El direct current motors of series excitation. Scheme of inclusion, characteristics and modes of operation of the engine of sequential excitation. Task and methodology for performing work
A complete mechanical characteristic of a DC motor allows you to correctly determine the main properties of the electric motor, as well as control their compliance with all the requirements for today's machines or devices of a technological type.
Design features
Represented by rotating injection elements that are placed on the surface of a statically fixed frame. Devices of this type have been widely used and are used when it is necessary to provide a variety of high-speed control in conditions of stability of the rotational movements of the drive.
From a constructive point of view, all types of DPT are represented by:
- rotor or anchor part in the form of a large number of coil elements coated with a special conductive winding;
- a static inductor in the form of a standard frame, supplemented by several magnetic poles;
- a functional cylindrical brush collector, located on the shaft and having copper lamellar insulation;
- statically fixed contact brushes used to supply a sufficient amount of electric current to the rotor part.
As a rule, PT electric motors are equipped with special brushes of graphite and copper-graphite type. The rotational movements of the shaft provoke the closing and opening contact group and also contribute to sparking.
A certain amount of mechanical energy is supplied from the rotor part to other elements, which is due to the presence of a belt-type transmission.
Operating principle
Synchronous inverted functional devices are characterized by a change in the performance of tasks by the stator and rotor. The first element serves to excite the magnetic field, and the second in this case converts a sufficient amount of energy.
Anchor rotation in a magnetic field is induced using EMF, and the movement is directed in accordance with the right hand rule. A 180° turn is accompanied by a standard change in the EMF movement.
The principle of operation of the DC motor
The collectors are connected to two turns by means of a brush mechanism, which provokes the removal of pulsating voltage and causes the formation of constant current values, and the reduction of armature ripple is carried out by additional turns.
Mechanical characteristic
To date, PT electric motors of several categories are operated, having various types of excitation:
- independent type, in which the winding power is determined by an independent energy source;
- serial type, in which the armature winding is connected in series with the excitation winding element;
- parallel type, in which the rotor winding is connected in the electrical circuit in a direction parallel to the power source;
- mixed type, based on the presence of several series and parallel winding elements.
Mechanical characteristic of a DC motor of independent excitation DPT
Mechanical motor characteristics subdivided into indicators of natural and artificial species. The undeniable advantages of DPT are represented by increased performance and increased efficiency.
Due to the special mechanical characteristics of devices with constant current values, they are able to easily withstand negative external influences, which is explained by a closed case with sealing elements that absolutely exclude moisture from entering the structure.
Models of independent excitation
PT NV motors have winding excitation connected to a separate type of source for electrical power. In this case, the winding excitation circuit of the NV DCT is supplemented with a regulating type rheostat, and the anchor circuit is supplied with additional or starting rheostat elements.
A distinctive feature of this type of motor is the independence of the current excitation from the armature current, which is due to the independent power supply of the winding excitation.
Characteristics of electric motors with independent and parallel excitation
Linear mechanical characteristic with independent type of excitation:
- ω - indicators of rotational frequency;
- U - voltage indicators on the operated anchor chain;
- Ф - parameters of the magnetic flux;
- R I and R d - the level of anchor and additional resistance;
- Α - engine design constant.
This type of equation determines the dependence of the rotational speed of the motor on the moment of the shaft.
Series Excitation Models
DPT with PTV is an electrical type device with constant current values having an excitation winding connected in series to the armature winding. This type of engines is characterized by the validity of the following equality: the current flowing in the armature winding is equal to the current of the winding excitation, or I \u003d I in \u003d I i.
Mechanical characteristics with series and mixed excitation
When using serial excitation type:
- n 0 - indicators of the shaft speed in conditions idle move;
- Δ n - indicators of change in rotational speed under mechanical load conditions.
The displacement of mechanical characteristics along the y-axis allows them to remain in a completely parallel arrangement to each other, due to which the regulation of the rotational frequency with a change given voltage U, brought to the anchor chain, becomes the most favorable.
Mixed excitation models
Mixed excitation is characterized by an arrangement between the parameters of parallel and sequential excitation, which easily ensures the significance of the starting torque and completely excludes any possibility of "spacing" of the sliding mechanism in idling conditions.
