JPH0458786A - Restarting method of inverter upon recovery from instantaneous power interruption - Google Patents

Abstract

PURPOSE:To avoid overcurrent state without interrupting operation of motor by recovering output frequency and output voltage of an inverter to levels prevailing immediately before instantaneous power interruption. CONSTITUTION:When the residual voltage of an induction motor 3, of which excitation is interrupted, can be damped to a level causing no trouble in control operation during an interval B and an interval C between times t2 and t3, i.e. an interval between times t1 and t3 the interval C is set as a pause interval and the time t3 is set as a control restarting time. Restart control is carried out such that the output frequency fo of inverter is equalized to a value fos at time t1 in the interval A immediately before occurrence of instantaneous power interruption after the time t1 and the inverter output voltage Vo is boosted from a predetermined low voltage Vo to a target value, i.e. a value at the time t1, with a predetermined time gradient.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for restarting an inverter that drives and controls an induction motor with a current limiting function when the power is restored from a momentary power failure.

[Conventional technology]

Conventional methods for restarting this type of inverter when the power supply is restored from an instantaneous power failure include waiting for the rotation of the load induction motor whose input has been cut off due to the stoppage of the inverter's output and then starting restart control; When the input is cut off, a dedicated circuit separately detects the rotational speed of the motor which is decelerating due to its inertia, and determines the detected rotational speed by determining the inverter output frequency that can draw the decelerating induction motor into power supply drive from the inverter as described above. There is a known method that calculates the calculated frequency and restarts the inverter operation using the calculated frequency.

[Problem to be solved by the invention]

However, in the above-mentioned method of restarting an inverter when power is restored from a momentary power failure, if restarting is performed after waiting for the load induction motor to stop rotating, it is necessary to interrupt the operation of the induction motor, which requires continuous operation. In addition, in the case of restarting the inverter operation using the retractable frequency as described above, a rotation speed detection circuit of the motor and a calculation circuit for the retractable frequency are separately required, and the retractable frequency also requires a dedicated circuit for calculating the retractable frequency. This leaves open the possibility of an overcurrent condition occurring when the inverter power supply is resumed.

In view of the above, it is an object of the present invention to provide a method for restarting an inverter when the power is restored after a momentary power failure, which does not require interruption of the motor operation as described above and avoids the occurrence of an overcurrent state.

[Means to solve the problem]

In order to achieve the above object, the method for restarting an inverter when the power supply is restored from a momentary power failure according to the present invention is a restart method with a current limiting function when the power supply is restored from a momentary power failure of an inverter that drives and controls an induction motor. The method includes restarting the motor at a point after the elapse of a downtime determined by including the time required for the residual voltage of the motor to decay to a predetermined value or less from the time of power restoration after the instantaneous power failure. from the start of control, the output frequency of the inverter is set to the same value as the value immediately before the momentary power failure occurred, and the output voltage is set from a predetermined low voltage to a constant value during normal operation of the inverter. The restart control is performed by increasing the output voltage at a predetermined time rate of increase with the target value being the output voltage corresponding to the output frequency immediately before the occurrence of the instantaneous power failure, which is determined according to the output voltage to output frequency ratio determined according to the output voltage to output frequency ratio. , and if the output current of the inverter exceeds a preset limit value during acceleration of the motor by boosting the inverter output voltage as described above, the constant output voltage vs. output frequency as described above. reducing the inverter output frequency at a predetermined temporal reduction rate until the inverter output current is reduced by a predetermined value from the limit value after the point in time when the inverter output current exceeds the limit value, with output voltage control according to the ratio, and further the reduction control Upon completion, the inverter output frequency is increased at a predetermined time increase rate with the value immediately before the instantaneous power failure occurring as a target value, and if the inverter output current exceeds the limit value again thereafter, as described above. The output frequency and output voltage of the inverter are repeatedly controlled to decrease and increase, respectively, and finally the output frequency and output voltage of the inverter are respectively restored to the values immediately before the momentary power failure occurred, and normal inverter operation is resumed from then on. shall be moved to.

