Three-phase asynchronous machine
A three-phase asynchronous machine ( three-phase induction machine ) is a three-phase machine in which the rotor (also runner ) runs in front of the rotating field of the stator as a generator and as an electric motor . It has a passive rotor, either continuously ( squirrel cage , cage rotor), or is occasionally short-circuited ( slipring ). When used as a generator, the rotor of this asynchronous machine can also be excited with a different frequency ( double-fed asynchronous machine ). Single-phase asynchronous motors are the capacitor motor , the shaded pole motor and the starter motor . The three-phase asynchronous machine was developed by Michail Ossipowitsch Doliwo-Dobrowolski at AEG in 1889 and is widely used in electrical drive technology.
The development of the asynchronous motor goes back to preliminary work by Galileo Ferraris (1885) and essential work by Michail von Dolivo-Dobrowolsky (1891). The latter built the first single cage rotor and later also the first double cage rotor.
The asynchronous motor is the most widely used electric motor today . Three-phase asynchronous machines with capacities of up to several mega watt produced. The advantage over other electric motors is the lack of a commutator and brushes . Brushes wear out and generate sparks (“ brush fire ”), which disrupts the line network with high-frequency vibrations. In addition, machines with brushes may not be used in explosion-proof areas because of the possible effects of brush fire as an ignition source . However, asynchronous motors - especially when operated on a frequency converter - also cause harmonics that affect the network.
The motor consists of two parts, the outer, fixed stator or stator and the rotor or rotor rotating in it. Alternatively, the rotor can also run around the stator, as is the case with a wheel hub motor . On both sides of the narrow air gap, electrical currents flow essentially in the axial direction. The currents are concentrated in coil wires that are surrounded by soft magnetic iron. The iron is laminated perpendicular to the axis .
When operating on three-phase current , the number of copper coils in the stator is three or a multiple thereof, see number of pole pairs , with a phase shift of the currents in adjacent coils of 120 degrees. The stator coils are connected to form three winding phases, the ends of which are led out.
There are two designs for the rotor of a three-phase asynchronous motor:
- In a squirrel-cage or squirrel-cage rotor , massive, highly conductive rods are short-circuited in a ring at both ends of the rotor. In mass production, the laminated core of the rotor is either provided with grooves or channels, which are then filled with aluminum. Fan blades that also serve as cooling fins are often cast at the same time. The number of bars is often different from the number of poles of the stator in order to reduce the pole sensitivity .
- All connections of the slip ring motor run either to a large motor connection box or to two separate motor connection boxes. In the large terminal box are the winding beginnings and ends of the stator windings with the designations U1 V1 W1 / U2 V2 W2, the ends of the rotor windings with the designations KLM (in large letters) and the protective conductor connection (PE for Protective Earth). In the other variant, the stator windings and the protective conductor are brought out in the first motor connection box and the rotor winding ends and also the protective conductor in the second. The designations of the connections are identical. The connection designations on the starting resistors are called (klm) in small letters. In addition, there is PE. Since this motor is operated with starting resistors, a star-delta connection is not used as with the squirrel-cage rotor . Starting resistors or star-delta connections are used because the starting current can reach approx. 10 times the rated current and the motor fuses could possibly trigger early. In addition, with these start-up circuits, a "soft" and slow start of the motor is guaranteed, as is desired in many cases.
The stator or stator consists of the housing, the stator core and the stator winding inserted in it, which is always designed as a multi-phase winding. The housing must support the torque against the foundation. Often the housing has cooling ribs on the outside , which are blown by the fan of the rotor.
The mode of operation of the three-phase asynchronous machine is based on the rotating field , which is directed radially in the air gap between stator and rotor.
If the rotor moves synchronously with the rotating field, the magnetic flux through the mesh of the cage is constant (apart from transients) and no voltage is induced. The torque is or becomes zero.
