Parallel excited motor connection. Characteristics of parallel excitation motors

Excitation of the motor direct current is a distinctive feature of such engines. The mechanical characteristics of direct current electric machines depend on the type of excitation. Excitation can be parallel, serial, mixed and independent. The type of excitation means in which 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 armature winding, the current flows in it is insignificant. In the circuit, both the rotor winding and the armature winding, adjusting resistances can be turned on.

Figure 1 - diagram parallel excitation DC machines

The excitation winding can also be connected to a separate current source. In this case, the excitation will be called independent. The performance of such a motor will be similar to that of a permanent magnet motor. The speed of rotation of a motor with independent excitation, as in a motor with parallel excitation, depends on the armature current and the main magnetic flux. The main magnetic flux is generated by the rotor winding.

Figure 2 - Scheme of independent excitation 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 be careful here. Since when it is excessively reduced or complete absence as a result of a break in the supply wire, the current in the armature can increase to dangerous values.

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

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

Figure 3 - Scheme of series excitation of a DC machine

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

Mixed excitation uses motors that have two windings at each pole. They can be turned on so that the magnetic fluxes are both added and subtracted.

Figure 4 - diagram mixed excitement DC machines

Depending on how the magnetic fluxes are related, the motor with such excitation can operate as a motor with a series and a motor with parallel excitation. It all depends on the situation, if a large starting moment is needed, such a machine operates in the mode of the coherent inclusion of windings. If a constant rotation speed is required, with a dynamically changing load, the opposite winding is used.

In DC machines, the direction of movement of the rotor can be changed. To do this, it is necessary to change the direction of the current in one of the windings. Anchor or excitation. By reversing the polarity, the direction of rotation of the motor can only be achieved in a motor with independent excitation or which uses a permanent magnet. In other switching schemes, you need to switch one of the windings.

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

Good day, dear readers! In this article I will talk about what excitation is in DC motors and "what it is eaten with."

Probably, each of us in childhood had toys with an electric drive. Those who were curious in those years did not miss the opportunity to disassemble these toys in order to see what was inside.

Looking inside such a toy, we found a small DC electric motor. Naturally, then we did not even think about why it works. Some of us, having found a motor in the toy, dared to disassemble it too. These curious comrades, having disassembled the motor, found there a permanent magnet (sometimes more than one), brushes and an anchor with a collector.

So, just a permanent magnet is the simplest system excitation for DC motors. After all, the motor armature rotates only when there is a constant magnetic field around it, which is created with the help of a permanent magnet.

DC motors industrial scale, as exciters, use special windings, called field windings.

The connection of these windings can be very different. They can be connected in parallel with the armature, in series with it, mixed and, even, independently of them.

By the way, motors that have a permanent magnet as an exciter are considered to be independently excited devices.

The exciting winding consists of significantly more turns than anchor. In this regard, the current of the armature winding is ten times higher than the current of the exciting one. The speed of rotation of such an engine can vary depending on the load and magnetic flux. Thanks to the connection properties, the engines parallel connection quite a bit subject to changes in speed.

Now let's consider the option of separate connection of the working and exciting windings. Such an engine is called an independent excitation motor. The speed of such an engine can be adjusted by changing the resistance of the anchor chain or magnetic flux.

There is a small nuance here: do not reduce the excitation current too much when the motor is turned on this way, since this is fraught with a very large rise in the armature current. The same is dangerous and an open circuit of the excitation of these motors. In addition, if the load of the motor with such an inclusion is small, or when it is turned on at idle speed, such a strong acceleration can occur that there is a danger to the engine.

As I already said, a type of DCT of independent excitation is considered to be a device that has permanent magnets as an exciter. I will say a few words about them as well.

Since DC motors and machines of the synchronous type can use permanent magnets instead of exciters, this option is considered quite attractive. And that's why:

• such a device has reduced current consumption by reducing the number of windings, as a result of which such indicators of such machines as efficiency are higher;
• with the use of permanent magnets instead of the exciter, the design of the exciting circuits of the engine is simplified, which increases its reliability, because the permanent magnet does not require power, therefore, such a motor does not have a current-collecting unit on the rotor.

Now about sequential inclusion windings (motors with series excitation).

In this connection option, the armature current will be also exciting. This causes the magnetic flux to change strongly depending on the load. This is the reason for the great undesirability of starting them at idle and at low load.

The application found such an inclusion where a significant starting moment is required, or the ability to withstand short-term overloads. In this regard, they are used as a means of traction for trams, trolleybuses, electric locomotives, metro and cranes. In addition, they are used as a starting means for internal combustion engines (as starters).

The last option for turning on DC motors is their mixed inclusion. Each of the poles of these motors is equipped with a pair of windings, one in parallel and the other in series. There are two ways to connect them:

• consonant method (in this case, the currents are added);
• the opposite option (subtraction of currents).

