The use of asynchronous motors in industry. Scope of synchronous electric motors
Question 13: Where are induction motors with squirrel-cage rotor?
Answer 13:
Asynchronous motors with a squirrel-cage rotor are used in electric drives (with speed control), conveyors, lifting mechanisms, fan installations, compressors, injection (liquid) pumps, various mixers (concrete, dough), ball mills, crushing plants, sawmills, machine tool drives .
Test questions
What is called a machine alternating current?
List the modes of operation of AC machines.
What indicators can be used to determine the mode of operation asynchronous machine?
What is an electromagnetic moment? Units.
What is the direction of the magnetic induction vector of a coil with current? Bring a drawing.
How Electric Energy consumed HELL from the network is converted into mechanical energy of rotor rotation?
What is called the number of pairs of poles of the machine?
The principle of operation of single-phase IM (with a starting winding
The principle of operation of single-phase (2-winding) IM with a phase-shifting capacitor. ?
Topic № 6. STUDY OF THE PARALLEL EXCITATION DC MOTOR
Goals of the work: 1) familiarize yourself with the device and principle of operation, start-up and methods of regulating the engine speed direct current parallel excitation;
2) to study the main characteristics of the engine and the method of their removal.
Work is performed on a universal stand (Fig. 47). As a DC motor load M 1 used three-phase asynchronous motor M 2 operating in dynamic brake mode. In order for an asynchronous motor to function as a brake, its stator winding is fed with direct current from a bridge rectifier connected to the secondary circuit of an autotransformer. T. By rotating the autotransformer motor, the brake current is set 
and, thereby, set the required braking torque on the motor shaft. An ammeter is used to measure the brake current. RA 1. The autotransformer is connected to the AC network by a switch Q 1.
In the armature circuit of the engine under study M 1 starting rheostat included 
, into the excitation winding circuit - adjusting rheostat 
and ammeter RA 3, measuring the drive current. The motor is connected to the DC network by a switch Q 2. Mains voltage U measured with a voltmeter PV, and the motor current 
- ammeter RA 4.
The electrical circuit of the stand is shown in fig. 46. The engine speed is measured with a tachometer not shown in the diagram. Scale this appliance calibrated in rpm (with a factor of 2/3).
test questions
Question 1. Explain the device and principle of operation of the parallel excitation motor.
Answer1: A DC motor is used to convert DC electrical energy into mechanical energy. Engine parallel excitation, consists of two main parts: a fixed one - the stator and a rotating one - the rotor. The design and electrical connection diagram are shown in Fig. 48 and Fig. 49, respectively.
The stator is a steel case - a frame, on the inner cylindrical surface of which the pole cores with pole tips are fixed. Coils are put on the cores, which make up the excitation winding connected to a direct current source. The excitation winding is located on the main (main) poles and creates the main magnetic flux of the motor. In addition to the main poles on the frame, there may be additional poles designed to improve switching.
The rotor consists of an armature and a collector, which are mounted on the same shaft and are mechanically one piece. The armature is a cylindrical core assembled from electrical steel sheets to reduce magnetic losses. In its grooves, a winding is laid, made of separate sections connected to each other and with collector plates.
The collector is a cylinder made up of separate copper plates isolated from each other and from the armature shaft. Fixed graphite (copper-graphite) brushes are superimposed on the collector, through which the armature winding is connected to a direct current source. The collector and brushes are designed to change the direction of the current in the conductors of the armature winding when they move from the zone of the magnetic pole of one polarity (for example, the north pole) to the zone of the pole of another polarity - (south pole). Due to this, the direction of rotation of the armature remains unchanged.
When the motor is connected to a DC source, currents appear in the field and armature windings ( 
and 
) As a result of the interaction of the armature current with the magnetic flux created by the excitation winding, an Ampere force arises and, accordingly, an electromagnetic torque:
,
where 
- coefficient depending on the design parameters of the engine; 
- armature current; 
is the magnetic flux of the machine.
Useful torque on the motor shaft M less electromagnetic torque by the value of no-load losses 
due to mechanical and magnetic losses.
In steady state, the torque is equal to the braking torque
.
When the armature rotates, its conductors cross the magnetic field and an EMF is induced in them 
, where 
- frequency of rotation of the anchor; 
- the value is constant for this machine.
Since the EMF is directed against the armature current, it is called counter-EMF.
