The present invention relates to an inverter for controlling
an operation of a servomotor, and more particularly, to a method of and an apparatus
for protecting a regenerative resistor connected to an inverter.
FIG. 12 shows a conventionally known inverter-type control
device for a servomotor. In FIG. 12, a DC link connects a power supply unit (not
shown) for rectifying an alternating current from an AC power source and for supplying
DC power, and an inverter 20 for inverting a DC voltage from the power supply unit
and for supplying it to a motor M. The inverter 20 is controlled by, for example,
a transistor PWM control circuit 21.
In a control of a servomotor by the inverter 20, a smoothing
capacitor 4 is connected to the DC link. A voltage applied to the capacitor 4 changes
depending on the operation modes of the motor, including acceleration, constant-speed,
and deceleration modes. In the acceleration mode, for example, electric power is
supplied from the power supply unit to the motor, so that the voltage drops. In
the deceleration mode, in contract with this, energy is returned from the motor
to the power-supply unit, so that the voltage rises.
While a field capacitor is used as the smoothing capacitor
4, for example, the capacitor used has a predetermined dielectric strength level.
If a voltage higher than the dielectric strength level is applied to such a capacitor,
the capacitor will be harmed.
Thus, the capacitor 4 is protected by connecting a regenerative
resistor 5 in parallel with the capacitor 4. In FIG. 12, a level detection circuit
7 turns on a transistor 14 to connect the regenerative resistor 5 with the DC link
when it detects that the voltage of the DC link has become equal to or higher than
a set voltage. By doing so, electric current is made to flow through the regenerative
resistor 5 so that the voltage applied to the capacitor 4 is lowered. The regenerative
resistor 5 generates heat corresponding to the applied power, thereby consuming
an excess power due to excess voltage.
If the regenerative resistor 5 consumes the excess power
due to excess voltage until over-regeneration occurs, the regenerative resistor
5 itself is thermally damaged to be fused, and becomes unable to protect the capacitor.
It is necessary, therefore, to protect the regenerative resistor and cut off a power
supply circuit from the servomotor, thereby protecting the motor and the supply
circuit, by detecting the over-regeneration before the regenerative resistor is
fused.
The voltage of the DC link and its fluctuation can be satisfactorily
coped-with by using a resistor with an adequately large capacity (wattage) as the
regenerative resistor 5. In order for the regenerative resistor to be increased
in capacity, however, its volume needs to be increased to withstand a large amount
of heat resulting therefrom. In view of the packaging space and cost of the regenerative
resistor, however, it is to be desired that the regenerative resistor for the inverter
should have as small a capacity as possible. In general, a regenerative resistor
has a fusing characteristic such as the one shown in FIG. 13, and will be fused
in a short time if an excessive electric power is applied to it.
Conventionally, therefore, the regenerative resistor is
protected by the following methods, for example.
- (1) Protecting method using a thermostat: FIG. 14 is a diagram for illustrating
protection of the regenerative resistor by means of a conventional thermostat, In
FIG. 14, a thermostat 2 is attached to the regenerative resistor 5 which is connected
with the DC link. The thermostat 2 is opened and closed in accordance with heat
generated by the regenerative resistor 5. A detection circuit 3 detects the opening
and closing of the thermostat 2. In the case of over-regeneration, the circuit 3
gives an alarm to stop the voltage application to the regenerative resistor, and
cuts off the DC power supply unit, thereby protecting the regenerative resistor
6 and the servomotor.
- (2) Protecting method using an analog simulation circuit: FIG. 15 is a diagram
for illustrating protection of the regenerative resistor by means of a conventional
analog simulation circuit. Referring to FIG. 15, a regenerative pulse signal delivered
from the level detection circuit 7 is guided to the analog simulation circuit 22
through an insulating element 9 such as a photocoupler. As shown in FIG. 16, the
analog simulation circuit 22 detects the pulse width and supply time of the regenerative
pulse signal supplied to a regenerative circuit 6, and estimates the regeneration
level of the regenerative circuit 6. If a set or higher regeneration level is reached,
the circuit 22 gives an alarm to stop the voltage application to the regenerative
resistor 5, and cuts off the DC link, thereby protecting the regenerative resistor
5 and the servomotor.
