How many types of motor windings
In this post, we'll cover a specific one Shape of the electric motor, the so-called Three-phase motor. You will learn that in the process Rationale a three-phase motor like him built up is and what roles each Components in a network take in. Below we go to the connection a three-phase motor and its functionality a. In addition, you will learn how to do that power and the Efficiency can be calculated and in what Synchronous and asynchronous motors distinguish.
We have all of this for you in one Video summarized so that you can find out the most important things in a nutshell!
Three-phase motor basic principle
In industry mostly high performance required that a DC motor cannot provide. For this mission there is the Three-phase motor. The three-phase motor is also characterized by a good synchronization, so one constant power transmission on the wave, off. As a Electric motor works and is set up, you already know from the first post on this playlist. The three-phase motor is a special type of electric motor.
The coils for the stator field can either be in Delta connection or Star connection operate.
The important thing here is Distinction between Three-phase machine and Three-phase motor. A three-phase machine can on the one hand the conversion of electrical into mechanical energy guarantee, i.e. as Three-phase motor act. On the other hand, a three-phase machine can also be used as a generator be used, which mechanical energy into electrical energy converts.
As already mentioned, the three-phase motor is operated with three-phase alternating current. The difference to direct current is that three phases, so there are three lines, their alternating voltage shifted by 120 ° against each other is. The sum of the three voltages is by the symmetrical distribution of stresses equals zero.
The three-phase alternating current leads to one in the three separate stator coils of the three-phase motor periodicallychanging tension. These windings are usually designated with the letters U, V and W. You can either in Star- or Delta connection be connected to the three-phase current. The functionality of the star and delta connection is also briefly explained in this article.
The Stator field is generated by energizing the three stator coils. If you look at the magnetic field of a coil, you can see that its schedule compared to the other magnetic fields offset by 120 ° and is sinusoidal. The sum of the Magnetic field always has the same strength and turns with the frequency of the three-phase current. The magnetic field is also called Rotating field designated.
Becomes a rotor in form of Bar magnets by doing Rotating field positioned, its north pole aligns with the south pole of the rotating field and its south pole aligns with the north pole of the rotating field. This results in the rotational movement. This principle is the same for all three-phase motors, the difference however lies with the Speeds. Therefore, three-phase motors can be used in Asynchronous and synchronous motors distinguish.
Three-phase motor structure
To understand how a three-phase motor works, it is useful to use the Structure of the three-phase motor with its individual Components to know.
The moving part of the three-phase motor is the rotor. The rotor is also called Anchor or runner designated. In general, the rotor consists of one movable, rotary axis and one Copper or aluminum coilthat is wrapped around a rotor core. The rotor becomes an electromagnet as soon as an electrical voltage is applied. The wave is firmly connected to the moving rotor and for the Power transmission responsible. Task of the rotor, in a conventional three-phase motor design, is the conversion the electrical energy in a mechanical movement through the Alignment with the magnetic field of the stator.
In the case of three-phase motors, the stator out three coils or a multiple thereof, each of which is supplied with current by one line voltage phase. The three-phase alternating current has the consequence that the Phases of the three coils by 120 degrees each time to each other postponed are. This results in a third delayed magnetic field in the three coils of the stator leading to the rotational movement in the rotor leads. The rotor and stator are the two central components of a three-phase motor structure.
The motor housing can be made of different materials depending on the performance. With small to medium power Die-cast aluminum used, whereas with more powerful engines Gray cast iron is used. There are often Cooling fins incorporated that the Surface of the housing increase. Thus, the warmth of the three-phase motor more efficient derived become.
The end shields are those A- and B-sided cover of the motor housing. The A and B sides of a motor housing are located orthogonal to the rotor shaft. you protect the three-phase motor and the inside of the machine against touch and are responsible for mounting the rotor shaft. Bearing shields are built into the housing with a very precise shape in order to ensure an even air gap between the stator and rotor.
The laminated rotor core is mounted on a steel shaft. The shaft passes through the end shields. On the A-side, the Output shaft the shaft operates one through the end shield and on the B side Fan or additional systems such as a mechanical brake.
The shaft operates a fan on the B side. This directs an air flow, which is generated by the rotary movement, back to the cooling fins of the motor housing through a hood.
The task of the bearings that are built into the end shields is that connection of rotational moving parts and the static components of the three-phase motor housing.
Three-phase motor connection
To connect the three-phase motor, there are basically two types of connection for the terminal board circuit: the Star connection and the Delta connection. In general it can be said: If there are two voltages on the nameplate of the three-phase motor, then for the highertension the Star connection and for the lower tension the Delta connection to be used when connecting.
