Electric machines perform the critical function of converting energy in operating mechanisms and generating stations. Such devices find their place in different areas, supplying executive bodies with sufficient power potential. One of the most popular systems of this type is represented by AC machines (MPT), which have several varieties and differences within their class.
General information about MPT
The segment of MPT or electromechanical converters can be divided into single-phase and three-phase systems. Also at the basic level, asynchronous, synchronous and collector devices are distinguished, while the general principle of operation and structural design have much in common. This classification of AC machines is conditional, since modern electromechanical conversion stations partially involve work processes from each group of devices.
As a rule, the MPT is based on the stator and rotor, between which an air gap is provided. Again, regardless of the type of machine, the duty cycle is based on the rotation of the magnetic field. But if in a synchronous installation the movement of the rotor corresponds to the direction of the force field, then in an asynchronous machine the rotor can move in a different direction and with different frequencies. This difference also determines the features of the use of machines. So, if synchronous can act both as a generator and as an electromechanical engine, then asynchronous are mainly used as motors.
As for the number of phases, single-phase and multiphase systems are distinguished. Moreover, from the point of view of practical use, representatives of the second category deserve attention. These are for the most part three-phase AC machines in which the magnetic field performs the function of the energy carrier. Single-phase devices, due to operational impracticality and large sizes, are gradually leaving the practice of application, although in some areas the decisive factor in their choice is low cost.
Differences from DC machines
The fundamental structural difference lies in the location of the winding. In AC systems, it covers the stator, and in DC machines, the rotor. In both groups, electric motors differ in the type of current excitation - mixed, parallel and serial. Today, AC and DC machines are used in industry, agriculture and in the domestic sphere, but the first option is more attractive in terms of its operational qualities. Generators and AC motors benefit from a more technological design, reliability and high energy efficiency.
The use of DC devices is common in areas where requirements for precision control of operating parameters come to the fore. It can be traction vehicles, processing machines and sophisticated measuring instruments. In terms of performance, AC and DC machines have high efficiency, but with different technical and design capabilities for specific application conditions. Working with direct current provides more options for controlling the speed, which is important when servicing servomotors and stepper motors.
Asynchronous MPT device
For the technical basis of this device in the form of a rotor and a stator, sheet steel is used, which is coated with an insulating oil-rosin layer on both sides before assembly. In low-power machines, the core can be made of electric steel without additional coating, since in this case the insulator is a natural oxide layer on a metal surface. The stator is fixed in the housing, and the rotor on the shaft. In high-power asynchronous AC machines, the rotor core can also be mounted on the rim of the housing with a sleeve mounted on a shaft. The shaft itself must rotate on the bearing shields, which are also fixed to the base of the housing.
The outer surfaces of the rotor and the inner surfaces of the stator are initially provided with slots for accommodating the winding conductors. At the stator of AC machines, the winding is more often carried out three-phase and connected to the corresponding network of 380 V. It is also called primary. Similarly, the rotor winding is performed, the ends of which usually form a connection in the configuration of a star. Contact rings are also provided, through which an additional rheostat for adjustment or a three-phase starting element can be connected.
It is important to note the parameters of the air gap, which performs the function of the damper zone, which reduces noise, vibration and heating during operation of the device. The larger the machine, the greater the clearance. Its value can vary from one to several millimeters. If it is structurally impossible to leave enough space for the air zone, an additional cooling system is provided for the installation.
The principle of operation of asynchronous MPT
The three-phase winding in this case is connected to a symmetric network with a three-phase voltage, as a result of which a magnetic field is formed in the air gap. Regarding the armature winding, special measures are taken to achieve a harmonious spatial distribution of the field for the damper gap, which forms a system of rotating magnetic poles. According to the principle of operation of an alternating current machine, a magnetic flux is formed at each pole, which crosses the contours of the winding, thereby provoking the generation of an electromotive force. In a three-phase winding, a three-phase current is induced to provide motor torque. Against the background of the interaction of the rotor current with magnetic fluxes, the formation of electromagnetic force on the conductors.
If the rotor is driven by an external force, the direction of which corresponds to the direction of the magnetic field fluxes of the AC machine, then the rotor will begin to overtake the rate of rotation of the field. This occurs in cases where the stator speed exceeds the nominal synchronous frequency. At the same time, the direction of movement of electromagnetic forces will be changed. Thus, a braking torque is formed with a reverse action. This principle of operation allows the machine to be used as a generator operating in the mode of transferring active power to the network.
The device and principle of operation of synchronous MPT
In terms of design and stator location, a synchronous machine is similar to an asynchronous one. The winding is called an anchor and is made with the same number of poles as in the previous case. An excitation winding is provided at the rotor, the energy supply of which is provided by slip rings and brushes connected to a direct current source. By source is meant a low-power exciter generator mounted on one shaft. In a synchronous AC machine, the winding acts as a primary magnetic field generator. In the design process, designers strive to create conditions so that the induction distribution of the field of excitation on the stator surfaces is as close to sinusoidal as possible.
With increased loads, the stator winding forms a magnetic field with rotation in the direction of the rotor with a similar frequency. Thus, a single rotation field is formed, in which the stator field will affect the rotor. This device of AC machines allows them to be used as electric motors, if the supply of three-phase current to the synchronous winding is initially provided. Such systems create the conditions for coordinated rotation of the rotor with a frequency corresponding to the stator field.