If the rotor rotates more slowly than the rotating field, the magnetic flux changes, which induces a voltage, which in turn creates a current. As long as the slip is small, the current is proportional to the rate of change of the flux, i.e. the slip. The field associated with the cage current is still small compared to the field of the stator and is 90 ° out of phase with it. The resulting torque is proportional to the slip.
If the opposing field of the cage is felt, the cage current no longer increases proportionally to the slip and the phase shift decreases. The torque reaches a maximum. The operating point lies between this maximum and the synchronous speed.
At the other extreme of the blocked rotor, the cage corresponds to the secondary winding of a (short-circuited) transformer . The power consumption is limited by the leakage flux and ohmic losses. In the start-up range, the motor has poor efficiency and heats up significantly. The high starting current can be reduced by an upstream starting resistor . In addition to the expense of additional components, you have to accept a longer start-up time.
Strong noises can occur during start-up ( magnetic Barkhausen effect ). There may be a persistence (sticking) at speeds below the nominal speed with loud noise, often at 1/7 of the synchronous speed. The grooves in the laminated cores of the stator and rotor generate harmonics in the power supply system (groove whistling).
First reason: When the rotor rotates past the stator slots, magnetic flux pulsations occur in the non-slot areas of the laminated core of the stator. If the rotor and stator have the same number of slots, the pulses interact and the motor can "stick".
Second reason: If harmonics get into resonance with the natural vibration of a machine part (rotor with bearing play), the rotor can stick. The rotor starts up briefly and stops humming or it creeps over this point.
The problem is avoided if the grooves of the rotor are arranged at an angle to the shaft axis. Although this makes production more expensive, phase-shifted magnetic fields can no longer come into resonance.
The motors are mostly controlled by contactors , depending on which operating mode is intended. One example is the star-delta connection . The motor speed can also be adjusted using converters such as B. Control frequency converters by increasing or decreasing the frequency. This is useful for systems that require a variable speed without using an adjustable gear. In the wood processing engines, for example, milling machines connected to a frequency converter in order to generate, for example, 200 Hz of the mains frequency of 50 Hz, the speed then to over 10,000 min -1 can be increased. The high centrifugal forces that act on the rotor require special machine designs.
Asynchronous motors have a high inrush current ; if this is not known, eight times the nominal current is assumed. In order to protect the network and connected gears, as well as to avoid triggering upstream fuses, special starting processes are used with asynchronous motors . The most frequently used method is the star-delta connection. When starting in star connection, the power and torque are reduced by a third. After the run-up time, the contactors are reversed to switch to delta operation. With the appropriate configuration or programming, frequency converters can start up asynchronous motors gently and according to the load. With more powerful engines, the respective starting procedure must be coordinated with the network operator.
With squirrel cage motors, the skin effect has a favorable effect when starting up . In the case of high slip, the current is concentrated at the edge of the short-circuit bars, which increases the resistance. The characteristic curve of power and torque versus speed can be influenced via the profile of the short-circuit rods.
Asynchronous machines can
- on the rigid network
- on the frequency converter or direct converter
- with pole changing
- as a slip ring rotor as a sub- or oversynchronous cascade
Different pole numbers and frequencies result in the following speeds for the rotating field:
|Number of poles||Number of pole pairs||n sync 50 Hz||n sync 60 Hz|
|2||1||50 s −1||60 s −1|
|4th||2||25 s −1||30 s −1|
|6th||3||16.6 s −1||20 s −1|
|8th||4th||12.5 s −1||15 s −1|
|10||5||10 s −1||12 s −1|
|12||6th||8.3 s −1||10 s −1|
|14th||7th||7.15 s −1||8.56 s −1|
|16||8th||6.25 s −1||7.5 s −1|
These are the stator rotating field speeds, i.e. the speed that the network impresses on the motor via the field windings in the stator. They are also known as synchronous speed.
In motor operation, the mechanical speeds are slightly below these values (mostly 1 - 8%) due to the principle- related slip, depending on the design and load. Due to the principle, because only the speed difference between the stator rotating field and rotor induces a voltage in the rotor through which a current flows in the rotor, the magnetic field of which generates the necessary torque in interaction with the magnetic field of the stator. As a result, the slip, i.e. the reduction in the rotor speed, is always dependent on the load torque.
