Induction motor

A three-phase induction motor, also known as three-phase induction machine, may be used as a motor or as a generator, either. It has a passive runner who always turned on ( squirrel cage, squirrel cage) or is short-circuited case by case basis ( slip-ring ). When used as a generator, the rotor of the induction machine can also be excited with a different frequency ( Double-fed asynchronous machine ). Single-phase asynchronous motors are operated motor capacitor, AC motor and shaded pole. The three-phase induction motor was developed in 1889 by Mikhail Ossipowitsch Doliwo - Dobrowolski at the AEG.

6.1 coil group
6.2 Sehnungsfaktor
6.3 winding factor

7.1 characteristic example

Basics

The development of the induction motor goes back to work of Galileo Ferraris, 1885, Nikola Tesla, 1887, and Mikhail Dolivo - of Dobrowolsky, 1889. Latter built the first single cage rotor and later a first double cage rotor.

Of the induction motor, the electric motor most widely used today. Three-phase induction motors are manufactured with capacities of up to several megawatts. The advantage compared to other electric motors is the absence of the commutator and brushes. Brushes wear and create sparks ( " brush fire " ), whereby the cable network is disturbed by high-frequency oscillations. In addition, machines may be used in explosion -proof areas with brushes because of possible effect of the brush fire as the ignition source is not. However, induction motors cause - especially when operated at a frequency - harmonics which react on the net.

Motor

Typical structure

The motor consists of two parts, the outer stationary stator or stator and the rotating rotor or rotor therein. On both sides of the narrow air gap electric currents flow substantially in the axial direction. The currents are concentrated in the coil wires, which are surrounded by a soft-magnetic iron. The laminated iron is perpendicular to the axis.

During the operation of three-phase, the number of the copper coils in the stator is a multiple of six, see number of pole pairs, with a phase shift of the currents in adjacent coils of 60 degrees. The stator coils are then connected to three phase windings, the ends of which are led out.

For the runner of a three-phase induction motor, there are two types:

If a short circuit or squirrel cage solid, well-conducting rods at both ends of the rotor are shorted annular. In the mass production of the laminated core of the rotor is provided with either grooves or channels which are then filled with aluminum. At the same time often fan blades are cast, which also serve as cooling fins. The number of rods is often different from the number of poles of the stator to reduce the pole sensitivity.

The stator or the stator consists of a housing, the stator core and the stator coil inserted therein, which is always carried out as a multi- phase winding. The housing must be supported against the foundation torque. Often, the housing has external cooling ribs which are blown by the fan of the rotor.

All connections to the slip-ring motor run either to a large motor terminal box or to two separate motor terminal boxes. In the large terminal box, the winding starts and ends of the stator windings are labeled U1 V1 W1/U2 V2 W2, the ends of the rotor windings with the names KLM ( in large letters ) and the protective earth terminal (PE for Protective Earth).

In the other variant, the first motor terminal box, the stator windings and the protective conductor and led out the rotor winding ends and also the protective conductor in the second. The names of the ports are the same. The terminal's names on the starting resistors are called ( klm ) in small letters. This in turn is PE. Since this motor is operated with starting resistors, one uses not as an induction motor, a star-delta circuit. Starting resistors or star-delta circuits are used, because the starting current can reach 10 times the rated current, and possibly the motor fuses could trigger early. In addition, a "soft " and slow start-up of the engine is ensured in these start-up circuits, as it is desired in many cases.

Function

The operation of the three-phase induction motor based on the rotary field, which is directed radially in the air gap between stator and rotor.

The rotor is moved in synchronism with the rotating field, then ( except for transients), the magnetic flux through the meshes of the cage constant and no voltage is induced. The torque is or becomes zero.

The rotor rotates slower than the rotating field, then the flow, which induces a voltage, which in turn induces a current change. As long as the slip is small, the current is proportional to the rate of change of the river, so they hatch. The cage associated with the current field is phase-shifted by 90 °, still small compared to the field of the stator and to this. The resulting torque is proportional to slip.

, The opposing field of the cage noticeable as current increases, the cage is no longer proportional to the slip, and the phase shift decreases. The torque reaches a maximum. The operating point is located between that maximum and the synchronous speed.

