Abstract
This study presents the challenges faced while implementing a new converter topology for switched reluctance motor (SRM) drives. A novel converter configuration that can excite the motor directly from the grid supply, i.e., alternating current supply, was proposed in earlier research. The proposed converter topology is designed to draw power directly from an AC grid. This converter is a modified version of the R-dump converter. There are many converter topologies available for excitation of SRM. The uniqueness of the converter topology developed in this research shows that it can use 4 switches per phase, which in turn increases the redundancy of the whole system. Moreover, this topology is well suitable for SRM excitation since the machine coil structure is constructed with 4 coils and 5 terminals. Since this novel converter is having the advantages of operating in AC input voltage, it improves the usefulness of SRM in household applications such as washing machines, hair dryers, and vacuum cleaners. An analysis of real-time implementation challenges faced while realizing the universal R-dump (URD) converter is presented with discussion to overcome the challenges. An experimental setup is made to verify the converter working.
1. Introduction
Switched reluctance motors are a special kind of stepping motor that operates on the principle of reluctance torque. Many kinds of literature encompass the inherent advantages of switched reluctance motors over other drive systems in motor construction and converter circuit. Windings of SRM are placed in such a way that they can be excited with both AC as well as DC supplies. Even though the coils have the capability, the converters proposed in the last two decades operated SRM as a DC machine. Only one research proposed the AC-operated machine [1]. Instead of using a rectifier to feed the DC supply to SRM, the converter proposed an AC supply. Matrix converter fed SRM is analyzed in [2], in which the supply is considered as an alternating DC supply provided by a full bridge diode rectifier.
Due to its rigid structure and absence of magnets or windings in the rotors, switched reluctance motors are desirable in high-speed applications such as vacuum cleaners. At the same time, its capabilities are limited in many home appliances because of its DC-operated converter topology. One such converter is presented in [3] for a photovoltaic-based pump (based on SRM) system. A novel L-dump converter is presented in [4] and discussed about the voltage stress and performance of the converter. A multilevel operating fault tolerant converter is proposed in [5] and challenges faced by a multilevel converter are discussed in [6]. A novel modular converter using six-pack IGBT modules is presented in [7], in which the advantages are compared with existing modular approaches. A similar integrated converter is proposed in [8], and analysis is made for electrical stress on power switches. Another novel converter is proposed in [9], in which a minimal switch converter is presented. One power switch is shared by two adjacent phases. Significant improvement in torque ripple reduction is achieved. A continuous toroidal winding SRM is proposed in [10], which can be driven with 6 switches unipolar current mode or 12 switches bipolar current mode. In the above said research bipolar DC current is exciting the coil. To reduce current rising and falling times, a quasi-three-level converter is presented in [11].
Buck converter-driven SRM is presented in [12] to reduce torque ripple and improve power quality at mains. Another power factor improvement research is presented in [13] with an active front end coupled with centralized control. The effect of fast switching speeds such as electromagnetic interference and current overshoot is discussed in [14] and possible solutions are listed. Generally, SRM is projected to induce power quality issues due to the converter used. The research proposed in this study deliberates the switching circuit that exhibits both the converter and rectifier roles, and because of this, the power quality issues do not arise comparably.
The implementation challenge of a novel converter called the universal R-dump converter consisting of bidirectional switches that can run on AC as well as DC supply are analyzed in this study. Despite more switches used in the design, the converter topology can gain better utilization due to the decrease in power quality issues and hence the SRM operating for both AC and DC power sources.
This research study is organized as follows. Section 2 gives the basic operation of SRM. Section 3 shows the universal R-dump converter and its basic operation. Section 4 presents the simulation platform used for the analysis. Section 5 elaborates on the challenges faced while real-time implementation of the above said noel converter. Possible ways to overcome the challenges are also discussed along with challenges. The experimental setup is detailed in section 6. A summary of the research and scope for future work is given in section 7.