Under conditions of mixed type of excitation:
Mixed excitation engine
Adjustment of the frequency of motor rotation in the presence of excitation of a mixed type is carried out by analogy with engines having parallel excitation, and varying the MDS windings contributes to obtaining almost any intermediate mechanical characteristic.
Mechanical characteristic equation
The most important mechanical characteristics of the DCT are presented by natural and artificial criteria, while the first option is comparable to the rated supply voltage in the absence of additional resistance on the motor winding circuits. Non-compliance with any of the specified conditions allows us to consider the characteristic as artificial.
ω \u003d U i / k Ф - (R i + R d) / (k Ф)
The same equation can be represented in the form ω = ω o.id. - Δω, where:
- ω o.id. \u003d U i / k F
- ω o.id - indicators of the angular velocity of the idle ideal stroke
- Δ ω = Mem. [(R i + R d) / (k Ф) 2] - a decrease in the angular velocity under the influence of a load on the motor shaft with a proportional resistance of the armature circuit
Characteristics of the equation mechanical type represented by standard stability, stiffness and linearity.
Conclusion
According to the applied mechanical characteristics, any DPT is distinguished by its design simplicity, accessibility and the ability to adjust the shaft speed, as well as the ease of starting the DPV. Among other things, such devices can be used as a generator and have compact dimensions, which well eliminates the disadvantages in the form of quickly worn out graphite brushes, high cost and the need to connect current rectifiers.
Related video
Creating a magnetic flux to generate a moment. The inductor must include either permanent magnets or excitation winding. The inductor can be part of both the rotor and the stator. In the engine shown in Fig. 1, the excitation system consists of two permanent magnets and is part of the stator.
Types of collector motors
According to the design of the stator, the collector motor can be and.
Scheme of a collector motor with permanent magnets
Collector motor direct current (KDPT) with permanent magnets is the most common among KDPT. This motor includes permanent magnets that create a magnetic field in the stator. Collector DC motors with permanent magnets (KDPT PM) are usually used in tasks that do not require high power. KDPT PM is cheaper to manufacture than collector motors with excitation windings. In this case, the moment of KDPT PM is limited by the field of permanent magnets of the stator. KDPT with permanent magnets responds very quickly to voltage changes. Due to the constant stator field, it is easy to control the speed of the motor. The disadvantage of a permanent magnet DC motor is that the magnets lose their strength over time. magnetic properties, resulting in a decrease in the stator field and reduced motor performance.
- Advantages:
- best value for money
- high moment on low revs
- fast response to voltage changes
- Disadvantages:
- permanent magnets lose their magnetic properties over time, as well as under the influence of high temperatures
Collector motor with excitation windings
- According to the stator winding connection scheme, collector electric motors with excitation windings are divided into motors:
Independent excitation scheme
Parallel Excitation Circuit
Series excitation circuit
Mixed excitation scheme
Engines independent And parallel excitation
In independent excitation motors, the field winding is not electrically connected to the winding (figure above). Usually, the excitation voltage U OB differs from the voltage in the armature circuit U. If the voltages are equal, then the excitation winding is connected in parallel with the armature winding. The use of independent or parallel excitation in the motor drive is determined by the drive circuit. The properties (characteristics) of these engines are the same.
In parallel excitation motors, the field winding (inductor) and armature currents are independent of each other, and the total motor current is equal to the sum of the field winding current and the armature current. During normal operation, with increasing voltage supply, the total current of the motor increases, which leads to an increase in the fields of the stator and rotor. With an increase in the total current of the motor, the speed also increases, and the torque decreases. When engine is loaded the armature current increases, resulting in an increase in the armature field. With an increase in armature current, the current of the inductor (field winding) decreases, as a result of which the field of the inductor decreases, which leads to a decrease in motor speed and an increase in torque.