The article discusses some areas of application of synchronous electric motors, which have excellent characteristics when rotating powerful drives. The synchronous electric machines themselves can develop power up to 20 thousand kW.
Synchronous motors differ from asynchronous motors in much greater power and payload. Changes in the excitation current allows you to adjust the load in them. Unlike induction motors in synchronous under shock loads, the speed remains constant, which allows them to be used in various mechanisms in the metallurgical and metalworking industries.
Motors with a synchronous type of action are capable of developing a power of up to 20 thousand kW, which is very important for actuating the actuators of powerful processing machines in mechanical engineering and other industries. For example, in high-performance guillotine shears, where there are large shock loads on the motor rotor.
Synchronous electric motors are successfully used as sources of reactive power in load nodes to maintain a stable voltage level. Quite often, motors with a synchronous principle of operation are used as power machines in high-capacity compressor units.
Powerful engines are made using a counter-ventilation system, in which the fan blades are located on the rotor. An economical and reliable synchronous motor ensures efficient and economical operation of pumping equipment.
An important characteristic of synchronous electrical machines is the maintenance of a constant speed of rotation, which is important for the rotation of drives in the form of pumps, compressors, fans, and various generators alternating current. It is also valuable to be able to regulate reactive current due to variations in the excitation current of the armature windings. Due to this, the cosine index φ increases in all operating ranges, which increases the efficiency of motors and reduces losses in electrical networks.
The motors themselves with a synchronous principle of operation are resistant to voltage fluctuations in the network, and provide a constant speed of rotation when they occur. Synchronous electric motors, when the supply voltage decreases, retain a greater overload capacity compared to asynchronous ones. The ability to boost the excitation current during voltage drops increases the reliability of their operation in case of emergency drops in the supply voltage in the electrical network.
Synchronous electrical machines are cost-effective at powers above 100 kW and are mainly used to rotate powerful fans, compressors and other power plants. As disadvantages of synchronous machines, one can note their design complexity, the presence of external excitation of the rotor windings, the difficulty of starting and rather high cost characteristics.
The principle of operation of a synchronous electric motor is based on the interaction of the rotation of the armature magnetic field with magnetic fields poles of the inductor. The armature is usually located on the stator, and the inductor on the movable rotor. At high powers, electromagnets serve as poles, while direct current is supplied to the rotor through sliding ring contacts.
Low power motors use permanent magnets located on the rotor. There are also synchronous machines with an inverted principle of operation, when the armature is placed on the rotor and the inductor on the stator. However, this design is used in engines of older designs.
Synchronous electrical machines can operate in generator mode, when the armature is located on the stator for easy selection of the generated electricity. Powerful generators operating in hydroelectric power plants are based on this principle.
Currently, almost all electric drives are unregulated drives with asynchronous motors. They are widely used in heat supply, water supply, air conditioning and ventilation systems, compressor units and other areas. Thanks to the smooth speed control, in most cases it is possible to dispense with chokes, variators, gearboxes and other control devices, which greatly simplifies the mechanical system, reduces its operating costs and increases reliability.
Starting the engine, when connected through a frequency converter, is carried out smoothly, without shocks and starting currents, which reduces the load on the mechanisms and the engine, increasing their service life. The use of an adjustable electric drive makes it possible to save up to eighty percent of electricity. Such savings are achieved due to the elimination of unproductive costs in control devices. In water supply systems, such regulation allows saving not only electricity, but also water, as well as reducing the number of accidents due to damage to pipelines.
Frequency converters are most successfully used in additional pumping pumps in heat and water supply systems. Such systems are characterized by uneven water consumption depending on the season, day of the week and time of day. With a constant amount of water supplied during the period of its increased analysis, the pressure significantly weakens, and with a decrease in flow in the line, pressure increases, which not only leads to water losses, but also increases the risk of pipeline rupture. The use of a frequency converter allows you to regulate the water supply in two ways - either in accordance with a certain schedule, or taking into account the actual water flow - this allows you to determine the pressure sensor or level gauge. Regulated water supply allows you to halve the cost of electricity, significantly reduce the consumption of heat and water.
Precise control of the rotation speed is necessary in the production of polymer threads, paper, wire, glass fabric. The use of a frequency converter in such processes makes it possible to obtain high quality products, increase productivity, eliminate breaks, while the material during winding will have an equal tension throughout the entire thickness of the roll. If the technological process requires the movement of products at a constant speed, several frequency converters are used, smooth start and stop, stepless speed change.
