In this motor, the field winding is connected in series to the armature circuit (Fig. 29.9, A), That's why magnetic fluxF it depends on the load current I = I a = I in . At small 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 = k Ф I a (k f- proportionality coefficient). In this case, we find the electromagnetic moment:
The rotation speed formula will take the form
. (29.15)
In Fig. 29.9, b performance characteristics presented M = F(I) And n= (I) series excitation motor. At heavy loads, the motor magnetic system becomes saturated. In this case, the magnetic flux practically does not change with increasing load and the characteristics of the motor become almost linear. The speed characteristic of a series-excited motor shows that the motor speed changes significantly with load changes. This characteristic is usually called soft.
Rice. 29.9. Series motor:
A- schematic diagram; b- performance characteristics; c - mechanical characteristics; 1 - natural characteristic; 2 - artificial characteristic
When the load on a series-excited motor decreases, the rotation speed increases sharply and, at a load less than 25% of the rated load, can reach values dangerous for the motor (“overrun”). Therefore, operating a series-excited motor or starting it with a shaft load less than 25% of the rated one is unacceptable.
For more reliable operation The shaft of the sequential excitation motor must be rigidly connected to the working mechanism through a coupling and gear drive. The use of a belt drive is unacceptable, since if the belt breaks or resets, the engine may “crawl.” Taking into account the possibility of engine operation at high rotation speeds, series-excited motors, according to GOST, are tested for 2 minutes to exceed the rotation speed by 20% above the maximum indicated on the nameplate, but not less than 50% above the nominal.
Mechanical characteristics of a series motor n=f(M) are presented in Fig. 29.9, V. Sharply falling mechanical characteristics curves ( natural 1 and artificial 2 ) provide the sequential excitation motor with stable operation under any mechanical load. The ability of these motors to develop high torque, proportional to the square of the load current, is important, especially under severe starting conditions and overloads, since with a gradual increase in the motor load, the power at its input grows more slowly than the torque. This feature of series excitation motors is one of the reasons for their wide application as traction motors in transport, as well as crane motors in lifting installations, i.e. in all cases of electric drive with difficult starting conditions and a combination of significant loads on the motor shaft with low rotation speed.
Nominal speed change of series-excited motor
, (29.16)
Where n - rotation speed at an engine load of 25% of the nominal.
The rotation speed of series-excited motors can be adjusted by changing either voltage U, or magnetic flux of the field winding. In the first case, an adjusting control is connected in series to the armature circuit rheostat R r (Fig. 29.10, A). As the resistance of this rheostat increases, the voltage at the motor input and its rotational speed decrease. This control method is used mainly in low-power engines. In the case of significant engine power, this method is uneconomical due to large energy losses in R rg . Besides, rheostat R r , calculated on the operating current of the motor, it turns out to be bulky and expensive.
When several engines of the same type operate together, the rotation speed is adjusted by changing their switching pattern relative to each other (Fig. 29.10, b). Thus, when the engines are connected in parallel, each of them is under full mains voltage, and when sequential connection two motors, each motor accounts for half the mains voltage. When working simultaneously more engines possible large quantity inclusion options. This method of speed control is used in electric locomotives, where several identical traction motors are installed.
Changing the voltage supplied to the motor is possible when the engine is powered from a source direct current with adjustable voltage (for example, according to a circuit similar to Fig. 29.6, A). As the voltage supplied to the motor decreases, its mechanical characteristics shift downward, practically without changing its curvature (Fig. 29.11).
Rice. 29.11. Mechanical characteristics of a series excitation motor when the input voltage changes
You can regulate the engine speed by changing the magnetic flux in three ways: bypassing the field winding with a rheostat r rg , sectioning the field winding and shunting the armature winding with a rheostat r w . Turning on the rheostat r rg , shunting the excitation winding (Fig. 29.10, V), as well as a decrease in the resistance of this rheostat leads to a decrease in the excitation current I in = I a - I рг , and consequently, to an increase in rotation speed. This method is more economical than the previous one (see Fig. 29.10, A), is used more often and is assessed by the regulation coefficient
.
Typically the rheostat resistance r rg is accepted so that k рг >= 50% .
When sectioning the field winding (Fig. 29.10, G) disconnection of part of the winding turns is accompanied by an increase in rotation speed. When shunting the armature winding with a rheostat r w (see Fig. 29.10, V) excitation current increases I in = I a +I рг , which causes a decrease in rotation speed. This method of regulation, although it provides deep regulation, is uneconomical and is used very rarely.