- Advantages:
- almost constant torque at low speeds
- good control properties
- no loss of magnetism over time (since there are no permanent magnets)
- Disadvantages:
- more expensive than KDPT PM
- the motor goes out of control if the inductor current drops to zero
The collector electric motor of parallel excitation has a decreasing torque on high revs and high, but more constant torque at low revs. The current in the inductor and armature windings are independent of each other, so the total motor current is equal to the sum of the inductor and armature currents. As a result, this type of engine has excellent performance speed control. The parallel field DC commutator motor is commonly used in applications that require power greater than 3kW, such as automotive and industrial applications. Compared with , shunt motor does not lose its magnetic properties over time and is more reliable. The disadvantages of a parallel excitation motor are higher cost and the possibility of the motor getting out of control if the inductor current drops to zero, which in turn can lead to motor failure.
In series excitation electric motors, the field winding is connected in series with the armature winding, while the excitation current is equal to the armature current (I c \u003d I a), which gives the engines special properties. At low loads, when the armature current is less than the rated current (I a < I nom) and the motor magnetic system is not saturated (Ф ~ I a), the electromagnetic torque is proportional to the square of the current in the armature winding:
- where M – , N∙m,
- c M - constant coefficient determined by the design parameters of the engine,
- F is the main magnetic flux, Wb,
- I a - armature current, A.
With increasing load, the magnetic system of the motor is saturated and the proportionality between the current I a and the magnetic flux F is violated. With significant saturation, the magnetic flux Ф practically does not increase with increasing Ia. The dependence graph M=f(I a) in the initial part (when the magnetic system is not saturated) has the shape of a parabola, then, when saturated, it deviates from the parabola and in the area of high loads it turns into a straight line.
Important: It is unacceptable to turn on series excitation motors in the network in idle mode (without a load on the shaft) or with a load of less than 25% of the nominal, since at low loads the armature speed increases sharply, reaching values at which mechanical destruction of the motor is possible, therefore, in drives with sequential excitation motors, it is unacceptable to use a belt drive, if it breaks, the engine goes into idle mode. The exception is series excitation motors with a power of up to 100-200 W, which can operate in idle mode, since their power of mechanical and magnetic losses at high speeds is commensurate with the rated power of the motor.
The ability of series excitation motors to develop a large electromagnetic torque provides them with good starting properties.
The series excitation commutator motor has a high torque at low speeds and develops high speed in the absence of load. This electric motor is ideal for applications that require high torque (cranes and winches) as both the stator and rotor current increase under load. Unlike shunt motors and shunt motors, the series motor does not have an accurate speed control characteristic, and in the event of a short circuit in the field winding, it may become uncontrollable.
The mixed excitation motor has two excitation windings, one of them is connected in parallel with the armature winding, and the second in series. The ratio between the magnetizing forces of the windings can be different, but usually one of the windings creates a large magnetizing force and this winding is called the main winding, the second winding is called the auxiliary. The excitation windings can be connected in coordination and counter, and accordingly the magnetic flux is created by the sum or difference of the magnetizing forces of the windings. If the windings are connected in accordance, then the speed characteristics of such a motor are between the speed characteristics of parallel and series motors. Counter windings are used when it is necessary to obtain a constant rotation speed or an increase in rotation speed with increasing load. Thus, the performance of a mixed excitation motor approaches that of a parallel or series excitation motor, depending on which of the excitation windings plays a major role.
17.Characteristics of the mixed excitation engine.
A schematic diagram of a mixed excitation motor is shown in fig. 1. This motor has two excitation windings - parallel (shunt, SHO), connected in parallel to the armature circuit, and serial (serial, CO), connected in series to the armature circuit. These magnetic flux windings can be connected in accordance with or counter.
Rice. 1 - Scheme of an electric motor of mixed excitation.
When the excitation windings are turned on consonantly, their MMFs are added and the resulting flux Ф is approximately equal to the sum of the fluxes created by both windings. With the opposite connection, the resulting flux is equal to the difference between the fluxes of the parallel and series windings. In accordance with this, the properties and characteristics of an electric motor of mixed excitation depend on the method of switching on the windings and on the ratio of their MMF.
speed characteristic n=f (Ia) at U=Uн and Iв=const (here Iв is the current in the parallel winding).
With an increase in the load, the resulting magnetic flux with the consonant inclusion of the windings increases, but to a lesser extent than that of a series excitation motor, therefore, the speed characteristic in this case turns out to be softer than that of a parallel excitation motor, but more rigid than that of a series excitation motor.
