Is induction motor and motor the same
Three-phase motors, asynchronous motors
Our modular system for three-phase motors enables you to have millions of drive combinations. And that worldwide: because the three-phase motors meet all efficiency classes up to IE4 and cover a power range from 0.09 kW to 225 kW. From a wide range of brakes, encoders, connectors, external fans, special coatings and paints, the modular system offers you the right drive for you.
What is a three-phase motor?
The group of induction machines includes electrical machines whose mode of operation is based on an air gap between the stator and rotor rotating magnetic field is based. The most important and most frequently used machine in this group is the asynchronous three-phase induction motor in the form of a squirrel-cage rotor. This is characterized by the following features:
- a simple and robust structure
- great operational reliability
- a low-maintenance operation
- a low price
The following electric motors are generally used in electrical drive technology:
- asynchronous three-phase motors (squirrel cage rotor, slip ring rotor, rotating field magnet)
- asynchronous single-phase AC motors
- asynchronous or synchronous servomotors
- DC motors
Since the speed of Three-phase motors can be controlled better, more easily and with less maintenance with frequency converters, DC motors and lose Three-phase motors with slip rings more and more important. Other types of three-phase asynchronous motors are of little importance in drive technology. Therefore a more detailed description is not given here.
If you combine an electric motor, like one Three-phase motor, with a gear, you get a so-called gear motor. Regardless of the electrical principle of the respective motor, the way in which it is attached to a gearbox is of particular importance for the mechanical construction of the motor. SEW-EURODRIVE uses for this specially adapted engines.
How does a three-phase motor work?
Runner or rotor
An injected or inserted winding (usually made of aluminum and / or copper) is located in the grooves of the laminated rotor core; traditionally, one turn corresponds to a rod. These rods are short-circuited at both ends by rings made of the same material. If you mentally remove the laminated core, the bars with the short-circuit rings are reminiscent of a cage. Hence the second common name for Three-phase motors: "Squirrel cage motor".
Stand or stator
The winding encapsulated with synthetic resin is inserted into the half-closed grooves of the stator core. The number of coils and the coil width are varied in order to achieve different numbers of poles (= speeds). Together with the motor housing, the laminated core forms the so-called stator.
Bearing shields made of steel, gray cast iron or die-cast aluminum close off the engine compartment on the A and B sides. The design in the transition to the stator determines, among other things, the degree of protection of the motor.
The laminated core on the rotor side is attached to a steel shaft. The two shaft ends extend through the end shields on the A and B sides. The A-side is the end of the output shaft (in the case of the geared motor, it is designed as a pinion pin); The fan with its blades for self-ventilation and / or additional systems such as mechanical brakes and encoders are installed on the B-side.
Motor housings can be made of die-cast aluminum for small to medium-sized outputs. Housings of all performance classes are also made of gray cast iron and welded steel. A terminal box is built onto the housing, in which the winding ends of the stator are connected to a terminal block for the customer's electrical connection. Cooling fins enlarge the surface of the housing and also increase the dissipation of heat to the environment.
Fan, fan cover
A fan on the B-side shaft end is covered by a hood. This hood directs the air flow, which is created when the fan rotates, over the ribs of the housing. As a rule, the fans are not dependent on the direction of rotation of the rotor. An optional protective cover prevents (small) parts from falling through the fan guard grille in the case of vertical designs.
The bearings in the A- and B-side end shields mechanically connect the rotating parts with the stationary ones. Usually deep groove ball bearings are used, more rarely cylindrical roller bearings. The bearing size depends on the forces and speeds that the respective bearing has to absorb. Various sealing systems ensure that the required lubricating properties remain in the bearing and that oils and / or greases do not escape.
How it works on the network
The symmetrical, three-strand winding system of the stator is connected to a three-phase three-phase network with the appropriate voltage and frequency. Flow in each of the three winding strands sinusoidal currents of the same amplitude, which are each offset from one another by 120 °. Due to the winding strands, which are also spatially offset by 120 °, the stator builds up a magnetic field that rotates at the frequency of the applied voltage.
This rotating magnetic field - in short Rotating field called - induces an electrical voltage in the rotor winding or in the rotor bars. Since the winding is short-circuited across the ring, flow Short circuit currents. Together with the rotating field, forces build up and form a torque over the radius of the rotor, which accelerates the rotor to speed in the direction of the rotating field. As the rotor speed increases, the frequency of the voltage generated in the rotor decreases because the difference between the rotating field speed and the rotor speed becomes smaller.