Accordingly, depending on the connection option (which also changes the ratio of magnetic fluxes), such a motor may be close either to a device with serial excitation, or to an engine with parallel excitation.

In most cases, they consider the serial winding as the main winding, and the auxiliary winding in parallel. Due to the parallel winding of such motors, the speed practically does not increase at low loads.

If a significant starting torque is required and the ability to adjust the speed on variable loads, a consonant connection is used. The opposite connection is used when it is necessary to receive constant speed with varying load.

If it becomes necessary to reverse the DCT (change the direction of its rotation), then change the direction of the current in one of its working windings.

By changing the polarity of connecting the motor terminals, it is possible to change the direction of only those motors that are switched on according to an independent circuit, or motors with a permanent magnet as an exciter. In all other devices, it is necessary to change the direction of the current in one of the working windings.

In addition, DC motors cannot be switched on by the connection method. full voltage... This is due to the fact that the value of their starting current is about 2 dozen times higher than the nominal (this depends on the size and speed of the motor). Engine start currents large sizes can exceed their rated operating current by fifty times.

Currents large quantities can cause the effect of circular arcing of the collector, as a result of which the collector is destroyed.

To turn on the DPT, the technique is used, or the use of starting rheostats. Direct type switching is possible only at low voltages and for small engines that have great resistance anchor winding.

Write comments, additions to the article, maybe I missed something. Take a look at, I will be glad if you find something else useful on my site. All the best.

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

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

DC motor design and maintenance

The main winding of the DC motor is anchor connecting to the power supply via brush apparatus... The armature rotates in a 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 is installed fan cooling, which drives the air flow through the internal cavities of the engine during its operation.

The brush set 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 constant effort. In the process of 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 motor also depend on the position of the brushes in space in the plane of rotation of the armature.

Over time, the brushes will wear out and be replaced. The collector at the points of contact with the brushes is also abraded. Periodically dismantle the anchor and grind the collector for lathe... After piercing, the insulation between the collector lamellas is cut to a certain depth, since it is stronger than the collector material and will destroy the brushes with further development.

DC motor switching circuits

Excitation windings - distinctive feature DC machines. The electrical and mechanical properties of the electric motor depend on the way they are connected to the network.

Independent excitement

The excitation winding is connected to an independent source. The performance of the motor is the same as that of a permanent magnet motor. The rotation speed is controlled by the resistance in the armature circuit. It is also regulated by a rheostat (control resistance) in the excitation winding circuit, but with an excessive decrease in its value or with a break, the armature current increases to dangerous values. Separately excited motors must not be started at idle speed or with light load on the shaft. The rotation speed will increase dramatically and the engine will be damaged.

The rest of the circuits are called self-excitation circuits.

Parallel excitation

The rotor and field windings are connected in parallel to the same power supply. With this connection, the current through the field winding is several times less than through the rotor. The characteristics of electric motors are tough, allowing them to be used to drive machines and fans.

Rotation speed control is provided by connecting rheostats to the rotor circuit or in series with the excitation winding.

Sequential excitement

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

Mixed excitement

In this scheme, two field windings are used, located in pairs at each of the poles of the electric motor. They can be connected so that their flows are either added or subtracted. As a result, the motor can have the characteristics of a series or parallel excitation circuit.

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

As in the case of a generator, the windings of the inductor and the armature of the motor can be connected either in series (Fig. 339) or in parallel (Fig. 340). In the first case, the motor is called a series-excited motor (or serial motor), in the second, a parallel-excited motor (or shunt motor). Motors with mixed excitation (compound motors) are also used, in which part of the inductor windings are connected in series with the armature, and part in parallel. Each of these types of engines has its own characteristics that make its use advisable in some cases and impractical in others.

1. Motors with parallel excitation. The circuit for connecting motors of this type to the network is shown in Fig. 361. Since here the armature and inductor circuits are independent of each other, the current in them can be controlled independently using separate rheostats included in these circuits. The rheostat included in the armature circuit is called the starting rheostat, and the rheostat included in the inductor circuit is called the control rheostat. When starting the engine with parallel excitation, the starting rheostat must be fully switched on; as the engine picks up speed, the resistance of the rheostat is gradually reduced and when the normal speed is reached, this rheostat is completely removed from the circuit. Motors with parallel excitation, especially of significant power, must in no case be switched on without a starting rheostat. In the same way, when you turn off the engine, you must first gradually introduce the rheostat and only then turn off the switch that connects the engine to the network.

Rice. 361. Scheme of switching on the motor with parallel excitation. The brass arc 1, along which the lever of the starting rheostat moves, is connected through the clamp 2 to the end of the adjusting rheostat, and through the clamp 3 to the starting rheostat. This is done so that when the starting rheostat is switched to idle contact 4 and the current is turned off, the excitation circuit does not break.