In the other extreme, corresponds to the locked rotor cage of the secondary winding of a ( short-circuited ), the transformer. The current consumption is limited by the leakage flux and ohmic losses. In the starting of the engine has a poor efficiency and heats up. The high starting current can be reduced by an upstream starting resistor. In addition to the cost of additional components you have to take a longer start-up time into account.

During startup, loud noises can occur ( Magnetic Barkhausen effect). It may be a persistence (bonding ) occur at speeds below the rated speed under strong noise, often at 1/ 7 of the synchronous speed. The grooves in the laminated cores of the stator and rotor harmonics in the power system generates ( Nutenpfeifen ).

The problem is avoided by arranging the grooves of the rotor at an angle to the shaft axis. While this increases the cost of manufacture of the motors, but phase shifted magnetic fields can no longer resonate so.

Control

The control is usually caused by contactors, depending on which mode is provided. An example is the star-delta circuit. Can be the engine speed via the inverter, such as Frequency control by increasing or decreasing the frequency. This is useful for systems that require a variable speed, without an adjustable gear to be used. In the wood processing, for example, motors are connected to milling machines via a frequency converter to generate for example 200 Hz from the mains frequency of 50 Hz, where the speed can be then increased to 10,000 min -1. The high centrifugal forces acting on the rotor, require special versions of the machines.

Start-up circuit

Induction motors have a high inrush current. If the inrush current is not known, one starts from the eight times the rated current. To protect the network and connected gear, as well as to avoid the trip upstream circuit fuses, using special tempering process for induction motors. The most commonly used is the star-delta circuit. During start-up in a star circuit power and torque are reduced to about one third. After the run- time of the contactor is switched to delta mode by reversing. Frequency induction motors can ramp up gently and load adjusted if configured or programming. With more powerful engines, the respective annealing process with the network operator needs to be tuned.

In cage motors of skin effect has a favorable effect when starting. At high slip, the current at the edge of the short-circuit rods concentrated, whereby the resistance increases. About the profile of the shorting bars, the characteristic line of power and torque vs. speed influence.

It used to be used ( including among rides ) starting resistors, especially water resistance to boot. The latter consist of a water tank, in the gradually electrodes are immersed.

In the refrigeration technique of the part -winding is an established standard methods for the reduction of the starting current.

Speed control

Asynchronous machines can

The fixed grid
The drive or cycloconverter
With pole-changing
A slip ring rotor as under-or over -synchronous cascade

Be operated.

Different pin counts and frequencies yield the following speeds for the spin box:

These are the stator rotating field speed, ie the speed that imparts the power to the motor through the field windings in the stator. It is also referred to as a synchronous speed.

During engine operation, all mechanical speeds are due to the Inherent slip depending on the design and current load in each case slightly below these values (usually 1-8 %). Due to the principle because only the difference in speed between stator and rotor induces a voltage in the rotor.

Important speeds are the idle speed (the motor runs without load), the rated speed ( engine delivers rated power as the product of the rated speed and rated torque ), breakdown speed (maximum torque, which is exceeded by the load, the motor stops ) and short-circuit speed ( engine stands, starting torque, starting current ).

If the three-phase induction motor driven at a higher than the synchronous speed, so it feeds power back into the grid ( generator mode ).

Dahlander ( Dahlander motor)

With the Dahlander the number of poles of the induction machine can ( pole-changing motors ) increases in the ratio 1:2, and thus their speed can be changed in a 2:1 ratio. Typical applications are: Lathes with two basic speeds: slow or fast running.
Two-speed blower drive housing for ventilation.

The Dahlander circuit offers the possibility of pole-changing and thus the switching speed for asynchronous machines in cage rotor design.

Pole-changing motors

It is possible to arrange on a shaft two completely separate engines. It is elegant if these motors are in an enclosure. Then both motors can have a common rotor ( cage). The stator windings are, however, carried out in duplicate. One stator is designed for low speed. Two stator is designed for a four or six times speed. A speed ratio of one to two is usually implemented with Dahlander circuit described above.