Important speeds are the idle speed (motor runs without load), the nominal speed (motor delivers nominal power as the product of nominal speed and nominal torque ), stall speed (maximum torque; if this is exceeded by the load, the motor stops) and short-circuit speed (motor stops, starting torque , Starting current).
If the three-phase asynchronous machine is driven to a higher speed than the synchronous speed, it feeds power back into the network (generator operation).
Dahlander circuit (Dahlander motor)
- With the Dahlander circuit , the number of poles of the asynchronous machine (pole-changing motors) can be increased in a ratio of 1: 2 and thus its speed can be changed in a ratio of 2: 1. Typical applications are:
- Lathes with two basic speeds: slow or fast running.
- Two-stage fan drive for housing ventilation.
With asynchronous machines in squirrel cage design, the Dahlander circuit offers the option of pole switching and thus speed switching.
It is conceivable to arrange two completely separate motors on one shaft. It is elegant when these motors are in a housing. Then both motors can have a common rotor (cage). However, the stator windings are designed twice. Stator one is designed for the low speed. Stator two is designed for four or six times the speed. A speed ratio of one to two is usually achieved with the Dahlander circuit described above.
Pole-changing motors have almost the same properties as Dahlander motors, with the difference that Dahlander motors have so-called "tapped windings" (the windings have three connections: beginning, end and a tap in the middle of the winding). So you only have three windings offset by 120 degrees in the stator core. Pole-changing motors are equipped with separate windings. That means: You have at least six windings in the stator core, i.e. not one pole pair like the Dahlander motor (three windings), but from two pole pairs upwards (six or more windings).
The point is not always to reduce the inrush current. In some cases it is also a question of the fact that too high a tightening torque, when switched on directly, has a disruptive effect on the system. The so-called KUSA circuit ( Ku rzschlussläufer- Sa nftanlauf) is a circuit for starting induction motors with cage rotors at about the half of the nominal torque.
With the KUSA circuit, a series resistor is placed in an outer conductor of the load circuit of the three-phase motor, which is bridged after an adjustable time or manually by means of a contact. It is often useful to tap into the series resistor in order to be able to set different amounts of the starting torque. This type of start-up is only possible with idling or a low counter-torque .
Advantages and disadvantages
The widespread embodiment as a three-phase motor with squirrel-cage rotor ( English squirrel cage induction motor ) is considered the "workhorse" of electric drive technology. Combined with a correspondingly controlled frequency converter , it is also able to start up against large counter-torques from working machines. The frequency converter assemblies are currently also increasingly taking on the task of motor protection. Motors with a built-in frequency converter are also available. This reduces the amount of wiring and interference suppression.
- robust, standardized, cost-effective as standardized equipment
- long service life, low maintenance, no brush wear on the squirrel cage rotor
- Property as a motor brake (generator operation) if (mechanical) speed is higher than electrical rotating field frequency
- Can be heavily overloaded for a short time (up to greater than 2 × rated torque , up to greater than 1.5 × rated power depending on motor cooling and overload duration)
- Starting against high counter torques without tools (also depending on the rotor design)
- almost constant speed, no "runaway" when idling
- Can be used in potentially explosive areas as there are no brushes or slip rings (no brush fire, no spark formation)
- comparatively low manufacturing costs
- IE3 and IE4 versions available as highly / highly efficient drives (efficiency η> 95%)
- the rotor is de-energized and can also run in liquids, gases or in a vacuum
- high speed capability, in operation with frequency converter consistently high efficiency
- High starting current according to the starting torque
- Always runs with slip , ie the (mechanical) speed of the motor shaft as drive (motor operation) is always less than n times the (electrical voltage) frequency (rotating field frequency). The speed is not stable, but cannot be changed at will due to voltage / current changes (see synchronous machine , reluctance motor )
- The speed can only be changed for special designs with pole changing or with an additional frequency converter
- Especially with small designs approx. 20 to 30% more volume with the same torque compared to permanent magnetized synchronous motors
- Three outer conductors are absolutely necessary for the supply (can be produced from single-phase alternating current with an electronic frequency converter, chopper motor (" chopper (electrical engineering) ") or operating capacitor ( capacitor motor ))
- If one of the three outer conductors is missing, the asynchronous motor cannot start; it hums when it comes to a standstill
- Complex theoretical calculation methods (compared to other electrical machines)
- Stepper and servo motors have advantages in positioning tasks and are lighter in comparison
- No holding torque at standstill
- No (electrical / electromagnetic) braking torque if the supply voltage is disconnected from the motor during operation
Norms and Categories
In the European Community, EN 60034 "Rotating electrical machines" must be observed.
