Do you have a specific enquiry and would like us to advise you? Send us a message with your questions.
The group of three-phase machines comprises electrical machines whose mode of operation is based on a magnetic field circulating in the air gap between the stator and rotor. rotating magnetic field rotor. The most important and most frequently used driven machine in this group is the asynchronous three-phase current induction motor in the squirrel-cage rotor design. This is characterised by the following features:
In the electrical drive technology the following electric motors are generally used:
As the rotational speed of AC motors can be controlled better, more easily and with less maintenance using frequency inverters, DC motors and AC motors with slip rings are becoming less and less important. Other types of AC asynchronous motor are only of minor importance in drive technology. For this reason, a more detailed description is not provided here.
If you combine an electric motor, such as a AC motorwith a gear unityou get a so-called gearmotor. Regardless of the electrical principle of the respective motor, the way in which it is attached to a gear unit is of particular importance for the mechanical design of the motoring operation. SEW-EURODRIVE uses specially adapted motors for this purpose. specially adapted motors.
Rotor or rotor
An injected or inlaid winding (usually made of aluminium and/or copper) is located in the grooves of the rotor laminated core; classically, one winding = one bar. These rods are short-circuited at both ends by rings made of the same material. If you remove the laminated core, the rods with the short-circuit rings resemble a cage. This is also the origin of the second common name for AC motorssquirrel-cage motor".
Stator or stator
The winding, encapsulated with synthetic resin, is inserted into the half-closed grooves of the stator laminated core. The number of coils and coil width are varied in order to achieve different numbers of poles (= rotational speeds). Together with the motor housing, the laminated core forms the so-called stator.
Endshields
Endshields made of steel, gray cast iron or die-cast aluminium seal off the motor interior on the A and B sides. The design of the transition to the stator determines, among other things, the degree of protection of the motor.
Rotor shaft
The laminated core on the rotor side is mounted on a steel shaft. The two shaft ends extend through the endshields on the A and B sides. The end of the output shaft is designed on the A-side (designed as a pinion shaft end in gearmotors); the fan with its blades for self-ventilation and/or supplementary systems such as mechanical brakes and encoders are attached on the B-side.
Motor housing
Motor housings can be made of die-cast aluminium for low to medium power ratings. However, housings of all power classes are also made of grey cast iron and welded steel. A terminal box is attached to the housing, in which the winding ends of the stator are connected to a terminal block for the customer's electrical connection. Cooling fins increase the surface area of the housing and also increase the heat dissipation to the environment.
Fan, fan guard
A fan on the B-side shaft end is covered by a hood. This bonnet releases the air flow generated by the rotation of the fan over the ribs of the housing. As a rule, the fans are not dependent on the direction of rotation of the rotor. An optional canopy prevents (small) parts from falling through the fan hood grille in vertical mounting positions.
Storage
The storages in the A- and B-side endshields mechanically connect the rotating parts with the upright ones. Groove ball bearings are usually used, more rarely cylindrical roller bearings. The size of the storage depends on the forces and rotational speeds that the respective storage has to absorb. Different sealing systems ensure that the required lubrication properties remain in the storage and that oils and/or greases do not escape.
The stator's symmetrical, three-strand winding system is connected to a three-phase three-phase mains supply of the appropriate stress and frequency. In each of the three winding strands flow sinusoidal currents of equal amplitude flow in each of the three winding strands.which are offset from each other by 120°. Due to the winding strands, which are also offset by 120°, the stator builds up a magnetic field that circulates with the frequency of the applied stress.
This rotating magnetic field - in short rotating field induces an electrical stress in the rotor winding or in the rotor bars. As the winding is short-circuited via the ring, short-circuit currents flow. short-circuit currents flow. Together with the rotating field, forces build up and form a torque over the radius of the rotor, which accelerates the rotor to rotational speed in the direction of the rotating field. As the rotor speed increases, the frequency of the stress generated in the rotor decreases, as the difference between the rotating field speed and the rotor speed becomes smaller.
