Abstract
Asynchronous or induction machines are among the leading devices in the electrical industry. They are well suited for motor and production applications with power ratings from a few kilowatts to megawatts. In addition, their use in renewable energy systems, especially in wind conversion systems, is increasing daily due to their advantages, such as low cost, brushless, and robust structure. In this work, the operation of a three-phase asynchronous motor with a winding rotor as an alternator, the variation of the voltage produced depending on the excitation current by keeping the number of revolutions constant, and the interpretation of the voltage produced according to the number of revolutions by keeping the excitation current constant have been investigated. The variation of the terminal voltage in ohmic, inductive, and capacitive loads has been experimentally investigated by observing the source voltage providing the rotational speed and excitation current constant.
1. Introduction
Using renewable energy sources such as wind, geothermal, biomass, wave energy, and small hydropower plays an essential role in meeting the increasing global energy demand. These sources bring many advantages due to their environmental friendliness [1–3]. Synchronous and asynchronous generators are used in power plants to generate electricity [4–6]. Induction generators (IG) are widely used in wind and minihydro applications [7, 8]. The advantages of asynchronous machines in power generation are well known [9]. The benefits of asynchronous machines are that they have lower unit costs, are brushless, are easy to maintain, and protect themselves against overloads and short circuits [4, 10–12]. Synchronous generators and induction generators are used in power plants for electricity generation. While synchronous generators are used in high-power power plants, asynchronous generators are preferred because of their advantages, such as low cost and ease of maintenance in places where power is low [13].
For a standard operating IG to operate in off-grid mode and as a generator, it must satisfy two states: (i) the capacitor bank provides the required reactive power and (ii) the rotor speed of the IG is above the synchronous speed [14, 15]. The second condition is the same for both on- and off-grid operations. Recently, asynchronous generators have attracted significant interest from researchers for stand-alone isolated applications. Various analyses are performed on steady-state, transient, and dynamic performances under stable/unstable conditions [16, 17]. When an induction motor is operated as an autonomous generator, although the rotational speed is kept at a constant value, the loads connected to the generator affect both voltage and frequency [18]. Therefore, the induction generator should maintain a continuous terminal voltage under varying loads. In practice, it is a characteristic that decreases terminal voltage and frequency with increasing load [19]. There is an armature reaction effect in the generator operation of induction motors, and there are deficiencies in the literature on the experimental investigation of this effect with ohmic, inductive, and capacitive loads. In this study, an asynchronous motor with a winding working rotor was operated as a generator and experimentally investigated in three sections. First, these sections examined the voltage variation produced depending on the excitation current by keeping the number of revolutions constant. Second, the voltage variation then depends on the number of revolutions at the constant and the variable excitation currents. In the last part, by keeping the source voltage constant, the output voltage variation in ohmic, inductive, and capacitive loads has been examined, and the results have been discussed.
2. Induction Generator Mode
If a three-phase asynchronous machine connected in parallel to a grid with constant voltage and frequency is driven above the synchronous speed toward the rotating field, the asynchronous machine switches from the motor to the generator-running state. In this case, the asynchronous machine starts to work as a generator by pulling the magnetizing current from the grid. Thus, with the shift , the torque and the sign of the mechanical force change direction and become negative. When is negative, the stator of the asynchronous machine starts to give active electrical energy to the grid. The interesting side of the IG is that if it does not produce the magnetizing current but draws it from the grid, this reactive current maintains the value and direction of the operating state as a motor.
According to the working principle of the induction motor, the number of rotations of the motor cannon reaches the number of synchronous rotations, and it rotates at a lower speed than this speed. The stator-rotating field cannon cuts the rotor windings if it reaches this speed. No voltage and current are induced in the rotor windings; the torque becomes zero. Therefore, there is a critical working size that defines the way the machine works. This magnitude is called shift and is denoted by , calculated by (1). In the given equation, is the synchronous speed, is the rotor speed, and is the number of machine revolutions.
is the number of rotations of the rotor-rotating field relative to the rotor. Considering that the machine has poles, the stator angular velocity () and rotor angular velocity () are given by
According to equations (2) and (3), the shift is found by (4). denotes the rotor frequency (Hz), and denotes the stator frequency. So, slip is the ratio of rotor current frequency to stator current.