The now lower induced voltages result in lower currents in the rotor cage and thus lower forces and lower torques. If the rotor reached the same speed as the rotating field, it would rotate synchronously and no voltage would be induced - the motor would consequently not be able to develop any torque. However, the load torque and the frictional moments in the bearings cause one Difference between rotor speed and rotating field speed and thereby a resulting balance between acceleration and load torque. The motor runs asynchronously.
Depending on the load on the motor, this difference is larger or smaller, but never zero, since there is always friction in the bearings even when the motor is idling. If the load torque exceeds the maximum acceleration torque that the motor can produce, the motor "tips" into an impermissible operating state, which can have a thermally destructive effect.
This is necessary for the function Relative movement between rotating field speed and mechanical speed is defined as slip s and is given as a percentage of the rotating field speed. In the case of low-power engines, the Slip Be 10 to 15 percent, Three-phase motors higher power have about 2 to 5 percent slip.
The operating behavior
The three-phase squirrel cage motor takes electrical power from the voltage network and converts it into mechanical power - that is, into speed and torque. If the engine were to work without losses, that would correspond mechanical power output Pfrom the consumed electrical power Pon.
As is unavoidable with any energy conversion, losses also occur in three-phase squirrel cage motors: Copper losses PCu and Rod losses PZ arise when a current flows through a conductor, Iron losses PFe are caused by the magnetization of the laminated core with mains frequency. Frictional losses PRb caused by friction in bearings; and ventilation losses from using the air for cooling. These copper, rod, iron and friction losses cause the motor to heat up. The ratio of power delivered to power consumed is defined as that Machine efficiency.
Efficiency is becoming more and more important
Due to legal requirements, in recent years more attention has been paid to the use of motors with higher degrees of efficiency. Corresponding normative agreements define this Energy saving classes, which have been included in the technical data by the manufacturers. In order to reduce the essential machine-dependent losses, this means for the construction of the electric motor:
- an increased use of copper in the motor winding (PCu)
- better sheet material (PFe)
- an optimized fan geometry (PRb)
- an energetically optimal storage (PRb)
If the torques and the current are plotted against the speed, the characteristic is obtained Torque-speed characteristic of the three-phase squirrel cage motor. The motor runs through this characteristic every time it is switched on until the stable operating point is reached. The number of poles, the design and the material of the rotor winding influence the course of the characteristics. Knowledge of these characteristics is particularly important for drives that are operated with counter torques (e.g. hoists).
Is the counter-torque of the driven machine higher than that Saddle torque, the rotor speed will "get stuck in the saddle". The motor no longer reaches its nominal operating point, i.e. the stable, thermally safe operating point. Is the counter-torque even higher than that Starting torque, the engine stops. If a running drive is overloaded (e.g. a conveyor belt overloaded), the speed decreases with increasing load. If the counter-torque exceeds that Overturning moment, The engine “tips over” and the speed drops to saddle speed or even to zero. All scenarios lead to very large currents in the rotor and stator, so that both heat up very quickly. If no suitable protective devices are available, this can lead to thermal destruction of the motor - it "burns out".
The heat classes
The heat generated in a current-carrying electrical conductor depends on the resistance of the conductor and the level of the current flowing through it. Frequent switch-on and starting with counter-torque put the three-phase squirrel-cage motor under high thermal conditions. 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's insulation material.
The maximum permissible overtemperature of the motors is limited by a Division into heat classes (formerly also called "insulation classes"). The motor must be able to be operated in the thermal class in which it was built with its rated output-related permanent excess temperature without being damaged. At a maximum coolant temperature of 40 ° C, the permissible excess temperature limit, for example in thermal class 130 (B): dT = 80 K.
These modes of operation are the most common
- The simplest operating mode is loading with a constant load torque. Due to the permanent load at the nominal point, the motor reaches the thermal steady state after a certain time. This company is called Continuous operation S1.
- in the Short-term operation S2 the motor is in operation for a certain period of time (tB) with constant load. During this period of time, the motor has not yet reached the thermal steady state. This is followed by a downtime that must be long enough for the engine to reach the temperature of the coolant again.
- in the Intermittent operation S3 the motor is in operation for a certain time (tB) with constant load. The start-up must not have any effect on the heating of the motor. This is followed by a certain downtime (tSt). In this operating mode, the relative duty cycle (ED) is specified. In the standard IEC 60034-1, the ratio of the operating time to a playing time (= operating time + downtime) of 10 minutes is given as an example.
Example: The operating mode S3 / 40% exists when the motor is switched on for 4 minutes and switched off for 6 minutes.
What is the switching frequency?
