6.2 Field Oriented Current Control Scheme
6.2.1 Description and features of the current control scheme
Fig. 6.2 illustrates a block diagram of the speed control system implemented with current vector control algorithm [52], which includes three parts: current command generator, current regulator, and flux-weakening voltage controller.
Current Limit
Ism
Ism
iq
id
ωb
A B
MTPA (Region I)
ωm
M Flux-weakening
(Region II)
Figure 6.1: The optimum current profile in the d-q coordinate plane forλm> LdIsm
Encoder PI
MTPA
PI Vd*2+Vq*2
Vsm 2
* 2
ds sm I I −
Current Regulator
*
Ids
*
Iqs
*
Id
*
Iq
ωs
*
ωs
*
Id
∆
VSI-PWM INVERTER
IPM
Vdc
Ia
Ib
∫ θ
dq abc
Va
Vc
2
*
Vd Vq*2
*
Vdq
Idq
ωs
Current Command Generator
FW Voltage Controller
Figure 6.2: Block diagram of current controlled IPMSM drive system
The current command generator provides the reference q-axis current Iq∗ and the reference d-axis currentId∗. Iq∗ is decided from the speed error (ωs∗−ωs) through the proportional-integral controller. The d-axis current command Id∗ is decided by (6.1) in the MTPA line (Region I) until the current regulator begins to saturate.
Id∗ = λm
2(Lq−Ld)−
v u u t
λ2m
4(Lq−Ld)2 +Iq∗2 (6.1)
Fig. 6.3 shows the details of current regulator. The d- and q-axis currents cannot be controlled independently by Vd and Vq because of the cross-coupling effects such asωsLdId andωsLqIq as shown in (2.12). The cross-coupling effects of the interior PMSM are dominant because the interior PMSMs have relatively large inductances. These effects increase as the speed increases. Therefore, it seems that the current responses as well as torque response are affected by the cross- coupling effects in the high-speed flux-weakening region. The cross-coupling effects can be cancelled by the feed forward compensation as shown in Fig. 6.3. The voltage commands are decided by the PI controller and decoupling feed forward compensation, thus the d- and q-axis current control loops can be linearized by the above decoupling current control.
Without a proper flux-weakening operation, the current regulator would be saturated and lose its controllability at a higher speed. Since the onset of the current regulator saturation varies according to load condition and the machine parameters, the beginning point of flux weakening and the flux level should be changed. A delay in flux-weakening may result in undesired output torque drop
PI
d d s m s
q L I
V0=ωλ +ω
q q s
d L I
V 0=−ω
PI
Id
Iq
*
Id
*
Iq
ωs
+
+
++
++ *
Vq
*
Vd
Figure 6.3: Current regulator with decoupling feedforward compensation
according to the saturation of the current regulator, but an early start deteriorates the acceleration performance. Therefore, it is desirable to change the onset point of flux-weakening according to load condition and the machine parameters.
The main idea of flux-weakening voltage controller in current control scheme is the use of the output reference voltage of the synchronous PI current regulator to identify the onset of the flux-weakening. As the speed of the interior PMSM gets higher during acceleration, the output of the current regulator, especially the q-axis current regulator, increases and approaches to the boundary of the pulse width modulator. Without a proper counter measure, the performance of the current regulator gets worse due to the reduced margin of the voltage and finally it loses its controllability. As shown in Fig. 6.2, the voltage regulator ensures the margin of the voltage and increases the d-axis current towards the negative direction to prevent saturation of the current regulators. With this outer voltage regulating loop the flux level is inherently adjusted and the flux-weakening operation is accomplished
automatically. At the low and intermediate speed region, the magnitude of the output voltage of the current regulator Vs is usually less than Vsm and thus flux- weakening controller is not activated. Even in this case, if the dc link voltage Vdcdrops suddenly, the flux-weakening operation can be carried out automatically.
The incremental d-axis current4Id∗ serves as the control input to the d-axis current for flux-weakening operation.
The basic action of q-axis current limiter is to decrease the q-axis current command Iq∗ in response to the presence of a growing d-axis current command Id∗, so that the total current is within the limit. By decreasing the q-axis current command Iq∗ in current limiter and increasing d-axis current commandId∗ towards the negative direction at the same time, the current regulators are able to regain the practical ability of regulating the d-axis and q-axis current (Id∗, Iq∗) along the current limit circle (B →M) in Fig. 6.1. The PI output ofIq∗ is limited by Iqmax, which is determined from (6.2) as shown in Fig. 6.2.
Iqmax =qIsm2 −Ids2 (6.2)
In the constant torque region, current limiter is not activated, the d- and q-axis current (Ids∗ , Iqs∗) are on the MTPA trajectory. If current limiter is activated, the current vector (Ids∗ , Iqs∗), which initially lies on the MTPA trajectory and inside the voltage limit ellipse at a given speed (Region I: A → B, in Fig. 6.1) is forced to move along the boundary of the current limit circle (Region II: B → M) as the speed increases. No additional feedback signal is required to implement the flux-weakening algorithm except the phase currents, the rotor position,Vdcand the
speed feedback, which are already available for the speed control in the constructed system as shown in Fig. 6.2.