The standard motors for which the major manufacturers publish lists with technical data are classified according to torque classes. Usually these motors can start against twice the nominal torque. The shaft height is a guide for the construction. The standard motor range starts at AH 56 and extends up to AH 315 (approx. 200 kW). Above AH 315, the trans-standard motor range begins with AH 355.
- Resistance rotor with a smoother start, but poor efficiency
- Slip ring motor with rotor winding led out via slip rings for the purpose of connecting a resistor (only when starting up)
- External rotor with stator inside and rotor outside
- Stator on both sides of the air gap, inside the rotor as an aluminum cylinder ( canned motor ) or disk ( Ferrari motor )
- Linear motor with "unrolled" geometry
- Linear motor with tubular stator for conveying liquid metals
In generator mode, the rotor rotates faster than the magnetic field and thus feeds energy into the network.
There are three different asynchronous machines that are used as generators.
- Asynchronous machine with squirrel cage rotor: asynchronous generator
- Asynchronous machine with slip ring rotor: Double-fed asynchronous machine
- Asynchronous machine with two stators: cascade machine
All three generator types are used in decentralized power plants.
Idealized consideration / equivalent circuit diagram
To understand the speed control processes, it is necessary to consider the equivalent circuit diagram of the asynchronous machine. The equivalent circuit diagram shows a circuit that is electrically equivalent to the machine, as seen by a frequency converter.
The stator winding is shown on the left-hand side, it consists of R s (copper resistance and equivalent series resistance of the magnetic reversal losses ) and the reactance of its inductance X s in asynchronous operation. The rotor or rotor is shown on the right: the inductance X r represents the inductance that appears when the motor is at a standstill; it results from the magnetic field lines passing the short-circuit cage. The effective resistance Rr is made up of
- the equivalent value of the real power delivered by the machine; this value changes with the change in torque or the load on the machine. It is very large when the machine is idling.
- Accordingly, the square of the step-up Statorwindungszahl ohmic resistance of the squirrel cage ; The squirrel cage consists of individual turns embedded in the iron, mostly made of aluminum.
When idling, the equivalent circuit diagram of the asynchronous motor essentially consists of R s and X s , which is why such a machine almost only consumes reactive power. The current consumed when idling is often as high as the rated current; due to the copper and magnetic reversal losses when idling, the machine often already has more than half the power loss at rated load. With increasing load, the active current through Rr and thus in the squirrel cage increases. The phase angle between current and voltage decreases from almost 90 ° to smaller values. In the case of highly magnetized asynchronous motors, as the torque rises, the total current often initially decreases, which only later rises again to the rated current as the torque rises.
The asynchronous machine consumes a reactive current with X s , which ensures the magnetization of the machine. In contrast to the three-phase synchronous machine , the magnetic flow in the asynchronous machine must first be built up by the reactive current in the stator winding.
The load-dependent active current generates a voltage drop in the cage part of the Rr, but only an insignificantly higher voltage drop in Rs. As a result, the losses increase faster with increasing load in the rotor than in the stator. The copper resistance Rs and the "copper" resistance of the squirrel cage portion of Rr cause increasing losses with the square of the power consumption, so the efficiency of the machine decreases with increasing load. In addition, there is their temperature dependency, which is why the efficiency of the warm machine decreases somewhat.
