A Novel Control Strategy for HEV Using Brushless Dual-Mechanical-Port Electrical Machine on Cruising Condition

Abstract

Brushless Dual-Mechanical-Port Electrical Machine (BLDMPEM) is a new type of motor designed for Hybrid Electric Vehicle (HEV), which contains two mechanical ports and two electric ports. Compared with Dual-Mechanical-Port Electrical Machine (DMPEM), the brushless structure brings higher reliability and easier maintenance. In this paper, the model of BLDMPEM is discussed. In Chapter 2, the energy flow and mathematical model of BLDMPEM are analyzed. Then a novel three-phase half-bridge controlled rectifier topology and its control strategy for cruising mode of HEV based on BLDMPEM are proposed in Chapter 3. Compared with the Field Oriented Control (FOC) strategy of BLDMPEM, the proposed method does not require accurate motor parameters, and it is much simpler and easier to be implemented. At last, simulation and experiment results show the feasibility and validity of the proposed strategy.

1. Introduction

HEV, as a transitional product from the conventional vehicle to electric vehicle, gets more and more attention. This type of vehicle, which inherits higher efficiency and lower pollution, has been an important field of the car’s development [1]. HEV usually contains generator, start/ generator, electric motor and a complex planetary gear, which is to operate the Internal Combustion Engine (ICE) to at optimum efficiency during all driving conditions with the controlled torque and speed of ICE. Nowadays, some of motor companies, such as Toyota and Ford, have produced their own HEVs [2]. Though the planetary gear is a very smart structure to deliver energy, the mechanical loss caused by the planetary gear can inevitably affect the efficiency and shorten the life of vehicle. Dual-Mechanical- Port Electrical Machine (DMPEM), a new type of motor, which has two mechanical ports and two electric ports, can just control both the torque and speed of ICE to meet the demand of ICE operating at optimum efficiency [3-10]. The working principle, winding characteristics and control strategies of DMPEM are analyzed [5, 6]. And some interesting conclusions and data are also given. Fig. 1 is the Electrical Variable Transmission (EVT) system based on DMPEM. The inner rotor usually connects to ICE, and the outer one connects to the automobile wheels. With independent control of the two electric ports, a good performance of both ICE and the automobile wheels’ speed and torque can be achieved, to meet the high efficiency of ICE and the requirements of the vehicle. An concrete control strategy of EVT based on DMPEM in HEV which mainly concerns the inner rotor torque control and the outer rotor speed control, is given in [10], to satisfy the optimal efficiency of ICE and the load demands. Besides, only a small part of the ICE’s energy is converted from the inverters, which reduces the size and cost of the system.

Although DMPEM has the above advantages, the brush structure is still a hidden problem in operation. A new type of DMPEM, which named brushless DMPEM (BLDMPEM), is proposed in [11-17].

The higher reliability and easier maintenance are realized because of the brushless structure. Different from DMPEM, BLDMPEM (in Fig. 2) uses another equivalent motor instead of the brushes and slip rings, so BLDMPEM can be seen as a combination of three motors. BLDMPEM is a very unique structure machine, and its magnetic properties are still complex, especially considering the impact of saturation. BLDMPEM can be equivalent to three motors, one more than DMPEM. In order to realize the Field Oriented Control (FOC) of BLDMPEM, more equivalent parameters are needed, and the impact of the variable parameters with the operating condition is also needed to be considered. Obviously, the control strategy based on FOC for BLDMPEM is too complicated.

Fig. 1.Control system with DMPEM

Fig. 2.Control system with BLDMPEM

This paper focuses on the two key issues of BLDMPEM system. Firstly, principle and structure of the system are proposed and analyzed; then the control strategy of HEV based on the system in cruising mode is given. In details, for first part, in chapter 2, the brushless system structure of Electrical Variable Transmission (BLEVT) based on BLDMPEM and the simplified equivalent circuit of the control port IV1, as shown in Fig. 3, is analyzed. Then the operating conditions of BLDMPEM based the HEV and a novel control strategy, named power direct control, which composes of a three-phase half-controlled rectifier circuit is proposed in Chapter 3. Finally, the feasibility and validity of the proposed topology and control strategy are verified using the simulation and experiment in Chapter 4.

Fig. 3.BLEVT based on BLDMPEM

 

2. Analysis of the system model

2.1 Principle of BLEVT system based on BLDMPEM

BLDMPEM can be a combination of three motors without considering the magnetic coupling among the stator and the inner and outer rotors, include electrical motor (EM) A, which contains the permanent magnet on the inner rotor and EM A stator winding, EM B, which contains EM B rotor winding on the outer rotor connected with EM A winding in reverse order, and EM C, which contains EM C rotor winding on the outer rotor and EM C stator winding on the stator, shown in Fig. 3.

