A new adaptive flux weakening method of PMSM

I. INTRODUCTION
Permanent magnet synchronous motors (PMSM) are more and more used because of their high power density, high power factors and high torque to current ratio. The PMSM are suitable for the application of electrical vehicle and traction systems on the purpose of reducing system volume and weight. In electrical vehicles, usually battery is used as the energy storage and power source whose output voltage is limited. To reduce energy loss, the gear box is replaced by a direct drive system. So the PMSM motors are usually needed to run in high speed and flux weakening region, especially in traction system.
Abstract — In this paper, an adaptive flux weakening method for PMSM is proposed. The demagnetization current can be obtained by an adaptive PI regulator according to the reference voltage output by the current loop controller and the voltage limitation of the inverter. This method can reduce the dependency on motor parameters and improve the robust and precision of flux weakening control. To avoid the damage on rotor permanent magnet by the flux-weakening current, a rotor flux observer based on extended Kalman filter is proposed to realize the real-time flux monitoring. Some simulation and experiments are implemented to verify the performance of the proposed method. Keywords: PMSM, adaptive flux weakening, flux observer, extended Kalman filter
(5b)
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The voltage limitation Umax is determined by the inverter
and the power sources of the system. Taking d and q-axes
q-axe currents in (2) increase the difficulty of real-time control.
The real-time calculation of the optimal d-axe and q-axe
currents to realize MTPA by (2) is very difficult. So even if in
uq = ω Ld id + ωψ d
(4b)
In PMSM control, the voltages limitation is:
u = ud 2 + uq2 ≤ U max
(5a)
That is to say:
( ) ( ) Lqiq
2+
Ld id +ψ d
2
≤
⎛ ⎜⎝
U max ω
⎞2 ⎟⎠
The electromagnetic torque of the PMSM is:
( ) Tem = pnψ diq + pn Ld − Lq idiq
(2)
Where pn is the number of poles pairs.
In salient PMSM, there is Ld < Lq . According to (2), the
salient motor with Ld = Lq , (2) becomes:
Tem = pnψ diq
(3)
So in non-salient motor the MTPA control is the same as
zero d-axe current control.
In salient motor, the crossing-coupling between d-axe and
axe current because of the current limitation in PMSM. So the
maximum torque per ampere (MTPA) is proposed to search the optimal d-axe and q-axe current selection [6-7]. In non-
flux weakening control of PMSM is usually needed. From (1),
if ignoring the effects of the stator resistances and inductances,
there are:
ud = −ω Lqiq
(4a)
+
Ld
did dt
− ω Lqiq
(1a)
uq
=
Riq
+
Lq
diq dt
+ ω Ld id
+ ωψ d
(1b)
Here ud , uq and id , iq are the stator voltages and currents on d
and q axes. R is stator resistance. Ld and Lq are stator inductances on d and q axes respectively. ω is rotor electrical angular speed. ψ d is the d-axe flux produced by the rotor permanent magnet.
of rotor magnet. Some simulation and experiments are implemented to verify the performance of the proposed adaptive flux-weakening control method and flux observer based on EKF.
electromagnetic torque will be increased if id < 0 which is a
very good character in flux weakening region. But the
increasing of the d-axe current will limit the amplitude of q-
the control of salient PMSM, the d-axe current is usually
controlled to be zero to simplify the control system.
In some of the applications, such as traction system, the
A New Adaptive Flux Weakening Method of PMSM
Zedong Zheng, Yongdong Li, Kui Wang, Lin Qiu
Dept. Electrical Engineering, Tsinghua University, Beijing, China E-mail: zzd@
II. PRINCIPLE OF THE FLUX WEAKENING METHOD
In rotor flux oriented coordinate (d-q axes), the stator voltage equations of the PMSM are as follows:
ud
=
Rid
In this paper, a simple adaptive flux weakening method is proposed. The demagnetization current can be calculated by an adaptive PI regulator according to the voltage and current limitation which is also adopted in [5]. In flux weakening control of PMSM, the demagnetization current will be carefully controlled to avoid damage to the rotor permanent magnet. So a rotor flux observer based on extended Kalman filter (EKF) is proposed also in this paper to monitor the state