Pole-changing motors have almost the same characteristics as the Dahlander motors, with the difference that Dahlander motors called " tapped windings " have ( the coils have three terminals: beginning, end and a tap in the middle of the winding). So you have the stator core only three windings offset by 120 degrees. Two-speed motors are equipped with separate windings. This means that you have at least six windings in the stator core, not a pair of poles, as the Dahlander motor ( three windings ) but from two pole pairs upstream ( six or more windings).

KUSA - circuit

Not always it comes to reduce the inrush current. In some cases, it is also important that an excessive torque, interferes with direct involvement on the plant. The so-called KUSA - circuit ( short-circuit armature soft-start ) is a circuit for starting of three-phase motors with squirrel-cage rotor, in about half of the nominal torque.

When KUSA - circuit resistor is connected in the load circuit of the induction motor in an outer conductor, which is bridged after a set time or manually by means of contact. It is often convenient to tap the series resistor to set different amounts of starting torque can. This type of startup is only at idle or low resistive torque into consideration.

Pros and Cons

With the advent of Power are now almost exclusively short - cage rotor motors (English squirrel cage induction motor ) used. This embodiment of the induction motor owes its designation as the "workhorse " of electric drive technology. Combined with a correspondingly controlled frequency converter it is also able to run against large counter moments of working machines. The Frequenzumrichterbaugruppen take currently increasingly the task of motor protection. In addition, engines are offered with mounted frequency inverter. This reduces the wiring and interference suppression.

Benefits

Long life, low maintenance, no brush wear the squirrel cage
Briefly strong overload capacity (up to greater than 2 × the rated torque )
Starting against high counter moments without tools (also depending on rotor design)
Nearly constant speed, no " runaway" idle
Used in hazardous area (potentially explosive atmospheres ), since no brushes or slip rings (avoiding the brush fire - sparking )
Relatively low manufacturing costs
The rotor is energized and can run in liquids, gases or in vacuum.

Disadvantages

Speed variation only in special designs with pole-changing or with additional frequency
Particularly with small models 20 to 30 % more volume at the same torque with respect to a permanent - magnetic synchronous motors,
Three outer conductor to supply required (alternatively, the drive or run capacitor ( capacitor motor ) for single-phase available)
Complex theoretical method to calculate ( in comparison to other electrical machines )
Step or servo motors have advantages in positioning and are compared more easily

Standards and categories

In the European Community the EN Note 60034 " Rotating electrical machines ".

Standard motors

Standardized mounting dimensions are given for Germany with the standards DIN 42673, 42676 and 42677. The power range up to 200 kW belongs to the low-voltage standard motors.

In the area of standard motors for which the major manufacturers publish lists of specifications, the motors are classified according to torque classes. Typically, these engines start against 2 times the nominal torque. For the construction of the shaft height is a standard gauge. The range starts with the standard motor AH56 and extends to the AH315 (approx. 200 kW). Above the AH 315 AH 355 begins with the non-standard motor area.

Special designs

Resistance runner with a softer start-up, but poor efficiency
Slip ring motor with slip rings lead-out rotor winding in order to connection of a resistance only at startup
External rotor with stator inside, outside rotor
Stator at both ends of the air gap in the rotor as an aluminum cylinder ( canned motor ) or disc (Ferrari motor)
Linear motor with " unrolled " geometry
Linear motor with stator in tubular form for the promotion of liquid metals

Induction

In the generator mode, the rotor rotates faster than the magnetic field and it supplies energy to the grid.

There are three different asynchronous motors, which are used as a generator.

Induction machine with squirrel-cage rotor: Asynchronous
Asynchronous slip- ring motors: Double-fed asynchronous machine
Asynchronous machine with two stators: Cascade machine

All three types of generators are used in decentralized power plants.

Idealized view / equivalent circuit

To understand the operations of a speed control, the consideration of the equivalent circuit of the induction machine is inevitable. The equivalent circuit diagram showing an electrical equivalent circuit for the machine, as it sees the drive.