2. Principle of Switched Reluctance Motor (SRM)
Switched reluctance has salient poles on both the rotor and the stator, but the windings are wrapped only on the stator. On the other hand, the rotor is built from a stack of salient pole laminations [15] with no windings, magnets, or cage windings. This simple construction of SRM makes it a unique type of a low-cost motor. Torque production is happening by the proneness of its rotor pole shifting to a position, where the inductance of the excited winding is maximized, and the reluctance of the excited pole gets minimized. The number of rotor poles is usually lesser than the number on the stator to avoid the occurrence of the rotor being in a state of producing no initial torque which occurs when all poles of the stator and rotor lock to one another. The stator windings are wound on the opposite poles and connected in series or parallel to make continuity within individual poles.
Figure 1 shows the cross-sectional view of an SRM which has 8 stator poles and 6 rotor poles drawn in finite element analysis (FEA) software called motor solve (Siemens Simcenter). Exciting each coil in a particular sequence, the direction of the movement of SRM can be controlled. For instance, if the clockwise rotation is desired, each coil is energized in the A-B-C-D sequence. For the counter-clockwise direction, the same sequence in the reverse is executed. During the movement, there are three relative positions between the stator and the rotor. The first is the unaligned position, the second is intermediate, and the third is the aligned position. When any pair of rotor poles is exactly aligned with the stator poles of a particular coil, that coil is said to be in an aligned position. Similarly, if the interpolar axis is aligned with the stator poles of a particular coil, that coil is said to be in an unaligned position. The position between the unaligned and aligned position is called the intermediate position.
3. The Universal R-Dump Converter
The modified version of the R-dump converter is shown in Figure 2. While operating in DC supply, the proposed topology has 2 switches replacing a single diode; thereby, each arm has the converter switching for both polarities of the signal input, making the converter adopted to energize SRM for both DC supply and AC supply. For positive and negative cycles, separate switches are provided. The control logic is made in such a way that depending on the polarity of the supply, the switching sequence is decided. Switching logic control is modeled in LabVIEW. FPGA and real-time target tools in LabView are used for the acquisition and data logging of real-time data like V, I, and rotor position acquired from the machine using NI cRIO 9048 controllers with analog and digital modules. The analog module reads the input voltage and digital modules provide the gating pulses for the converter. A mixed signal oscilloscope is used to visualize the current waveforms of the coil.
The converter is verified to function for both DC as well as AC power supplies. For the AC power supply, switches in coils A, B, C, and D switch correspondingly whenever each enters forward bias.
The four switches used by each coil provide the continuity of switching for every polarity reversal of the signal, such that at any given time, two switches keep the coil in excitation with the remaining two switches deenergizing the energy stored in the coil and dump to the load (resistor here).
4. Simulation Platform
The switched reluctance motor is modeled in the MATLAB Simulink platform by configuring parameters (rated power, 0.5 kW; rated current, 3 A; resistance per phase, 2.67 Ω) using an inbuilt block set model. The motor model is excited with an alternating current supply. Converter topology triggering logic uses supply voltage as one of the measurements to control the switching so that automatic switching between positive and negative occurs. Variations in coil voltage and current are proportional to the supply voltage. During zero crossing, a sudden reverse voltage buildup is occurring in the phase coils that get energized during the previous energizing mode.
5. Implementation Challenges
Any physical system will have its own limitations and challenges while implementing in real-time. The universal R-dump converter is also having some implementation challenges. In an 8/6 SRM, the stator and rotor coils are set apart at 45° and 60° mechanical angles, respectively. For a clear visualization, the coil placement can be drawn in a straight line as in Figure 3.