However, the conventional protecting methods for the regenerative
resistor of the inverter for the servomotor involves a problem that the regenerative
resistor can not be satisfactorily protected from a thermal breakage by the over-regeneration.
For example, the protecting method using a thermostat can
protect the regenerative resistor from thermal breakage resulting from the over-regeneration
by detecting the over-regeneration in a case where a low regenerative power continues
for a relatively long time, but it cannot respond to thermal breakage in a case
where a high regenerative power is generated in a short time, thus failing to protect
the regenerative resistor.
In contrast, the protecting method using an analog simulation
circuit can protect the regenerative resistor from thermal breakage resulting from
the over regeneration by detecting the over-regeneration in a case where a high
regenerative power is generated in a short time, but it cannot respond to the thermal
breakage in a case where a low regenerative power continues for a relatively long
time, thus failing to protect the regenerative resistor.
FIG. 17 is a diagram for illustrating protective regions
according to the conventional regenerative resistor protecting methods. In FIG.
17, the abscissa axis represents regenerative power Wd, while the ordinate axis
represents an alarm operation time Ts, that is, the time required before an alarm
is given in each protecting method. Also, a broken line indicates a regenerative
resistor fusing curve, a dashed line and a hatched region based on the dashed line
indicate a protective region covered by the thermostat, and a full line and a hatched
region based on the full line indicate an alarm line and a protective region covered
by the analog simulation circuit, respectively.
As shown FIG. 17, the protective region in which an alarm
can be given before the regenerative resistor fuses corresponds to the case where
the regenerative power and alarm operation time for the thermostat are low and long,
respectively, or to the case where the regenerative power and alarm operation time
for the analog simulation circuit are high and short, respectively. Thus, each protecting
method involves a region in which the regenerative resistor cannot be protected.
Thus, it could be proposed to enlarge the protective regions
for the regenerative resistor by combining the two protecting methods. If these
methods are simply combined, however, there will be a region that cannot be covered
by either of the two protecting methods like the case of the portion corresponding
to the regenerative resistor fusing curve indicated by A in FIG. 17 depending on
the situation. If regenerative operation in this region occurs, the regenerative
resistor will suffer thermal breakage.
In order to protect the regenerative resistor throughout
its fusing curve, therefore, the alarm curve must be adjusted in the manner shown
in FIG. 18. With this adjusted alarm curve the regenerative resistor can be protected,
but, since an alarm is actuated within a range in which the respective values of
the regenerative power and alarm operation time are smaller than values represented
by the fusing curve of the regenerative resistor, the servomotor operates in a range
lower than its rating, thereby giving rise to a problem that the characteristics
of the servomotor will inevitably be lowered.
Also, the parameter adjustment for the conventional. analog
simulation circuit is complicated, and fine adjustment for each regenerative resistor
is difficult.
The present invention is based on the prior art of employing
a thermostat which protects a regenerative resistor independence on its temperature.
Such prior art is disclosed in JP-A-2 211 083 and JP-A-63 161 886.
Also JP-A-60 013 485 discloses a regenerative power discharging
unit comprising a resistor, a transistor, and a controller.
The object of the present invention is to provide a protection
method and a protection circuit for a regenerative resistor in an inverter for a
servomotor, which is capable of satisfactorily protecting the regenerative resistor
from thermal breakage by an over-regeneration.
More specifically, the object is to provide a protection
method and a protection circuit for a regenerative resistor in an inverter of a
servomotor, which is capable of coordinating protective regions for the regenerative
resistor by a thermostat and by an analog simulation circuit when these two protections
are combined.