On the left side of the stator winding is shown, it consists of Rs ( copper resistance and equivalent series resistance of the core losses ) and the reactance Xs its inductance for asynchronous operation. Law is the rotor or rotor shown: the inductance Xr represents the appearing with the motor inductance, it results from the passing of standing squirrel cage magnetic field lines. The resistance Rr is composed of

The equivalent value of the output of the machine active power; This value changes with the change in the torque or load on the engine. He is very tall in the machine is idle.
Which according to the square of the step-up Statorwindungszahl ohmic resistance of the squirrel cage; the short-circuit cage consists of individual, embedded in the iron turns, usually made of aluminum.

At idle, the equivalent circuit of the induction motor is essentially therefore consists of Rs and Xs, which is why such a machine receives almost only reactive power. The recorded idle power is often similar to or higher than the rated current, the machine due to the copper and core losses at idle often already more than half the power loss at rated load. As the load increases, the active current increases by Rr and thus in the squirrel cage. The phase angle between current and voltage is reduced from almost 90 ° to smaller values . In hochmagnetisierten induction motors associated with increasing torque even at first often a decrease of the total current place, which then rises again later with increasing torque up to nominal current.

Of the induction machine that is a reactive current is added with Xs, which provides for the magnetization of the machine. In contrast to the three-phase synchronous machine, the magnetic flux in the induction motor must be built only by the reactive current in the stator winding.

The load-dependent active current produces a voltage drop in the cage component of Rr, but only a slightly higher voltage drop in Rs Consequently, the losses increase with increasing load in the rotor more quickly than in the stator. The copper resistor R and the " copper " resistance of the cage rotor portion Rr cause the square of the power consumption increasing losses, therefore increases the efficiency of the engine with increasing load. On top of their temperature dependence, which is why the efficiency of the machine warm drops something.

In the inverter reactance Xs is also getting smaller with ever smaller frequency. In compliance with the rated current so the voltage supplied by the inverter must fall. Thus the ratio of the voltage divider Rs Xs is always unfavorable and Rs leads to rising relative to the available engine power losses. In continuous operation, can only approximate the nominal torque can be generated, since the cooling of the rotor and stator is not given sufficient. At higher than the rated speed or rated frequency induction may, however, - in consideration of the insulation - working on higher voltages and is more effective.

Modern frequency can measure even Rs / Rr and are thus able to configure itself automatically for any connected motor and so protect it from overload. A holding torque or speed close to zero can be achieved with a vector control. Again, missing cooling since the fan on the rotor then this itself, no longer cools the outstanding stator windings and the air gap.

Complex phasor model of the asynchronous motor with squirrel cage rotor

The model is subject to the assumption of a rotationally symmetric structure of the machine as well as the lack of a Streufeldreluktanz. To this model can be extended. It is here, however, (initially) not taken into account in order to keep the model as simple and understandable. The same applies to the number of turns of the stator winding.

Here, the entries of a vector ( x, y) shown in the plane of rotation as a complex number x iy. The field as well as the supply voltage and the stator are the rotating pointer sizes of the stand is the pointer of the rotor current. Connected to the three phases of the electricity network, the pointer can be used as shown. ( Triangle)

The mesh equation of the stator circuit is taking into account the law of induction:

As the rotor rotates forward " sees" it, the magnetic field rotate backwards.

Thus, the mesh equation of the rotor circuit results in co-rotating coordinates:

The magnetic field is the result of rotor and stator current multiplied by the Hauptfeldreluktanz:

Replaced by one results in the system of equations in the unknowns and.

Considering Streufeldreluktanzen in the form of the inductances and as well as the number of turns of the stator is obtained very similar equations:

The torque generated results from the cross product of and rotor current. Shown here is analogous to the pointer model in complex numerical calculation that.

Winding arrangement

Coil group

To smooth all of the exciter field winding of a coil in a groove to be concentrated in the non- normally, but distributed in a plurality of adjacent grooves.

This distribution is the voltage amplitude of the fundamental wave, which is accounted for by the zone factor decreased.

Sehnungsfaktor

As chording the displacement of the winding layers is referred to in a multi- layer winding. This displacement causes a smoothing of the excitation curve, and thus a reduction of the harmonic of the induced voltage.

