Mechanical characteristic of parallel excitation dpt. Types of excitation and switching circuits for DC motors

A DC motor with parallel excitation is an electric motor in which the armature and excitation windings are connected to each other in parallel. Often, in terms of its functionality, it surpasses the aggregates of mixed and sequential types in cases where it is necessary to set a constant speed of operation.

Parallel Excited DC Motor Characteristics

The formula for the total current coming from the source is derived according to the first Kirchhoff law and has the form: I \u003d I i + I c, where I i is the armature current, I c is the excitation current, and I is the current that the motor consumes from the network. It should be noted that in this case I in does not depend on I I, i.e. the excitation current is independent of the load. The value of the current in the excitation winding is less than the armature current and is approximately 2-5% of the mains current.

In general, these electric motors are distinguished by the following very useful traction parameters:

• High efficiency (since the armature current does not pass through the field winding).
• Stability and continuity of the working cycle with load fluctuations over a wide range (since the magnitude of the torque is maintained even if the number of shaft revolutions changes).

In case of insufficient torque, starting is carried out by switching to a mixed type of excitation.

Engine Applications

Since the speed of such motors remains almost constant even when the load changes, and can also be changed using an adjusting rheostat, they are widely used in work with:

• fans;
• pumps;
• mine lifts;
• overhead electric roads;
• machine tools (turning, metal-cutting, weaving, printing, straightening, etc.).

Thus, this type of motor is mainly used with mechanisms that require a constant speed of rotation or its wide adjustment.

Speed ​​control

Speed ​​control is a purposeful change in the speed of an electric motor forcibly with the help of special devices or devices. It allows you to ensure the optimal mode of operation of the mechanism, its rational use, as well as reduce energy consumption.

There are three main ways to control the speed of a motor:

1. Change in the magnetic flux of the main poles. It is carried out using an adjusting rheostat: with an increase in its resistance, the magnetic flux of the main poles and the excitation current Iv decrease. This increases the number of revolutions of the armature at idle, as well as the angle of inclination of the mechanical characteristic. The rigidity of the mechanical characteristics is maintained. However, increasing the speed can lead to mechanical damage to the unit and poor commutation, so it is not recommended to increase the speed by more than a factor of two by this method.
2. Changing the resistance of the armature circuit. An adjusting rheostat is connected in series to the armature. The rotation speed of the armature decreases with increasing resistance of the rheostat, and the slope of the mechanical characteristics increases. Adjusting the speed in the above way:

• helps to reduce the rotational speed relative to the natural characteristic;
• associated with a large amount of losses in the regulating rheostat, therefore, uneconomical.

1. Rheostatic change in the voltage applied to the armature. In this case, a separate voltage regulated power source, such as a generator or a controlled valve, is required.

Motor with independent excitation

The independent excitation DC motor implements the third principle of speed control. Its difference is that the excitation winding and the magnetic field of the main poles are connected to different sources. The excitation current is a constant characteristic, and the magnetic field changes. In this case, the number of revolutions of the shaft at idle changes, the rigidity of the characteristic remains the same.

Thus, the principle of operation of a DPT with independent excitation is rather complicated due to the independent operation of two sources, however, its main advantage is its high efficiency.

DC motors are not used as often as AC motors. Below are their advantages and disadvantages.

In everyday life, DC motors have found application in children's toys, since batteries serve as sources for their power. They are used in transport: in the subway, trams and trolleybuses, cars. In industrial enterprises, DC electric motors are used in drives of units for uninterrupted power supply of which batteries are used.

DC motor design and maintenance

The main winding of a DC motor is anchor connected to the power supply via brush apparatus. The armature rotates in the magnetic field created by stator poles (field windings). The end parts of the stator are covered with shields with bearings in which the motor armature shaft rotates. On the one hand, on the same shaft, fan cooling, which drives the flow of air through the internal cavities of the engine during its operation.

The brush apparatus is a vulnerable element in the design of the engine. The brushes are rubbed against the collector in order to repeat its shape as accurately as possible, they are pressed against it with a constant force. During operation, the brushes wear out, conductive dust from them settles on stationary parts, it must be removed periodically. The brushes themselves sometimes need to be moved in the grooves, otherwise they get stuck in them under the influence of the same dust and “hang” over the collector. The characteristics of the engine also depend on the position of the brushes in space in the plane of rotation of the armature.

Over time, the brushes wear out and need to be replaced. The collector at the points of contact with the brushes is also worn out. Periodically, the anchor is dismantled and the collector is machined on a lathe. After turning, the insulation between the collector lamellas is cut off to a certain depth, since it is stronger than the collector material and will destroy the brushes during further development.