It is not difficult to understand the considerations that give rise to these rules for turning motors on and off. We have seen (see formula (172.1)) that the current in the armature

,

where is the mains voltage, and - e. d. s., induced in the armature windings. At the first moment, when the engine has not yet had time to spin up and gain sufficient speed, e. etc. with. is very small and the current through the armature is approximately equal to

The armature resistance is usually very low. It is calculated so that the voltage drop across the armature does not exceed 5-10% of the mains voltage for which the motor is designed. Therefore, in the absence of a starting rheostat, the current in the first seconds could be 10-20 times higher than the normal current for which the motor is designed at full load, and this is very dangerous for it. With the introduced starting rheostat with resistance starting current through anchor

. (173.1)

The resistance of the starting rheostat is selected so that the starting current exceeds the normal one by no more than 1.5-2 times.

Let us explain what has been said numerical example... Suppose that we have a 1.2 kW motor, rated for 120 V and having an armature resistance. Armature current at full load

.

If we connected this motor to the network without a starting rheostat, then in the first seconds the starting current through the armature would have a value

,

10 times the normal operating current in the armature. If we want the starting current to exceed the normal one by no more than 2 times, that is, it was equal to 20 A, then we must choose the starting resistance so that the equality takes place

,

where ohm comes from.

It is also clear that for a shunt motor it is very dangerous to suddenly stop it without turning it off, for example, due to a sharp increase in the load, since in this case e. etc. with. drops to zero and the current in the armature increases so much that the excess of Joule heat released in it can lead to melting of the insulation or even the winding wires themselves (the motor "burns out").

An adjusting rheostat included in the inductor circuit serves to change the engine speed. By increasing or decreasing the resistance of the inductor circuit using this rheostat, we change the current in the inductor circuit, and thereby the magnetic field in which the armature rotates. We saw above that for a given load of the motor, the current in it is automatically set such that the resulting torque balances the braking torque created by the load of the motor. This is due to the fact that the induced e. etc. with. reaches the corresponding value. But induced e. etc. with. is determined, on the one hand, by magnetic induction, and on the other, by the frequency of rotation of the armature.

The greater the magnetic flux of the inductor, the lower the engine speed must be in order to obtain a certain value of e. etc., and, conversely, the weaker the magnetic flux, the higher the rotation frequency should be. Therefore, in order to increase the rotational speed of the shunt motor at a given load, it is necessary to weaken the magnetic flux in the inductor, i.e. introduce more resistance into the inductor circuit using an adjusting rheostat. On the contrary, in order to reduce the rotational speed of the shunt motor, it is necessary to increase the magnetic flux in the inductor, that is, to reduce the resistance in the inductor circuit, bringing out an adjusting rheostat.

With the help of an adjusting rheostat, it is possible to set the normal engine speed at normal voltage and no load. With an increase in the load, the current in the armature should increase, and the e. etc. with. - decrease. This is due to a slight decrease in the frequency of rotation of the armature. However, the decrease in speed due to an increase in the load from zero to normal engine power is usually very small and does not exceed 5-10% of the normal engine speed. This is mainly due to the fact that in motors with parallel excitation, the current in the inductor does not change when the current in the armature changes. If, with changes in the load, we wanted to maintain the same speed, then this could be done by slightly changing the current in the inductor circuit with the help of an adjusting rheostat.

Thus, from an operational point of view, DC motors with parallel excitation (shunt motors) are characterized by the following two properties: a) their rotational speed remains almost constant when the load changes; b) the frequency of their rotation can be changed over a wide range using an adjusting rheostat. Therefore, such motors are quite widely used in industry where both of these features are important, for example, for driving lathes and other machines, the speed of which should not be strongly dependent on the load.

173.1. In fig. 362 shows a diagram of a shunt motor with a so-called combined starting and adjusting rheostat. Understand this circuit and explain what role the individual parts of this rheostat play.

Rice. 362. To exercise 173.1

173.2. The shunt motor needs to be put in motion. For this, two rheostats are given: one of a thick wire with low resistance, the other of a thin wire with high resistance. Which of these rheostats should be included as a trigger and which one as an adjustment one? Why?

2. Motors with series excitation. The circuit for connecting these motors to the network is shown in Fig. 363. Here the armature current is at the same time the current of the inductor, and therefore the starting rheostat changes both the current in the armature and the current in the inductor. At no-load or very low loads, the current in the armature, as we know, must be very small, that is, the induced emf. etc. with. should be almost equal to the mains voltage. But with a very small current through the armature and the inductor, the field of the inductor is also weak. Therefore, at a low load, the required e. etc. with. can only be obtained through a very high engine speed. As a consequence, at very low currents (light load), the speed of the series-excited motor becomes so high that it can become dangerous from the point of view of the mechanical strength of the motor.