[Effect]

When the electrical input is cut off and power is applied again to the rotating induction motor due to its inertia, the steady-state value of the input current of the motor is proportional to the power supply voltage, and as a function of the slip, the slip decreases, that is, the motor It decreases as the speed increases. In addition, the inrush transient value of the input current is determined based on factors such as a composite voltage that takes into account the polarity of the residual voltage of the motor that decays over time and the power supply voltage at the moment when the power supply is applied.
It is difficult to detect the polarity relationship between the two types of pressure for each phase of the motor in advance and determine the optimal application timing of the power source. Therefore, it is desirable that the power supply is reapplied as described above after the motor residual voltage has sufficiently attenuated, the power supply voltage is changed from the low voltage state, and the motor rotational speed is as high as possible.

In accordance with the above, the present invention relates to restarting an inverter that serves as a variable voltage variable frequency power source for an induction motor to be controlled when the power is restored from a momentary power failure, and the present invention sets the timing for resuming power supply to the motor from the time when the power is restored to the residual voltage of the motor. After the pause time (usually around 1 second for small induction motors) that is considered sufficient for the current to decay to a predetermined value, the upper limit value is set for limiting control of the output current of the inverter, which is the input current of the motor. Set a current limit band having a predetermined current as a predetermined inverter current limit value, and then set target values of the output frequency and output voltage of the inverter to be taken at the time of completion of the restart, respectively, to the values immediately before the instantaneous power failure occurs. The output voltage is increased to the target value from a predetermined low voltage at a predetermined time gradient while maintaining the output frequency at the target value, and if the output current of the inverter is lower than the current limit during the step-up process. If the upper limit of the band is exceeded, the inverter output frequency is reduced at a predetermined time gradient, and the inverter output current is reduced by increasing the speed of the motor by increasing the output torque of the motor; When the output current reaches the lower limit of the current limit band, the inverter output frequency is increased again toward its target value at a predetermined time gradient, and such frequency increase/decrease control is no longer necessary within the current limit band. This is done repeatedly depending on the situation.

Incidentally, the output voltage of the inverter is controlled so as to keep the ratio of the two constant during the above-mentioned increase/decrease control of the output frequency.

As described above, the inverter accelerates the electric motor with reduced voltage after its residual voltage attenuates, and further performs the current limiting control as described above regardless of the load required torque characteristics of the electric motor, thereby ensuring smooth and safe restart. Start-up can be performed.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a system block diagram of an inverter showing an embodiment of the present invention;
2 and 3 are operation waveform diagrams of various input and output quantities of the inverter.

In FIG. 1, 1 is a three-phase AC power supply whose line voltage is vS, 2 is a main circuit and a control circuit, and 3 receives the voltage V5 as input and outputs a line voltage V0 of frequency f0.
3 is a three-phase induction motor, 4 is a current transformer CT, and the inverter device 1 receives input from CT3.
This is a current detection circuit that performs required current calculations from each phase output current detection signal of No. 3.

Next, in Fig. 2, figure (a) indicates the voltage of the AC power supply ■
1 over time, where the voltage v1 undergoes a power outage at time t, becomes zero, returns to the same value at time t2, and the period B between the two times is extremely short. Figures (B) and (C) show the restart control state of the inverter device 2 corresponding to the instantaneous power failure state shown in Figure (A). Now, the sum of the period C between times t2 and t and the period B, that is, the times t1 and t
If it can be considered that the residual voltage of the induction motor 3 whose excitation is cut off for 3 hours can be attenuated to a value that does not interfere with control, the period C is set as a rest period, and time t is set as a restart control start time. After the time t3, the restart control changes the inverter output frequency f0 to the value f at time t1 in the period A before the momentary power failure occurs, as shown in Figure (b). , and the inverter output voltage v0 is set to a predetermined low voltage V as shown in FIG. A predetermined time gradient tal from S,
At θ1, the pressure is increased to the target value at time t1. Note that during the period A, the ratio of the output voltage to the output frequency of the inverter is controlled to be a constant value, so that the inverter output voltage at the time t is k −fO.

becomes. Now, at time t,■. = -f, , then the period between times t3 and t7 becomes a voltage reduction acceleration period for the induction motor 3, and if the inverter output current does not exceed its limit value during this period, At the time t7, the restart control is completed and normal inverter operation is started in the subsequent period F.