DC motor switching circuits

The presence of excitation windings is a distinctive feature of DC machines. The electrical and mechanical properties of the electric motor depend on how they are connected to the network.

Independent arousal

The excitation winding is connected to an independent source. The characteristics of the motor are the same as those of a permanent magnet motor. The rotation speed is controlled by the resistance in the armature circuit. It is also regulated by a rheostat (regulating resistance) in the excitation winding circuit, but if its value is excessively reduced or if it breaks, the armature current increases to dangerous values. Motors with independent excitation must not be started at idle or with a small load on the shaft. The rotation speed will increase sharply and the motor will be damaged.

The remaining circuits are called circuits with self-excitation.

Parallel excitation

The rotor and excitation windings are connected in parallel to the same power source. With this inclusion, the current through the excitation winding is several times less than through the rotor. The characteristics of electric motors are tough, allowing them to be used to drive machine tools, fans.

Adjustment of the rotation speed is provided by the inclusion of rheostats in the rotor circuit or in series with the excitation winding.

sequential excitation

The excitation winding is connected in series with the anchor winding, the same current flows through them. The speed of such an engine depends on its load, it cannot be turned on at idle. But it has good starting characteristics, so the series excitation circuit is used in electrified vehicles.

mixed excitement

This scheme uses two excitation windings located in pairs on each of the poles of the motor. They can be connected so that their flows either add up or subtract. As a result, the motor can have characteristics similar to series or parallel excitation.

To change the direction of rotation change the polarity of one of the excitation windings. To control the start of the electric motor and the speed of its rotation, stepwise switching of resistances is used.

The shunt motor is the best choice among DC motors for driving applications that require almost constant speed and at the same time economical speed control. The diagram of this engine is shown in fig. 4-25.

Rice. 4-25. Motor of parallel excitation.

The clamps of the starting rheostat are designated: L - connected to the line (mains supply); M - to the excitation winding terminals and I - to the armature terminals. The black circles (Fig. 4-25) indicate the working contacts, and the gaps between them correspond to the resistance sections of the rheostat. When the engine is running, a metal arc 3 constantly connects the L terminal to the terminals of the shunt rheostat that regulates the excitation current. extreme left position, at which the resistance of the rheostat is minimal.

When the switch is closed and the lever of the starting rheostat is moved to the first of the working contacts, the motor current branches into the armature current and the excitation winding current

Thus, the current in the supply circuit

The first current surge depending on the value of the starting resistance Under the influence of the initial torque, the armature begins to rotate and with increasing speed, the armature current decreases. Then the lever of the starting rheostat can be transferred to the second contact. In this case, the armature current, having increased by a throw, will cause an increase in torque and a further increase in speed, and then again begins to decrease. Then the rheostat lever is transferred to the next contact, etc. The start ends when all the resistance is removed and full voltage is applied to the armature. The resistance of the starting rheostat is usually designed for short-term start operation and it is impossible to leave the rheostat handle on the intermediate contacts for a long time.

Rice. 4-26. Speed ​​characteristics of the motor of parallel excitation.

The faster counter-e. d.s. armature, the sooner the current decreases and the less heating of the armature winding. Therefore, the start-up is always carried out at the highest excitation current, short-circuiting the resistance of the regulating rheostat (Fig. 4-25). Then the magnetic flux of the machine F and counter-e. d.s. will be maximum. In addition, the electric motor at start-up must develop an increased torque, and this can also be with the highest magnetic flux formula (4-8)].

Before turning off the engine, the starting rheostat lever is switched to the zero contact, and then the knife switch is opened. This prevents burning of the switch contacts.

The speed characteristic of the engine at is shown in fig. 4-26 Curve 1. When there is no mechanical load, no-load current and speed are the highest:

With an increase in the load (resistance torque) on the motor shaft, the rotational speed drops slightly, since the automatic increase in torque occurs due to an increase in the current in the armature circuit, which, according to equation (4-14a), increases sharply with a slight decrease in counter-e. d.s. due to the small value of the resistance of the armature circuit. This characteristic is called rigid.

Rice. 4-27. Operating characteristics of the parallel excitation motor.

With a constant excitation current, the magnetic flux F can be considered approximately constant, since the influence of the armature reaction is insignificant.

Then the motor torque

approximately proportional to the current Therefore, if we plot M along the x-axis in Fig. 4-26, then the mechanical characteristic of the engine will be obtained, i.e.

