How the linear induction motor works

Asynchronous machine

The asynchronous machine belongs to the family of Three-phase machines. In this post we explain why it behaves asynchronously and how it works. Like most electric motors, the asynchronous machine can also be used as a generator and as engine be used. We'll mainly focus on the in this post Engine operation a.

Asynchronous machine basic principle

An asynchronous machine, often too Three-phase asynchronous machine or Three-phase induction machine called, is a three-phase machine in the rotor the rotating field of the stator either runs ahead or behind.

If the rotor turns slower than the rotating field of the stator, it is called Engine operation. In the opposite case, the asynchronous machine is called generator used.

Asynchronous machine structure

An asynchronous motor, like any other, possesses Electric motor also, one stator and one rotor. The stator consists of three coils or a multiple thereof. These are charged with three-phase current and produce a rotating magnetic field. If you want to know more about it, you can watch the video on the topic again Three-phase motor look at.

Squirrel cage

For the rotor an asynchronous motor is available two various Embodiments. One design is the Squirrel cage or squirrel cage. The conductor bars have a low resistance and are short-circuited to one another in a ring at the ends. The grooves in the stator core are usually filled with aluminum or copper as conductor bars.

Slip ring rotor

Another possible design of the rotor is that of the Slip ring rotor. The rotor winding is not short-circuited, but is led to the outside via slip rings. External resistors can then be switched on via step switches, which influence the speed and torque.

Asynchronous machine functionality

The threeWindings of the stator be at aThree-phase three-phase network connected with the appropriate voltages and frequencies. Sinusoidal currents of the same amplitudes flow through the three coils. TheCurrent amplitudes are in the coils at 120 degrees to each otheroffset. Since the stator coils of a three-phase asynchronous motor are also offset by 120 degrees and arranged in a circle, aMagnetic fieldgeneratedthat with the frequency of the applied voltage dieThe axis of the rotor revolves. Let us first assume an initial speed of the rotor that corresponds to that of the rotating field. In this case it would be magnetic flux in the conductor bars of the rotor constantbecause the area would not change. It would no voltage be induced. The consequence would be that Zero torque is. If the rotor Indeed slower rotates than the rotating field of the stator, changes also the magnetic flux. That leads to a induced voltagethat creates a current in the conductor bars. As a result, the rotor itself generates a magnetic field. Since the rotor is turning, it is also a rotating field. But how does this produce a torque? On the one hand, the rotating field of the rotor interacts with the rotating field of the stator, as a result of which a force acts on the rotor Torque results. On the other hand, the current-carrying conductor bars in the rotating field act on the Lorentz force and consequently a torque.

The smaller the difference between the rotor speed and the rotating field speed, the lower the change in magnetic flux. This leads to a reduction in the Lorentz force. At the same speed, the Lorentz force, as already mentioned, is zero and the rotor becomes slower. This behavior gives us information about the naming, because the Asynchronous motor can only with an asynchronous speed run and develop torque. That means we need one difference between Rotor and rotating field speed. This difference will too Slip s and is calculated as follows:

  • = Rotating field speed
  • = Engine speed

The Slip so is a measure for that Reduction of the rotor speedwhich is dependent on the load moment. As already mentioned, asynchronous motors can also be used as generator operate. In this case, the rotor speed is greater than the rotating field speed (). In motor operation, the rotor speed is therefore lower than the rotating field speed ().

Speed-torque characteristic curve of asynchronous machine

This leads us to Torque-speed characteristic an asynchronous machine! The torque-speed curve for a two-pole asynchronous motor looks like this:

During start-up, the asynchronous motor has a relatively low torque, which can be increased up to Tilt speed increases. The Maximum of the characteristic is this Overturning moment . The Nominal operating point is located between the tilt speed and the idle speed. If the engine is running with the Idle speed runs, then it is operated without load. In the Short circuit speed equals 0, the motor and the starting torque are at standstill works. The synchronous idle speeds correspond to those of a three-phase motor. Synchronous would mean that the rotor rotates at the same speed as the rotating field. The synchronous speed is the stator rotating field speed that is induced in the stator windings by the power supply system. As a result of the slip, the motor speed is approx. 1% to 8% lower than the synchronous speed.

