DC motors
Lecture outline: 1. Basic concepts. 2. Start the engine. 3. Parallel excitation motor. 4. Series motor. 5. Mixed excitation motor.
1. Basic concepts Collector machines have the property of reversibility, i.e. they can operate in both generator and engine modes. Therefore, if a DC machine is connected to a DC energy source, then currents will appear in the field winding and in the armature winding of the machine. The interaction of the armature current with the excitation field creates an electromagnetic moment M on the armature, which is not braking, as was the case in the generator, but rotating.
Under the influence of the electromagnetic torque of the armature, the machine begins to rotate, i.e. the machine will operate in engine mode, consuming electrical energy from the network and converting it into mechanical energy. During engine operation, its armature rotates in a magnetic field. An emf Ea is induced in the armature winding, the direction of which can be determined by the “right hand” rule. By its nature, it does not differ from the EMF induced in the armature winding of the generator. In a motor, the EMF is directed against the current Ia, and therefore it is called the counter-electromotive force (back-EMF) of the armature (Fig. 1).
Rice. 1. The direction of the back EMF in the armature winding of the motor. The direction of rotation of the armature depends on the directions of the magnetic flux F and the current in the armature winding. Therefore, by changing the direction of any of the indicated quantities, you can change the direction of rotation of the armature. When switching the common terminals of the circuit, the switch does not change the direction of rotation of the armature, since this simultaneously changes the direction of the current in both the armature winding and the field winding.
2. Starting the motor When the motor is directly connected to the network, a starting current arises in its armature winding: Ia’ = U/ = Σr. Typically, the resistance Σr is small, so the starting current reaches unacceptably high values, 10 to 20 times the rated motor current. Such a large starting current is dangerous for the engine, it can cause a circular fire in the car; with such a current, an excessively large starting torque develops in the engine, which has a shock effect on the rotating parts of the engine and can mechanically destroy them.
Rice. 2. Scheme for switching on the starting rheostat Before starting the engine, it is necessary to place the rheostat lever P on idle contact 0 (Fig. 2). Then the switch is turned on, moving the lever to the first intermediate contact 1 and the motor armature circuit is connected to the network through the greatest resistance of the rheostat rп р = r1 + r2 + r3 + r4.
To start engines of higher power, it is not advisable to use starting rheostats, since this would cause significant energy losses. In addition, starting rheostats would be bulky. Therefore, the motors have a high voltage start-up power. Examples of electric locomotive traction motors are switching them from a series connection during start-up to a parallel connection during normal operation or starting an engine in a “generator-motor” circuit. apply in this way rheostat-free reductions are start
3. Parallel excitation motor The circuit diagram for connecting a parallel excitation motor to the network is shown in Fig. 3, a. A characteristic feature of this motor is that the current in the field winding does not depend on the load current. The rheostat in the excitation circuit rрг serves to regulate the current in the excitation winding and the magnetic flux of the main poles. The motor is determined by its control characteristics, which are understood as the dependence of the rotation speed n, current I, useful torque M2, rotating torque M on the power on the motor shaft P2 at U = const and Iв = const (Fig. 3, b). Performance properties
Rice. 3. Diagram of a parallel-excitation motor (a) and its performance characteristics (b) The change in engine speed during the transition from rated load to idling load, expressed as a percentage, is called the nominal change in speed:
is a straight line. If we neglect the armature reaction, then (since Iв = const) we can take Ф = const. Then the mechanical characteristic of the parallel excitation motor is slightly inclined to the abscissa axis (Fig. 4, a). The greater the value of resistance included in the armature circuit, the greater the inclination angle of the mechanical characteristic. with mechanical absence of additional resistance in the armature circuit 1). The mechanical characteristics of the engine obtained by introducing additional resistance into the armature circuit are called artificial (direct lines 2 and 3). natural characteristic of the engine line is called (straight
Rice. 45.4. Mechanical characteristics of a parallel-excitation motor: a – when additional resistance is introduced into the armature circuit; b – when the main magnetic flux changes; c – when the voltage in the armature circuit changes, the type of mechanical characteristic also depends on the value of the main magnetic flux F. Thus, with an increase in F, the rotation speed XX n0 increases and at the same time Δn increases.
4. Series excitation motor In this motor, the excitation winding is connected in series to the armature circuit (Fig. 5, a), therefore the magnetic flux Ф in it depends on the load current I = Ia = Iв. At the required loads, the magnetic system of the machine is not saturated and the dependence of the magnetic flux on the load current is directly proportional, i.e. Ф = kфIa. In this case, we find the electromagnetic moment: M = cmkфIaIa = cm’ Ia2.