The ratio between the MMF of the windings can vary over a wide range. Motors with a weak series winding have a slightly decreasing speed characteristic (curve 1, Fig. 2).
Rice. 2- Speed characteristics mixed excitation motor.
The greater the proportion of the series winding in the creation of the MDS, the closer the speed characteristic approaches the characteristic of the series excitation motor. In Fig. 2, line 3 depicts one of the intermediate characteristics of the mixed excitation motor, and for comparison, the characteristic of the sequential excitation motor is given (curve 2).
When the series winding is turned on in the opposite direction, the resulting magnetic flux decreases with increasing load, which leads to an increase in the motor speed (curve 4). With such a speed characteristic, the operation of the engine may turn out to be unstable, because. the flux of a series winding can greatly reduce the resulting magnetic flux. Therefore, motors with opposite windings are not used.
Mechanical characteristic n=f (M) with U=Un and Iv=const. mixed excitation motor is shown in Fig. 3 (line 2).
Rice. 3 - Mechanical characteristics of the mixed excitation engine.
It is located between the mechanical characteristics of engines of parallel (curve 1) and series (curve 3) excitation. By appropriately selecting the MMF of both windings, it is possible to obtain an electric motor with a characteristic close to that of a parallel or series excitation motor.
Scope of engines of sequential, parallel and mixed excitation.
Therefore, for series excitation motors, torque overloads are less dangerous. In this regard, series excitation motors have significant advantages in the case of difficult starting conditions and changes in the load torque over a wide range. They are widely used for electric traction (trams, metro, trolleybuses, electric locomotives and diesel locomotives on railways) and in lifting and transport installations.
Natural high-speed and mechanical characteristics, scope in engines of parallel excitation.
Natural high-speed and mechanical characteristics, scope in engines of mixed excitation.
In this motor, the field winding is connected in series to the armature circuit (Fig. 29.9, but), that's why magnetic fluxF it depends on the load current I = I a = I in . At low 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. F = k f I a (k f- coefficient of proportionality). In this case, we find the electromagnetic moment:
The rotation frequency formula will take the form
On fig. 29.9, b performance data presented M = F(I) And n= (I) series excitation motor. At high loads, saturation of the magnetic system of the engine occurs. In this case, the magnetic flux practically does not change with increasing load, and the characteristics of the motor become almost rectilinear. The series excitation motor speed characteristic shows that the motor speed changes significantly with load changes. This characteristic is called soft.
Rice. 29.9. Sequential excitation motor:
but- circuit diagram; b- performance characteristics; c - mechanical characteristics; 1 - natural characteristic; 2 - artificial characteristic
With a decrease in the load of the sequential excitation motor, the rotational speed increases sharply and, at a load of less than 25% of the nominal value, it can reach values \u200b\u200bthat are dangerous for the engine (“overshoot”). Therefore, the operation of a series excitation motor or its start-up with a shaft load of less than 25% of the nominal is unacceptable.
For more reliable operation the shaft of the sequential excitation motor must be rigidly connected to the working mechanism by means of a coupling and a gear. The use of a belt drive is unacceptable, since if the belt is broken or reset, the engine may “run out”. Considering the possibility of operating the engine at increased speeds, series excitation engines, according to GOST, are tested for 2 minutes to exceed the speed by 20% above the maximum indicated on the nameplate, but not less than 50% above the nominal.
Mechanical characteristics of a series excitation motor n=f(M) are presented in fig. 29.9, in. Sharply falling curves of mechanical characteristics ( natural 1 and artificial 2 ) provide the sequential excitation motor with stable operation under any mechanical load. The property of these motors to develop a large torque proportional to the square of the load current is important, especially in difficult conditions starting and during overloads, since with a gradual increase in the load of the engine, the power at its input increases more slowly than the torque. This feature of series excitation motors is one of the reasons for their widespread use as traction motors in transport, as well as crane motors in lifting installations, i.e. in all cases of an electric drive with difficult starting conditions and a combination of significant loads on the motor shaft with low rotation frequency.
Rated speed change of series excitation motor
where n - rotational speed at an engine load of 25% of the nominal.
