EVS 25 Shenzhen , China , Nov 5-9 , 2010 Flux Weakening Control for Stator-Doubly-Fed Doubly Salient Motor

To satisfy the demands of electric vehicles (EVs) of fast torque response in wide range of velocity, the principle of flux weakening of the stator-doubly-fed doubly salient (SDFDS) motor which is designed for EVs has been studied and analyzed. And its advantage of regulating the field flux independently has been fully utilized. The flux weakening strategy is proposed and high performance of SDFDS motor drive is acquired at high speed. Simulation and experiment are carried out based on Matlab/Simulink and experiment platform. The simulation and experimental results with good speed adjusting performance verify the effectiveness of the flux weakening control strategy of SDFDS motor drive.


Introduction
As energy crisis and environment pollution are increasingly intensified over the world, the electric vehicles (EVs) become the competitive substitute for gasoline vehicles which consume lots of oil and exhaust emissions [1].In order to enable EVs directly competing with gasoline vehicles, the aims of the EV motor drive are to pursue high efficiency over wide speed range, high power density, high controllability, high reliability and maintenance free operation [2]- [6].As so far, four types of motor drives have been applied to EVs.They are brushed DC motor drives, induction motor (IM) drives, PM synchronous or PM brushless motor drives, and switched reluctance motor (SRM) drives [2]- [3].As a novel motor designed for EVs, the stator-doubly-fed doubly salient (SDFDS) motor [4]- [5], [7] has the feature of regulating the field flux independently and effectively, which makes the implementations of the flux weakening for expanding the speed range and on-line efficiency optimization easier than other motors [8]- [9].Fig. 1 shows the cross-section of a 3-phase 12/8-pole SDFDS motor.
The back electromagnetic force (EMF) becomes so big when the motor is at high speed that the flux has to be weakened for speed increasing.The rotor flux of induction motor is in inverse proportion to speed for flux weakening [10].As for PM synchronous motors [11], the demagnetizing current i d is obtained by flux-weakening characteristics as a function of the operating speed.Prior angle control is used for PM brushless motors to increasing the motor speed [12].And the conduction angle is adjusted for high speed running as to the switched reluctance motor [13].The advantage of the SDFDS motor of regulating the field flux independently has been fully utilized in this paper.The flux weakening strategy is proposed and high performance of the SDFDS motor drive is acquired at high speed.Both simulation and experiment are carried out.Good speed adjusting performance is achieved, verifying the effectiveness of the flux weakening control system of the SDFDS motor drive.
T T According to the operation principles of the SDFDS motor, the electromagnetic torque of phase p ( p =A, B, C) can be expressed as: where, 2 1 2 is the field torque.
where, e T is the total electromagnetic torque, is the friction coefficient and T l is the load torque.
According to (3), the motor speed the steady state, namely the steady state motor speed depends on the total electromagnetic torque e T .Based on the above analysis, the increase of back electromagnetic force makes the decrease in phase current when the motor works at high speed.Thus the total electromagnetic torque e T minishes, which restrain the rise of motor speed.
Hence, the motor flux has to be weakened in order to extend the range of speed.As for the SDFDS motor, the flux weakening control can be attained easily by regulating the field current f i , as compared to other motors.

SDFDS Motor
Fig. 3 shows the simulated output torque characteristics versus field current i f at the speeds of 2000，2500，3000 r/min.It can be seen that as the field current decreases, the output electromagnetic torque first increases and then decreases.Thus the speed also first increases and then decreases when the field current decreases.Hence, the key of flux weakening control for the SDFDS motor is to get the field current Based on the above analysis, the flux weakening control strategy of the SDFDS motor can be developed as follows: I.When the reference speed of motor ref n is less than or equal to the rated speed N n , the standard motor control of regulating the speed is adopted; II.When the reference speed of motor ref n is higher than the rated speed N n ， the flux weakening control strategy is adopted for regulating the speed: ① When the feedback speed fb n is less than or equal to the rated speed N n , the standard motor control of regulating the speed is adopted； ② When the feedback speed fb n is higher than the rated speed n N , the flux weakening control strategy is adopted.
The control configuration of flux weakening control strategy of the SDFDS motor drive at high speed is shown in Fig. 4. ;

Simulation
In order to verify the effectiveness of the proposed flux weakening control scheme, a simulation model is developed in Matlab/Simulink software, as shown in Fig. 5.
When the reference speed is 3000 r/min at no load, the armature current, field current, and step response curve of speed are shown in Fig. 6 under the flux weakening control strategy.
As illustrated in Fig. 6, the speed of SDFDS motor can reach the reference value rapidly and there is no steady speed error.The good speed adjusting performance is achieved when the proposed field weakening control strategy is used.

Experimental Results
In order to further investigate the effectiveness of the proposed field weakening control strategy and to verify the theoretical simulation, a prototype of the SDFDS machine has been designed and fabricated, as shown in Fig. 7.The main design data of the prototype are listed in Table 1.The experimental setup consists of a 3-phase 12/8-pole SDFDS motor with its stator connected to an IGBT-based PWM converter.A TMS320F2812 DSP based digital control platform is designed for implementing efficiency optimal algorithm.The block diagram of flux weakening control and experimental system are shown in Fig. 8 and Fig. 9, respectively.The experiments on the prototype machine were carried out at the same operating point as the simulation.Comparing Fig. 10 with Fig. 6 shows that the experimental results agree with simulated results well, verifying the effectiveness of the proposed scheme.

Figure 2 :
Figure 2: Three phase full bridge circuit and its conduction principles the electromagnetic torque reach the peak value.Then the flux weakening component f I conventional PI regulator can be used to realize flux weakening for extending the range of speed.

Figure 3 :
Figure 3: Curve of output torque and field current under different speed

Figure 4 :Figure 5 :
Figure 4: Control configuration of flux weakening control strategy of SDFDS motor drive