Rice. 29.10. Regulating the rotation speed of series-excited motors
[document]1.doc
Homework No. 2(module 5)
“DC motor with series excitation. Purpose of elements. Principle of operation"
gr.TP-07
Asmolkova O. A.
I semester 2009
DC motor with series excitation. Purpose of elements. Principle of operation
1. Design and purpose of DC motor elements
.
DC motor - electric car , DC machine, converting direct current electrical energy into mechanical energy. It consists, like all direct current machines, of a stationary stator with poles and a rotating rotor (armature) with a commutator.
Stator A DC machine consists of a cylindrical frame (housing), poles with an excitation winding and bearing shields ( rice. 2.1.). The main (main) poles are strengthened on the frame to excite the main magnetic flux and additional ones to improve commutation in the motor. The main pole consists of a pole core made of sheet steel and bolted to the frame, and an excitation winding coil. The core at the free end is equipped with a pole piece to create the required distribution of magnetic induction along the armature circumference. bed 3 is the yoke of the machine, that is, the part that closes the magnetic circuit of the main flux F. It is made of cast steel, since the magnetic flux in it is relatively constant. Additional poles are installed on the frame between the main ones. Their winding is connected in series with the armature winding. The purpose of these poles is to create an additional magnetic field. This is necessary to ensure that the brushes on the commutator do not spark.
Anchor (rotor) is the part of the machine in the winding of which, when it rotates relative to the main magnetic field EMF is induced. Anchor 5 A DC motor consists of a steel shaft, a steel toothed core, a winding laid in its slots, and a commutator mounted on the armature shaft ( rice. 2.1.). Field windings are necessary to ensure optimal interaction between the magnetic fields of the rotor and stator (i.e., to create maximum torque on the rotor). A characteristic part of the engine (or any electric machine) DC is the collector. This is a hollow cylinder assembled from wedge-shaped copper plates isolated from each other. The commutator plates are also isolated from the engine shaft. They are connected by conductors to the winding threads located in the grooves of the armature. The rotating winding is connected to the external circuit by sliding contact between the brushes and the commutator. The collector in DC machines serves to rectify the alternating EMF induced in the rotating armature winding and to obtain a constant in direction electromagnetic moment.
Rice. 2. 1. DC motor design:
1 - excitation winding;2 - poles;3 - bed;4 - pole piece;5 - anchor;6 - conductors of the armature winding;
7 - gear armature core;8 - air gap of the machine
2. Working principle of DC motor
2.1 General information
When the armature winding rotates in a stationary magnetic field, an alternating EMF is induced in it, changing with frequency:
Where n- speed of rotation of the armature.
When the armature rotates, a variable emf acts between any two points of the armature winding. However, between the stationary contact brushes there is a constant EMF in magnitude and direction E, equal to the sum instantaneous EMF values induced in all series-connected armature turns located between these brushes.
EMF dependence E from the magnetic flux of the machine and the rotation speed of the armature has the form:
When connecting the armature winding to a network with voltage U, emf E will be approximately equal to the voltage U, and rotor speed:
Consequently, due to the presence of a collector, when a DC machine operates in motor mode, the rotor speed is not strictly related to the network frequency, but can be varied within wide limits by changing the voltage U and magnetic flux F. The axis of symmetry separating the poles of a direct current machine is called its geometric neutral.
When the external circuit is open, the current will not flow in the armature winding, since the EMF induced in two parts of the armature winding, located on both sides of the geometric neutral, are directed counter and are mutually compensated. In order to supply the maximum voltage from the armature winding to the external circuit, this circuit must be connected to two points of the armature winding, between which there is the greatest potential difference, where the brushes should be installed. When the armature rotates, the points shift from the geometric neutral, but more and more new winding points will approach the brushes, between which the EMF acts E, therefore the EMF in the external circuit will be unchanged in magnitude and direction. To reduce EMF pulsations when brushes move from one commutator plate to another, at least 16 active conductors are usually connected to each parallel branch of the armature winding.
On the armature, through the winding of which current flows I, the electromagnetic moment acts:
When the machine operates in motor mode, the electromagnetic torque is rotating.
2.2 DC motor armature reaction
At idling magnetic flux in the motor is created only by NS ^F into the field windings. In this case, the magnetic flux F V with a constant air gap between the armature and the core of the main pole (which is typical for many DC machines), it is distributed symmetrically relative to the longitudinal axis of the machines.