The rotational speed of series excitation motors can be controlled by changing either voltage U, or the magnetic flux of the excitation winding. In the first case, an adjusting rheostat R rg (Fig. 29.10, but). With an increase in the resistance of this rheostat, the voltage at the input of the engine and the frequency of its rotation decrease. This control method is mainly used in small power engines. In the case of a significant engine power, this method is uneconomical due to large energy losses in R rg . Besides, rheostat R rg , calculated on the operating current of the motor, it turns out to be cumbersome and expensive.
When several engines of the same type are working together, the rotational speed is regulated by changing the scheme of their inclusion relative to each other (Fig. 29.10, 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 accounts for half the mains voltage. With the simultaneous operation of a larger number of engines, a greater number of switching options are possible. This method of speed control is used in electric locomotives, where several identical traction motors are installed.
It is possible to change the voltage supplied to the motor when the motor is powered from a DC source with regulated voltage (for example, according to a circuit similar to Fig. 29.6, but). With a decrease in the voltage supplied to the motor, its mechanical characteristics shift down, practically without changing their curvature (Fig. 29.11).
Rice. 29.11. Mechanical characteristics of a series excitation motor with a change in the input voltage
There are three ways to regulate the engine speed by changing the magnetic flux: by shunting the excitation winding with a rheostat r rg , sectioning the excitation winding and shunting the armature winding with a rheostat r w . Turning on the rheostat r rg , shunting the excitation winding (Fig. 29.10, in), as well as a decrease in the resistance of this rheostat leads to a decrease in the excitation current I in \u003d I a - I rg , and consequently, to an increase in the rotational speed. This method is more economical than the previous one (see Fig. 29.10, but), is used more often and is estimated by the regulation coefficient
Usually the resistance of the rheostat r rg taken so that krg >= 50% .
When sectioning the field winding (Fig. 29.10, G) turning off part of the turns of the winding is accompanied by an increase in the rotational speed. When shunting the armature winding with a rheostat r w (see fig. 29.10, in) excitation current increases I in \u003d I a + I rg , which causes a decrease in rotational speed. This method of regulation, although it provides deep regulation, is uneconomical and is used very rarely.
Rice. 29.10. Regulation of rotational speed of sequential excitation motors.
DC motors, depending on the methods of their excitation, as already noted, are divided into motors with an independent, parallel(by shunt), consistent(serial) and mixed (compound) excitation.
Motors of independent excitation, require two power sources (Fig. 11.9, a). One of them is needed to power the armature winding (conclusions Z1 And Z2), and the other - to create a current in the excitation winding (winding terminals Ш1 And SH2). Additional resistance Rd in the armature winding circuit is necessary to reduce the starting current of the motor at the moment it is turned on.
With independent excitation, mainly powerful electric motors are made in order to more conveniently and economically regulate the excitation current. The cross section of the excitation winding wire is determined depending on the voltage of its power source. A feature of these machines is the independence of the excitation current, and, accordingly, the main magnetic flux, from the load on the motor shaft.
Motors with independent excitation are practically identical in their characteristics to motors of parallel excitation.
Parallel excitation motors are switched on in accordance with the scheme shown in Fig. 11.9, b. clamps Z1 And Z2 refer to the armature winding, and the clamps Ш1 And SH2- to the excitation winding (to the shunt winding). Variable resistance Rd And Rv designed respectively to change the current in the armature winding and in the excitation winding. The excitation winding of this motor is made of a large number of turns of copper wire of relatively small cross section and has a significant resistance. This allows you to connect it to the full mains voltage specified in the passport data.
A feature of this type of motors is that during their operation it is forbidden to disconnect the excitation winding from the anchor chain. Otherwise, when the excitation winding opens, an unacceptable EMF value will appear in it, which can lead to engine failure and damage to the operating personnel. For the same reason, it is impossible to open the excitation winding when the engine is turned off, when its rotation has not yet stopped.
With an increase in the speed of rotation, the additional (additional) resistance Rd in the armature circuit should be reduced, and when the steady speed is reached, it should be removed completely.
Fig.11.9. Types of excitation of DC machines,
a - independent excitation, b - parallel excitation,
c - sequential excitation, d - mixed excitation.
