Electricity: motive power systems – Induction motor systems – Primary circuit control
Reexamination Certificate
2001-12-12
2003-11-11
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Induction motor systems
Primary circuit control
C318S809000, C318S757000
Reexamination Certificate
active
06646411
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an inverter which drives a motor-driven compressor for use in a vehicle air conditioning apparatus. More particularly, this invention relates to a control method that effectively reduces the frequency with which the compressor motor is stopped due to an overload condition. Moreover, the present invention relates to an inverter configured to perform the control method of the present invention.
2. Description of Related Art
In
FIG. 1
, a known motor-driven compressor
1
and an inverter
2
for use in a vehicle air conditioning system are shown. Motor-driven compressor
1
comprises a compressor
11
and a three-phase, direct current (DC) motor
12
, which drives compressor
11
. Inverter
2
comprises a smoothing condenser
22
, three pairs of switching elements
21
a
-
21
f
(NPN-type transistors), a shunt resister
23
, and a motor drive controller
24
. Inverter
2
is supplied with DC electrical power from a DC battery
3
. Motor drive controller
24
comprises a central processing unit (CPU)
26
, an analog-to-digital (A/D) converter
25
, a memory
27
, a motor position detector
29
, and a control signal generator
28
. When a rotational frequency command signal
31
is input to CPU
26
from an external device (not shown), CPU
26
outputs a signal
32
to control signal generator
28
in accordance with a program stored in memory
27
. Control signal generator
28
activates specific combinations of switching elements
21
a
-
21
f
sequentially in accordance with a specific order.
When combinations of switching elements
21
a
-
21
f
are activated sequentially, three-phase, DC current flows into the coils of motor
12
, and motor
12
begins to rotate. As motor
12
rotates, a counter-electromotive force (back-emf) is generated on the terminals of motor
12
. The back-emf is input to motor position detector
29
, and then is translated into a signal
33
which provides an indication of a rotational position of a rotor of motor
12
. When CPU
26
receives signal
33
from motor position detector
29
and calculates a rotational position of the rotor, CPU
26
generates a new signal
32
for controlling switching elements
21
a
-
21
f.
Thus, CPU
26
may calculate rotational frequency of motor
12
based on signal
33
which is output from motor position detector
29
. When a current flows into motor
12
, it also flows through shunt resister
23
, which is located on a return path of the current. This current Ip flowing through shunt resister
23
is called a phase current, and a potential difference develops across shunt resister
23
. This potential difference is proportional to the phase current and is input to A/D converter
25
. Thus, by monitoring the amplitude of current Ip, CPU
26
controls signal
32
, which further controls switching elements
21
a
-
21
f,
so that the current Ip does not exceed a certain level.
A known, main control program of inverter
2
may be stored in memory
27
of motor drive controller
24
. This known control program operates as follows. As mentioned above, CPU
26
monitors the amplitude of the current Ip. When a load corresponding to an air conditioning parameter increases, e.g., when the ambient temperature rises, a load on compressor
11
and on motor
12
also increases. Generally, the current Ip, which flows through motor
12
, is proportional to a rotational load on motor
12
.
By monitoring the amplitude of the current Ip, CPU
26
may detect that a rotational load on motor
12
has increased. The amplitude of the current Ip, which may flow through motor
12
and inverter
2
, is limited by (1) a rated current of motor
12
; (2) a rated current of switching elements
21
a
-
21
f;
(3) a rated current of conducting wires, which connect these devices; and (4) a rated current of the connectors between these devices. When an amplitude of current Ip approaches the rated current of motor
12
and inverter
2
, CPU
26
then lowers the rotational frequency of motor
12
. However, because the magnitude of the load on motor
12
varies, after a period of delay, in response to a change in the rotational frequency of motor
12
, the motor load continues to increase for some period after the rotational frequency of motor
12
is lowered. As a result, the amplitude of the phase current Ip may continue to increase, and it may exceed a rated current of inverter
2
or motor
12
.
When the amplitude of the phase current Ip exceeds a rated current of inverter
2
or motor
12
, CPU
26
ceases to activate control signal generator
28
and, thus, switching elements
21
a
-
21
f.
This action by CPU
26
stops drive motor
12
. If motor
12
is stopped abruptly during the operation of the vehicle air conditioning system, warm air may be discharged from a port of the air conditioning system. In addition, motor
12
may be restarted after a predetermined period of time elapses from the time at which motor
12
was first stopped. Thus, according to known methods for controlling an inverter, motor stoppage may occur more frequently whenever an amplitude of the current Ip approaches a limit value, i.e., a rated value of inverter
2
or motor
12
.
SUMMARY OF THE INVENTION
A need has arisen to provide an improved control method for an inverter that drives a motor-driven compressor. A further need has arisen for a control method that reduces the frequency with which a motor is stopped upon the approach of an overload condition. Moreover, a need has arisen to provide an inverter configured to perform this control method. According to the control method of the present invention, at least five set points related to the amplitude of the phase current may be preset. The first set point among the five set points is one of (1) the rated current of the switching elements, (2) the rated current of the motor, or (3) the rated current of the conducting wires and connectors which connect these devices. According to the method of the present invention, when the amplitude of the phase current exceeds this first or greatest set point, activation of the motor is stopped. When the amplitude of the phase current falls below the lowest of the five set points, the rotational frequency of the motor is accelerated by a predetermined degree of acceleration.
The remaining three intermediate set points are located between the first set point and the lowest set point. The intermediate set points define subintervals between the first set point and the lowest set point. When the amplitude of the phase current enters one of the subintervals defined by these three intermediate set points, the motor is accelerated by various negative degrees of accelerations (i.e., the motor is decelerated) predetermined and defined for each of the subintervals. Because the control method of the present invention steadily decreases rotational velocity of the motor by a certain deceleration before the amplitude of the phase current exceeds the first or largest set point, the method of the present invention effectively reduces the occurrence of motor stoppage as an overload condition approaches.
In an embodiment of this invention, a control device of an inverter includes a plurality of switching elements for driving a three-phase DC motor in response to a rotational frequency, command signal received from an external device. The rotational velocity and the rotational acceleration of the motor are determined from signals which are in response to a back-emf received from the DC motor as it rotates. The amplitude of the phase current of the motor is determined, as well. When the amplitude of the phase current exceeds a first set point, activation of the motor is stopped. When the amplitude of the phase current is less than the first set point but greater than a second set point, which is less than the first set point, the rotational velocity of the motor is decelerated at a predetermined rate. When the amplitude of the phase current is less than a third set point, which is smaller than the second set point, the r
Hirono Daisuke
Kuwabara Ichirou
Yoshii Yuuji
Baker & Botts L.L.P.
McCloud Renata
Nappi Robert E.
Sanden Corporation
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