Today, the scope of electric motors is very extensive, and one of the most popular and used types of motor is asynchronous Electrical engine. But the asynchronous electric motor itself is divided into two types:
- with short-circuited rotor winding (squirrel-cage rotor), phase rotor;
- Schrage-Richter motor (powered from the rotor side).
Application of asynchronous electric motors
Asynchronous motors can operate in two modes of operation: as a generator and as an electric motor. This shows that they can be used as a source electric current in autonomous mobile power sources.
The use of asynchronous motors as a traction force is more extensive and affects many areas of human life. They have found wide application both in household electrical appliances of low power, and in the technological equipment of enterprises and agriculture.
Types of main faults, their diagnostics and necessary repair of an asynchronous electric motor
Although asynchronous electric motors have high reliability and low manufacturing cost, which led to their popularity, they, nevertheless, fail. Some malfunctions of electric motors can only be diagnosed on specialized equipment and require repair in a factory for the production and repair of electric motors. However, there are malfunctions that you can diagnose yourself and eliminate which is possible in the conditions of your production.
One of these faults is that the electric motor does not pick up normal speed at start-up or does not rotate. The causes of this malfunction may be electrical or mechanical in nature. Electrical causes include an internal break in the rotor or stator winding, broken connections in the starting equipment, or a break in the supply network. If there is a break in the internal windings of the motor, if they are connected according to the “triangle” scheme, then you must first open them. After that, using a megohmmeter, the phase in which the break occurred is determined. After determining the breakage, the motor winding is rewound and reassembled and installed in place.
Undervoltage in the network, poor contacts in the rotor winding, or high resistance in the rotor circuit of a wound rotor motor causes the motor to rotate at full load below the rated speed. Bad contacts in the winding are detected by applying voltage (20 -25% of the nominal) to the motor stator. At the same time, the locked rotor is turned manually and the current strength in all phases of the stator is checked. In a healthy rotor, the current strength in all positions is the same. In the event that contact is broken in the soldering of the frontal parts, a voltage drop will be noted. The maximum allowable difference in readings should not exceed 10%.
Deployment of the electric motor with an open circuit of the phase rotor. The cause of such a malfunction is a short circuit in the rotor winding. This malfunction is a careful external examination, as well as a measurement of the insulation resistance of the rotor winding. In the event that the inspection does not give results, then it is determined by determining the uneven heating of the rotor winding. In this case, the rotor is braked, and a reduced voltage is applied to the stator.
Uniform heating of the electric motor above the permissible norm occurs due to prolonged overload and deterioration of the cooling system. This fault leads to premature wear of the winding insulation.
Local heating of the stator winding occurs due to a short circuit of the winding to the housing in 2 places, an erroneous connection of the coils in any phase, a short circuit between 2 phases, or a short circuit between the turns of the winding in one of the phases of the stator winding. You can diagnose this malfunction by reducing the speed of rotation of the electric motor, a strong hum or the smell of overheated insulation. Determination of a damaged winding is carried out by measuring the resistance (the damaged phase has less resistance), or by measuring the current strength when a low voltage is applied.
When connecting the windings according to the "star" scheme, the current strength in the damaged phase will be higher than in the rest. In the case of using a "triangle", the line current in the healthy wires will have a higher value.
Burnout or melting of steel that occurs when short circuit winding of the stator, the shorting of steel sheets due to the contact of the stator with the rotor or due to the destruction of the insulation leads to local heating of the active steel of the rotor. In this case, smoke appears, the smell of burning, sparks, the buzz of the engine intensifies. This malfunction occurs due to wear or improper installation of bearings, strong vibration or one-sided attraction of the rotor to the stator (turn shorts in the stator winding).
Asynchronous machines
Lecture 5
At present, asynchronous machines are used mainly in motor mode. Machines with a power of more than 0.5 kW are usually three-phase, and with a smaller power - single-phase.
For the first time, the design of a three-phase asynchronous motor was developed, created and tested by our Russian engineer M. O. Dolivo-Dobrovolsky in 1889-91.
The demonstration of the first engines took place at the International Electrical Exhibition in Frankfurt am Main in September 1891. Three three-phase motors of different power were presented at the exhibition. The most powerful of them had a power of 1.5 kW and was used to drive a DC generator. The design of the asynchronous motor proposed by Dolivo-Dobrovolsky turned out to be very successful and is the main type of design of these motors to date.