When the machine operates under load, current flows through the armature winding, and the armature NS creates its own magnetic field. The effect of the armature field on the magnetic field of the machine is called anchor reaction. Magnetic flux F aq, created by NS anchors F aq in a two-pole machine, when the brushes are installed on the neutral, it is directed along the transverse axis of the machine, therefore the magnetic field of the armature is called transverse. As a result of the flow F aq the symmetrical distribution of the machine's magnetic field is distorted, and the resulting flux F res turns out to be concentrated mainly at the edges of the main poles. At the same time, the physical neutral b-b(the line connecting the points of the armature circle at which the induction is zero) is displaced relative to the geometric neutral a-a at some angle β (Fig.2.2). In engines, the physical neutral is shifted against the direction of rotation.
Based on the law total current The armature NS acting in the air gap at a distance x from the axis of the main poles is determined by the expression:
Therefore, NS anchors F aq varies linearly along its circumference; below the middle of the main pole it is zero, and at the points where the brushes are installed it has a maximum value. Magnetic induction in air
^ Fig2.2 - Magnetic field of a DC motor: a) from the field winding; b) from the armature winding; c) resultant (F V - magnetic flux at idle; F aq - magnetic flux created by the NS armature; F res - resulting flow; a-a - geometric neutral; b-b - physical neutral; β – neutral displacement angle b-b)
Gap with an unsaturated magnetic system:
Where is the size of the air gap at point x.
2.3 DC motor torque
If the field winding and motor armature are connected to a DC network with voltage ^U then an electromagnetic torque occurs M Em. Net torque M on the motor shaft is less than the electromagnetic one by the value of the counteracting torque created in the machine by friction forces and equal to the torque M X in x.x. mode, i.e. M = M Em -M X .
The starting torque of the motor must be greater than the static braking torque M t when the rotor is at rest, otherwise the motor armature will not begin to rotate. In steady state (at n = const) there is an equilibrium of the rotating M and braking M t moments:
M = M Em – M X = M T
From mechanics it is known that mechanical power motor can be expressed in terms of torque and angular velocity
Therefore, the useful engine torque is ^M(N m), expressed in terms of useful power R(kW) and rotational speed n(rpm),
M =9550P/n
Let's discuss some important questions starting and operating DC motors. From Eq. electrical state engine it follows that
I I = (U -- E)/R I
In operating mode, the armature current I I is limited by e. d.s. E, if n is approximately equal n nom. At the moment of start-up n = 0, e. d.s. E = 0 And starting current I P = U/ R I 10-30 times more than nominal. Therefore, direct starting of the engine, i.e. direct connection of the armature to the mains voltage, is unacceptable. To limit the large starting current of the armature, before starting, a starting rheostat is turned on in series with the armature. R P with little resistance. In this case, when E = O
I P =U/(R I –R P ) << U/R I
Rheostat resistance RP selected according to the permissible armature current.
As the engine accelerates to rated speed, e. d.s. E increases, and the current decreases and the starting rheostat is gradually and completely removed (starting rheostats are designed for short-term activation). Regulating rheostat R reg in the excitation circuit with a relatively high resistance (tens and hundreds of Ohms) before starting the engine, it is completely removed so that when starting, the excitation current and stator magnetic flux F were nominal. This leads to an increase in starting torque, which ensures quick and easy engine acceleration.
After start-up and acceleration, a steady state of engine operation occurs, in which the braking torque on the shaft ^ Mt will be balanced by the torque developed by the engine M Em , i.e. M Em == M T ( at n = withnst. )
DC electric motors can restore the steady state of operation disrupted by a change in braking torque, i.e. they can develop torque M, equal to the new braking torque value M T at a correspondingly new speed n".
Indeed, if the load braking torque Mt turns out to be greater than the engine torque M Em, then the armature rotation speed will decrease. At constant voltage U and flow F this will cause a decrease in e. d.s. E armature, increasing the armature current and torque until equilibrium occurs, at which M Em = M T And n" < n. When the braking torque is reduced to Mt, a steady state of operation occurs in the same way. M Em = M T" And n"> n" . Thus, DC motors have the property of self-regulation - can develop a torque equal to the braking torque.
2.4 Frequency regulation
The armature rotation speed of a DC motor is determined based on the electrical equation of state U= ER I I I after substituting e into it. d.s. E = sFn:
Armature voltage drop R I I I small: at rated load it does not exceed 0,03 - 0,07 U nom .
Thus, the rotational speed of a DC motor is directly proportional to the applied line voltage and inversely proportional to the stator magnetic flux . You can regulate the engine speed in two ways: by changing the stator flux F or the voltage U supplied to the engine. The rotation speed is controlled by changing the magnetic field of the machine using an adjusting rheostat in the engine excitation circuit. The voltage supplied to the motor is changed by regulating the source voltage.