In converter operation, the reactance Xs also becomes smaller and smaller as the frequency decreases. If the nominal current is adhered to, the voltage supplied by the frequency converter must therefore decrease. As a result, the ratio of the voltage divider R s to X s becomes more and more unfavorable and Rs leads to increasing losses relative to the available motor power. In continuous operation, only approximately the nominal torque can be generated because the rotor and stator are not adequately cooled. On the other hand, if the speed or frequency is higher than the nominal, an asynchronous motor may work on higher voltages - taking into account the insulation - and is more effective.
Modern frequency converters can measure R s / R r themselves and are thus able to automatically configure themselves for any connected motor and thus protect it from overload. A holding torque or speeds close to zero can be achieved with vector control . Here, too, there is no cooling, as the fan wheel on the rotor then no longer cools the rotor itself, the protruding stator windings and the air gap.
Complex pointer model of the asynchronous motor with squirrel cage
The model is subject to the assumption of a rotationally symmetrical structure of the machine and the absence of a leakage field reluctance . The model can be expanded to include these. However, it is not taken into account here (initially) in order to keep the model as simple and understandable as possible. The same applies to the number of turns of the stator winding.
The entries of a vector (x, y) in the plane of rotation are represented as a complex number x + iy. The field as well as the supply voltage and the stator current are the rotating pointer variables of the stator, it is the pointer of the rotor current. Connected to the three phases of the electricity network, the pointer can be represented as. (Delta connection)
As the runner rotates forwards, he “sees” the magnetic field rotating backwards.
This results in the mesh equation of the rotor circle in co-rotating coordinates:
The magnetic field is the result of the rotor and stator current multiplied by the main field reluctance :
Replacing with results in the system of equations with the unknowns and .
If one takes into account stray field reluctances in the form of the inductances and the number of turns of the stator, one obtains very similar equations:
The generated torque results from the cross product of and rotor current. Analogous to the pointer model, this is represented here in complex number calculation.
- (* for the conjugate complex values of )
To smooth out the excitation field, not all turns of a coil are usually concentrated in one slot, but rather distributed in several adjacent slots.
This distribution reduces the voltage amplitude of the fundamental wave, which is taken into account by the zone factor.
- with number of holes (number of slots per pole per strand) and number of strands .
In a multilayer winding, the displacement of the winding layers is referred to as tension. This shift smoothes the excitation curve and thus reduces the harmonics of the induced voltage.
The tension reduces the induced stress amplitude, which is taken into account by the tension factor. He calculates too
with the number of pole pairs , the number of slots and the winding pitch . The winding step describes the ratio of the coil width to the slot pitch.
The product of the chord factor and the zone factor is called the winding factor.
Characteristic values / characteristics
The terms rated power, rated speed and rated torque result from the information on the technical data of the motor and the associated nameplate . The design values are also used in this context.
The nominal torque is usually not noted on the nameplate. It can be calculated from the following formula. See also performance in technical applications .
- Torque M in Newton meters (Nm)
- Power P in kilowatts (kW)
- Speed n in revolutions per minute (min −1 )
- 9549 is a rounded number
The associated synchronous speed (or rotating field speed) is always just above the nominal speed, which results from
- Speed n in revolutions per minute (min −1 )
- Mains frequency f in Hertz (s −1 , given on the nameplate)
- Number of pole pairs p (always integer)
At 50 Hz, this results in values of 3000, 1500 or 750 revolutions per minute with the number of pole pairs 1, 2 or 4.
The example shown for a nameplate relates to a motor that is only planned for star operation. With a mains frequency of 50 Hertz and a nominal power of 5000 kW and a nominal speed of 1480 / min the following results:
- Number of pole pairs = 2
- Synchronous speed = 1500 / min
- Nominal torque approximately 32.3 kNm
The Ossanna circle represents another method for the visual representation of the power, torque and loss of an asynchronous machine in generator and motor operation depending on the slip .