Over the years, asynchronous motors have found a very wide application in various industries and agriculture.
They are used in the electric drive of metal-cutting machines, hoisting and transport machines, conveyors, pumps, fans. Low-power motors are used in automation devices.
The widespread use of induction motors is due to their
advantages compared to other motors: high reliability, the ability to work directly from the AC mains, ease of maintenance.
5.2. The device of a three-phase asynchronous machine
The fixed part of the machine is called stator, mobile - rotor. The stator core is made of sheet electrical steel and pressed into the frame. On fig. 5.1 shows the stator core assembly. The frame (1) is made of cast, non-magnetic material. Most often, the bed is made of cast iron or aluminum. On the inner surface of the sheets (2), from which the stator core is made, there are grooves in which three-phase winding(3). The stator winding is made mainly of insulated copper wire of round or rectangular cross section, less often of aluminum.
The stator winding consists of three separate parts called phases. The beginnings of the phases are indicated by letters from 1, from 2, from 3, the ends - from 4, from 5, from 6.
The beginnings and ends of the phases are displayed on the terminal block (Fig. 5.2 a), fixed on the frame. The stator winding can be connected according to the star (Fig. 5.2 b) or delta (Fig. 5.2 c) scheme. The choice of the stator winding connection scheme depends on the line voltage of the network and the nameplate data of the motor. In passport three-phase motor line voltages of the network and the connection diagram of the stator winding are set. For example, 660/380, Y/∆. This motor can be connected to a network with Ul = 660V according to the star scheme or to a network with Ul = 380V - according to the triangle scheme.
The main purpose of the stator winding is to create a rotating magnetic field in the machine.
Rotor core(Fig. 5.3 b) is recruited from sheets of electrical steel, on the outer side of which there are grooves into which the rotor winding is laid. The rotor winding is of two types: short-circuited and phase. Accordingly, asynchronous motors come with a squirrel-cage rotor and a phase rotor (with slip rings).
Rice. 5.3
The short-circuited winding (Fig. 5.3) of the rotor consists of rods 3, which are laid in the grooves of the rotor core. From the ends, these rods are closed with end rings 4. Such a winding resembles a “squirrel wheel” and is called a “squirrel cage” type (Fig. 5.3 a). The squirrel-cage motor has no moving contacts. Due to this, such engines have high reliability. The rotor winding is made of copper, aluminum, brass and other materials.
Dolivo-Dobrovolsky was the first to create an engine with a squirrel-cage rotor and explore its properties. He found that such engines have a very serious drawback - limited starting torque. Dolivo-Dobrovolsky called the reason for this shortcoming - a strongly shorted rotor. He also proposed the design of an engine with a phase rotor.
On fig. 5.4 shows a sectional view of an asynchronous machine with a phase rotor: 1 - frame, 2 - stator winding, 3 - rotor, 4 - slip rings, 5 - brushes.
At the phase rotor, the winding is three-phase, similar to the stator winding, with the same number of pole pairs. The winding turns are laid in the grooves of the rotor core and connected according to the star scheme. The ends of each phase are connected to the contact rings fixed on the rotor shaft, and through the brushes they are brought out into the external circuit. Slip rings are made of brass or steel and must be insulated from each other and from the shaft. As brushes, metal-graphite brushes are used, which are pressed against the slip rings with the help of brush holder springs fixed motionless in the machine body. On fig. 5.5 given symbol asynchronous motor with squirrel-cage (a) and phase (b) rotor.
On fig. 5.6 shows a sectional view of an asynchronous machine with a squirrel-cage rotor: 1 - frame, 2 - stator core, 3 - stator winding, 4 - rotor core with a squirrel-cage winding, 5 - shaft.
On the shield of the machine, fixed on the bed, the data are given: R n, U n, I n, n n, as well as the type of machine.
- P n is the rated net power (on the shaft)
- U n and I n - nominal values of line voltage and current for the specified connection scheme. For example, 380/220, Y/∆, InY/In∆.
- n n - rated frequency rotation in rpm.
The machine type, for example, is given as 4AH315S8. This is an asynchronous motor (A) of the fourth series of protected design. If the letter H is absent, then the engine is of a closed design.
- 315 - height of the axis of rotation in mm;
- S - installation dimensions (they are set in the reference book);
- 8 - the number of poles of the machine.