According to a first aspect of the present invention, a
protecting apparatus for a regenerative resistor comprises: level detecting means
for detecting a voltage produced in a DC link by regenerative power from a servomotor
and for outputting a regenerative pulse signal when the detected voltage exceeds
a first reference voltage; switching means for applying a DC link voltage to a regenerative
resistor in response to the regenerative pulse signal; first protecting means for
protecting the regenerative resistor by stopping power supply to the DC link when
the temperature of the regenerative resistor is increased to a predetermined value
or greater; and a second protecting means having a charge-discharge circuit to be
charged and discharged in response to the regenerative pulse signal outputted from
the level detecting means and a comparator circuit for comparing the voltage of
the charge-discharge circuit with a second reference voltage, for protecting the
regenerative resistor by stopping power supply to the DC link when the voltage of
the charge-discharge circuit exceeds the second reference voltage, wherein a fusing
region of the regenerative resistor is adjusted to be within at least one of a protective
region by the first protecting means and a protective region by the second protecting
means.
The first protecting means can be realized by a thermostat
as a heat-responsive switching device, for example.
In the second protecting means, the protective region of
the analog simulation means can be adjusted by the charge-discharge time constant
of the charge-discharge circuit, and the adjustment of the charge time constant
of the charge-discharge circuit can be achieved by varying the resistance value
of a resistor connected in series with a capacitor that constitutes the charge-discharge
circuit.
According to a second aspect of the present invention,
a protecting method for a regenerative resistor comprises the steps of: performing
a first protecting operation of stopping power supply to a DC link when the temperature
of a regenerative resistor is increased to a first predetermined value or greater;
performing a second protecting operation of estimating a quantity of heat accumulated
in the regenerative resistor by a charge-discharge circuit to be charged and discharged
in response to a regenerative pulse signal and for stopping power supply to the
DC link when the estimated quantity of heat exceeds a second predetermined value;
and adjusting the charge-discharge circuit so that a fusing region of the regenerative
resistor is continuously covered by the first protecting operation and the second
protecting operation.
According to a development of the second aspect of the
present invention, a method of protecting a regenerative resistor comprises the
steps of: setting a protective region of first protecting means for stopping power
supply to the DC link when the temperature of the regenerative resistor is increased
to the first predetermined value or greater; setting a protective region of second
protecting means for estimating the quantity of heat accumulated in the regenerative
resistor by the charge-discharge circuit to be charged and discharged in response
to the regenerative pulse signal and for stopping power supply to the DC link when
the estimated quantity of heat exceeds the second predetermined value so that a
fusing region of the regenerative resistor which is not covered by the protective
region of the first protecting means is covered; and determining the circuit constant
of the charge-discharge circuit having the set protective region.
According to a further development of the second aspect
of the present invention, a protecting method for a regenerative resistor comprises
the steps of: providing first protecting means for stopping power supply to the
DC link when the temperature of the regenerative resistor is increased to the first
predetermined value or greater; providing second protecting means having a capacitor
and a resistor connected in series with the capacitor, for estimating the quantity
of heat accumulated in the regenerative resistor by a charge-discharge circuit to
be charged and discharged in response to the regenerative pulse signal and for stopping
power supply to the DC link when the estimated quantity of heat exceeds the second
predetermined value; setting a plurality of charge-discharge characteristics of
the charge-discharge circuit based on the capacity of the capacitor of the second
protecting means and the resistance value of the resistor as parameters, and selecting
a charge-discharge characteristic for coveting a fusing region of the regenerative
resistor which is not covered by the protective region of the first protecting means
from among a plurality of charge-discharge characteristics; and obtaining the value
of the parameter corresponding to the selected charge-discharge characteristic to
determine the charge-discharge circuit.