From Figure 3, it is clearly understood that each pole is having a definite distance from the other poles. Especially, that distance will change in any normal operating condition of SRM. Each dot in a straight line represents a pole. Poles A and A′ are diametrically opposite poles which will make the magnetic flux lines when the conductor wrapped on that pole gets excited. This is also true for other poles BB′, CC′, and DD′. Since the rotor is having a smaller number of poles compared to the stator, the poles aa′, bb′, and cc′ move to the minimum reluctance position when the stator coil gets energized. The rotor pole with minimum distance will be attracted towards the excited stator pole. Distances between various poles are given in Table 1.
The minimum position variation between stator pole pairs and rotor pole pairs occurs in two positions AA′–aa′ and CC′–bb′. From this point, if AA′ gets excitation means the pole aa′ in the straight-line graph moves to the left by 15° and gets aligned with AA′. Similarly, if CC′ gets excitation means the pole aa′ moves to the right by 15° and gets aligned with CC′ (reverse direction of AA′). The direction of rotation depends on the excitation given to the pole pairs. For one pole pair excitation, a 15° mechanical rotation occurs. Similarly, for every coil energized there will be a 15° mechanical rotation. Energizing all four sets of coils in sequence will result in 60° of mechanical rotation. For a complete revolution, each set of coils must energize 6 times. In other words, we can say a machine running at 1 rpm will make a mechanical movement of 6° per second.
Gate pulses to the converter are derived with the help of encoder pulses (signal). Position pulses depend on the speed of the rotor. The change of the encoder pulse from one phase to the other is directly proportional to the speed of the rotor. An example of the position pulse train is shown in Figure 4.
The position signal in Figure 4 is given by a generalized poly phase model of SRM [16]. For instance, if we consider the rotor is rotating at 10 rpm, i.e., 60 deg/sec for every 250 ms, encoder pulse change will occur. AC supply is at 50 Hz (20 ms for one cycle); within 250 ms, the supply will complete 12.5 cycles. In this condition, the same coil will go under positive and negative supplies without deenergizing sequence. This is because the time to switch the sequence is not sufficient to build the flux. For various values of speed, the time taken for 15° rotation (i.e., the time taken for encoder pulse change) and the number of cycles covered are given in Table 2.
Considering a different scenario of a higher speed of 1000 rpm, i.e., 6000 deg/sec, the rotor will take 2.5 ms for 15° of mechanical rotation. After every 2.5 ms, coil switching occurs. All four coils will get energized twice (one in positive supply and another one in negative supply) in one cycle of supply. This condition is shown in Figure 5. It is observed that for a higher speed of 1000 rpm, the switching time is nearly 2.5 ms.
From Table 2, speeds below 250 rpm will result in exciting the same coil with positive and negative supplies continuously without deenergizing the coil. For various values of supply voltage, the energizing and deenergizing timing of the prototype SRM is given in Table 3.
From Table 3, it is clearly clarified that each coil needs a minimum of 3 ms of energizing time and 3 ms of deenergizing time for a given voltage range. Mode of operation in the URD converter clears those deenergizing switches for positive and negative supplies, is different [2], and will never conduct at the same. Changes in supply polarity will also change the deenergizing switches. This will open the coil without deenergizing it. Considering this condition, the coil needs a gap in excitation between positive and negative cycles. Since the supply voltage cannot be altered, the excitation after a small threshold in the supply voltage can solve this problem. For example, exciting the coils only if the supply is more than, say, 30% of the peak voltage in both directions. This leads to a zero torque production zone from 30% to −30% peak voltage values with a smooth transition from positive to negative sequence of supply.
Another challenge in implementation is starting the machine with an AC supply because during the starting of the motor, the pole changing time is very much smaller than the supply frequency. Within one pole change, many positive and negative cycles will pass through. This means the encoder pulse change is very slow, which in turn affects the coil excitation. This is because one coil cannot energize with both positive and negative cycles without time to deenergize the coil.