OVSH - shunt excitation winding, OVS - serial excitation winding, "OVN - independent excitation winding, Rd - additional resistance in the armature winding circuit, Rv - additional resistance in the excitation winding circuit.
The absence of additional resistance in the armature winding at the time of starting the motor can lead to a large starting current that exceeds the rated current of the armature in 10...40 times .
An important property of the parallel excitation motor is its almost constant rotational speed when the load on the armature shaft changes. So when the load changes from idling to the nominal value, the speed decreases by only (2.. 8)% .
The second feature of these engines is economical speed control, in which the ratio of the highest speed to the lowest can be 2:1 , and with a special version of the engine - 6:1 . The minimum rotational speed is limited by the saturation of the magnetic circuit, which does not allow increasing the magnetic flux of the machine, and the upper limit of the rotational speed is determined by the stability of the machine - with a significant weakening of the magnetic flux, the engine can go "peddling".
Sequential excitation motors(serial) are switched on according to the scheme (Fig. 11.9, c). conclusions C1 And C2 correspond to the serial (serial) excitation winding. It is made from a relatively small number of turns of mainly large-section copper wire. The field winding is connected in series with the armature winding.. Additional resistance Rd in the circuit of the armature and excitation windings, it allows to reduce the starting current and regulate the engine speed. At the moment the engine is turned on, it should have such a value at which the starting current will be (1.5...2.5)In. After the engine reaches a steady speed, additional resistance Rd output, i.e. set to zero.
These motors develop large starting torques at start-up and must be started at a load of at least 25% of its rated value. Turning on the engine with less power on its shaft, and even more so in idle mode, is not allowed. Otherwise, the engine may develop unacceptably high speed, which will cause it to fail. Engines of this type are widely used in transport and lifting mechanisms, in which it is necessary to change the rotation frequency over a wide range.
Mixed excitation motors(compound), occupy an intermediate position between parallel and series excitation motors (Fig. 11.9, d). Their greater belonging to one or another type depends on the ratio of parts of the main excitation flow created by parallel or series excitation windings. At the moment the engine is turned on, to reduce the starting current, additional resistance is included in the armature winding circuit Rd. This engine has good traction characteristics and can idle.
Direct (non-rheostatic) switching on of DC motors of all types of excitation is allowed with a power of not more than one kilowatt.
Designation of DC machines
At present, the most widely used general-purpose DC machines of the series 2P and most new series 4P. In addition to these series, engines are produced for crane, excavator, metallurgical and other drives of the series D. Engines and specialized series are manufactured.
Series engines 2P And 4P subdivided along the axis of rotation, as is customary for asynchronous AC motors of the series 4A. Machine series 2P have 11 dimensions, differing in the height of rotation of the axis from 90 to 315 mm. The power range of the machines of this series is from 0.13 to 200 kW for electric motors and from 0.37 to 180 kW for generators. Motors of the 2P and 4P series are designed for voltages of 110, 220, 340 and 440 V. Their nominal speeds are 750, 1000, 1500,2200 and 3000 rpm.
Each of the 11 machine dimensions of the series 2P has two lengths (M and L).
Electric Machine Series 4P have some better technical and economic indicators in comparison with the series 2P. the complexity of manufacturing a series 4P compared with 2P reduced by 2.5...3 times. At the same time, copper consumption is reduced by 25...30%. According to a number of design features, including the method of cooling, protection from atmospheric influences, the use of individual parts and assemblies of the machine of the series 4P unified with asynchronous motors series 4A And AI .
The designation of DC machines (both generators and motors) is presented as follows:
ПХ1Х2ХЗХ4,
where 2P- a series of DC machines;
XI- execution according to the type of protection: N - protected with self-ventilation, F - protected with independent ventilation, B - closed with natural cooling, O - closed with airflow from an external fan;
x2- height of the axis of rotation (two-digit or three-digit number) in mm;
HZ- conditional length of the stator: M - first, L - second, G - with tachogenerator;
An example is the designation of the engine 2PN112MGU- DC motor series 2P, protected version with self-ventilation H,112 height of the axis of rotation in mm, the first dimension of the stator M, equipped with a tachogenerator G, used for temperate climates At.
