the electromagnetic state of the motor. To do this, it is necessary to have a very exact model of the engine together with a very high calculation capacity.

Contenido

La parte fija del circuito magnético (estator) es un anillo cilíndrico de chapa magnética ajustado a la carcasa que lo envuelve. La carcasa tiene una función puramente protectora. En la parte interior del estator van dispuestos unas ranuras donde se coloca el bobinado(correspondiente).

En el interior del estator va colocado el rotor, que es un cilindro de chapa magnética fijado al eje. En su periferia van dispuestas unas ranuras en las que se coloca el bobinado correspondiente.

El entrehierro de estos motores es constante en toda su circunferencia y su valor debe ser el mínimo posible.

electrical circuits

The two electrical circuits are located, one in the slots of the stator (primary) and the other in the slots of the rotor (secondary), which is short-circuited.

The short-circuited rotor can be formed by coils that are short-circuited outside the machine directly or through rheostats; or, it can be formed by copper bars placed in the slots, which must be carefully soldered to two rings of the same material, called short-circuit rings. This set of bars and rings forms the squirrel cage motor.

There are also single-phase asynchronous motors, in which the stator has a single-phase winding and the rotor is a squirrel cage. They are small power motors and in them, by virtue of Leblanc's Theorem, the magnetic field is equal to the sum of two equal rotating fields that rotate in opposite directions. These single-phase motors do not start on their own, so some auxiliary means must be provided for starting (split phase: resistor or capacitor, shielded pole).

The rotation speed of the magnetic field or synchronism speed is given by:.

Where is the system frequency, in Hz, and is the number of pairs of poles in the machine. The speed being thus given in revolutions per minute (rpm).

What produces the induced voltage in the rotor bar is the relative motion of the rotor compared to the magnetic field of the stator, this can be seen in the following equation:

Where:.

: Bar speed in relation to the magnetic field.

: Magnetic flux density vector.

: Length of the conductor in the magnetic field.

: Represents the "vector product" operation.

The squirrel cage motor consists of a rotor "Rotor (electrical machine)") made up of a series of metal conductors (usually aluminum) arranged parallel to each other, and short-circuited at their ends by metal rings, this is what forms the so-called squirrel cage due to its graphic similarity to a squirrel cage. This 'cage' is filled with material, usually stacked sheet metal or aluminum. In this way, a -phase system of conductors is achieved (the number of conductors being, commonly 3) located inside the rotating magnetic field created by the stator, which results in a very effective, simple, and very robust physical system (basically, it does not require maintenance as it lacks brushes).

The wound rotor motor "Rotor (electrical machine)") has a rotor made up, instead of a cage, of a series of conductors wound on it in a series of slots located on its surface. In this way we have a winding inside the magnetic field of the stator, with the same number of poles, and in motion. This rotor is much more complicated to manufacture and maintain than the squirrel cage one, but it allows access to it from the outside through rings that short-circuit the windings. This has advantages, usually such as the possibility of using a starting rheostat that allows the starting speed and torque to be modified, as well as reducing the starting current.

In either case, the rotating magnetic field produced by the stator inductor coils generates induced currents in the rotor, which are what produce the movement.

The asynchronous motor works on the principle of Faraday mutual induction. By applying three-phase alternating current to the inductor coils, a rotating magnetic field is produced, known as a rotating field, whose frequency will be equal to that of the alternating current that is fed to the motor. This field, when rotating around the rotor in a state of rest, will induce electrical tensions that will generate currents in it. These will in turn produce a magnetic field that will follow the movement of the stator field, producing a couple or motor torque that makes the rotor rotate (principle of mutual induction). However, since induction in the rotor only occurs if there is a difference in the relative speeds of the stator and rotor fields, the speed of the rotor never reaches that of the rotating field. Otherwise, if both speeds were equal, there would be no induction and the rotor would not produce torque. This difference in speed is called "slip" and is measured in percentage terms, so this is the reason why induction motors are called asynchronous, since the rotor speed differs slightly from that of the rotating field.

The slip differs with the mechanical load applied to the rotor, being maximum with the maximum load applied to it. However, despite this, the motor varies its speed little, but the motor torque or coupling increases (and with it the intensity of current consumed) so it can be deduced that they are constant speed motors.