Brief Description of the Drawings
- FIG. 1 is a block diagram showing a regenerative resistor protecting apparatus
for a servomotor inverter according to one embodiment of the present invention;
- FIG. 2 is a block diagram showing an example of an arrangement of an analog
simulation circuit of the present invention;
- FIG. 3 is a flowchart of a regenerative resistor protecting method for a servomotor
inverter of an embodiment of the present invention;
- FIG. 4 is a diagram for illustrating a duty factor of regenerative operation;
- FIG. 5 is a diagram showing a charge-discharge characteristic based on the regenerative
operation of a charge-discharge circuit;
- FIG. 6 is a diagram for illustrating the dependence of regenerative operations
represented by alarm curves on the duty factor;
- FIG. 7 is a diagram for illustrating setup of alarm curves based on the analog
simulation circuit;
- FIG. 8 is a diagram for illustrating a protective region of the regenerative
resistor using the analog simulation circuit;
- FIG. 9 is a diagram for illustrating the adjustment of the alarm curves based
on the analog simulation circuit of an embodiment of the present invention;
- FIG. 10 shows another example of the arrangement of the analog simulation circuit
of an embodiment of the present invention;
- FIG. 11 illustrates a simulation effect of the analog simulation circuit of
an embodiment of the present invention;
- FIG. 12 is a diagram showing conventionally 'known inverter control for a servomotor;
- FIG. 13 is a diagram showing the fusing characteristic of a regenerative resistor;
- FIG. 14 is a diagram showing the protection of a regenerative resistor by means
of a conventional thermostat;
- FIG. 15 is a diagram showing the protection of the regenerative resistor by
means of a conventional analog simulation circuit;
- FIG. 16 is a diagram showing the operation of the analog simulation circuit;
- FIG. 17 is a diagram showing protective regions according to a conventional
regenerative resistor protecting method; and
- FIG. 18 is a diagram showing the adjustment of an alarm curve based on the conventional
analog simulation circuit.
In FIG. 1, a DC link 10 connects a DC power supply unit
30 and an inverter 20, and supplies power to a servomotor. The DC link 10 is connected
with a smoothing capacitor 4 such as a field capacitor, and a regenerative circuit
6 for protecting the capacitor 4 in case of over-regeneration is connected to the
DC power supply unit 10 in parallel with the capacitor 4. The regenerative circuit
6, which may be composed of an IGBT, for example, converts energy produced in case
of over-regeneration into heat by means of a regenerative resistor 5.
A level detection circuit 7 detects the voltage of the
DC link 10, and delivers a regenerative pulse signal when the detected DC link voltage
exceeds a reference value. In response to the regenerative pulse signal from the
level detection circuit 7, the regenerative circuit 6 energizes the regenerative
resistor 5. A voltage VrefL from a DC link reference voltage source 8 may be used
as the reference value for the level determination by the level detection circuit
7.
The regenerative resistor protecting apparatus according
to the present embodiment is provided with a first protecting device, which includes
a thermostat 2 as a heat-responsive switching device attached to the regenerative
resistor 5. The thermostat 2 opens or closes its contacts depending on a temperature
change that is attributable to the heat generated in the regenerative resistor 5
by regenerative operation. The open-close operation of the thermostat 2 is detected
by means of a detection circuit 3, and an alarm or some other signal is outputted
when the temperature is not lower than a set point, for example. In response to
this alarm signal, the power for the DC link 10 is cut off to protect the regenerative
resistor, and stopping the motor and other processes are carried out. Sources of
electric power supply include the DC power supply 30, which supplies electric power
from a three-phase AC source of, for example, 200 volts through a diode bridge,
and electric power produced by the regenerative energy of the motor. When an alarm
is given to indicate over-regeneration, both these sources of supply are disconnected
electrically from the DC link 10, so that electric power is cut off from the DC
link 10. In this state, only energy accumulated in the capacitor 4 of the DC link
10 is discharged through the regenerative resistor 5.
In the regenerative resistor protecting apparatus In the
regenerative resistor protecting apparatus according to the present embodiment,
moreover, a regenerative pulse signal delivered from the regenerative circuit 6
is inputted to an analog simulation circuit 1, for use as a second protecting device,
through' an insulating element 9 such as a photocoupler. The analog simulation circuit
1 detects the pulse width and supply time of the regenerative pulse signal supplied
to the regenerative circuit 6, and estimates the regeneration level of the regenerative
circuit 6. If a set or higher level is reached by the estimated regeneration level,
the circuit 1 gives an alarm to cut off the electric power from the DC link 10,
thereby protecting the regenerative resistor, and carrying out stopping of the motor
and other processes.