In order to achieve a desired control performance of HEV, BLDMPEM based BLEVT system is set up (in Fig. 3). The system includes ICE, BLDMPEM, bidirectional converter, and battery. Using this system, the demands of HEV operation can be achieved while ICE is working at its optimal efficient operation condition.

2.2 Model of BLDMPEM

BLDMPEM is a non-linear coupling system. In order to focus on the main characteristics of machine, we assume that: (1) Windings on the stator and outer rotor are threephase symmetrical sinusoidal. (2) Permanent magnet flux in the air gap is sinusoidal. (3) The effects of hysteresis, eddy current, and magnetic saturation are all ignored.

As shown in Fig. 2, BLDMPEM can be equivalent to three motors. And we consider motor A and B as generators, and motor C as a motor. Then three motor coordinates are defined as follows: EM A: The electrical angular velocity of the coordinate is ωA = PA *nO *2π / 60 . And the positive direction of the magnetic field rotation is counterclockwise. EM B: The electrical angular velocity of the coordinate is ωB = PB *nO *2π / 60 . And the positive direction of the magnetic field rotation is clockwise. EM C: The electrical angular velocity of the coordinate is ωC = PC *nO *2π / 60. And the positive direction of the magnetic field rotation is counterclockwise.

The synchronously rotating frames of the three equivalent motors are all fixed on the outer rotor. Therefore, the flux expressions and voltage expressions can be obtained.

Then, combing the flux and voltage expressions, the mathematic model can be given in matrix form as:

Here nO is the speed of the outer rotor. ΩO is the mechanical angle speed of the outer rotor. PA , PB , PC are the number of pole pairs of EM A, B and C, respectively. udsB , uqsB are peak values of EM B stator input voltage. udsC , uqsC are peak values of EM C stator input voltage. idsB , iqsB are peak values of EM B stator current. idsC , iqsC are peak values of EM C stator current. idrB , iqrB are peak values of EM B rotor current. idrC , iqrC are peak values current of EM C rotor. lsA , lsB , lsC are inductances of EM A, EM B, and EM C stator. lmB , lmC are coupling inductances of EM B and EM C. rrB , rrC are resistances of EM B’s and EM C’s rotor. p is d/dt , and φf1 is peak value of permanent magnet flux linkage.

The equation of torque is

Here idsA , iqsA are peak values of EM A stator current, idrA, iqrA are peak values of EM A rotor current, φdsA , φqsA are peak values of EM A’s air gap flux linkage, and TA , TB, TC are peak values of torque of EM A, B, C.

Equation of speed:

Here TICE is peak value of ICE’s torque, TA , TB , TC are peak values of EM A, EM B and EM C, TL is load torque, and JI , JO are inner and outer rotor inertias.

Assume that the direction of the rotation of the motor is as shown in the following figure.

The mechanical speed of the inner and outer rotor is nI and nO . As shown in Fig. 4, EM A stator winding and EM B rotor winding are connected reversely in phase, so that:

Fig. 4.Direction of the rotation of BLDMPEM

The frequency of EM B stator winding can be expressed as:

The slip of EM B is:

The positive direction of electrical angle of the EM B stator and the inner rotor is opposite. If we take the equivalent rotation direction of EM B as the positive direction, the slip rate of EM B can be given as:

Converting the parameters of EM A and EM B to EM B stator, then the expression of EM A is given in the form:

And , ,

Here is peak value of EM A phase voltage, is peak value of EM A back Electromotive force (Back-EMF), and XσsA is leakage reactance of EM A.

The model of EM B can be given as:

Here is peak value of EM B stator phase voltage, is peak value of EM B stator Back-EMF, is peak value of EM B rotor equivalent phase voltage, peak value of EM B rotor equivalent Back-EMF, is peak value of EM B rotor equivalent current.

Because EM A stator winding and EM B rotor winding are connected in reverse, the currents and voltages of the two parts are equal.

The equivalent circuit can be obtained as shown in Fig. 5 by combining the circuits of EM A and EM B in Fig. 2.

shows the equivalent power from ICE, and it expresses the mechanical port connected with the internal combustion engine. shows the equivalent power of the mechanical energy passing from the inner rotor to outer rotor through EM A.

Here . is Back-EMF of EM A, which is proportional to the slip speed between the inner and outer rotors. represents a voltage source, which delivers the mechanical power of EM B stator to the outer rotor.

Because of , and when ignoring , EM A and EM B can be simplified to a simple circuit, which contains an equivalent voltage source, an equivalent resistance and an equivalent series inductance, as shown in Fig. 6.