You can introduce an additional rheostat into the armature circuit. In this case, the starting rheostat is replaced by a ballast R etc Such a rheostat performs the functions of both a starting rheostat and a control rheostat. The equation for the rotation frequency of the DC motor armature in this case has the form
It follows that the engine speed can be controlled by changing the mains voltage, the resistance of the ballast rheostat or the stator flux.
Reversing engines. From the engine torque equation M Em = kFI I it follows that reversal, i.e. changing the direction of rotation of the armature, can be carried out by changing the direction of the current in the field winding (flux F) or armature current.
To reverse the motor “on the fly,” the direction of the armature current is changed (by switching the armature terminals), but the field winding is not switched, since it has a high inductance and breaking its circuit with current is unacceptable. Reversing a switched-off motor is also accomplished by changing the direction of the current in the field winding (switching its terminals).
3. Series-wound motor
In a motor with sequential excitation ( Fig.2.3a) the excitation current is equal to the armature current: I V =I A, therefore magnetic flux Ф is a function of load current I A. The nature of this function varies depending on the load size. At I a <(0,8...0,9) I nom, when the magnetic system is unsaturated, F=k f I A, and the proportionality coefficient TO f remains practically constant over a wide range of loads. With a further increase in load, the flow F growing slower than I a >I nom) we can assume that Ф=const. In accordance with this, dependencies also change n=f(I a ), M=f(I a) (rice. 2.3.b).
Rice. 2.3. - a) circuit of a motor with sequential excitation; b) the dependence of its torque and rotation speed on the armature current (I I – armature current; I V – excitation current;r n
– load resistance;
n- rotational speed; 1 – natural characteristic; 2.3 - rheostatic characteristics corresponding different meanings additional resistance r n ).
In addition to natural characteristics 1, it is possible, by including additional resistances r n in the armature circuit, to obtain a family of rheostatic characteristics 2, 3, and 4. The larger the value of r n, the lower the characteristic is located.
At low loads, the speed n increases sharply and can exceed the maximum permissible value(the engine goes into overdrive). Therefore, such engines cannot be used to drive mechanisms operating in idle mode and at light load.
With a rigid characteristic, the rotation speed n is almost independent of the torque M, so the power is:
, Where WITH 4
- constant.
At soft characteristic engine n is inversely proportional to , as a result of which:
, where is a constant.
Therefore, when the load torque changes over a wide range, the power R 2 , and, therefore, power R 1 and current I a change in motors with series excitation within smaller limits than in motors with parallel excitation, in addition, they tolerate overloads better.
They are determined mainly by the way the excitation winding is turned on. Depending on this, electric motors are distinguished:
1. With independent excitation : the field winding is powered by an external DC source (exciter or rectifier),
2. with parallel excitation: the field winding is connected in parallel with the armature winding,
3.: the field winding is connected in series with the armature winding,
4. with mixed excitement: It has two field windings, one connected in parallel with the armature winding and the other in series with it.
All these electric motors have the same design and differ only in the design of the field winding. The excitation windings of these electric motors are made in the same way as those.
Electric motor direct current with independent excitation
In this electric motor (Fig. 1, a), the armature winding is connected to the main source of direct current (DC network, generator or rectifier) with voltage U, and the field winding is connected to an auxiliary source with voltage UB. An adjusting rheostat Rрв is included in the field winding circuit, and a starting rheostat Rn is included in the armature winding circuit.
The control rheostat is used to regulate the rotation speed of the motor armature, and the starting rheostat is used to limit the current in the armature winding during startup. Characteristic feature electric motor is that its excitation current Iв does not depend on the current Iа in the armature winding (load current). Therefore, neglecting the demagnetizing effect of the armature reaction, we can approximately assume that the motor flux F does not depend on the load. The dependences of the electromagnetic torque M and rotational speed n on the current Iа will be linear (Fig. 2, a). Consequently, the mechanical characteristic of the engine will also be linear - the dependence n (M) (Fig. 2, b).
If there is no rheostat with resistance Rn in the armature circuit, the speed and mechanical characteristics will be rigid, i.e., with a small angle of inclination to the horizontal axis, since the voltage drop IаΣRя in the machine windings included in the armature circuit at rated load is only 3-5 % of Unom. These characteristics (straight lines 1 in Fig. 2, a and b) are called natural. When a rheostat with resistance Rn is included in the armature circuit, the slope angle of these characteristics increases, as a result of which it is possible to obtain a family of rheostatic characteristics 2, 3 and 4, corresponding to different values of Rn1, Rn2 and Rn3.