Rice. 5. Series excitation motor: a – schematic diagram; b – performance characteristics; c – mechanical characteristics, 1 – natural characteristics; 2 – artificial characteristic The engine torque when the system is unsaturated is proportional and the rotation speed is inversely proportional to the magnetic square state of the load current. current,
5, b In Fig. the operating characteristics of M = f(I) and n = f(I) of a series-excited motor are presented. At heavy loads, the motor magnetic system becomes saturated. In this case, the magnetic flux will hardly change as the load increases, and the characteristics of the motor become almost linear. The frequency characteristic of the sequential excitation rotation shows that the engine speed changes significantly with load changes. This characteristic is usually called soft. engine
2) provide n excitation characteristics of a mechanical motor = f(M) sequential are shown in Fig. 5, c. Sharply falling curves of mechanical characteristics (natural 1 and artificial sequential excitation motor stable operation under any mechanical load. The property of these motors to develop large torque, proportional to the square of the load current, is important, especially under difficult starting conditions and overloads, since with a gradual As the engine load increases, the power at its input increases more slowly than the torque.
Rice. 6. Regulating the speed of motors 2) provide sequential excitation characteristics of the motor excitation Mechanical f(M) = sequential are presented in Fig. 5, c. Sharply falling curves of mechanical characteristics (natural 1 and artificial motor sequential excitation stable operation n
The rotational speed of series-excited motors can be adjusted by changing either the voltage U or the magnetic flux of the field winding. In the first case, the adjusting rheostat Rрг is connected in series to the armature circuit (Fig. 6, a). As the resistance of this rheostat increases, the voltage at the motor input and its rotational speed decrease. This control method is used in low-power engines. In this case, the method of significant engine power is uneconomical due to large energy losses in Rрг. In addition, the rheostat Rрг, designed for operating and current, is expensive. this engine is cumbersome, it turns out
When several engines of the same type operate together, the rotation speed is adjusted by changing their switching pattern relative to each other (Fig. 6, b). So, when the motors are connected in parallel, each of them is under full mains voltage, and when two motors are connected in series, each motor is exposed to half the mains voltage. With the simultaneous operation of more motors, a greater number of switching options is possible. This method of speed control is used in electric locomotives, where several traction motors of the same type are installed. on
Changing the voltage supplied to the motor is also possible when the motor is powered from a DC source with adjustable voltage (for example, according to a circuit similar to Fig. 7, a). When the voltage supplied to the motor decreases, its mechanical characteristics shift downward, practically without changing its curvature (Fig. 8). rotation speed rрг; You can regulate the motor by changing the magnetic flux in three ways: by shunting the field winding of the winding with a rheostat of the field armature; bypassing with a rheostat rsh. sectioning the winding
“Static electricity” - Excess electricity must be removed from the body by grounding. Cloth. Grounding results. For thousands of years, our ancestors walked the earth barefoot, grounding themselves naturally. Normalization of pressure. “Excess” electricity can lead to serious malfunctions in the functioning of organs and systems.
“Body forces” - Force acts on the connection, and the reaction of the connection acts on the body. Circle. A surface on which friction can be neglected is considered smooth. D'Alembert's principle. Theorem on the speed of a point in complex motion. Force is a sliding vector. Cylindrical hinge. Varignon's theorem. Theorem on the addition of pairs of forces. Hard seal.
“History of Electricity” - 20th century - the emergence and rapid development of electronics, micro/nano/pico technologies. History of the development of electricity. 19th century - Faraday introduces the concept of electric and magnetic fields. XXI century - electrical energy has finally become an integral part of life. XXI century - power outages in household and industrial networks.
“Atomic nuclei” - Diagram of the structure of a nuclear power plant. Superheavy nuclei (A > 100). Kernel sizes. Nuclear forces. Nuclear fission. The magnetic field is created by superconducting windings. N? Z diagram of atomic nuclei. Scattering of a particle in the Coulomb field of a nucleus. Rutherford's experience. Models of atomic nuclei. Nuclear synthesis. Mass and binding energy of the nucleus.
“What physics studies” - Introductory speech by the teacher. Rocket launch. Technique. What does physics study? Eruption. Combustion. Physics. Aristotle is the greatest thinker of antiquity. Thermal phenomena of nature. Magnetic phenomena of nature. Aristotle introduced the concept of “physics” (from the Greek word “fusis” - nature). Introducing students to a new school subject.