Fig. 5.The equivalent circuit of EM A and EM B

Fig. 6.The equivalent circuit of the control port IV1

And the equivalent voltage source is shown as:

The equivalent resistance is:

The equivalent reactance can be given as:

So EM A, EM B and the control port IV1 of EM B can be simplified as the following equivalent circuit diagram:

 

3. Control Strategy

3.1 Analysis of BLDMPEM on the cruising condition

There are four main operating conditions of HEV, which are starting engine, low speed, braking and cruising. And cruising is one of the most important HEV’s operating conditions. When the vehicle is cruising, ICE is on, and both EM A and EM B are generating, and EM C is motoring. Fig. 7 is a diagram of the energy flowing for cruising condition. The input power PICE of EM A, contains the winding copper losses of EM A stator and EM B rotor, the electrical power PemB , and a direct mechanical power PmA of the outer rotor. The electrical power without the mechanical power PmB is the feed back power to the battery. The mechanical power of the outer rotor can be equivalent with the sum of three motors.

Fig. 7.Energy distribution with the cruising condition

Obviously, the main purpose of IV1 part is to achieve part of the energy of EM A and EM B back to the battery on the cruising condition.

3.2 Control of BLDMPEM on the cruising condition

EM A and EM B are running as generators, and EM C is running as a motor, when the vehicle with BLDMPEM is on the cruising condition. In order to control the speed of EM C, vector control method (normally, the Field Oriented Control (FOC)) of the conventional induction motor can be used. Here we use both inside and outside motors’ encoder position signals, to make sure the outer rotor’s speed satisfy the operating conditions.

For the control of the power ports (in Fig. 1), a control strategy to achieve the optimal efficiency operating of ICE and meet the operating requirements is proposed in reference (10), which gives the power reference of EM A and EM B back to the DC energy, according ICE’s best efficiency curve. Different from BLEVT, EVT in reference (10) can be equaled to two permanent magnetic synchronous motors (PMSMs), and the exact position of the flux linkage can be obtained by the encodes set on the inner and outer rotors, so FOC is used in EVT for the desirable performance.

BLDMPEM can be equaled to three motors (one PMSM and two asynchronous motor) that is different from DMPEM. The model of BLDMPEM shown in equations (1), (2) and (3) are very complex, based on which the control is complex too. The exact parameters, which are nonlinear with the temperature and current and difficult to obtain, are needed to be known. The control performance of BLEVT is affected by the precision of the magnetic chain position which is decided by the precision of the magnetic chain position. Hence FOC for BLEVT is very more difficult to be realized.

With the simplified model for EM A and B in Chapter 2.2, the phase angle of Ėeq is the key factor to achieve the precise decoupling control of the two strong coupling motors (EM A and EM B), with one electric port, when EM A and EM B are generating. The exact parameters of the motor, which are varying with the operating conditions, are needed to obtain the exact phase angle. Considering the equivalent simplified model according to the previous analysis, a simple circuit topology shown in Fig. 8, is proposed, which can meet the requirements of BLDMPEM based on HEV during the cruise condition.

Fig. 8.Three-phase half-controlled rectifier circuit

Using the topology shown in Fig. 8, the energy of EM A and EM B back to the DC bus can be controlled to achieve a reasonable distribution of HEV. The three upper switches are not controlled, and the same drive signal is given to the three lower switches. The analysis of the three-phase halfcontrolled rectifier circuit on different operating conditions is as follows:

There are six cases of the directions of the three-phase currents without considering the zero current case, since the AC current of the inductor can not change suddenly, The operating states of the current according to the phase angle of the voltage source can be as Fig. 9 when the threebridge lower arms have drive signals.

The operating states of the current according to the phase angle of the voltage source can be as Fig. 10 when the lower switches are off.

The solid lines in Figs. 9 and Fig. 10 represent the current flowing through the circuit. The current paths of the on and off states of the switches with the same current directions are shown in Figs. 9 (a) and Fig. 10 (a). The current directions of the B and C phases are negative (defining the positive direction as the flowing out the threephase voltage source). Hence, even if there are drive signals for the lower switches, the current still flows from the freewheeling diode of the B and C phases. The direction and size of the AC currents are determined by the voltage of the voltage source and the states of the switches.

Fig. 9.The circuits with the lower switch on

Fig. 10.The circuits with the lower switch off

Fig. 11.The overall control diagram of BLDMPEM on the cruising condition

The states of the lower switches in Figs. 9 and 10 show that i0 is zero while the lower switches are on, and i0 is positive while the lower switches are off, without considering the operating condition is. The DC current i0 can be controlled through the control of the lower switches, so that the energy which transferred from the AC side to the DC side can be controlled. The energy can be fast tracked without the phase angle of the three-phase voltage with three symmetrical phases. And the control strategy of the three-phase half-controlled rectifier circuit, which named as power direct control strategy, is obtained. The overall control block diagram is shown in the Fig. 11.