By chording the induced voltage amplitude, which is taken into account by the Sehnungsfaktor reduced. It is calculated as

With the pole number, number of grooves and the winding step. Here, the winding pitch is the ratio of coil width to slot pitch.

Winding factor

The product of Sehnungs and zone factor is called the winding factor.

Characteristics / characteristics

The terms rated power, rated speed and rated torque arising from the information on the technical data of the motor and the associated label. In this context, reference is also made to the design values.

The nominal torque is not indicated generally on the nameplate. It can be calculated from the following formula. See also performance in technical applications.

Torque M in Newton meter (Nm)
Power P in kilowatts ( kW)
Rotational speed n in revolutions per minute ( min-1 )
9549 is a rounded value

The corresponding synchronous speed ( or rotating field speed) always lies just above the rated speed, the off

Results.

Speed n in revolutions per minute (min -1)
Mains frequency f in Hertz or (s-1 ) ( indicated on the type plate)
Number of pole pairs p ( always an integer )

At 50 Hz, so values of 3000, 1500 or 750 revolutions per minute result with the numbers of pole pairs 1, 2 or 4

The example shown for a nameplate refers to a motor, which is scheduled only for the star operation. At a mains frequency of 50 Hz and a rated power of 5000 kW and a rated speed of 1480/min follows:

Number of pole pairs = 2
Synchronous speed = 1500 rpm
Rated torque about 32.3 kNm

Another method for the visualization of power, torque and loss of an asynchronous machine in the generator and motor operation as a function of slip represents the Ossanna Circle

Characteristic example

The picture shows the typical torque curve as a function of the speed. In delta mode, the motor has about three times the torque compared to the star operation. The operating points B1 or B2 are beyond the tipping moment K1 or K2.

P ( as pump) is shown as an example of the curve of the required torque of a centrifugal pump.

It depends on that the speed range of zero is passed through as quickly as possible to the tipping point, because in this area the motor has a poor efficiency and heated accordingly. The (critical ) start-up time depends on the inertia of the machine and on the ratio of starting torque.

The example shows that the pump is also apparent in a star connection runs smoothly, because the operating points B1 and B2 are close together. Nevertheless, it is possible that the motor under constant load in star connection refers to a high current to deposit as required by the driven machine moment. The engine thus heats up, because in the calculation of heat losses of the absorbed current is a square. A heated above the manufacturer's specified allowable temperature will shorten the life of the engine greatly. Often the required design moment for the operation is in a delta connection but so great that the engine can not muster in a star configuration. The start-up and switching to delta connection must therefore be carried out without load or to load torques, the motor can still cope with star connection, without inadmissible to warm up.

In the example, the driving torque (asterisk) in the starting range is about two to four times greater than the required torque of the pump. The difference is the accelerating portion. Therefore could be here a restart of the pump with open spool. Technical standard is the start of a pump with the slide closed. Then, the required torque is considerably smaller and the critical start-up range is passed through as quickly as possible.

Fan with long wings (eg in a cooling tower ) have a large moment of inertia. Furthermore, the start-up is possible only under load. This long lead times and planning engine result - fan requires regular, careful interpretation.

Application Examples

Small power motors

Pump drives in all industries
Compressors (eg, refrigerant compressors for cooling smaller rooms)
Fans for all industries
Drives for industrial trucks

Medium-power engines

Pumps, fans, compressors for all industries
Press drives ( flywheel, spindle, eccentric )
Extruder drives
Traction drives for cars and buses ( electric or hybrid vehicles)
Machine tool drives (eg main spindle drives )
Auxiliary drives on ships, locomotives, etc.

High-performance engines

Pumps, fans, compressors for all industries
Power plant auxiliary drives
Traction drives for lifts
Rope / Kettenzugantrieb

Legal regulations and other rules

EN 60 034 Part 1 General requirements for rotating electrical machines
EN 60 034 Part 8 Terminal markings and direction of rotation for electrical machines
DIN IEC 34 Part 7 types rotating electrical machines
EN 60034-5 protection of rotating electrical machines
EN 60034-6 types of cooling for rotating electrical machines