Assuming that the frequency of the stator currents is constant, the increase in the frequency of the rotor currents causes the slip to increase and the motor rotation speed to decrease, and the decrease of this frequency causes the rotor rotation speed to increase. The frequency of the rotor currents is directly proportional to the rotor cutting the speed of the stator-rotating field. When the rotor is stopped, and , and theoretically, when the rotor rotates at synchronous speed, and . The number of revolutions of asynchronous motors cannon reach the synchronous speed. If the asynchronous motor is rotated from the outside, will be because . This mode is the operation of the asynchronous motor as a generator, and it is . In this case, the slip is given by
When , the rotor field opposes the rotating field, and when , the braking effect occurs in this operation. These operating modes of the induction motor are shown in Figure 1.
In Figure 2, the asynchronous machine is connected parallel to the grid. While no torque is applied to its shaft, the asynchronous machine connected to the mains draws P active and Q reactive powers from the grid and operates as a motor. When a torque is applied from the turbine to the asynchronous machine shaft, the machine continues to work as a motor if the rotational speed is below the synchronous speed. However, when the speed exceeds the synchronous speed, the asynchronous machine continues to draw Q reactive power from the grid but gives the active power P to the grid. In this case, the asynchronous machine provides the excitation by using the reactive power it receives from the grid, creates the necessary magnetization, and works as a generator.
Using the equivalent circuit given in Figure 3, the active power flowing from the rotor of the IG to the stator, the losses in the rotor, the shaft input power that must be applied to the rotor shaft, the active power losses in the stator, iron losses, rotational and friction losses, and the output from the generator to the load or the active grid power can be determined. In Figure 3, the expressions , , , , , and are stator resistance, stator reactance, rotor resistance, rotor reactance, iron losses, and magnetizing reactance, respectively. In addition, the reactive power consumed in leakage and magnetization reactance, the total reactive power consumed in the machine, the total apparent power produced by the generator, the stator phase current, the power coefficient, and the generator’s efficiency can be calculated.
Since n > ns in the generator operation with s < 0. Therefore, the result of the term in the equivalent circuit is negative for the generator. This means negative resistance and indicates that the power flow is from the rotor to the stator. In the generator-running state of the asynchronous machine, since ; the stator current phase is changed by 180°. The generator supplies the electrical power shown in (6) to the grid and changes the direction according to the motor operation.
It receives mechanical power from the machine shaft and gives electrical energy to the grid to which it is connected to from its electrical ends and .
3. No-Load Experimental Part
The experimental setup is shown in Figure 4; universal and induction motors were used to rotate the induction motor. Ohmic, inductive, and capacitive loads have been employed in the loaded operation.
3.1. Investigation of the Terminal Voltage Depending on the Excitation Current at Constant Speed
In this part, the wound-rotor induction motor was rotated by another asynchronous motor with a constant speed of 1500 revolutions per minute (RPM), and a direct current (DC) was passed through the rotor windings. The terminal voltage change was examined at various excitation current values passing through the rotor windings while the number of revolutions was kept constant. In Figure 5, the connection diagram of the device in which the terminal voltage is examined depending on the excitation current at fixed speed has been given. The connection diagram of the rotor circuit is shown in Figure 6.
When there is no load in the setup, the phase voltage values of induced in the stator windings, rotor excitation current at constant speed, and load current are given in Table 1. The standard deviation of the excitation current is 698.09, while the standard deviation of the induced voltage is 80.19.
Figure 7 shows the change in the terminal voltage depending on the excitation current in the no-load condition and at a constant speed; increasing the excitation current increases the lead voltage in asynchronous machine generator operation.
3.2. The Investigation of the Terminal Voltage Depending on the Speed of the Driving Machine by Keeping the Excitation Current Constant
Figure 8 shows the connection of the setup for the examination of the end voltage depending on the speed of the universal motor drive machine by keeping the excitation current constant at 1.2 amperes.
In this research, the DC excitation current applied to the rotor of the wound-rotor induction motor was kept constant at 1.2 A, a universal motor and a wound-rotor asynchronous motor were rotated, and the terminal voltage was observed at various revolutions. Table 2 shows the terminal voltage values depending on the amount of speed at constant excitation current. The standard deviation value of the speed is 353.5, while the standard deviation value of -induced voltage is 59.54.
In Figure 9, the variation of the terminal voltage is given depending on the number of revolutions at constant excitation current.