Very easy to use performance data (Fig. 4-27) given in the catalogs and descriptions of the electric motor. it

at , where is the efficiency of the engine, and is the net power on the shaft.

Motor power developed on the shaft

and the torque

At a constant rotation frequency, the dependence would be a straight line passing through the origin. However, the speed decreases with increasing and the torque is not proportional. The current at a constant U is proportional to the power in the power circuit. Since the losses of the motor are small, the current is approximately proportional to .

Speed ​​control of a shunt motor is usually done by varying the field current. This method gives an economical smooth control within 1: 1.5, and in a special version - up to 1: 8. The regulation is as follows. The motor torque at Ф = const is proportional to the current and the current

Due to the small value, the voltage drop in the armature circuit is small. Therefore, at constant values ​​of U and armature, it can increase significantly with a slight decrease in counter-e. d.s.

For example, at and at armature current counter-e. d.s. . If counter-e. d.s. decreases by only 10 V (approximately 5%) and will, then the armature current, i.e., will increase by 3 times.

Thus, if, at a certain constant load and speed, the excitation current is reduced, for example, by 5%, then. the magnetic flux Ф and counter-e will immediately decrease by the same amount. d.s. E. This will cause a sharp increase in armature current and torque, and the excess torque will be used to accelerate the rotation of the armature. However, as the speed of the anchor increases, the counter-e. d.s. will increase again, the armature current will decrease to a value at which the torque will take its previous value. Thus, if equal, a new constant speed will be established, greater than the previous one.

With this method of regulation, the energy losses in the regulating rheostat (loss power Gvgv) are very small, since it is only

This method allows you to change the engine speed in the direction of its increase above the nominal.

If, with a constant load on the motor shaft, an additional resistance ch is connected in series with the armature winding, then at the first moment the armature current will decrease, which will reduce the torque and, since the moment of resistance will be greater, the speed will decrease. However, due to the decrease in speed and counter-e. d.s. the armature current will increase, the torque will increase, and if the torques are equal, the further decrease in speed will stop.

The engine will continue to run at a constant but reduced speed. This method of regulation is uneconomical due to significant energy losses in the resistance of the rheostat.

DC motor excitation is a distinctive feature of such motors. The mechanical characteristics of DC electrical machines depend on the type of excitation. Excitation can be parallel series mixed and independent. The type of excitation means in what sequence the armature and rotor windings are turned on.

With parallel excitation, the armature and rotor windings are connected in parallel to each other to the same current source. Since the excitation winding has more turns than the anchor winding, the current flows in it is negligible. In the circuit, both the rotor winding and the armature winding, adjusting resistances can be included.

Figure 1 - parallel excitation circuit of a DC machine

The excitation winding can also be connected to a separate current source. In this case, the excitation will be called independent. Such a motor will have characteristics similar to a motor that uses a permanent magnet. The speed of rotation of a motor with independent excitation, like that of a motor with parallel excitation, depends on the armature current and the main magnetic flux. The main magnetic flux is created by the rotor winding.

Figure 2 - independent excitation circuit of a DC machine

The rotation speed can be adjusted using a rheostat included in the armature circuit, thereby changing the current in it. You can also adjust the excitation current, but here you need to be careful. Since if it is excessively reduced or completely absent as a result of a break in the supply wire, the current in the armature can increase to dangerous values.

Also, with a small load on the shaft or in idle mode, the rotation speed may increase so much that it can lead to mechanical destruction of the engine.

If the excitation winding is connected in series with the anchor, then such excitation is called serial. In this case, the same current flows through the armature and the excitation winding. Thus, the magnetic flux changes with the motor load. Therefore, the speed of the engine will depend on the load.

Figure 3 - series excitation circuit of a DC machine

Motors with such excitation must not be started at idle or with a small load on the shaft. They are used in the event that a large starting torque or the ability to withstand short-term overloads is required.

Mixed excitation uses motors that have two windings on each pole. They can be turned on so that the magnetic fluxes both add up and subtract.

Figure 4 - mixed excitation circuit of a DC machine

Depending on how the magnetic fluxes correlate, a motor with such excitation can operate as a motor with serial or parallel excitation. It all depends on the situation, if you need a large starting torque, such a machine operates in the mode of consonant switching on of the windings. If a constant rotation speed is required, with a dynamically changing load, the windings are turned on in opposite directions.

In DC machines, you can change the direction of the rotor. To do this, you need to change the direction of the current in one of the windings. Anchor or excitation. By changing the polarity, the direction of rotation of the motor can only be achieved in a motor with independent excitation, or in which a permanent magnet is used. In other switching schemes, one of the windings must be switched.