The picture opposite shows the typical torque curve depending on the speed. In delta operation, the motor has about three times the starting torque compared to star operation. The operating points B 1 or B 2 are beyond the breakdown torque K 1 or K 2 .
With P (like pump) the curve for the required torque of a centrifugal pump is drawn as an example.
It is important that the speed range from zero to the tipping point is passed as quickly as possible, because in this range the motor has a poor efficiency and warms up accordingly. The (critical) start-up time depends on the inertia of the machine and the ratio of the start-up torques.
The example shows that the pump apparently also runs without problems in star connection, because the operating points B1 and B2 are close together. Nevertheless, it is possible that the motor draws too high a current under continuous load in star connection to generate the torque required by the driven machine. The motor heats up considerably as a result of the fact that the power consumed is included in the calculation of the heat losses as a square. A heating above the permissible temperature specified by the manufacturer shortens the life of the motor significantly. Often the required rated torque for operation in delta connection is so great that the motor cannot produce it in star connection. Starting up and switching to delta connection must therefore take place without load or up to load torques that the motor can still handle in star connection without heating up excessively.
In the example, the driving torque (star) in the start-up area is around two to four times greater than the required torque of the pump. The difference is the accelerating part. Therefore, the pump could start up with the slide open. The technical standard is starting a pump with a closed slide. Then the required torque is considerably smaller and the critical start-up area is passed through as quickly as possible.
Fans with long blades (e.g. in a cooling tower) have a large moment of inertia. Furthermore, starting is only possible under load. This potentially results in very long start-up times, so that the motor-fan combination must be carefully designed.
Statutory provisions and other regulations
- EN 60 034 Part 1 General provisions for rotating electrical machines
- EN 60 034 part 8 Terminal designations and direction of rotation for electrical machines
- DIN IEC 34 Part 7 Types of rotating electrical machines
- EN 60034-5 Degrees of protection of rotating electrical machines
- EN 60034-6 Types of cooling, rotating electrical machines
- Günter Boy, Horst Flachmann, Otto Mai: The master's examination in electrical machines and control technology . 4th edition. Vogel Buchverlag, Würzburg 1983, ISBN 3-8023-0725-9 .
- Gregor D. Häberle, Heinz O. Häberle: Transformers and electrical machines in power engineering systems. 2nd Edition. Verlag Europa-Lehrmittel, Haan-Gruiten 1990, ISBN 3-8085-5002-3 .
- Andreas Kremser: Electrical machines and drives, fundamentals, motors and applications. 2nd Edition. Teubner Verlag, Stuttgart 2004, ISBN 3-519-16188-5 .
- Detlev Roseburg: Electrical machines and drives. Fachbuchverlag Leipzig in Carl Hanser Verlag, 1999, ISBN 3-446-21004-0 .
- Günter Springer: Expertise in electrical engineering. 18th edition. Verlag Europa-Lehrmittel, Wuppertal 1989, ISBN 3-8085-3018-9 .
- Germar Müller, Karl Vogt, Bernd Ponick: Calculation of electrical machines . 6th edition. WILEY-VCH Verlag, Weinheim 2008, ISBN 978-3-527-40525-1 .
- Institute for Electrical Power Engineering and Energy Systems: Electrical power engineering, asynchronous machine
- Basic internship at the University of Stuttgart: Three-phase asynchronous machine (DAM) (accessed on Feb. 18, 2014)
- Rotary field animation of an asynchronous machine (YouTube video)
- Internship documents (Christian-Albrechts-Universität Kiel) : Single-phase equivalent circuit diagram of the asynchronous machine, locus of the stator current (Der Heylandkreis), power and torque of the asynchronous machine (including derivation of the complete torque curve) (accessed on May 20, 2015)
- General Electricity Society in Berlin: Anchor for AC motors . German Imperial Patent 51.083 from March 8, 1889.
- Any cage current that is still present decays due to ohmic losses.