In addition, in FIG. 3, figures (A) to (C) correspond to figures (A) to (C) in FIG. 2, respectively,
This shows the control state when the inverter output current exceeds its limit value during the voltage reduction acceleration period of the period shown in FIG. 2, and is in the middle of voltage reduction acceleration from time t as described above.

If it is assumed that the inverter output current exceeds the upper limit value at time t4 after the elapse of time, the frequency f0 is controlled to be reduced by a predetermined time gradient t,..., θ2 from time t4, and the voltage v0 is As mentioned above, the frequency f0 is being reduced.
With respect to the ratio k, the voltage increase continues until time t5 when the values become equal to k -f, and after the transfer time t5, as the frequency f0 decreases, V, = -f
, and if the inverter output current further decreases to the lower limit value of the current limit band that is lower than the upper limit value by a predetermined value at time t at the end of period E, then at time t6 After that, the predetermined time gradient t, ゎθ
, the frequency f0 is controlled to increase toward the target frequency fos, and at the same time the voltage ν is increased. Also ■. = -f, and after the period D3 has elapsed, the voltage is increased to f. =f0. ,
At time t7 when V, = -f, , the restart control as described above is completed, and thereafter normal inverter operation in period F is made to proceed.

If the inverter output current exceeds the upper limit value again in the period D3, the same control as in the rainy periods E and D is repeated.

〔Effect of the invention〕

According to the present invention, regarding an inverter that drives and controls an induction motor, when the power is restored from an instantaneous power failure, the output voltage of the inverter is increased from a predetermined low voltage at a predetermined time gradient after the residual voltage of the motor is attenuated. In addition to performing voltage reduction acceleration control on the electric motor, in order to limit the output current of the inverter as necessary, the output frequency increase/decrease control of the inverter is combined with voltage control to keep the ratio of output voltage to output frequency constant. By doing so, it is possible to restart the motor smoothly by avoiding inrush current and overcurrent during acceleration, and it is also possible to reduce the cost of the inverter device by eliminating the need to add a rotation speed detection means for the motor. .

[Brief explanation of the drawing]

FIG. 1 is a system block diagram of an inverter showing an embodiment of the present invention, and FIGS. 2 and 3 are operation waveform diagrams of various input and output quantities of the inverter. 1... AC power supply, 2... Inverter device, 3...
Induction motor, 4...Current detection circuit, CT, ~CT,・
...Variation Figure 1 υC Figure 2

Claims (1)

[Claims] 1) A restart method with a current limiting function when the power supply of an inverter that drives and controls an induction motor is restored from a momentary power failure, wherein the residual voltage of the motor is attenuated to a predetermined value or less from the time of power restoration from the instantaneous power failure. The time after the pause time determined, including the time required for the same value, and the output voltage corresponds to the output frequency immediately before the instantaneous power failure, which is determined according to the output voltage to output frequency ratio, which is a constant value during normal operation of the inverter, from a predetermined low voltage. Restart control is performed by increasing the output voltage to a target value at a predetermined rate of increase over time, and if the output current of the inverter is set in advance while the motor is being accelerated by increasing the inverter output voltage as described above. If the limit value is exceeded, the inverter output current is reduced by a predetermined value from the limit value after the limit value is exceeded, with output voltage control according to the constant output voltage to output frequency ratio as described above. The inverter output frequency is reduced at a predetermined time reduction rate until the reduction control is completed, and the inverter output frequency is increased at a predetermined time increase rate with the value immediately before the instantaneous power failure occurring as a target value. If the inverter output current exceeds the limit value again, the inverter's output frequency and output voltage are repeatedly reduced and increased as described above, and finally the inverter's output frequency and output voltage are reduced and increased. A method for restarting an inverter when the power is restored from a momentary power failure, the method comprising: recovering the output voltage to the value immediately before the momentary power failure occurred, and thereafter transitioning to normal inverter operation.