Multi-stage Drive of Switched Reluctance Motor Based on Ring Winding Structure

A new ring winding structure can drive switched reluctance motors (SRMs). It uses a full-bridge power converter circuit to drive SRMs and is significant for developing SRMs. For this structure, this paper proposes an optimization strategy. A voltage regulator is used to boost the winding current during the motor starting phase to ensure the average starting and low-speed operation of the motor. After the motor speed reaches a specific value, the passive device replaces the voltage regulator, which ensures the stable operation of the SRM while reducing the frequency of the voltage regulator and improving the service life of the regulator. This paper simulates the optimization strategy of the ring winding structure and derives the calculation method for switching speed.


Introduction
The asymmetric H-bridge circuit (AHB) is a typical circuit (as shown in Fig. 1) that is used in switched reluctance motor (SRM) drive systems [1].For a three-phase SRM, this topology requires three transistor-diode bridge arms with three diode-transistor bridge arms.AHB circuits are usually used only for SRMs, with limited applications and high usage costs [2] [3].

Fig. 1 Asymmetric H-bridge circuit
Unlike the AHB circuit, the full-bridge power conversion circuit (as shown in Fig. 2) is in high demand.It has a wide range of applications and has a low cost as a standard motor drive structure [4].We can use this topology in motor driving systems such as BLDC motors, electrically excited synchronous motors, and induction motors.However, this topology will output positive and negative currents during operation and cannot be used directly in SRM drive systems.
Using full-bridge power conversion circuits to drive SRMs is of great importance for developing SRMs and has been studied by increasing numbers of scholars in recent years.In [5] and [6], two full-

Fig. 2 Full-bridge power conversion circuit
There are two traditional ideas to drive SRM by full-bridge circuits, one is to change the internal winding structure of the motor [7], and the other is to drive SRM equivalent to a synchronous reluctance motor [8].The former will increase the motor manufacturing cost, and the latter fails to achieve the desired effect due to the biconvex machine of SRM.
Our team designed a ring structure (as shown in Fig. 3) that can drive the SRM based on a fullbridge power conversion circuit to avoid changes in the motor structure.We connect the SRM windings in series according to the phase order to form a ring structure and then insert a voltage between two windings to ensure that the motor winding current is always positive [9].
The Windings Voltage Fig. 3 Full-bridge power conversion circuit with ring winding structure In the ring structure, the voltage source is always operating.When the bridge arm current at the output of the full-bridge power converter is positive, the voltage source can make the motor winding current too high and generate unnecessary losses.In subsequent studies, our team used DC-DC converters to replace the voltage source, control the circulating current in the ring structure, and reduce power losses [10].
The above structure still has the possibility of optimization.When the motor speed is low, the DC-DC converter keeps working to ensure the starting and running of the motor.When the motor speed reaches a specific value, passive devices replace the DC-DC converter, reducing losses in DC-DC converters.

Operating principle
In the ring structure mentioned in Fig. 3, the SRM winding current (Fig. 4) contains two parts: the trapezoidal current supplied by the full-bridge power converter with a peak value of  and the circulating current  supplied by the voltage source.The former alternates between positive and negative, and the latter is always positive, and the superposition of the two ensures that the SRM winding current is always positive.The maximum current value of the SRM winding is  , the minimum value of it is  , and the relationship among  ,  , and  is: This paper uses a DC-DC half-bridge converter circuit as a voltage regulator to construct a ring structure (Fig. 5).The circuit requires only two power devices with low power consumption and simple control, and the circulating current  can be controlled by controlling the turn-on and turn-off of the two power devices.

DC-DC Half-bridge Converter Circuit
The Windings of SRM Fig. 5 Ring structure with DC-DC half-bridge converter circuit When the trapezoidal periodic current provided by the full-bridge power conversion circuit is negative, the DC-DC half-bridge converter circuit will provide positive  to raise the SRM winding current and make it positive.The DC-DC half-bridge circuit with reasonable control guarantees the SRM winding current is positive at all times, ensuring the smooth starting and low-speed operation of the SRM.
When the SRM runs at high speed, the motor's requirement for winding current will reduce, allowing a certain amount of non-positive current in the winding.Replacing the voltage source with a passive device such as a diode can meet the winding current requirement of the SRM at high-speed operation, reduce the power loss of the DC-DC converter circuit and extend the service life of the converter circuit.
The DC-DC half-bridge circuit has two symmetrical diodes on its load side, and they can become passive devices for the high-speed operation of the SRM.After the speed of the SRM reaches a specific value, we provide a turn-off signal for the power device of the DC-DC half-bridge converter circuit on its power side.Then, the symmetrical diodes on the load side ensure that the SRM winding current is positive.Due to the symmetrical structure of the two diodes, current flows through both diodes simultaneously when the motor runs at high speed.

