Electric power conversion systems – Current conversion – Using semiconductor-type converter
Reexamination Certificate
2003-01-10
2004-10-12
Han, Jessica (Department: 2838)
Electric power conversion systems
Current conversion
Using semiconductor-type converter
C363S098000, C363S056030, C363S041000
Reexamination Certificate
active
06804130
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an inverter apparatus in which the suppression of an excess current is intensified when an induction motor is to be driven by V/f control.
BACKGROUND ART
Conventionally, a current is increased if a rapid acceleration is carried out or a load is suddenly changed when the induction motor is to be V/f controlled. In that case, when a current exceeding a tolerance flows to the semiconductor device of an inverter apparatus, the device is broken down. Consequently, an excess current level is set. When a current exceeding the excess current level flows, an excess current protecting function is fulfilled to prevent the breakdown of the device by gate breaking.
Referring to the gate breaking fulfilled by the excess current protecting function, moreover, restarting is required. For this reason, a current is limited and a semiconductor is protected by using a gate breaking circuit having the function of carrying out an automatic reset at a lower level than a previous excess current level or a current limiting circuit for outputting a 0 voltage pattern to a much lower level.
Furthermore, there has been performed control for paying attention to only the magnitude of a current, for example, a method of detecting the magnitude of a current to correct a frequency, and a method of stopping an acceleration if the current is increased during the acceleration and reducing a frequency if the current is increased during a stationary operation.
Next, description will be given to a specific example in which conventional V/f control is actually carried out.
FIG. 5
is a control block diagram showing the conventional V/f control.
FIG. 8
shows an example of a voltage command Vq* and a current I in a certain power state in the V/f control shown in
FIG. 5
, and a voltage component of an electric motor, in which an axis d indicates a reference phase of a control output, illustrating a control configuration for controlling a voltage of an axis q set into a position at 90 degrees from the axis d.
In the V/f control shown in
FIG. 5
, &thgr;represents a position of the axis d seen from a certain reference position (for example, a U phase). A frequency command calculating section
1
inputs a frequency command Fref and calculates an acceleration frequency from an acceleration time set by acceleration command calculating means
2
, and integrates the acceleration frequency by acceleration frequency integrating means
3
, stops an acceleration when a frequency command value set by command value limiting means
4
is obtained, and thus creates a frequency command at the present time.
Moreover, when a speed integrated by the acceleration frequency integrating means
3
is set to have a lower limit value to be a frequency command value by the command value limiting means
4
during a deceleration, the deceleration is stopped. Slip frequency means
5
calculates the slip frequency of an electric motor from a current detection value for a torque, thereby obtaining an output frequency
6
.
A V/f calculating section
7
obtains the voltage command Vq* from the output frequency based on a frequency—voltage pattern shown in FIG.
7
. Moreover, the output frequency is integrated by phase calculating means
8
to obtain an output phase &thgr;, and a three-phase (UVW phase) voltage command is obtained from voltage commands Vq* and Vd* (a value of 0) and &thgr; by a PWM command calculating section
9
and is converted into a PWM pattern, and the PWM pattern is output to a gate driver circuit
10
and a voltage is applied to an electric motor 1M.
Conventionally, a magnitude I1 of a current is detected by current detecting means
12
b
as stall (stop state) preventing measures, and an acceleration is delayed if any and the acceleration (deceleration) is carried out with a negative value during a stationary operation when the magnitude I1 is increased by acceleration correcting means
11
b
. In some cases in which a rapid acceleration is carried out or a load fluctuates suddenly, however, an increase in the current cannot be suppressed so that a stall is carried out due to the excess current protection of hardware.
For this problem, there are taken measures for creating a current limiting circuit shown in
FIG. 6
to control a current, thereby preventing the stall.
As shown in
FIG. 6
, the current limiting circuit is added to an ordinary structure in which a conversion into a voltage having a UVW phase is carried out from the dq axes by voltage converting means
9
a
in the PWM command calculating section
9
and a PWM modulation is carried out over the same voltage by a triangular wave comparator
9
b
, and a gate drive signal is thus created through an inverting circuit and a non delay circuit
26
. Consequently, the breakdown of a semiconductor device can be prevented. In the drawing, an excess current level is divided into three stages of:
Ioc>Ic1b>Ic1a, and
a current detection value I1 is first compared with an excess current level Ioc by a comparator
21
. When I1 is greater than Ioc, the result of the comparison is latched into a latch circuit
24
and a gate breaking signal is selected and output by a gate breaking selection circuit
27
. The latch circuit
24
is reset in response to a reset signal sent from a controller in a predetermined timing.
If a current is smaller, a result obtained by comparing the current detection value I1 with a next current limitation level Iclb by a comparator
20
is latched into a latch circuit
23
and a gate breaking signal is output by the gate breaking selection circuit
26
. The comparator
20
and the latch circuit
23
will be referred to as a CLB circuit.
If the current is further smaller, the current detection value I1 is compared with a subsequent current limitation level Ic1a by a comparator
19
. If I1 is greater, an on signal is latched into a latch circuit
22
and is sent to a 0 voltage switching circuit
25
, and a 0 voltage pattern created by a 0 voltage pattern generator
18
is output. The comparator
19
and the latch circuit
22
, and the 0 voltage pattern generating circuit
18
and the 0 voltage switching circuit
25
will be generally referred to as a CLA circuit.
The latch circuits
22
and
23
are automatically reset in a certain set timing CLK. Consequently, in the case in which the current detection value I1 is smaller than the excess current level Ioc and is greater than the current limitation level Ic
1
a, a gate drive signal having a fixed pattern is obtained. However, since gate breaking is not carried out, it is possible to continuously perform the operation while limiting the current. In the excess current preventing measures to be taken depending on only the magnitude of a current, the current is increased if a voltage is dropped in the regeneration state of an electric motor, and furthermore, a power is not supplied to the electric motor while the CLA and CLB circuits are operated. For this reason, it cannot be denied that an efficiency is reduced.
Examples of a method other than the correction to be carried out depending on the magnitude of a current include a method of correcting a voltage by using a voltage limitation vector in a reverse direction to the magnitude of a detected current and PI controlling the magnitude of the voltage limitation vector, thereby correcting a speed command.
In the case of the method in
FIG. 6
, however, the magnitude of the voltage is corrected from the magnitude of the current to carry out a current limitation. Therefore, if the voltage is reduced when the electric motor is set in a regeneration state, the current is increased. Thus, the current cannot be limited depending on the state of the electric motor but is increased so that the excess current protecting function is fulfilled and a stall is caused by gate breaking in some cases.
In a method using a 0 voltage, moreover, a current is distorted so that the supply of a power to the electric motor is eliminated. As a result, the efficiency is reduced. For this reason, there is a problem in that the demand
Han Jessica
Kabushiki Kaisha Yaskawa Denki
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