The resulting lower induced stresses result in lower currents in the rotor cage and therefore lower forces and lower torques. If the rotor were to reach the same rotational speed as the rotating field, it would rotate synchronously and no stress would be induced - the motor would therefore not be able to develop any torque. However, the load torque and the frictional torques in the storages cause a difference difference between rotor and rotating field speed and thus a resulting balance between acceleration and load torque. The motoring operation is asynchronous.
Depending on the load on the motor, this difference is greater or smaller, but never zero, as there is always friction in the storage even in no-load operation. If the load torque exceeds the maximum acceleration torque that can be produced by the motor, the motor "tilts" into an impermissible operating state, which may have a thermally destructive effect.
This relative movement between relative movement between the rotating field speed and the mechanical rotational speed is defined as slip s and is specified as a percentage of the rotating field speed. For motors with a low power rating, the slip 10 to 15 per cent, AC motors motors with a higher power rating have a slip of approx. 2 to 5 per cent.
The three-phase current squirrel-cage motor absorbs electrical power from the mains supply and converts it into mechanical power rating - i.e. rotational speed and torque. If the motoring operation were to be loss-free, the mechanical power rating Pfrom the absorbed electrical power rating Pon.
As is inevitable with any energy conversion, however, losses also occur in the three-phase current squirrel-cage motor: Copper losses PCu and rod losses PZ occur when a current flows through a conductor, Iron losses PFe occur when the laminated core is remagnetised at line frequency. Friction losses PRb are caused by friction in storage; and fan losses due to the use of air for cooling. These copper, rod, iron and friction losses cause the motoring operation to heat up. The ratio of the power rating output to the power rating input is defined as the efficiency of the machine.
Due to legal requirements, the use of motors with higher efficiencies has become increasingly important in recent years. Corresponding standardisation agreements define energy saving classes, which manufacturers have included in the TD. In order to reduce the main machine-dependent losses, this means that the design of the electric motor must be optimised:
Recording the torque and the current versus the rotational speed gives the characteristic speed-torque characteristics of the three-phase current squirrel-cage motor. Until the stable operating point is reached, the motor runs through this characteristic every time it is switched on. The number of poles, the design and the material of the rotor winding influence the characteristics. Knowledge of these characteristics is particularly important for drives that are operated with counter-torques (e.g. vertical drives).
If the counter-torque of the driven machine is higher than the pull-up torque, the rotor speed will "get stuck in the saddle". The motor no longer reaches its nominal operating point, i.e. the stable, thermal safe operating point. If the counter-torque is even higher than the starting torque, the motor stops. If a running drive is overloaded (e.g. a conveyor belt is overloaded), the rotational speed decreases as the load increases. If the counter-torque exceeds the breakdown torque, the motor "tilts" and the rotational speed drops to saddle speed or even to zero. All scenarios lead to very large currents in the rotor and stator, causing both to heat up very quickly. If no suitable protective devices are in place, this can lead to the thermal destruction of the motor - it "burns out".
The heat generated in a current-carrying conductor depends on the resistance of the conductor and the level of the current flowing through it. Frequent switching on and startup with counter-torque place a very high thermal load on the three-phase current squirrel-cage motor. The permissible heating of the motor depends on the temperature of the surrounding cooling medium (e.g. air) and the heat resistance of the winding insulation material.
The maximum permissible overtemperatures of the motors are defined by a categorisation into thermal classes (formerly also known as "insulation classes"). The motor must be able to operate in the thermal class in which it was built at its rated continuous excess temperature without suffering failure. With a coolant temperature of max. 40 °C, the approval limit for thermal class 130 (B), for example, is: dT = 80 K.
The permissible switching frequency indicates how often a motor can be switched on in an hour without thermal overload. It depends on
The permissible switching frequency of a motor can be increased by the following measures:
Three-phase current squirrel-cage motors can be operated at different speeds by changeover switch of windings or winding parts. can be operated at different rotational speeds. different speeds. Inserting several windings into the grooves of the stator or reversing the direction of current flow in individual winding parts results in different numbers of poles. With separate windings, the power rating per number of poles is less than half that of a single-speed motor of the same size.