For a motor with 230 / 400V according to the nameplate, the Star connection be used.
The Star connection will also be Y connection called because the circuit looks like a "Y". To the three different Connection or terminal points one coil connection of each of the three coils is on one ladder each connected. The other three coil connections are connected at the common star point. The three points are designated with the numbers 1, 2 and 3 and the star point with 0. The three winding phases are located on one side in the star point. The voltage of 230V can be measured between the star point and the phase.
For a motor with 400 / 690V, on the other hand, the Delta connection can be used.
The Delta connection will also be π circuit called. You can see three different points or terminal points here and there is neither a series nor a parallel connection. The resistors are - as the name suggests - in the triangle between points 1, 2 and 3. Three-phase motors with 400 / 690V can start up either directly or via a star-delta connection. The latter is often just with more powerful three-phase motors used. To the Direction of rotation of the rotor of a three-phase motor to reverse, just have to two of the three conductor phases interchanged become.
Three-phase motor mode of operation
The three Windings of the stator be at a Three-phase three-phase network connected with the appropriate voltages and frequencies. In the first chapter it was already mentioned that the three coils are traversed by sinusoidal currents of the same amplitudes. The Current amplitudes are in the coils at 120 degrees to each other offset. Since the stator coils of a three-phase motor are also offset by 120 degrees and arranged in a circle, a Magnetic field generated that with the frequency of the applied voltage die The axis of the rotor revolves. The size of the Magnetic field remains in operation the same amount, but changes in Unison with the created frequencyof the three-phase network.
The mains voltage in Germany has a Frequency of 50 Hz. The three-phase current changes its period 50 times per second. The same thing happens with the magnetic field of a coil. The Period duration of the stator field is thus 20ms.
The Magnetic field of the three-phase motor, also Rotating field called, induced in the coil of the rotor one electrical voltagewith which the rotor to a Electromagnets becomes. The Rotor aligns itself consequently in the direction of the rotating field out, is based on the speed of the three-phase motor accelerated and builds as a result Torque on. The speed of the rotor is not always identical to the speed of the rotating field, which is due, among other things, to frequency drops in the rotor coil.
As with Brushless DC motor can with three-phase motors several pairs of coils be used.
The pairs face each other and are distributed around the perimeter. The number of pole pairs is determined by the Number of pole pairs p set. One switches several pairs of coils for each phase one after the other and evenly around the circumference of the stator distributed, the engine speed changes. This results in the Formula for the idle speed:
- f = frequency
- p = number of pole pairs
In the table are the Idle speeds for one Mains frequency of 50Hz in depending on the Number of pole pairs listed. If we the Increase the number of pole pairs, so we can do the Change speed. At low speeds, the result is a quieter, more even run with heavy loads. Do we want them Standard speed for a three-phase motor with p = 1 increase, then this is only possible with the help of one Transmission or the Increase in frequency. Since the mains frequency is specified, the frequency can only be increased with a frequency converter.
Three-phase motor power and efficiency
The power and the Efficiency a three-phase motor can be best described using a specific Example clarify.
Consider a three-phase motor that is operated with three-phase alternating current. The technical data can be found on the nameplate of the three-phase motor:
- 3 ~ motor
- Line voltage: U: 400V
- Conductor current I: 19 A
- Output power at the shaft (power output): 9 kW
- Active factor cos : 0,8
In addition to the key figures, the type plate usually also shows the speed and the frequency of the three-phase current.
With the key figures, the Power consumption of the three-phase motor calculated become:
The Factor root 3 must because of the Three phase alternating current must be taken into account. In the example, the result is input power:
The Power dissipation, i.e. the energy that is lost through waste heat or friction, is calculated from the difference between Power consumption and power output. For the three-phase motor in the example:
In this context, the Three-phase motor efficiency be determined.
Three-phase motor types
The basic structure of a three-phase motor also results in the Differentiation between synchronous and asynchronous motors.
The synchronous motor is characterized by the fact that the rotor exactly with same speed runs like that Magnetic field of the stator coils and thus also in sync with the Frequency of the three-phase network. Synchronous three-phase motors can basically be used as engine, as well as a three-phase motor generator Find application. Synchronous machines can only use a so-called Excitation field operate. This excitation field is generated by applying a direct current to the pole wheel of the rotor or by using permanent magnets. Without the Excitation field could no torque be built on the shaft.