According to the power, DC electrical machines can conditionally be divided into the following groups:
Micromachines ………………………...less than 100 W,
Small machines ……………………… from 100 to 1000 W,
Low power machines…………..from 1 to 10 kW,
Medium power machines………..from 10 to 100 kW,
Large machines……………………..from 100 to 1000 kW,
High power machines……….more than 1000 kW.
By rated voltages electrical machines are conditionally subdivided as follows:
Low voltage…………….less than 100 V,
Medium voltage ………….from 100 to 1000 V,
High voltage……………above 1000V.
According to the rotational speed of a DC machine, it can be represented as:
Low-speed…………….less than 250 rpm.,
Medium speed………from 250 to 1000 rpm,
High-speed………….from 1000 to 3000 rpm.
Super high speed…..above 3000 rpm.
Task and method of work performance.
1. To study the device and the purpose of individual parts of DC electrical machines.
2. Determine the conclusions of the DC machine related to the armature winding and to the excitation winding.
The conclusions corresponding to one or another winding can be determined with a megohmmeter, an ohmmeter, or with an electric light bulb. When using a megohmmeter, one of its ends is connected to one of the terminals of the windings, and the other is touched in turn to the rest. The measured resistance, equal to zero, will indicate the correspondence of the two terminals of one winding.
3. Recognize the armature winding and the excitation winding by the conclusions. Determine the type of excitation winding (parallel excitation or series).
This experiment can be carried out using an electric light bulb connected in series with the windings. Constant voltage should be applied smoothly, gradually increasing it to the specified nominal value in the machine's passport.
Given the low resistance of the armature winding and the series excitation winding, the light bulb will light up brightly, and their resistances measured with a megohmmeter (or ohmmeter) will be almost zero.
A light bulb connected in series with a parallel excitation winding will burn dimly. The resistance value of the parallel excitation winding must be within 0.3...0.5 kOhm .
The armature winding leads can be recognized by attaching one end of the megohmmeter to the brushes while touching the other end to the winding leads on the electrical machine panel.
The conclusions of the windings of the electrical machine should be marked on the conditional label of the conclusions shown in the report.
Measure winding resistance and insulation resistance. Winding resistance can be measured using an ammeter and voltmeter circuit. The insulation resistance between windings and windings relative to the housing is checked with a megohmmeter rated for 1 kV. The insulation resistance between the armature winding and the excitation winding and between them and the housing must be at least 0.5 MΩ. Display the measurement data in the report.
Depict conditionally in a cross section the main poles with the excitation winding and the armature with the turns of the winding located under the poles (similar to Fig. 11.10). Independently take the direction of the current in the field and armature windings. Specify the direction of rotation of the motor under these conditions.
Rice. 11.10. Double Pole DC Machine:
1 - bed; 2 - anchor; 3 - main poles; 4 - excitation winding; 5 - pole pieces; 6 - armature winding; 7 - collector; Ф - main magnetic flux; F is the force acting on the conductors of the armature winding.
Control questions and tasks for self-study
1: Explain the structure and principle of operation of the motor and DC generator.
2. Explain the purpose of the collector of DC machines.
3. Give the concept of pole division and give an expression for its definition.
4. Name the main types of windings used in DC machines and know how to implement them.
5. Indicate the main advantages of parallel excitation motors.
6.What are design features parallel excitation windings compared to series excitation windings?
7. What is the peculiarity of starting DC motors of series excitation?
8. How many parallel branches do simple wave and simple loop windings of DC machines have?
9. How are DC machines designated? Give an example of a notation.
10. What is the allowed insulation resistance between the windings of DC machines and between the windings and the housing?
11. What value can the current reach at the moment of starting the engine in the absence of additional resistance in the armature winding circuit?
12. What is the allowed motor starting current?
13. In what cases is it allowed to start a DC motor without additional resistance in the armature winding circuit?
14. Due to what can the EMF of an independent excitation generator be changed?
15. What is the purpose of the additional poles of the DC machine?
16. At what loads is it allowed to turn on the series excitation motor?
17. What determines the value of the main magnetic flux?
18. Write expressions for the EMF of the generator and the engine torque. Give an idea of their components.
LABORATORY WORK 12.