As shown in FIG. 2, the analog simulation circuit 1 is
provided with a charge-discharge circuit 12, which is charged and discharged in
response to the regenerative pulse signal from the level detection circuit 7, and
a comparator 11 for comparing the voltage charged by the charge-discharge circuit
12 with a reference voltage VrefA.
The charge-discharge circuit 12 includes a charge circuit
section, which is formed by, for example, connecting a capacitor C1 and resistors
R2 and R3 in series, grounding one end, and applying a predetermined voltage Vcc
to the other end, and a discharge circuit section, which is formed by a resistor
R1 connected between the ground and a contact between the capacitor C1 and the resistor
R2. The output end of a transistor Tr1 is connected to the contact between the capacitor
C1 and the resistor R2 and to one end of the resistor R1, and the charge-discharge
circuit 12 is charged and discharged responding to the regenerative pulse signal
as a trigger.
The.voltage of the capacitor C1 is applied to the input
of the comparator 11 through a reversely-connected diode 15. Voltage comparison
is effected based on the reference voltage VrefA as a threshold value, and an alarm
signal is outputted when the reference voltage VrefA is exceeded by the voltage
with which the capacitor G1 is charged.
A switch SW1 can be connected in parallel with the resistor
R3 in the charge circuit section. The charge-discharge time constant can be changed
by opening or closing the switch SW1.
FIG. 3 is a flowchart for illustrating the operation of
an embodiment of the present invention, and FIGS. 4 to 9 are diagrams for illustrating
the operation according to the embodiment of the present invention. Hereinafter,
an description will be made referring to the flowchart of FIG. 3. According to the
present embodiment, the first protecting device based on the thermostat and the
second protecting device based on the analog simulation circuit are used in combination
with each other, and protective regions for the regenerative resistor are continuously
protected by adjusting the circuit characteristies of the analog simulation circuit.
Step S1: The circuit configuration of the analog simulation
circuit as the second protecting device is settled. The following is a description
of the analog simulation circuit that employs the circuit configuration shown in
FIG. 2.
Step S2: A relational expression indicative of the charge-discharge
characteristic is obtained from the circuit configuration of the charge-discharge
circuit of the employed analog simulation circuit. This relational expression for
the charge-discharge characteristic is given by f1.
The relational expression f1 for the charge-discharge characteristic
can be given by the following equation (1) as a function for determining the alarm
operation time Ts, using the capacitor C1 and the resistors R1, R2 and R3, which
constitute the charge-discharge circuit, alarm operation reference voltage VrefA
and a duty D for the regenerative operation as parameters.
where the duty factor D for regenerative operation is given by the ratio (= Ton/T)
of a regeneration time Ton to each period T of regenerative operation, as shown
in FIG. 4.
In FIG. 2, when the transistor Tr1 is turned off in response
to the regenerative pulse signal delivered from the level detection circuit 7 to
the transistor Tr1, the voltage Vcc is applied to the capacitor C1, and charging
is performed (as indicated by the broken-line arrow in FIG. 2) with a time constant
that is dependent on the resistor R2 (or series resistor formed of the resistors
R2 and R3) and the capacitor C1. On the other hand, when the transistor Tr1 is turned
on, electric charge accumulated in the capacitor C1 is discharged through the resistor
R1 and the transistor Tr1 (as indicated by the dashed-line arrow in FIG. 2) with
a time constant that is dependent on the resistor R1 and the capacitor C1.