The following section discusses the loading effect in the transformer which is another main challenge in the implementation of the URD converter. While running the machine at high speeds, a high switching frequency is needed. This fast switching will make the transformer experience very fast on and off switching. This will make the transformer make a loading noise during operation. For an ideal transformer, the impedance is given aswhere R represents the resistance of the winding and XL represents the inductive reactance:
From relation (2), it can be understood that XL is proportional to the frequency f. If the frequency f increases, XL is also increasing correspondingly and the impedance increases.
X L α (1/I), P α I; so, XLα (1/P). An increase in impedance reduces the useful power compared to the rated power. There are two main losses in the machine, namely, hysteresis and eddy current loss. An increase in loss reduces the efficiency of the machine. I represents the current and P represents the useful power.
Hysteresis loss can be calculated from
Similarly, Eddy’s current loss iswhere Ph α f and Peα f2. Bm represents the maximum flux density.
From relations (3)-(4), the hysteresis loss and eddy current losses will become predominant compared to the useful power.
Figure 6 shows the modeling of an energizing SRM using transformer source supply with an RL load via a switch operating at a very high frequency as modeled in MATLAB Simulink.
This models that the input voltage is pure AC with 50 Hz frequency. A machine rotating at 1000 rpm will make a switching changes every 2.5 ms (Table 2). The frequency of the sine will dictate the position switch in such a way that it will experience a switching for every interval of 2.5 ms.
High-speed switching creates high losses; thus, the machine gets heated up. This will reduce the life of the transformer [17]. However, if connected to a direct grid supply, the poor THD in current will affect the overall efficiency and stability of the grid to which the converter and motor setup are connected.
Emulation output voltage due to high switching is shown in Figure 7. This graph shows the stress seen by a transformer connected to a high switching speed load. A high-power high-frequency transformer may solve this issue and consume a small place compared to conventional low-frequency transformers.
6. Experimental Validation
The experimental setup is made for the prototype machine along with the equipment mentioned in Section 3 of this study. Figure 8 shows the stator and rotor of the prototype machine.
Experiments are carried out with AC supply. As discussed in Section 5, the machine could not start and run because of positive and negative excitation to the same coil without deenergization and switches are damaged. To verify that the converter can work in both the directions of supply, the same converter circuit is excited with positive DC supply and negative DC supply without altering any of the connections in the converter. The overall experimental setup is shown in Figure 9. The modular converter and controller setup along with the motor is presented.
Figure 10 shows current waveforms for positive supply captured in the mixed signal oscilloscope.
Easy visualization currents for two coils are shown in figure. Figure 11 shows the current waveforms for negative supply without altering the converter circuit.
In both the cases, the direction of current alone changes and the direction of rotation remains the same. The direction of speed can only be changed by changing the excitation sequence. It is verified that the converter can excite the motor both in positive and negative supplies.
A comparison of various performance factors for different converter topologies is given in Table 4.
7. Conclusion
In this research, a universal R-dump converter proposed for SRM is analyzed and the implementation challenges faced are discussed. An experimental setup is made for the analysis of the proposed converter. Three major challenges, namely, coil excitation near zero, starting the motor (very slow speed operation), and the effect of high-frequency switching in the transformer (or grid-connected) are analyzed and possible solutions are proposed. The URD converter is preferable because it improves the timing for deenergizing during the switching of the converter from positive to negative or vice versa. Therefore, increasing the energy transfer from coil to dump load is required for high-speed applications. From reference [18], it is observed that SRM has improved torque to volume ratio and ease of control compared to other motors. The URD converter is a good candidate for achieving high speed and torque. Such converter drive with SRM can be used for drone applications that need high torque for its cruise control and axis tilting. The future work is to analyze the performance of ZC (R + XC) dump converter topology. A detailed experimental analysis incorporating all the abovementioned solutions will be presented in the future work as continuation.
Data Availability
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that there are no conflicts of interest.
Acknowledgments
This work was supported by RUSA (R&I), MHRD, Government of India, and State Project Directorate, RUSA, Government of Tamil Nadu.