Electricity: motive power systems – Constant motor current – load and/or torque control
Reexamination Certificate
2001-03-21
2003-02-25
Fletcher, Marlon T. (Department: 2837)
Electricity: motive power systems
Constant motor current, load and/or torque control
Reexamination Certificate
active
06525497
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus and method for controlling the speed of a 3-phase motor using four switching elements, and more particularly to a phase distortion compensating apparatus and method for reducing a torque ripple in a 3-phase motor using four switching elements, which are capable of directly or indirectly detecting a voltage difference between upper and lower DC link capacitors connected to respective switch legs of an inverter including the switching elements, and adjusting respective switching times of phase voltages, based on the detected voltage difference to reduce a torque ripple generated in the 3-phase motor.
2. Description of the Related Art
FIG. 1
is a block diagram illustrating a conventional 3-phase motor controller using four switching elements. As shown in
FIG. 1
, the 3-phase motor controller includes a pair of DC link capacitors, that is, an upper DC link capacitor
3
and a lower DC link capacitor
4
, respectively adapted to receive a DC voltage rectified from an AC voltage, and to store the DC voltage therein. The 3-phase motor controller also includes a B4 inverter
2
configured to turn on or off in response to a switch control signal when the DC voltage from each of the capacitors
3
and
4
is supplied, thereby supplying a 3-phase voltage adapted to rotate a 3-phase motor
1
. The 3-phase motor
1
is coupled to respective switch legs of the B4 inverter
2
and coupled to a connection node between the upper and lower DC link capacitors
3
and
4
.
Generally, inverters are known which use six switching elements to control a 3-phase motor. However, such inverters are expensive. In order to reduce the costs of such inverters, an inverter has been proposed which uses four switching elements to control a 3-phase motor. An example of such an inverter is the B4 inverter
2
shown in FIG.
1
.
Now, the operation of the conventional 3-phase motor controller using four switching elements as mentioned above to control the 3-phase motor will be described.
When an AC voltage is inputted to the 3-phase motor controller, it is rectified by a rectifier means (not shown) which, in turn, generates a DC voltage. This DC voltage is supplied to the upper and lower DC link capacitors
3
and
4
connected in parallel to each other.
As a result, the upper and lower DC link capacitors
3
and
4
conduct charge and discharge operations in an alternating fashion. The alternating charge and discharge operations are controlled in accordance with respective status changes of switching elements composing the B4 inverter
2
.
The B4 inverter
2
has four switching status, as shown in
FIGS. 3
a
to
3
d
. The following description will be made in association with the case in which the 3-phase motor has a Y-connection. In the following description, “1” means an ON state of the upper switching elements in the B4 inverter
2
whereas “0” means an ON state of the lower switching elements. Where only the upper ones of the switching elements respectively corresponding to four voltage vectors of the B4 inverters are switched on, that is, in a status <1, 1>, the voltage V
1
charged in the upper DC link capacitor
3
is supplied to the 3-phase motor
1
. In this status, no voltage is supplied from the lower DC link capacitor
4
to the 3-phase motor
1
.
On the other hand, when only the lower switching elements are switched on, that is, in a status <0, 0>, the voltage V
2
charged in the lower DC link capacitor
4
is supplied to the 3-phase motor
1
. In this status, no voltage is supplied from the upper DC link capacitor
3
to the 3-phase motor
1
.
In statuses <0, 1>and <1, 0>, both the upper and lower DC link capacitors
3
and
4
supply the voltages V
1
and V
2
to the 3-phase motor
1
, respectively.
In order to allow the 3-phase motor
1
to rotate, it is necessary to generate voltages of three phases each exhibiting a phase difference of 120° from one another, Va, Vb and Vc, as shown in FIG.
2
.
In order to generate these voltages of three phases, one of three nodes in the B4 inverter
2
respectively corresponding to voltage vectors of three phases applied to the 3-phase motor
1
is connected to the connection node between the upper and lower DC link capacitors
3
and
4
, and the remaining two nodes are connected to respective legs between the upper switching elements and the associated lower switching elements.
Also, all 3-phase voltage vectors have not direction, and conclusively “Vu” and “Vw” is only generated, as depicted
FIG. 2
when a voltage-Vc is applied to the voltage Va, Vb, Vc in order to generate voltage having the same effect as balanced 3-phase voltages.
The voltage vectors Vu and Vw serve to generate balanced 3-phase voltages along with a voltage of zero-phase. That is, it is possible to obtain 3-phase balanced voltages using four switches.
Two voltage vectors Vu and Vw generated by the B4 inverter
2
have a phase difference of 60° therebetween, as shown in FIG.
2
. In the case in which the c-phase of the 3-phase motor is connected to the connection node between the upper and lower DC link capacitors
3
and
4
, as mentioned above, the phase of the voltage vector Vu is retarded from the a-phase voltage Va by 30°.
Therefore, where the B4 inverter
2
is controlled using pulse width modulated (PWM) pulses, it is possible to control the 3-phase motor
1
using a switching logic of the B4 inverter
2
expressed by the following Equation 1:
V
u
=
V
a_dc
=
[
1
2
+
1
2
·
ma
·
sin
(
θ
-
π
6
)
]
·
T
samp
V
w
=
V
b_dc
=
[
1
2
+
1
2
·
ma
·
sin
(
θ
-
π
2
)
]
·
T
samp
(
1
)
where, “&thgr;” represents a rotor position, “ma” represents a modulation rate, and “Tsamp” represents a switching sampling time.
The above Equation 1 is associated with the case in which the c-phase of the 3-phase motor is connected to the connection node between the upper and lower DC link capacitors. Referring to Equation 1, it can be found that the voltages Vw and Vu have a phase difference of 60° therebetween, and the voltage Vu is retarded in phase from the voltage Va by 30°.
In accordance with the above mentioned conventional method, the supply of a voltage to the motor has two statuses, that is, a status, in which the voltage is supplied based on only one of the upper and lower DC link capacitors is used, and a status, in which the voltage is supplied based on both the upper and lower DC link capacitors are used, in accordance with the switching statuses of the switching elements composing the B4 inverter. For this reason, there is a voltage difference between the upper and lower DC link capacitors, so that it is impossible to apply a balanced 3-phase voltage to the 3-phase motor. As a result, a torque ripple occurs. Due to such a torque ripple, it is impossible to achieve a reliable speed control.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above mentioned problems involved in the related art, and an object of the invention is to provide a phase distortion compensating apparatus and method for reducing a torque ripple in a 3-phase motor using four switching elements, which are capable of adjusting respective switching times of phase voltages, to be supplied to the 3-phase motor by an inverter including the switching elements, based on a voltage difference between upper and lower DC link capacitors respectively adapted to supply voltages to the inverter, thereby reducing a torque ripple generated in the 3-phase motor.
Another object of the invention is to provide a phase distortion compensating apparatus and method for reducing a torque ripple in a 3-phase motor using four switching elements, in which a torque ripple generated in the 3-phase motor is reduced, based on a voltage difference between upper and lower DC link capacitors supplied to an inverter including the switching elements and detected using a current flowing through the 3-phase mot
Cheong Dal Ho
Lee Dong Myung
Oh Jae Yoon
Birch & Stewart Kolasch & Birch, LLP
Fletcher Marlon T.
LandOfFree
Phase distortion compensating apparatus and method for... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Phase distortion compensating apparatus and method for..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Phase distortion compensating apparatus and method for... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3137992