The starting current in a DC machine is large enough, so it should be started with an additional rheostat to avoid damaging the windings.

Ministry of Science and Education of the Russian Federation

Federal Agency for Education

State educational institution

Higher professional education

National Research

IRKUTSK STATE TECHNICAL UNIVERSITY

Department of Power Supply and Electrical Engineering

Parallel Excitation DC Motor

Lab Report #9

in the discipline "General electrical engineering and electronics"

Fulfilled

Student SMO-11-1 ________ Dergunov A.S. __________

(signature) Surname I.O. (the date)

Associate Professor E and ET ________ Kiryukhin Yu.A. __________

(signature) Surname I.O. (the date)

Irkutsk 2012

Purpose of work 3

Task 3

Brief theoretical information 3

Electrical installation equipment 5

Work order 6

Answers security questions 9

Objective

Familiarize yourself with the structure and operation of a parallel excitation DC motor and investigate its characteristics.

Exercise

Familiarize yourself with the design and principle of operation of a parallel excitation DC motor. Familiarize yourself with the connection diagram of the parallel excitation motor. Familiarize yourself with the conditions for starting a parallel excitation motor. Familiarize yourself with the methods of controlling the engine speed. Check the engine at idle. Build an adjustment characteristic. Check the engine under load. Plot operating and mechanical characteristics.

Brief theoretical information

In a parallel excitation motor, the field winding is connected in parallel with the armature winding (see Fig. 1). The magnitude of the current in the field winding is less than the armature current and is 2 - 5% of .

The operational properties of engines are evaluated by operating, mechanical and control characteristics.

Rice. one

On fig. 8 shown workers parallel excitation motor characteristics: speed dependence , armature current values , torque
, efficiency and power consumed from the network from net power at constant voltage and excitation current .

Rice. 2

Mechanical motor characteristic is the dependence of the armature speed on the torque on the shaft at constant voltage and resistance of the excitation circuit . It shows the influence of the mechanical load on the motor shaft on the speed, which is especially important to know when choosing and operating motors. Mechanical characteristics can be natural or artificial. Motor characteristic at rated
,
and resistance
called natural. Formula for engine speed:

Mechanical characteristic equation:

, (1)

where
- speed at ideal idling (
);

– change in rotational speed caused by the action of the load.

Since for DC motors, the resistance of the armature winding
small, then with an increase in the load on the shaft, the rotational speed n changes slightly. Characteristics of this type are called rigid.

If we neglect the demagnetizing effect of the armature reaction and take
, then the natural mechanical characteristic of the parallel excitation motor has the form of a straight line, slightly inclined to the abscissa axis (Fig. 3, straight line 1).

If a ballast rheostat is introduced into the motor armature circuit
, then the dependence
will be determined by the expression

. (2)

RPM at perfect idle remains unchanged, and the change in rotational speed
increases, and the angle of inclination of the mechanical characteristic to the x-axis increases (Fig. 3, straight line 2). The resulting mechanical characteristic is called artificial .

A forced change in the engine speed at a constant load torque on the shaft is called regulation. Rice. 3

Speed ​​control in parallel excitation motors is possible in two ways: by changing the magnetic flux and by changing the resistance in the armature circuit.

R
speed control by changing the resistance in the armature circuit is carried out using a starting-adjusting rheostat
. With increasing resistance
the rotational speed decreases according to the formula (2). This method is uneconomical, as it is accompanied by significant losses for heating the rheostat.

Speed ​​control by changing the magnetic flux is carried out by means of a rheostat , included in the excitation winding (see Fig. 1). Rice. ten Rice. four

With an increase the current in the excitation winding decreases , the magnetic flux decreases
, which causes an increase in rotational speed.

At low values ​​of the excitation current, and even more so when the excitation circuit is broken (
), that is, with a small magnetic flux
, the rotational speed increases sharply, which leads to the "spacing" of the engine and to its mechanical destruction. Therefore, it is very important to ensure that all electrical connections in the excitation circuit are secure.

The dependence of the rotational speed on the excitation current is called regulating motor characteristic (see Fig. 4).

Speed ​​control by changing the magnetic flux
very economical, but not always acceptable, since when changing
the rigidity of the mechanical characteristics changes significantly.

Parallel excitation motors, due to the linearity and "rigidity" of mechanical characteristics, as well as the possibility of smooth regulation of rotation speed over a wide range, have become widespread both in power electric drives (for mechanisms and machine tools) and in automatic control systems.