“Igor Vasilyevich Kurchatov” - His mother was a teacher, his father was a land surveyor. The Beloyarsk NPP is named after Kurchatov. I.V. Kurchatov is a deputy of the Supreme Soviet of the USSR of the third and fifth convocations. Biography of Kurchatov I.V., as an outstanding Soviet physicist. The Institute of Atomic Energy, founded by him, was named after Kurchatov in 1960. Who is I.V. Kurchatov?
There are 19 presentations in total
Electric motor - electric machine
(electromechanical converter), in which the electrical
energy is converted into mechanical energy as a side effect
is the release of heat.
Electric motors
Alternating current
Synchronous
Asynchronous
Direct current
Collector
Brushless
Universal
(can eat
both types
current)
principle of electromagnetic induction.
The electric machine consists of:
fixed part - stator (for asynchronous and synchronous
AC machines) or inductor (for machines
direct current)
moving part - rotor (for asynchronous and synchronous
AC machines) or anchor (for DC machines
current). Typically, a rotor is an arrangement of magnets in the shape of a cylinder,
often formed by coils of thin copper wire.
The cylinder has a central axis and is called a “rotor” because
that the axis allows it to rotate if the motor is built
Right. When passed through the rotor coils
electric current, the entire rotor is magnetized. Exactly
you can create an electromagnet. 8.2 AC motors According to the principle of operation, AC motors are divided
for synchronous and asynchronous motors.
Synchronous electric motor - electric motor
alternating current, the rotor of which rotates synchronously
with the magnetic field of the supply voltage. These engines
usually used at high powers (from hundreds of kilowatts
and higher).
Asynchronous electric motor - electric motor
alternating current, in which the rotor speed is different
on the frequency of the rotating magnetic field created by the supply
tension. These engines are most common in
present time. Operating principle of a three-phase asynchronous electric motor
When connected to the network, a circular rotating
magnetic field that penetrates a short-circuited winding
rotor and induces an induction current in it. From here, following the law
Ampere, the rotor begins to rotate. Rotor speed
depends on the frequency of the supply voltage and the number of pairs
magnetic poles. Difference between speed
stator magnetic field and rotor speed
characterized by sliding. The motor is called asynchronous
since the rotation frequency of the stator magnetic field does not coincide with
rotor speed. A synchronous motor has a difference in
rotor designs. The rotor is either constant
a magnet, or an electromagnet, or has part of a squirrel
cages (for launching) and permanent or electromagnets. IN
synchronous motor rotation frequency of the stator magnetic field and
rotor speeds are the same. To start use
auxiliary asynchronous electric motors, or a rotor with
short-circuited winding. Three-phase asynchronous motor To calculate the characteristics of an asynchronous motor and
research of various modes of its operation is convenient to use
substitution schemes.
Moreover, a real asynchronous machine with electromagnetic
connections between the windings are replaced by a relatively simple
electrical circuit, which makes it possible to significantly simplify
calculation of characteristics.
Considering that the basic equations of an asynchronous motor
similar to the same transformer equations,
The equivalent circuit of the motor is the same as that of the transformer.
T-shaped equivalent circuit of an asynchronous motor When calculating the characteristics of an asynchronous motor with
using an equivalent circuit, its parameters must be
known. The T-shaped diagram fully reflects the physical
processes occurring in the engine, but difficult to calculate
currents Therefore, there is great practical application for analysis
Another scheme finds the operating modes of asynchronous machines
substitution, in which the magnetizing branch is connected
directly at the input of the circuit, where voltage U1 is supplied.
This circuit is called an L-shaped equivalent circuit. L-shaped scheme
replacing asynchronous
engine(s) and its
simplified version (b) For different mechanisms, it serves as an electric drive.
asynchronous motor, which is simple and reliable. These engines
easy to manufacture and cheap compared to others
electric motors. They are widely used in both
industry, agriculture and construction.
Asynchronous motors are used in electric drives
various construction equipment, in lifting countries.
The ability of such an engine to operate in intermittent mode makes it possible to use it in
construction cranes. During disconnection from the mains, the engine does not
It cools down and does not have time to heat up during operation. 8.3. Electric motors
direct current Commutator motor
The smallest motors of this type (units of watt)
used mainly in children's toys (working
voltage 3–9 volts). More powerful motors (tens of watts)
used in modern cars (operating voltage
12 volts): fan drive for cooling systems and
ventilation, wipers. Brushed motors can convert like
electrical energy into mechanical energy and vice versa. From this
it follows that it can work as an engine and as a generator.
Let's consider the principle of operation on an electric motor.