The current passing through the armature windings of the alternator creates a magnetic field. This magnetic field (or rotating fields) affects the main magnet field formed by the poles; the armature field disrupting the polar field is known as the armature reaction [21]. Since the no-load current is zero, no current flows through the armature, and the armature magnetic field is zero; that is, no armature reaction occurs at idle work.
4. Loaded Working Experiment
In the loaded operation test, the phase voltage of the universal motor was adjusted to 220 V by changing the excitation current applied to the rotor windings. The DC voltage applied to the rotor windings was kept constant; however, as the wound-rotor asynchronous motor operated as an alternator was loaded, the RPM changed, and the speed was kept constant by changing the voltage applied to the universal motor. In ohmic, inductive, and capacitive loads, as the load increases, the number of revolutions is adjusted to the number of idle revolutions, and numerical values are taken. In other words, the change between the load current and the terminal voltage has been observed by keeping the rotational speed and the DC voltage applied to the rotor constant. Figure 10 shows the connection diagram of the loaded operation experiment of the induction motor in the generator mode.
4.1. Operating at Ohmic Load
Ohmic loads do not draw reactive power from the grid, and their needs are toward active power. An incandescent lamp was used as the ohmic load. In the case of ohmic load, it is seen in Table 3 that the end voltage decreases due to the increase in the load current, and the speed is fixed at 1900 RPM. is the load current and is the phase current passing through the rotor device.
In the case of ohmic load, the variation of the terminal voltage depending on the increase in the load current is seen in Figure 11; the excitation current is kept constant at around 1.2 amperes.
The armature (rotor) magnetic field weakens the pole field (stator winding magnetic field), and the resultant magnetic field in the air gap weakens, along with the ohmic load, reducing the terminal voltage.
4.2. Working on Inductive Load
In the case of inductive load, it is seen in Table 4 that the end voltage decreases depending on the increase in the load current; the speed is fixed at 1900 RPM. is the load current, is the phase current through the rotor circuit, and the speed is set at 1900 RPM.
In the case of inductive load, the variation of the end voltage depending on the increase in the load current is seen in Figure 12; the excitation current was kept constant at around 1.2 amperes.
Armature reaction is present in ohmic load, but armature reaction is more present in inductive load than in the ohmic load. In the case of inductive load, due to the armature reaction effect and the weakening of the resultant flux, as in the alternator, the voltage drop becomes greater.
4.3. Capacitive Load Operation
The field created by the armature current weakens or strengthens the main area formed by the poles, depending on the type of load. In the capacitive load, the armature reaction reacts toward supporting the polar field, and with this effect, the valuable flux increases. In the case of capacitive load, it is seen in Table 5 that the end voltage increases as the load current increases.
The variation of the terminal voltage under a constant speed of 1900 RPM, depending on the increase in the load current in the capacitive load condition, is seen in Figure 13.
5. Conclusion
In this study, a DC excitation current is applied to the rotor winding of the wound-rotor induction motor; the induction motor and the universal motor rotate the motor shaft. The operation of a three-phase asynchronous motor with a wound rotor similar to a three-phase alternator and, first of all, the variation of the voltage produced depending on the excitation current by keeping the speed constant are investigated. The voltage variation produced according to the number of revolutions was experimentally investigated by keeping the excitation current constant. Since the no-load current is zero and no current flows through the armature, the zero-armature magnetic field does not cause an armature reaction. It has been observed that the winding rotor induction motor behaves like an alternator in this operating mode. In no-load operation, the generated terminal voltage increases depending on the excitation current and the number of revolutions per minute.
The loaded study used varying values of ohmic, inductive, and capacitive loads. The field created by the armature current weakens or strengthens the main field formed by the poles, depending on the type of load. This situation manifests itself as a decrease or increase in the terminal voltage. At ohmic and inductive loads, the armature reaction reduces the useful flux in the air gap, and the end voltage drops. In capacitive loads, the useful flux increases with the armature reaction effect. As in synchronous generators, it has been observed that the terminal voltage decreases depending on the increase in the load current in the case of ohmic and inductive loads, but the terminal voltage increases with the load current in the capacitive load. It has been concluded that a wound-rotor induction motor can act as an alternator in DC excitation, and theoretical predictions can be experimentally proven in the loaded operation. The deficiencies in showing the effects of the inductive reaction on the generator operation of the wound-rotor induction motor and the ohmic, inductive, capacitive, and no-loaded effects have been considered experimentally.
Data Availability
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The author declares that there are no conflicts of interest regarding the publication of this paper.