Electricity: motive power systems – Synchronous motor systems – Armature winding circuits
Reexamination Certificate
2000-04-19
2001-09-11
Ip, Paul (Department: 2837)
Electricity: motive power systems
Synchronous motor systems
Armature winding circuits
C318S716000, C318S807000
Reexamination Certificate
active
06288515
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to electric vehicles and, more particularly, to a system and method for controlling a surface-mounted permanent magnet synchronous machine drive used in electric vehicles over a wide speed range using a reference voltage.
The assignee of the present invention designs and develops electric vehicles employing surface-mounted permanent magnet synchronous machine drives. One operation that is required when controlling such machine drives is flux weakening, which is when the machine drive operates above base speed.
One straight-forward approach is to employ a number of look-up tables to resolve flux weakening operation. In order to achieve results and performance similar to those of the present invention, the straight-forward approach mentioned above requires creation of numerous and cumbersome data structures, such as look-up tables, to handle all possible situations in the system and its environment.
A method presented in a paper authored by J. H. Song, J. M. Kim and S. K. Sul, entitled “A New Robust SPMSM Control to Parameter Variations in Flux Weakening Region”, in
Proc. IECON'
96, pp. 1193-1198, 1996, attempts to provide a solution to the problem solved by the present invention. The Song et al. approach requires measurement of DC bus voltage.
It would be desirable to have a method that does not rely on look-up up tables and wherein automatic transition is accomplished at any operating conditions. It would also be desirable to have a method that does not require measurement of the DC bus voltage.
It is, therefore, an objective to provide for a system and method for controlling a surface-mounted permanent magnet synchronous machine drive over a wide speed range using a reference voltage. It is also an objective to provide for a system and method for controlling a surface-mounted permanent magnet synchronous machine drive used in electric vehicles.
SUMMARY OF THE INVENTION
The present invention comprises a surface-mounted permanent magnet synchronous machine drive and a method of controlling the machine drive. An exemplary surface-mounted permanent magnet synchronous machine drive comprises a motor, a voltage source inverter coupled between a battery and the motor that drives the motor, and a control system coupled to the inverter for controlling the inverter and the motor. The heart of the control system is a flux weakening circuit which functions to reduce the effective back emf reflected on a stator winding by injecting a negative current in the north rotor pole direction, and therefore weakening the rotor magnet effect on the stator winding voltage. In the invention, a feedback-based flux weakening strategy is used which is independent of motor and system parameters.
An exemplary control system comprises a first coordinate transformation circuit for processing measured motor phase current signals (i
a
, i
b
) and rotor position signals (&thgr;
r
) to generate motor current signals in the synchronous frame (i
d
, i
q
). A flux weakening circuit processes torque command (T
ref
), and d-axis and q-axis modulation index signals (d
d
, d
q
) calculated in the previous sampling interval to generate reference current signals (i
d
ref
, i
q
ref
). A current regulating loop processes the reference current signals (i
d
ref
, i
q
ref
) and the motor current signals (i
d
, i
q
) to generate new d-axis and q-axis modulation index signals (d
d
, d
q
). A second coordinate transformation circuit transforms the d-axis and q-axis modulation index signals (d
d
, d
q
) into modulation index signals in the stationary coordinate frame (d
&agr;
, d
&bgr;
). A space vector modulator having a continuous transition into the six-step mode of inverter operation modulates the modulation index signals in the stationary coordinate frame signals (d
&agr;
, d
&bgr;
) to produce outputs that drive the voltage source inverter.
An exemplary method comprises the following steps. A torque command comprising a q-axis current reference signal (i
q
ref
) is supplied. The q-axis current reference signal (i
q
ref
) is limited. A d-axis current reference signal (i
d
ref
) is generated in a flux weakening loop. The current reference signals (i
d
ref
, i
q
ref
) are compared with d-axis and q-axis motor current signals (i
d
, i
q
) to produce current error signals. The current error signals are compensated to generate modulation index signals (d
d
, d
q
). The sum of the squares of the modulation index signals (d
d
, d
q
) is generated. The sum of the squares of the modulation index signals (d
d
, d
q
) is compared with the square of a modulation index (d) to produce a modulation index error signal. The modulation index error signal is adjusted (using proportional-integral regulation, for example) to generate the d-axis current reference (i
q
ref
).
The present invention does not rely on look-up tables; the automatic transition is accomplished at any operating conditions. When compared to the method disclosed in the Song et al. paper, a major improvement provided by the present invention is that the measured value of the DC bus voltage is not needed to determine the on-set point for the flux weakening mode. In the present invention, the on-set point for the flux weakening mode is uniquely determined by the modulation index (d) at the boundary of six-step operation, and is the same for all values of DC bus voltage.
This aspect of the present invention provides for the ability to adjust the on-set point by changing the modulation index (d
m
2
). For instance, when six-step operation should be disabled, the flux weakening on-set point (d
m
2
) may be simply adjusted using software to a lower value, so that the flux weakening loop becomes active at lower speeds and/or lower torque values. In this way, sufficient voltage margin is provided automatically by injecting negative i
q
ref
, and the inverter does not need to operate in full six-step mode to assure the current control. The present invention may also be used for speed or position control, if desired.
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Hiti Silva
Maric Dragan S.
Stancu Constantin C.
DeVries Christopher
General Motors Corporation
Ip Paul
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