Electrically speaking, the asynchronous motor can be defined as an electrical transformer whose stator windings represent the primary, and the rotor windings are equivalent to the secondary of a short-circuited transformer.

At the moment of start-up, as a result of the rotor's resting state, the relative speed between the stator and rotor fields is very high. Therefore, the current induced in the rotor is very high and the rotor flux (which always opposes that of the stator) is maximum. As a consequence, the stator impedance is very low and the current absorbed from the network is very high, reaching values ​​of up to 7 times the nominal current. This value does not cause any damage to the motor since it is transient, and the strong starting torque makes the rotor turn immediately, but it causes abrupt and momentary drops in voltage that manifest themselves mainly as flickering of the lamps, which is annoying, and can cause damage to sensitive electronic equipment. Induction motors are all prepared to withstand this starting current, but repeated and very frequent starts without rest periods can progressively raise the stator temperature and compromise the useful life of the stator windings until causing failures due to melting of the insulation. That is why electronic "soft start" devices are used in medium and large powers, which minimize the motor starting current.

As the rotor gains speed, its current decreases, the rotor flux also decreases, and with it the impedance of the stator windings, remember that it is a phenomenon of mutual induction. The situation is the same as connecting a transformer with the secondary shorted to the AC network and then with a variable resistor inserted, progressively increasing the load resistance until reaching the nominal current of the secondary. Therefore, what happens in the stator circuit is a reflection of what happens in the rotor circuit.

Asynchronous motors are essential in various applications due to their robustness and efficiency. Next, we analyze the industrial applications of these motors:.

Las técnicas de control más comunes son las 4 siguientes:.

Control of line voltage applied to the stator

Line voltage control is carried out by varying the voltage supplied to the stator winding. This voltage can be regulated by varying the firing angle of the thyristors. This system introduces harmonics into the network and a good power factor is not achieved.

Line voltage-frequency control. Scalar control

A drop in frequency would produce a drop in the inductive impedance and if the voltage is maintained it causes a rise in current, which could cause the motor to burn out. This effect is avoided by modifying both parameters, but keeping the V/Hz ratio constant.

The technique used to vary the voltage applied on the stator and the applied frequency in proportion is pulse width modulation or PWM through DC/AC conversion. Using PWM modulation, a DC input voltage is converted to a symmetrical AC output voltage with the desired magnitude and frequency.

Depending on the supply network, the regulation is different:

ç.

Field Oriented Control (FOC)

The purpose of vector modulation or oriented field control applied to asynchronous machines is to achieve a type of linear control, making the current that produces the magnetic flux independent of the current that produces the motor torque. To achieve this, because asynchronous machines do not have two decoupled windings, a fictitious and equivalent circuit reference is created, with two windings arranged in quadrature (at 90° electrical angle) in the stator, replacing the three real windings.

In this way, the three-phase system of stator currents is transformed into a two-phase system of quadrature currents, non-stationary, that rotates synchronously with the rotor's magnetic field.

Consequently, these two currents represent the two decoupled windings and therefore can be controlled independently. In this new reference system the two stator currents are processed as rotating vectors, hence the name Vector Control or Spatial Vector Modulation (SVM) or Field Oriented Control (FOC).

There are two types of field-oriented control:

Direct Torque Control (DTC)

In direct torque control it offers a very fast torque response and good dynamic behavior. The instantaneous values ​​of torque and flux are calculated from the machine stator variables.

Torque and flux are directly and independently controlled by the optimal section of the inverter switching states. The values ​​are compared to the reference and generates an error.

The error enters a controller and generates a logic signal that modifies the space vector of the inverter voltage so that it adopts the most suitable value. Thus it forces the air gap flux vector to vary according to the reference value.

The motor torque is controlled by the rotation of the stator flux vector using appropriate switching states. At the same time, the magnitude of the stator flux vector is controlled in the same way, that is, with the use of the switching state of the inverter. This value can be changed according to the flow setpoint requirements. The calculated torque and flux values ​​are compared with their setpoints, the errors enter the hysteresis controllers. Its outputs are logic signals of discrete values ​​± 1.0 that are applied to the switching table that chooses one of the eight possible states of the inverter voltage space vector.

Its main difference from the other previous control methods is that in DTC there is no separate PWM modulator, but rather the position of the power converter switches is determined directly by the electromagnetic state of the motor. To do this, it is necessary to have a very exact model of the engine together with a very high calculation capacity.