The energy flowing back to the battery through the electrical port IV1, is needed to be controlled on the cruising condition. The control strategy, which realizes the closed loop control of the DC current i0 based on proposed three-phase half-controlled rectifier circuit, can be used to control the power back to the DC bus through the IV1 port. And the reference power is gotten by the total control system which mainly concerned the whole vehicle efficiency. In order to ensure the outer rotor speed to meet the cruising condition, a conventional FOC for the equivalent motor C is used.

With the proposed control strategy in Fig. 11, the energy back to the battery can be controlled without the equivalent parameters, to meet demand of the cruising condition, as the same time, to make sure ICE operating on the optimum efficiency curve.

 

4. Control Analysis Using Simulation and Experiment

In order to verify the proposed methods, a three-phase half-bridge rectifier control model as shown in Fig. 12 is built up.

The parameters of BLDMPEM are shown in Table 1.

The simulation parameters are shown in Table 2 in order to make the simulation model closer to a reality system.

When the feedback power to DC side is PeB = 300W , the simulation wave is shown in Fig. 13.

As shown in Fig. 13, the voltage of the DC bus is around 101.5V, and the circuit is on the non-controlled rectifier condition before the rectifier work.

The purpose is to prevent the excessive inrush current. And then the three-phase half-bridge rectifier works. Therefore, the energy transformation can be achieved by using the proposed three-phase half-bridge rectifier and its control strategy in the paper.

Fig. 12.Simulation model of the three-phase half-bridge rectifier

Table 1.BLDMPEM designing parameters

Table 2.Performance three-phase half-bridge rectifier

Fig. 13.The simulation results of the three-phase halfbridge rectifier

Fig. 14.LDMPEM

Fig. 15.The hardware of the experiment platform

BLDMPEM and rotors are shown in Fig. 14.

The controller hardware and whole experimental platform are shown in Fig. 15. The experimental platform contains two permanent magnet synchronous motors, one as simulation engine and the other as load.

To simulate the operating condition, we use a permanent magnet synchronous motor as ICE connected with the inner rotor, and use another permanent magnet synchronous motor as a load connected with the outer rotor. And the speed of the inner rotor (N1) is 600r/min, and the speed of the outer rotor (N2) is 20r/min. The experiment waves are shown in Fig. 16, in which we apply the control method as shown in Fig. 9. Here the power back to DC side is 300W.

In Fig. 16, wave 1 and 4 are the currents of motor B’s stator, and wave 2 is pulse drive signal. We can see that the energy transformation can be achieved by using the topology and control strategy proposed in this paper.

Fig. 16.The currents of the electrical port of the motor B’s Stator

Fig. 17.I0*=3A, N2=20rpm, N1 rises from 0 to 600rpm, without load

In Fig. 17, The motor B is used as generator back to DC bus, and the current on the DC bus () is set as 3A. The control strategy is shown in Fig. 11.Wave 1 is the A phase current of the motor B’s stator, wave 3 is the voltage of the DC bus, The load permanent magnet machine’s stator is open, in other words, the BLDMPEM is not loaded. Wave 4 is the A phase current of the motor C’s stator. Here, N2 is 20r/min and N1 is 0 to 600r/min.

In Fig. 18, I0* is also set as 3A, N2 is 100r/min and N1 is 0 to 700r/min. The control strategy shown in Fig. 11 is also used. The load permanent magnet machine’s stator has 1Ω resister load for each phase. Wave 1 is A phase current of Motor B’s stator, the unit is 2A/Div; Wave 2 is A phase current of Motor C’s stator, the unit is 5A/Div.

In Fig. 18, when N1 is higher than a certain speed, the DC bus current can fit the given value I0*, so in cruising mode, the speed difference between two rotors should large enough.

Obviously, a good performance can be achieved by using the topology and control strategy proposed in this paper, when the HEV with BLDMPEM is during the cruising condition.

Fig.18.I0*=3A, N2=100rpm, N1 rises from 0 to 700rpm, the load PM machine stator has 1Ω resister load waveforms

 

5. Conclusion

A novel circuit topology and its control strategy are proposed to achieve good performance, for BLDMPEM based HEV during the cruising condition. The feasibility of the strategy is verified by the simulation and experiment results. And the proposed circuit and its control strategy can also be used to the similar double-fed motor, for other special working condition, or failure running.

BLDMPEM can also be applied to some other applications, such as offshore wind generator, mining, and so on.