Selection of switching speed
Taking the three-phase SRM in Fig. 5 as an example, where the three-phase windings are A, B, and C in order from top to bottom.When the SRM is in the high-speed operation stage, the diode will connect in series with the C-phase winding, which only ensures that the C-phase winding current  is always positive.At the same time,  and  may have negative currents.
Fig. 6 shows the winding current provided by the full-bridge power converter for the SRM.Half of the winding current in this cycle is positive, and the other half is negative.The diode can act as a forward conduction device in the cycle change when the C-phase winding current is positive.At this time, there is a tendency for the A-phase and B-phase winding currents to drop from the current cycle change in Fig. 4 to the cycle change in Fig. 6, and this tendency also causes the A-phase and B-phase windings to produce negative currents.

Fig. 6 Windings current provided by full-bridge power converter
The SRM three-phase winding currents in the ring structure (Fig. 7) are six-pulse currents, and  ,  , and  are the three-phase winding currents of the motor.In the current cycle, the C-phase winding currents change in the order of 0, 0.5 , 1.5 , 2 , 1.5 , 0.5 , and 0. When the Cphase winding current is 0 A, the diode is in reverse cut-off status.After the C-phase winding current transforms from 0 A to 0.5 ,  is larger than 0 A due to the connection among the three-phase windings.Thus, the diode is in the forward conduction stage, and there is a decreasing trend for the Aphase and B-phase currents.In the subsequent phase, the minimum current of the B-phase appears later than that of the A-phase, so the maximum negative current appears in B-phase in one current cycle.
where n is the motor speed,  is the number of motor rotor poles, and R is the motor B-phase winding resistance. , is the motor winding current provided by the full-bridge power conversion circuit for B-phase (as shown in Fig. 6). ,  is the winding current in the ideal case,  of it is 0 A with the period variation of Fig. 4, and  of it is related to the motor load torque.Y represents the tendency of the A-phase and B-phase winding currents to drop.
We can obtain the maximum negative currents that could occur in the B-phase winding by substituting  and specific switching speeds for the above equation.Similarly, we can invert the appropriate switching speed according to the maximum negative current allowed.When the motor speed exceeds the switching speed, the diode will replace the voltage regulator and the maximum negative current of the motor winding will be maintained within the appropriate range.

Simulation analysis
This paper uses Simulink/MATLAB as the simulation tool for the proposed driving strategy.Fig. 8 shows the block diagram of the proposed control strategy.In the proposed control strategy, when the motor speed is less than the switching speed, we use a DC-DC converter to boost the winding current.When the motor speed is greater than the switching speed, we use a diode to ensure proper operation of the motor.
The paper uses a DC-DC half-bridge converter circuit as a DC-DC converter.When the motor speed reaches the switching speed, a shutdown signal is an input for the power device of the halfbridge converter circuit, and the diode on its load side ensures the stable operation of the motor.
The paper used a three-phase SRM model in the simulation.When the motor runs at a low speed, the paper uses a DC-DC converter to ensure that  is always greater than 0.5 A. With a 300-rpm speed and a 5 N*m load, Fig. 9 compares the C-phase winding current before and after switching.Fig. 9 Comparison of C-phase winding currents in two operating states of SRM From Fig. 9, the winding currents of SRM in high-speed and low-speed operation are both sixpulse trapezoidal currents, and they change phases every 60 degrees.The two current waveforms are the same.However, the winding current of SRM in the low-speed operation state is about 0.5 A larger than that in the high-speed passive operation state.Fig. 10 Three-phase winding current in high-speed active state Fig. 10 shows the three-phase winding currents of SRM in high-speed passive operation.From Fig. 10, we can see that the winding current varies from 0 A to 3.3 A. The C-phase winding current is positive, but there is also a tiny negative current due to the diode's buffer circuit.
As mentioned before, the A-phase and B-phase winding currents are not directly in series with the diode, so there is a significant negative current.The two phases can generate a smaller negative current with a higher speed.The formula mentioned before can select the appropriate switching speed to ensure that the negative current is within the acceptable range when the motor runs at high speed.
The simulation results demonstrate that the optimization of the ring structure in this paper works well and achieves the expected results.

Conclusion
This paper proposes an optimization strategy based on the SRM drive system with a ring winding structure, in which a DC-DC converter is used to lift the winding current during the motor starting and low-speed stages to ensure average motor starting.A diode ensures motor operation after the motor speed reaches the switching speed.For this optimization strategy, the paper derives the switching speed calculation method, makes the strategy simulation, and gets the expected results.

Fig. 4
Fig. 4 SRM winding current in the ring structure

Fig. 7
Fig. 7 Three-phase winding current in ring structureIn the current cycle, the B-phase winding minimum current is:

Fig. 8
Fig. 8 Block diagram of the multi-stage control based on ring winding structure