Pole-changing AC gearmotors are used for example used as travel drives. The travelling speed is high when operating with a low number of poles. For positioning, a changeover is made to the low-speed low-pole winding. During the changeover switch, the motor initially retains its high rotational speed due to the inertia. The AC motor works as a generator in this phase and brakes. The kinetic energy is converted into electrical energy and fed back into the grid. The disadvantage is the large torque surge during the changeover switch, but this can be reduced by suitable switching measures.
The current development of inexpensive converter technology favours the technological replacement of pole-changing motors with single-speed, frequency-controlled motors in many applications.
A single-phase motor is a good choice for applications where
Fans, pumps and compressors are typical application examples. There are two fundamental design differences can be found here:
On the one hand, the classic asynchronous AC motor is only connected to one phase and the neutral conductor. The third connection is made via the phase shift with the help of a capacitor is used. As the capacitor cannot generate a phase shift of 120°, but only 90°, this type of single-phase motor is usually only rated at two thirds of the power rating of a comparable AC motor.
The second way of building a single-phase motor is to adapt the winding adaptation. Instead of the three-phase winding, only two phases are realised, and these are also differentiated as the main and auxiliary phases. The coils, which are now offset by 90° in space, are also energised at 90° offset in time by means of a capacitor, which creates the rotating field. The unequal current ratios of the main and auxiliary windings usually only allow two thirds of the power rating of an AC motor of the same size. Typical motors for single-phase operation are capacitor motor, canned motor and starting motorwhich does not require a capacitor.
SEW-EURODRIVE has both types of single-phase motors in its range - the DRK. the DRK..motors. Both are supplied with an integrated running capacitor. As this is housed directly in the terminal box, interference contours are avoided. With a running capacitor, approx. 45 to 50 per cent of the rated torque is available for startup.
For customers who require a higher starting torque of up to 150 % of the nominal torque, SEW-EURODRIVE can supply the capacitance values of the required starting capacitors, which are available from well-stocked specialised dealers.
Rotary field solenoids are special versions of AC motors with squirrel-cage rotors. They are designed in such a way that, even at rotational speed 0, they only have such a high current consumption that they do not destroy themselves thermally. This is important, for example, when opening doors, setting points or with press tools when a position has to be reached and held safely by motoring operations.
Another common operating mode is the so-called counter-current braking operation: An external load is able to spin the rotor in the opposite direction of rotation to the rotating field. The rotating field "brakes" the rotational speed and draws generator mode energy from the system, which is fed back into the grid - a kind of rotary braking without mechanical braking work.
With the DRM../DR2M.. 12-pole rotary field solenoids, which are designed for use with a rated torque at idle state and are thermally durable. SEW-EUODRIVE's rotary field solenoids are suitable for different requirements and speeds and are available with up to three rated torques depending on the operating mode.
If electric motors are used in potentially explosive atmospheres (in accordance with Directive 2014/34/EU (ATEX)), certain protective measures must be taken on the drives. Depending on the area and region of application, SEW-EURODRIVE offers different explosion-protected AC motors motors.
For applications that are operated directly on the mains and must also have a synchronous rotational speed or have this feature without a sensor on a simple inverter, SEW-EURODRIVE offers the so-called LSPM motors motors. LSPM is the abbreviation for Line Start Permanent Magnet. The LSPM motor is an AC asynchronous motor with additional permanent magnets. permanent magnets in the rotor. It starts up asynchronously, then synchronises to the supply frequency and from then on runs in synchronous mode in slip-free synchronous mode with the line frequency. A motor technology that opens up new, flexible application possibilities in drive technology e.g. the transfer of loads without a drop in speed.
These compact hybrid motors have no rotor losses during operation. no rotor losses during operation and impress with their high high efficiency. Energy saving classes of up to IE4 are achieved.
The size of a DR..J motor with LSPM technology is two stages smaller than a series motor with the same performance and the same efficiency class. By contrast, motors of the same size achieve twice the efficiency class of asynchronous motors.
By activating the postcode search, data is transferred to the USA by Google. For more information, see our privacy policy.