The charge-discharge characteristic based on the regenerative
operation of the charge-discharge circuit can be represented by the characteristic
shown in FIG. 5, for example. In FIG. 5, a full line represents a charge-discharge
characteristic curve of the capacitor C1. The capacitor C1 is charged so that the
voltage across it rises during the regeneration time Ton for the regenerative operation,
and is discharged so that the voltage across it drops during a non-regeneration
time Toff for the regenerative operation. When the regenerative operation is continued,
the voltage across the capacitor C1 gradually rises as the rise and drop of the
voltage are repeated. The voltage of the capacitor C1 represents the regeneration
level of the regenerative circuit 6 obtained in response to the regenerative pulse
signal supplied to the regenerative circuit 6. The analog simulation circuit 1 compares
the voltage across the capacitor C1 with the alarm operation reference voltage VrefA,
and generates an alarm at the alarm operation time Ts when the alarm operation reference
voltage VrefA is exceeded.
Thus, the alarm operation time Ts varies depending on the
respective values of the capacitor C1 and the resistors R1, R2 and R3, which constitute
the charge-discharge circuit, regenerative operation duty factor D, and alarm operation
reference voltage VrefA, as indicated by the expression (1).
Step S3: Then, the relational expression (1), which is
indicative of the charge-discharge characteristic and is obtained in Step S2, is
transformed to obtain a relational expression indicative of an alarm curve that
represents a characteristic of the analog simulation circuit that generates an alarm.
In the description to follow, this relational expression for the alarm curve is
given as f2.
Regenerative power Wd consumed by the regenerative resistor
during the regenerative operation can be given by the following expression (2).
where VDC is the voltage of the DC link 10 applied to the regenerative
resistor 5, and Rd is the resistance value of the regenerative resistor 5.
Based on the expression (2) for the regenerative power
Wd and the expression (1), the alarm operation time Ts can be given by the following
expression (3) as a function of the regenerative power Wd having the duty factor
D as a variable.
In the case of the analog simulation circuit shown in FIG.
2, for example, the alarm operation time Ts is given by the following expression
(4).
where
In the expression (3), therefore, f2 is a function of Wd,
and varies with R0 (= R2 + R3) as a parameter.
Since Wd is simply proportional to the duty factor D; the
function f2 is also a function of D. That is, in FIG. 6, the axis of abscissa can
be replaced directly with the duty factor D. Accordingly, the alarm curve can be
moved by varying the parameter R0. Thus, the alarm curve moves away from the origin
as the parameter R0 increases.
Steps S4 and S5: Then, alarm curves CA based on the analog
simulation circuit are set compared with the fusing curve indicating the fusing
characteristic of the regenerative resistor and the operation curve of the thermostat.
FIG. 7 is a diagram for illustrating setup of the alarm curves CA of the analog
simulation circuit 1. The fusing characteristic of the regenerative resistor 5 is
inherent in the regenerative resistor 5 itself, and this fusing characteristic is
primarily dependent on the employed regenerative resistor. In FIG. 7, this fusing
characteristic of the regenerative resistor is represented by a broken-line curve.
The operation characteristic of the first protecting device
using the thermostat 2 is also inherent in the thermostat 2 itself, and this operation
characteristic is univocally determined by the employed thermostat. In FIG. 7, this
operation characteristic of the thermostat is represented by a dashed-line curve.
On the other hand, the characteristic of the analog simulation
circuit, as the second protecting means used for the regenerative resistor protection
according to the present embodiment, can be adjusted in the aforesaid manner, whereby
fusing regions of the regenerative resistor that cannot be protected by the first
protecting device can be adjusted so as to be within the protective regions of the
second protecting device.
In FIG. 7, an alarm curve CA that can cover the region
of the regenerative resistor fusing curve which is not covered by the thermostat
operation curve is selected from among a plurality of alarm curves CA represented
by full lines. In FIG. 7, of the four alarm curves CA, those three situated nearer
to the axis of ordinate are selectable alarm curves CA.
In this example, the alarm curve CA0 is set as shown in
the figure so that it covers the region of the regenerative resistor fusing curve
which cannot be covered by the thermostat operation curve plus some margin.
Then, it is determined whether or not the set alarm curve
CAO is an appropriate alarm curve. If an appropriate alarm curve CA cannot be selected,
the program returns to Step S1, whereupon the circuit configuration of a different
analog simulation circuit is set, and the aforesaid processes of Steps S2 to S5
are executed.