It is known from the laws of physics that if through a conductor,
being in a magnetic field to pass a current, then it will start
act force.
Moreover, according to the right hand rule. The magnetic field is directed from
north pole N to south S, if the palm of the hand is directed towards
side of the north pole, and four fingers in the direction of the current
in the conductor, then the thumb will indicate the direction
acting force on the conductor. Here's the basics of how it works
commutator motor. But how do we know the small rules and create the right things. On
On this basis a frame was created rotating in a magnetic field.
For clarity, the frame is shown in one turn. Just like in the past
example, two conductors are placed in a magnetic field, only the current in
these conductors are directed in opposite directions,
therefore the forces are the same. In total, these forces give a torque
moment. But this is still a theory. The next step was to create a simple brushed motor.
It differs from the frame in the presence of a collector. It provides
the same direction of current over the north and south poles.
The disadvantage of this engine is uneven rotation and
inability to work on alternating voltage.
The next step was to eliminate the unevenness of the stroke by
placing several more frames (coils) at anchor, and from
constant voltage removed by replacing permanent magnets
on coils wound on the stator pole. When leaking
alternating current through the coils, the direction of the current changes as
in the stator and armature windings, therefore, the torque,
both at constant and alternating voltage there will be
directed in the same direction, as required to be proven. Commutator motor design Brushless motor
Brushless DC motors are also called
valve Structurally, the brushless motor consists of
consisting of a rotor with permanent magnets and a stator with windings. IN
In a commutator motor, on the contrary, the windings are on the rotor.
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Slide captions:
COLLECTOR ELECTRIC MOTOR Developed by a technology teacher of the highest category, Honorary Worker of Primary Vocational Education of the Russian Federation MBOU "Secondary School No. 7" in Kaluga Gerasimov Vladislav Alexandrov
What do these electrical appliances have in common?
COLLECTOR ELECTRIC MOTOR
STORY. The first commutator electric motor was designed in Russia by the Russian scientist Jacobi Boris Semenovich in 1838. By the 70s of the 19th century, the electric motor had already been improved so much that it has been preserved in this form to this day.
Boris Semenovich Jacobi
Purpose: Conversion of electrical energy into mechanical energy. Mechanical energy sets in motion the working parts of machines and mechanisms.
Operating principle: Electric current from the source (battery of galvanic cells) is supplied to the winding through special sliding contacts - brushes. These are two elastic metal plates that are connected by conductors to the poles of the current source and pressed against the collector. When electric current flows through the armature winding, the rotor begins to rotate under the influence of the magnet.
General structure of the electric motor: 1-bearings, 2-rear stator cover, 3-winding, 4-armature, 5-core, 6-armature winding, 7-collector, 8-front cover, 9-shaft, 10-impeller.
The smallest engines of this type. three-pole rotor on plain bearings; a collector unit of two brushes - copper plates; bipolar stator made of permanent magnets. They are mainly used in children's toys (operating voltage 3-9 volts).
Powerful motors (tens of watts), as a rule, have: a multi-pole rotor on rolling bearings; collector unit of four graphite brushes; four-pole stator made of permanent magnets. This is the design of most electric motors in modern cars (operating voltage 12 or 24 Volts): driving fans of cooling and ventilation systems, windshield wipers, washer pumps.
Commutator wheel motor, 24 volts 230 watts.
Motors with a power of hundreds of watts Unlike the previous ones, they contain a four-pole stator made of electromagnets. The stator windings can be connected in several ways: in series with the rotor (so-called series excitation), advantage: large maximum torque, disadvantage: high idle speed, which can damage the engine.
in parallel with the rotor (parallel excitation) advantage: greater speed stability when the load changes, disadvantage: lower maximum torque; some of the windings are in parallel with the rotor, some are in series (mixed excitation) to some extent combines the advantages of previous types, for example - car starters. separate power supply (independent excitation) the characteristic is similar to parallel connection, but can usually be adjusted.
DC shunt motor
Series-wound DC motor
Methods for changing the rotation speed of an electric motor shaft By changing the magnitude of the stator excitation current. The greater the current in the stator, the higher the speed of the electric motor shaft.
Advantages of electric motors. No harmful emissions during operation Do not require constant maintenance Can be installed anywhere Work in vacuum conditions Do not use flammable substances (gasoline, diesel fuel) Easy to use
Failures in the operation of a commutator electric motor. Operating conditions and service life of motors in household machines are different. The reasons for their failure are also different. It has been established that 85-95% of operation failures are due to damage to the insulation of the windings, distributed as follows: 90% of interturn short circuits and 10% of damage and breakdowns of insulation to the housing. Then comes wear of the bearings, deformation of the rotor or stator steel and bending of the shaft.