Rice. 1. Schematic diagrams of electric motors direct current with independent (a) and parallel (b) excitation
Rice. 2. Characteristics of electric motors direct current with independent and parallel excitation: a - high-speed and torque, b - mechanical, c - working more resistance Rn, the greater the angle of inclination the rheostatic characteristic has, i.e., the softer it is.
The adjusting rheostat Rpv allows you to change the motor excitation current Iv and its magnetic flux F. In this case, the rotation speed n will also change.
No switches or fuses are installed in the field winding circuit, since when this circuit is broken, the magnetic flux of the electric motor sharply decreases (only the flux from residual magnetism remains in it) and a emergency mode. If the electric motor is running at idle or with a small load on the shaft, the rotation speed increases sharply (the engine starts to spin). In this case, the current in the armature winding Iа greatly increases and a circular fire may occur. To avoid this, the protection must disconnect the motor from the power source.
A sharp increase in rotation speed when the excitation winding circuit is broken is explained by the fact that in this case the magnetic flux F (to the value of flux Fost from residual magnetism) and e decrease sharply. d.s. E and the current Iа increases. And since the applied voltage U remains unchanged, the rotation speed n will increase until e. d.s. E will not reach a value approximately equal to U (which is necessary for an equilibrium state electrical circuit anchor, at which E= U - IаΣRя.
When the load on the shaft is close to the rated one, the electric motor will stop if the excitation circuit breaks, since the electromagnetic torque that the motor can develop with a significant decrease in the magnetic flux decreases and becomes less than the load torque on the shaft. In this case, the current Iа also sharply increases, and the machine must be disconnected from the power source.
It should be noted that the rotation speed n0 corresponds to ideal idle speed, when the engine does not consume power from the network electrical energy and its electromagnetic moment is zero. In real conditions, in idling mode, the engine consumes the idling current I0 from the network, which is necessary for compensation internal losses power, and develops a certain torque M0 required to overcome the friction forces in the machine. Therefore, in reality, the idle speed is less than n0.
The dependence of the rotation speed n and the electromagnetic torque M on the power P2 (Fig. 2, c) on the motor shaft, as follows from the relationships considered, is linear. The dependences of the armature winding current Iya and power P1 on P2 are also almost linear. The current Iya and power P1 at P2 = 0 represent the no-load current I0 and the power P0 consumed during no-load. The efficiency curve has a character common to all electric machines.
Electric motor direct current with parallel excitation
In this electric motor (see Fig. 1, b), the excitation and armature windings are powered by the same source of electrical energy with voltage U. A control rheostat Rpv is included in the excitation winding circuit, and a starting rheostat Rp is included in the armature winding circuit.
In the electric motor under consideration, there is essentially separate power supply to the armature and excitation winding circuits, as a result of which the excitation current Iв does not depend on the armature winding current Iв. Therefore, a shunt-excited electric motor will have the same characteristics as a separately excited motor. However, a shunt-wound motor operates normally only when supplied from a constant voltage DC source.
When the electric motor is powered from a source with a variable voltage (generator or controlled rectifier), a decrease in the supply voltage U causes a corresponding decrease in the excitation current Iв and magnetic flux Ф, which leads to an increase in the armature winding current Iа. This limits the ability to regulate the armature rotation speed by changing the supply voltage U. Therefore, electric motors intended to be powered by a generator or controlled rectifier must have independent excitation.
Electric motor direct current with sequential excitation
To limit the current at start-up, a starting rheostat Rп is connected to the armature winding circuit (Fig. 3, a), and to regulate the rotation speed, an adjusting rheostat Rрв can be connected in parallel to the excitation winding.
Rice. 3. Schematic diagram electric motor direct current with sequential excitation (a) and the dependence of its magnetic flux Ф on the current Iа in the armature winding (b)
Rice. 4. Motor characteristics direct current with sequential excitation: a - high-speed and torque, b - mechanical, c - working.
A characteristic feature of this electric motor is that its excitation current Iв is equal to or proportional (when the rheostat Rpв is turned on) to the armature winding current Iа, therefore the magnetic flux Ф depends on the motor load (Fig. 3, b).