Step S6: After the alarm curve CAO is set, the combined
resistor R0 (= R2 + R3), which is used as the parameter for determining the relational
expression f2, is obtained from the relational expression f2 corresponding to the
alarm curve CA0.
Step S7: An alarm operation time TsO for the protection
of the regenerative resistor is set. This alarm operation time Ts0 determines a
characteristic for the protection of the regenerative resistor. A preset value is
used for the alarm operation voltage VrefA.
Step S8: The circuit constant of the charge-discharge circuit,
which performs the alarm operation at the alarm operation time Ts0 with the characteristic
of the alarm curve CA0, is obtained from R obtained in step 6 and the alarm operation
time Ts0 set in Step S7.
In order to obtain the circuit constant of the charge-discharge
circuit, the capacitor C1 and the resistors R1, R2 and R3 are set by substituting
values of D0 and Ts0 for D and Ts in the aforesaid relational expression f1 for
the charge-discharge characteristic.
Step S9: It is determined whether or not the circuit constants
of the analog simulation circuit, including values of the capacitor C1, resistors
R1, R2 and R3, etc. set in the aforesaid process of Step S8, have appropriate and
practical values.
If the set circuit constants have inappropriate values,
the program returns to Step S1, whereupon the circuit configuration of a different
analog simulation circuit is set, and the aforesaid processes of Steps S2 to S9
are executed.
Step S10: If the set circuit constants have appropriate
values, the analog simulation circuit is constructed based on these circuit constants.
The analog simulation circuit can be changed in configuration
in Steps S5 and S9 by changing the switch SW shown in FIG. 2 to vary the charge
time constant.
FIG. 8 shows an example of a protective region for the
regenerative resistor determined using the analog simulation circuit set in accordance
the aforesaid flowchart. Referring to FIG. 8, a regenerative resistor fusing curve
is within at least one of protective regions covered by the thermostat and the analog
simulation circuit, so that the regenerative resistor can be continuously protected
by the thermostat and/or the analog simulation circuit.
According to the analog simulation circuit configuration
of the present embodiment, the alarm curve CA can be adjusted by R0 (= R2 + R3)
as the parameter, and the movement of the alarm curve CA in FIG. 9 for adjustment
can simultaneously be made with respect to the two axes for the regenerative power
Wd and the alarm operation time Ts. This can be achieved by the configuration of
the analog simulation circuit according to the present embodiment.
This indicates that the characteristic of the analog simulation
circuit of the present embodiment can be adjusted with respect to both the axes
for the regenerative power Wd and the alarm operation time Ts, and that the characteristic
of the analog simulation circuit can be adjusted easily for the fusing characteristic
of the regenerative resistor.
FIG. 10 shows another example of the configuration of the
analog simulation circuit according to the present invention. In a charge-discharge
circuit 13, a plurality of resistors R3(1) to R3(n) are connected in series with
a resistor R2, and the charge-discharge time constant can be varied by changing
over switches SW(1) through SW(n).
This analog simulation circuit operates substantially in
the same manner as the analog simulation circuit shown in FIG. 2, except for the
charge time constant of the charge-discharge circuit 13.
FIG. 11 shows a simulation result in the case where R1
= 750 k&OHgr;, R2 = R3 = 390 k&OHgr;, R4 = 82 k&OHgr; , C1 = 33 µ F, VrefA
= 10 V, and Vcc = 15 V are used.
The present invention can provide a protecting method and
a protection circuit for a regenerative resistor in an inverter for a servomotor
in which a regenerative resistor is satisfactorily protected from thermal damage
or breakage caused by over-regeneration. Moreover, the present invention can provide
a regenerative resistor protecting method and a protection circuit for a regenerative
resistor in an inverter for a servomotor in which protective regions for a regenerative
resistor are coordinated when combining a protection of the regenerative resistor
by a thermostat and a protection of the regenerative resistor by an analog simulation
circuit.