The repair process includes the following main operations:
Pre-repair tests External cleaning of dirt and dust Disassembly into components and parts Removal of windings Washing components and parts Troubleshooting components and parts Repair and production of components and parts Assembling the rotor Manufacturing and laying of windings Drying and impregnation work Mechanical processing of the assembled rotor and its balancing Assembling components and parts Assembly of electric motors Tests after repair External finishing
Summing up the lesson. What is an electric motor? What devices use commutator motors? What parts does a commutator motor consist of? What principle underlies the operation of a commutator motor?
Electric motors
- Purpose: to study the device and principle of operation of electronics. engines of various designs; become familiar with the operating principle of an asynchronous motor (single-phase)
- Where are electric motors used in everyday life and industry?
- Electric drill
- Washing machine
- Vacuum cleaner
- Electric shaver
- Sewing machine
- Electric transport, etc.
- Electric drill
- The electric drill uses a commutator motor.
- Electric motor
- Washing machine
- Washing machines use an asynchronous single-phase electric motor.
- electric motor
- vacuum cleaner
- Vacuum cleaners use a commutator motor.
- electric motor
- electric transport
- To move trams, trolleybuses, and electric trains, high-power electric motors are used.
- Commutator motor design
- The commutator motor is universal and can operate on both direct and alternating current.
- anchor
- collector
- bed
- inductor
- Features of the operation of a commutator electric motor.
- By changing the voltage on the motor brushes, you can adjust the rotor speed. Due to this, the commutator motor is used in those machines where it is necessary to change the rotation speed of the mechanisms. (kitchen electrical appliances; electric drill; electric razor; hair dryer; tape recorders; sewing machine; electric carpentry tools, etc., as well as electric transport)
- brushes
- collector
- Rotor winding
- How does a commutator motor work?
- The operating principle of the engine is based on interaction
- conductor ( anchors) with electric current and magnetic field,
- created by an electromagnet (inductor). Mechanical force
- arising from such interaction causes rotation
- anchor (rotor).
- Such engines are divided into:
- AC motors, frame and core of which are made of sheets of electrical steel;
- DC motors in which the named parts are made solid.
- The field winding of the electromagnet in AC motors is connected in series with the armature winding, which provides a large starting torque.
- The device of an asynchronous electric motor
- Next, let's look at the operating principle of an asynchronous motor.
- rotor
- stator
- Operation of an asynchronous motor
- The operating principle of an asynchronous motor is based on the interaction of a rotating magnetic field with currents that are induced by the field in the conductors of a short-circuited rotor.
- The rotor is mounted on bearings and therefore moves in the direction of the rotating rotor.
- Structurally, an asynchronous motor consists of two main parts:
- - stationary – stator;
- - movable – rotor.
- The stator has three windings wound at an angle of 120°. The rotor has a winding in the form of a squirrel wheel.
- Operation of an asynchronous motor
- Asynchronous motors have their own:
- * advantages - simple in design, reliable in operation and used in all sectors of the national economy;
- * disadvantages – inability to obtain a constant number of revolutions (compared to collector); When starting, it carries a large current and is sensitive to voltage fluctuations in the network.
- Of the total number of electric motors produced, 95% are asynchronous.
- Features of the operation of an asynchronous electric motor
- Unlike a commutator motor, where carbon brushes rub against the commutator, in an asynchronous motor the windings are located in the stator, therefore, without rubbing parts, the service life of an asynchronous motor is much longer than that of a commutator, and its range of applications is much wider. (washing machines, vacuum cleaners, woodworking and metalworking machines, fans, pumps, compressors, etc.
- Anchor
- windings
- Using a three-phase motor at home
- To use a three-phase motor in everyday life, where there is single-phase electrical wiring, a capacitor must be connected to the circuit. The disadvantage of this method is the use of expensive paper capacitors. (for every 100W of power 10 microfarads for a voltage of 250-450V.
- Connecting an asynchronous single-phase motor to the network
- In household machines, single-phase asynchronous motors are used that have two windings:
- # working; # launcher; The windings are located at an angle of 90°. When connected to the network, a rotating magnetic field is formed, and the squirrel-cage rotor begins to rotate, after which the starting winding is turned off.
- starting winding
- ~ 220V
- Determine what type of electric motor is used in this household appliance.
- Determine what type of electric motor is used in industrial equipment.