Rice. 363. Scheme of switching on the motor with sequential excitation

The engine is said to be "racing". This is unacceptable and therefore series-excited motors must not be started up without load or at light load (less than 20-25% of the normal motor power). For the same reason, it is not recommended to connect these motors to machine tools or other machines with belt or cable drives, as a break or accidental release of the belt will lead to "runaway" of the motor. Thus, in motors with series excitation, when the load increases, the current in the armature and the magnetic field of the inductor increase; therefore, the engine speed drops sharply, and the torque it develops sharply increases.

These properties of motors with sequential excitation make them the most convenient for use in transport (trams, trolleybuses, electric trains) and in lifting devices (cranes), since in these cases it is necessary to have large torques at the moment of starting at a very high load at low speeds , and at lower loads (at normal stroke) lower torques and higher frequencies.

The regulation of the speed of the motor with series excitation is usually carried out by an adjusting rheostat connected in parallel with the inductor windings (Fig. 364). The lower the resistance of this rheostat, the greater part of the armature current is branched into it and the less current flows through the inductor windings. But with a decrease in the current in the inductor, the motor speed increases, and with its increase, it decreases. Therefore, in contrast to what was the case for the shunt motor, in order to increase the rotational speed of the serial motor, it is necessary to reduce the resistance of the inductor circuit by bringing out the adjusting rheostat. In order to reduce the rotational speed of a serial motor, it is necessary to increase the resistance of the inductor circuit by introducing an adjusting rheostat.

Rice. 364. Scheme for switching on a rheostat for regulating the speed of a serial motor

173.3. Explain why a serial motor cannot be started up with no load or with a low load, but a shunt motor can.

Table 8. Advantages, disadvantages and fields of application of engines different types

In conclusion, we will compare in the form of a table. 8 main advantages and disadvantages of various types of electric motors discussed by us in this chapter, and their areas of application.

The current flowing in the excitation winding of the main poles creates a magnetic flux. Electric cars direct current should be distinguished by the method of excitation and the circuit for switching on the excitation winding.

DC generators can be performed with independent, parallel, series and mixed excitation. It should be noted that the use of DC generators as energy sources is now very limited.

Excitation winding DC generator with independent excitation receives power from an independent source - a direct current network, a special exciter, a converter, etc. (Fig. 1, a). These generators are used in powerful systems when the excitation voltage must be selected different from the generator voltage, in systems that are powered by generators and other sources.

The value of the excitation current of powerful generators is 1.0-1.5% of the generator current and up to tens of percent for machines with a capacity of the order of tens of watts.

Rice. 1. Circuits of DC generators: a - with independent excitation; b - with parallel excitation; c - with sequential excitement; d - with mixed excitation P - consumers

Have G generator with parallel excitation the excitation winding is connected to the voltage of the generator itself (see Fig. 1, b). The armature current I I is equal to the sum of the load currents I p and the excitation current I in: I I = I p + I in

Generators are usually made for medium powers.

Excitation winding series-excited generator connected in series to the armature circuit and flowed around by the armature current (Fig. 1, c). The process of self-excitation of the generator is very rapid. Such generators are practically not used. At the very beginning of the development of the energy sector, power transmission system with series-connected generators and sequential excitation motors.

The generator with mixed excitation has two excitation windings - parallel ORP and series ORP usually with a consonant inclusion (Fig. 1, d). The parallel winding can be connected before the series winding ("short shunt") or after it ("long shunt"). The MDS of a series winding is usually small and is designed only to compensate for the voltage drop in the armature under load. Such generators are now also practically not used.

The excitation circuits for DC motors are similar to those for generators. high power are usually performed independently excited... With parallel field motors, the field winding is powered from the same power source as the motor. The excitation winding is connected directly to the voltage of the energy source so that the influence of the voltage drop in the starting resistance is not affected (Fig. 2).

Rice. 2. Diagram of a DC motor with parallel excitation

Mains current Ic is made up of the armature current I I and the excitation current I in.

Sequential excitation motor circuit is similar to the diagram in Fig. 1, c. Due to the series winding, the torque under load increases more than with parallel excitation motors, while the rotation speed is reduced. This property of motors determines their widespread use in electric locomotive traction drives: in main-line electric locomotives, urban transport, etc. Voltage drop in the excitation winding at rated current is a few percent of the rated voltage.

Mixed excitation motors due to the presence of a series winding, to some extent have the properties of series excitation motors. Currently, they are practically not used. Parallel excitation motors sometimes they are made with a stabilizing (series) winding, connected in accordance with the parallel field winding, to ensure more quiet operation at peak loads. The MDS of such a stabilizing winding is small - a few percent of the main MDS.