The permissible switching frequency indicates how often a motor can be switched on in one hour without thermal overload. It depends on:
- the mass moments of inertia to be accelerated
- the static load
- the type of deceleration
- the duration of the startup
- the ambient temperature
- the duty cycle
The permissible switching frequency of a motor can be increased by taking the following measures:
- by increasing the thermal class
- by choosing the next largest motor
- by adding an external fan
- by changing the gear reduction and thus the mass inertia ratios
- by choosing a different type of braking
What are pole-changing three-phase squirrel cage motors?
Three-phase squirrel cage motors can be switched by switching windings or winding parts operated at different speeds become. By inserting several windings in the slots of the stator or by reversing the direction of current flow in individual winding parts, different numbers of poles result. With separate windings, the power per number of poles is less than half the power of the single-speed motor of the same size.
Pole-changing three-phase gear motors are for example used as travel drives. The travel speed is high when operating with a low number of poles. For positioning, a switch is made to the multi-pole winding at low speed. When switching, the motor initially retains its high speed due to the inertia. The Three-phase 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 big one is disadvantageous Moment shock when switching, but this can be reduced by suitable switching measures.
The current development of inexpensive converter technology favors the technological replacement of pole-changing motors with single-speed, frequency-controlled motors in many applications.
Single phase motors
A single phase motor is a good choice when in applications
- no high starting or starting torque is required
- the motors are connected to a single-phase AC network and
- a rather small power (<= 2.2 kW) is used.
Fans, pumps and compressors are some of the typical application examples. Two fundamental constructive differences can be found here:
For one, the classic asynchronous Three-phase motor only connected to one phase and the neutral conductor. The third connection is about the phase shift using a capacitor re-enacted. Since the capacitor cannot produce a 120 ° phase offset, but only 90 °, this type of single-phase motor is usually rated with only two thirds of the output of a comparable three-phase motor.
The second way to build a single phase motor is by technical development adjustment. Instead of the three-phase winding, only two phases are implemented, and these also differ as main and auxiliary phases. The coils, which are now offset by 90 °, are energized by means of a capacitor offset by 90 ° in time, which creates the rotating field. The unequal current ratios of the main and auxiliary windings usually allow only two thirds of the power of a three-phase motor of the same size. Typical motors for single-phase operation are Capacitor motor, gap and starter motorthat works without a capacitor.
SEW-EURODRIVE has both types of construction of single-phase motors in its range - the DRK..motors. Both are supplied with an integrated operating capacitor. Since this is housed directly in the terminal box, interfering contours are avoided. With an operating capacitor, approx. 45 to 50 percent of the nominal torque is available for starting.
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 starting capacitors required for this, which are available from well-stocked specialist dealers.
Rotating field magnets
Are rotating field magnets Special designs of Three-phase motors with squirrel cage. In the design, they are dimensioned so that, even at speed 0, they only have such a high power consumption that they do not thermally destroy themselves. This is for example the Open doors, switch points or at Press tools makes sense when a position has to be reached and held safely by motor and electrical means.
Another common operating mode is the so-called Countercurrent braking operation: An external load is able to turn the rotor against the direction of rotation of the rotating field. The rotating field "brakes" the speed and removes regenerative energy from the system, which is fed back into the network - so to speak rotary braking without mechanical braking work.
With the DRM ../ DR2M .. 12-pole rotating field magnets, which are thermally permanently designed for use with rated torque at standstill.The rotating field magnets from SEW-EUODRIVE are suitable for different requirements and speeds and are offered with up to three rated torques depending on the operating mode.
Explosion-proof three-phase motors
If electric motors are used in potentially explosive areas (in accordance with Directive 2014/34 / EU (ATEX)), certain protective measures must be taken on the drives. For this purpose, SEW-EURODRIVE offers different explosion-proof three-phase motors depending on the area and region of application.
Hybrid motors: "asynchronous" and "synchronous" in one motor
For applications that are operated directly on the mains and must also have a synchronous speed or have this feature on a simple converter without an encoder, SEW-EURODRIVE offers the so-called LSPM motors at. LSPM is the abbreviation for L.ine S.tart P.permanent M.agnet. The LSPM motor is a three-phase asynchronous motor with additional permanent magnets in the rotor. It starts up asynchronously, then synchronizes itself to the supply frequency and from then on runs in synchronous operation, synchronously with the mains frequency. An engine technology that new, flexible application options in drive technology opened, e.g. B. the transfer of loads without a drop in speed.
These compact hybrid engines show in operation no rotor losses up and impress with one high efficiency. Energy saving classes up to IE4 are achieved.
The size of a DR..J motor with LSPM technology is two stages smaller than a series motor of the same power and with the same efficiency class. Motors of the same size, on the other hand, achieve an efficiency class that is twice as high as that of asynchronous motors.
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