When the armature winding current Iya is less (0.8-0.9) rated current Inom, the magnetic system of the machine is not saturated and we can assume that the magnetic flux Ф changes in direct proportion to the current Iа. Therefore, the speed characteristic of the electric motor will be soft - with an increase in current I, the rotation speed n will sharply decrease (Fig. 4, a). The decrease in rotation speed n occurs due to an increase in the voltage drop IаΣRа. in internal resistance Rya. armature winding circuit, as well as due to an increase in magnetic flux F.
The electromagnetic moment M will increase sharply with increasing current Iа, since in this case the magnetic flux Ф also increases, i.e. the moment M will be proportional to the current Iа. Therefore, at a current Iya less than (0.8 N - 0.9) Inom, the speed characteristic has the shape of a hyperbola, and the torque characteristic has the shape of a parabola.
At currents Iа > Inom, the dependences of M and n on Iа are linear, since in this mode the magnetic circuit will be saturated and the magnetic flux Ф will not change when the current Iа changes.
The mechanical characteristic, i.e., the dependence of n on M (Fig. 4, b), can be constructed on the basis of the dependences of n and M on Iа. In addition to natural characteristic 1, it is possible, by including a rheostat with resistance Rp in the armature winding circuit, to obtain a family of rheostatic characteristics 2, 3 and 4. These characteristics correspond to different values of Rn1, Rn2 and Rn3, and the greater Rn, the lower the characteristic is located.
The mechanical characteristics of the engine in question are soft and hyperbolic in nature. At low loads, the magnetic flux Ф decreases greatly, the rotation speed n increases sharply and can exceed the maximum permissible value (the engine starts to spin). Therefore, such motors cannot be used to drive mechanisms operating in idle mode and at low load (various machines, conveyors, etc.).
Usually minimal permissible load for engines large and medium power is (0.2 .... 0.25) Inom. To prevent the engine from running without load, it is connected rigidly to the drive mechanism ( gear transmission or blind clutch), the use of a belt drive or friction clutch is unacceptable.
Despite this disadvantage, motors with sequential excitation are widely used, especially where load torque changes within wide limits and difficult conditions start-up: in all traction drives (electric locomotives, diesel locomotives, electric trains, electric cars, electric forklifts, etc.), as well as in drives of lifting mechanisms (cranes, elevators, etc.).
This is explained by the fact that with a soft characteristic, an increase in load torque leads to a smaller increase in current and power consumption than in motors with independent and parallel excitation, therefore motors with series excitation are better able to withstand overloads. In addition, these motors have a higher starting torque than motors with parallel and independent excitation, since when the armature winding current increases during starting, the magnetic flux increases accordingly.
If we accept, for example, that the short-term starting current can be 2 times the rated operating current of the machine, and neglect the influence of saturation, the armature reaction and the voltage drop in the circuit of its winding, then in a motor with series excitation the starting torque will be 4 times the rated (in Both current and magnetic flux increase 2 times), and in motors with independent and parallel excitation - only 2 times more.
In fact, due to the saturation of the magnetic circuit, the magnetic flux does not increase in proportion to the current, but still the starting torque of a motor with series excitation, all other things being equal, will be significantly greater starting torque the same motor with independent or parallel excitation.
The dependences of n and M on the power P2 on the electric motor shaft (Fig. 4, c), as follows from the provisions discussed above, are nonlinear; the dependences of P1, Iа and η on P2 have the same shape as for motors with parallel excitation.
Electric motor direct current with mixed excitement
In this electric motor (Fig. 5, a) the magnetic flux Ф is created as a result of the combined action of two excitation windings - parallel (or independent) and series, through which excitation currents Iв1 and Iв2 = Iя pass
That's why
where Fposl is the magnetic flux of the series winding, depending on the current Iya, Fpar is the magnetic flux of the parallel winding, which does not depend on the load (determined by the excitation current Ib1).
The mechanical characteristics of an electric motor with mixed excitation (Fig. 5, b) are located between the characteristics of motors with parallel (straight line 1) and series (curve 2) excitation. Depending on the ratio of the magnetomotive forces of the parallel and series windings in the nominal mode, it is possible to bring the characteristics of a motor with mixed excitation closer to characteristic 1 (curve 3 at low ppm of the series winding) or to characteristic 2 (curve 4 at low ppm). c. parallel winding).
Rice. 5. Schematic diagram of an electric motor with mixed excitation (a) and its mechanical characteristics (b)
Advantage of the engine direct current with mixed excitation is that it, having a soft mechanical characteristic, can operate at idle when Fseq = 0. In this mode, the rotation frequency of its armature is determined by the magnetic flux Fpar and has a limited value (the engine does not spin).