For the Speed ​​control there are different procedures. This goes over the Number of pole pairs, about the Frequency of the supply voltage with a frequency converter via which Slip control, about the Voltage frequency control and about the Field weakening. The course of the torque-speed characteristic changes in all methods.

It is important to mention that you cannot simply connect the motor to the three-phase network and it will start running without further ado. The inrush current is very high with asynchronous motors. Therefore, other start-up procedures are used. Most often the engine is used in Star connection switched, since the power and the torque are reduced to about a third in this case. As soon as the engine has started properly, the Triangle operation be switched. Another start-up procedure is the des Stand starter. For this purpose, starting resistors are switched into the stator windings. As soon as the engine has started, the starting resistors are bridged.

Equivalent circuit diagram asynchronous machine

A Equivalent circuit diagram is a visualization of a Equivalent circuit, which electrically equivalent to the machine behaves. The rotating field induces voltages in the stator and rotor windings of the asynchronous machine. An equivalent circuit diagram can be derived from the given quantities of stator current, rotor current, stator voltage and rotor voltage. With the Equivalent circuit diagram can in turn be further Calculations hire. An asynchronous motor is operated with three-phase current. The stator windings are identical in construction and arranged symmetrically, which means that you can use the asynchronous motor with a single-strand equivalent circuit can describe.

From this equivalent circuit diagram, for example, the torque, the breakdown torque, the idling speed or the slip of the asynchronous machine can be calculated.

Asynchronous machine basic equations

The Torque an asynchronous machine is calculated using the following formula:

The formula for that Overturning moment, i.e. the maximum torque of the asynchronous motor in operation, is:

The Idle speed the three-phase asynchronous machine can be calculated using the following relationship:

The Slip, which is fundamentally important for the function of the asynchronous machine, can also be mapped using the rotational frequencies:

Alternatively, the specific Characteristic values also above the Nameplate derived attached to each asynchronous motor. For example, the nominal torque is not noted on the nameplate of the asynchronous machine, but can be calculated from the information on the nameplate using the following relationship:

  • = Nominal power
  • = Speed ​​in revolutions per minute

Further conventional information on a nameplate is the nominal voltage, the Rated current, the Power factor and the Nominal frequency.

Double fed asynchronous machine

The double fed asynchronous machine is a special design the three-phase asynchronous machine. It differs from the conventional design in that the speed and the reactive power rotor side about a frequency converter can be controlled. The asynchronous machine is connected to the power grid at the stator, whereas the converter is sinusoidal Currents variable frequency in the rotor circle be fed. In the case of double-fed asynchronous machines, the Slip performance from the rotor circuit via a line inverter fed back into the network become. This condition is also called subsynchronous motor operation designated. The double-fed asynchronous machine is used, for example, in wind turbines.

Asynchronous machine application

Three-phase asynchronous machines are available in various designs and are therefore also used in a wide variety of areas of application. They are distinguished by one robust and simple construction and are therefore widely used as standard drives with constant speed. You come in Conveyor technology, in Fans, pump and also Household appliances for use. When fed via a frequency converter, asynchronous machines are also suitable for demanding areas of application. To be mentioned here Electric locomotives, vehicles or Machine tools.

Summary

Let's summarize: the asynchronous motor is operated with three-phase current, which is a Rotating field evokes. In a short-circuited squirrel cage, a Speed ​​difference by changing the magnetic flux a Voltage induced. This causes a current, which on the one hand causes the Lorentz force acts and acts on the other Magnetic field in the rotor is generated, which interacts with the rotating field. These forces act on the rotor Torque.