Explicit and implicit synchronous machines
The main difference between explicit pole systems is the presence of protruding poles in the structure, which are attached to special shaft protrusions. In typical mechanisms, fixation is performed using T-shaped tail fasteners to the rim of the cross or the shaft through the sleeve. In the design of low-power alternating current machines, this same problem can be solved by bolted connections. As the material of the winding, strip copper is used, which is wound on the rib, insulating with special gaskets. At the tips with poles in the grooves are placed the rods of the winding for starting. In this case, a material with high resistivity like brass is used. The ends of the windings are welded to short-circuiting elements, forming common rings for short circuiting. Explicit machines with a power potential of 10-12 kW can be performed in the so-called reversed design, when the armature rotates, and the poles of the inductor remain stationary.
For non-polar machines, the design is based on a cylindrical rotor made of steel forging. In the rotor there are grooves for the formation of the field winding, the poles of which are designed for high speeds. However, the use of such a winding in electric machines with alternating current of high power is impossible due to the high degree of wear of the rotor in harsh operating conditions. For this reason, even in medium power plants for rotors, high-strength components from solid forgings based on chromium-nickel-molybdenum or chromium-nickel steels are used. In accordance with the technical requirements for strength, the maximum diameter of the working part of the rotor of an implicit pole synchronous machine cannot be higher than 125 cm. This explains the unusual form factor of the rotor with an elongated case, although there are limitations on this parameter associated with increased vibrations for too long elements. The maximum rotor length is 8.5 m. Various turbogenerators can be attributed to non-polar units used in industry. With their help, in particular, they connect the working moments of steam turbines with thermal power plants.
Features of vertical hydrogenerators
A separate class of explicit pole synchronous MPTs provided with a vertical shaft. Such installations are connected to hydraulic turbines and are selected according to the power of the serviced flows in terms of speed. Most AC machines of this type are slow moving, but at the same time have a large number of poles. Among the critical working components of a vertical hydrogenerator, one can note a thrust bearing and a thrust bearing, which bear the load from the rotating parts of the engine. In particular, the pressure from water flows, which acts on the turbine blades, is superimposed on the thrust bearing. In addition, a brake is provided to stop rotation, and guide bearings that absorb radial forces are also present in the working structure.
In the upper part of the machine, along with the hydrogenerator, auxiliary units can be placed - for example, the generator exciter and the regulator. By the way, the latter is an independent AC machine with a winding and poles of permanent magnets. This setting provides power to the motor to provide automatic controller function. In large vertical hydrogenerators, the exciter can be replaced by a synchronous generator, which, together with excitation nodes and mercury rectifiers, provides power to power devices that serve the working process of the main hydrogenerator. The vertical shaft machine configuration is also used as a drive mechanism for powerful hydraulic pumps.
Collector MPT
The presence of a collector assembly in the MPT design is often determined by the need to perform the function of converting the rotational speed in electrical communication of different frequency circuits on the rotor and stator windings. This solution allows you to give the device additional operational properties, including automatic regulation of operating parameters. AC collector machines, which are connected to three-phase networks, receive three brush fingers in each segment of the double pole division. The brushes are interconnected in parallel with jumpers. In this sense, collector MPTs are similar to DC motors, but differ from them in the number of brushes used at the poles. In addition, the stator in this system may have several additional windings.
The closed armature winding when using a collector with three-phase brushes will be a three-phase complex winding with a connection in the form of a triangle. During the rotation of the armature, each phase of the winding remains unchanged, however, the sections alternately move from one phase to another. If a six-phase set of brushes with a shift of 60 ° relative to each other is used in an AC collector machine, a six-phase winding is formed with a polygon connection. On brushes of a multiphase machine with a collector group, the current frequency is determined by the rotation of the magnetic flux with respect to the fixed brushes. The direction of rotation of the rotor can be either counter or consistent.
The use of MPT
Today MPT are used everywhere where in one form or another the generation of mechanical or electrical energy is required. Large production units are used in the maintenance of engineering systems, power plants and hoisting and transport units, and low-power units are used in ordinary household appliances from fans to pumps. But in both cases, the purpose of alternating current machines comes down to generating enough energy potential. Another thing is that structural differences, the implementation of the internal configuration of the stator and rotor, as well as the control infrastructure are of fundamental importance.
Although the general design of the MPT for a long time retains the same set of functional components, the increasing requirements for the operation of such systems force developers to introduce additional control and management bodies. At the present stage of technological development, especially in the context of the use of AC machines in the manufacturing sector, it is difficult to imagine the operation of such engines and generators without high-precision means of regulating operating parameters. For this, a variety of control methods are used - pulse, frequency, rheostatic, etc. The introduction of automation in the regulatory infrastructure is also a characteristic feature of the modern operation of MPT. Control electronics is connected to the power plant on the one hand, and on the other, to software controllers, which, according to a given algorithm, give commands for setting specific parameters of the mechanism.
Conclusion
Current generators and electric motors are a mandatory power component in modern industry. Due to their function, machines, transport, communication installations and other electrical units and devices that require power supply work. At the same time, there is a huge array of types and subspecies of AC and DC electric machines, the features and characteristics of which ultimately determine the niche for their operation. The technical and operational features of MPT include a simpler design device and relatively low maintenance requirements. On the other hand, DC machines prove to be a more attractive solution to the problems of energy supply in complex critical power systems. The domestic production segment of power industrial equipment has vast experience in the design and manufacture of both types of electric machines. Large enterprises are increasingly focusing on the development of individual solutions with structural and operational features. Deviations from typical projects are often associated with the need to connect auxiliary functional units and equipment such as cooling systems, protective equipment against overheating and network fluctuations, additional and backup power. In addition, a part of the structural properties of electric machines is greatly influenced by the external operating environment, which is also taken into account at the design and engineering stages.