In contrast to the synchronous motor, the speeds of the asynchronous motor differ. The further the Speed of the rotor to the Speed of the rotating field approximatesthe stronger sinks the frequency the voltage applied in the rotor. In order to sinks also the electricity in the rotor cage. The result are a lower torque and lower forces on the shaft. However, this relationship only applies in theory, since it applies to the bearings of the shaft in practice Power losses are to be observed. This results in a difference between the speed of the Rotor and the speed of the magnetic field the Stator coils. The three-phase motor therefore runs asynchronous. The difference between the rotor speed and the speed of the rotating field is important for the function of the three-phase motor. This difference is also called the Slip designated. If the asynchronous motor is used as an electric motor, the rotor follows the rotating field, whereas when the three-phase motor is used as a generator, the rotor is ahead of the rotating field.
At Synchronous motors can basically between Internal and external pole machines can be distinguished. In both cases, however, the generation of an excitation field is central to the function of the three-phase motor or generator. With the help of the stator coil, energy is fed into the grid in generator mode and taken from the grid when it is used as a motor. The spatial design of the stator corresponds to the structure of a classic three-phase motor, as described above.
At a Internal pole machine lies the rotor Furthermore centered in the rotating field of the stator, where the excitation field is also induced. This exciter field generates Alternating voltages in the coils of the stator. application finds the Internal pole machine for example as a generator in power plants.
From one External pole machine however, when the coils are used to generate the Excitation field on the stator are located. The magnetic field lines emanating from the stator influence the coil windings of the rotor and generate an alternating voltage in it. application Find External pole machines as auxiliary generators or excitation machines.
Asynchronous motors also basically consist of the two components rotor and stator. For asynchronous motors, too, there is a configuration that the rotor runs around the stator. A distinction can be made between two designs:
Is the Rotor as so-called Squirrel cage designed, this is enclosed by a permanently short-circuited cage made of massive windings. The rotor in this modification consists of one Sheet iron package, in the Metal bars (Turns) made of non-ferrous metal are incorporated. Instead of in the rotor coils, the electrical voltage and thus the magnetic field is induced in the squirrel cage in the rotor cage.
In contrast, there is the modification in which the rotor winding protrudes Slip rings is led to the outside. The Windings are there in the grooves of the rotor core embedded. This type is called Slip ring motor designated.
Modes of operation
The Stress of electrical machines and motors can be described with the so-called Nominal operating mode describe. The nominal operating mode there Information about the reasonable burden of electric motors. Three operating modes are listed below as examples:
- Continuous operation describes the operation with a constant load, which lasts until the machine reaches the thermal steady state. The standardized marking corresponds to the abbreviation S1.
- If the duration of the constant load is not sufficient to achieve thermal equilibrium, one speaks of Short-term operation S2.An additional criterion is that there is a downtime that must last until the engine temperature again approaches the coolant temperature to a precisely defined difference.
- A motor operation that is made up of periodically constant games in which the start-up has no influence on the motor temperature Periodic intermittent operation Called S3.
A example the designation would be S3 - 25 percent, in which the motor alternately runs for 2.5 minutes and then remains at a standstill for 7.5 minutes.
In addition to the nominal operating modes, there is also the characteristic value of Switching frequency. This gives information about how often a engine can be switched on in an hour, without in a state of thermal overload to pass over. In the state of thermal overload, the function of the three-phase motor is at risk. The switching frequency depends on several parameters. This includes, for example, the ambient temperature or the period of operation.
Three-phase motor application
The application of three-phase motors is often one of them dependent, if one configuration as a synchronous motor or as an asynchronous motor.
Synchronous motors are, for example, more efficient and known for their comparatively high level of efficiency. In return, they usually have high acquisition costs and high maintenance intensity. This is why synchronous motors are used in high-priced machines, for example in ship drives or industrial choppers.
Asynchronous motors are very robust and inexpensive compared to synchronous motors. Because of these characteristics, induction motors also find Comprehensive application. You can find them in all areas of industry, for example in tools, conveyor belts, industrial robots or large machines.
Also for them mobility Electric motors and especially three-phase motors play a major role. For example, several series of the ICE are operated by three-phase asynchronous motors. In practically all modern electric cars, three-phase motors are also used on the drive side.
The three-phase motor is one special type of electric motor. He will be with Three phase electricity operated, the one voltage offset by 120 degrees in three separate ones Ladders generated. This will be in the stator coils Induced magnetic fieldsthat add up to a common magnetic field, the so-called Rotating field. This rotating field revolves around a central area in the stator coils rotor, which aligns with the rotating field and thereby the speed of the rotating field accelerated becomes. One statically connected to the rotor wave takes up this rotary movement, with what a Torque generated becomes. The three-phase motor therefore transforms electrical energy into mechanical movement.
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