JP4584570B2 - Switching device for induction motor drive power supply - Google Patents

Description

  The present invention relates to a system for driving an induction motor by two power sources, an inverter and a commercial power source, and relates to a drive power source switching method when an inverter is abnormal.

  2. Description of the Related Art A method of performing energy saving operation of an induction motor by supplying AC power from an inverter that performs constant control of a speed / voltage ratio (V / f) is widely used. In this case, when a failure occurs in the operating inverter, the inverter is disconnected and the commercial power source and the induction motor are directly connected. For example, this is disclosed in Patent Document 1.

JP-A-57-151275

  In the above prior art, when an excessive output current of the inverter is detected, the inverter is disconnected and the AC power source and the induction motor are directly connected. However, when the short circuit on the induction motor side is the cause, a short circuit current flows in from the commercial power supply, which causes a disadvantage of increasing the damage of the induction motor.

  An object of the present invention is to provide a switching device for an induction motor driving power source that overcomes the above-described problems of the prior art and can prevent the damage of the induction motor from being expanded in a system driven by two power sources, an inverter and a commercial power source. .

  To achieve the above object, the present invention provides an AC power source, an inverter, an induction motor driven from the AC power source through the inverter, a failure detector for detecting a failure of the inverter, and the inverter as the AC power source. And an induction motor drive comprising: switching means for connecting to the induction motor and disconnecting the inverter when the inverter fails; and connection means for directly connecting the AC power source to the inverter when the inverter is disconnected. In the power supply switching device, when the inverter is disconnected, the terminal voltage of the induction motor is detected, and it is determined whether the terminal voltage is instantaneously reduced or normally lowered. It is characterized by that.

  The terminal voltage of the induction motor is an induced residual voltage, and the connection is performed by the connection means when the normal drop is reduced to a predetermined level according to the time constant of the induction motor.

  The terminal voltage is detected on the inverter side or the induction motor side of the switch means between the inverter and the induction motor.

  According to the present invention, in an induction motor drive system driven by an inverter and a commercial power source, a short circuit failure of the induction motor is determined when an inverter overcurrent is detected, and switching to the commercial power source is not performed when the short circuit is detected. It is possible to prevent the motor from being damaged and expanded.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a configuration diagram of an induction motor drive power source switching device according to an embodiment. Connected between the AC power source PS, the AC power source PS and the direct insertion type induction motor IM via the switches S1 and S2, and the inverter INV for driving the induction motor IM at a variable speed, and the induction motor IM to the AC power source PS / A switch S3 for disconnecting is provided. The inverter INV has a current sensor CT that detects its output current, an overcurrent detector (OC) 1 that detects an excessive output current of the inverter INV by the output of CT, and an inverter that stops by the output of the overcurrent detector 1 A stop circuit 2 is provided.

  The output of the overcurrent detector 1 is input to the timer TD4 and the S1S2 off circuit 3 that turns off the switches S1 and S2 and disconnects the inverter from the AC power supply PS and the induction motor IM. TD4 generates an output after a certain time limit, and is given to an S3 input circuit 5 which generates an input signal of the switch S3 via an AND circuit 14. The timer 4 is set to a time that is about three times the decay time constant so that it is turned on after a period in which the induced voltage of the induction motor is sufficiently attenuated.

  By the way, when the inverter is stopped by the stop circuit 2, the induced voltage of the induction motor IM attenuates in a short time when the short circuit on the induction motor side is the cause. Therefore, when the switch is switched to the commercial power source by turning on the switch S3 after the delay time of the timer 4, a large short circuit current determined by the short circuit impedance between the power source side and the induction motor IM flows to the induction motor.

  Therefore, in this embodiment, the conditions for inputting to the S3 input circuit 5 are determined as follows. First, after the output of the switch S1S2 off circuit 3, the output of the one-shot multi 10 that generates a pulse having a certain time width and the output of the overcurrent detector 1 are input to the AND circuit 11. Further, the voltage detector PT6 that detects the output voltage of the inverter INV, the function generator 8 that outputs an abnormality determination value of the terminal voltage of the induction motor IM, and the magnitude of the function generator 8 output and the voltage detector 6 output are determined. The comparator 9 is provided, and the output of the comparator 9 is input to the AND circuit 11. The function generator 8 is set lower than the normal terminal voltage according to the normal speed-voltage characteristic when the induction motor is controlled by the inverter. The output of the AND circuit 11 is held by the holding circuit 12. The output of the holding circuit 12 is inverted by the NOT circuit 13 and input to the AND circuit 14.

  Next, the operation of this embodiment will be described with reference to the time chart of FIG. When the effective value I1 of the inverter output current detected based on the current sensor CT exceeds a predetermined overcurrent level Ioc, the overcurrent detector 1 outputs an OC output. The inverter stops in response to the OC output, and immediately after that, the inverters are disconnected by turning off the switches S1 and S2. One condition of the AND circuit 14 is satisfied after the delay time (TD1) of the timer 4 from the OC output. As the delay time TD1, a time (about three times the time constant τ) necessary for the induction voltage of the induction motor IM to sufficiently attenuate (time constant τ) is selected.

  The one-shot multi 10 generates a pulse having a certain time width at the falling edge of the switch S1, S2 off signal. The pulse width of the one-shot multi 10 is TD2, and since the induction voltage of the induction motor remains sufficiently during this short time TD2, the comparator 9 does not detect the terminal voltage determination value P or less, and the output is Absent. Accordingly, the AND circuit 11 and the holding circuit 12 remain at the low level, the NOT circuit 13 is at the high level, and both conditions are satisfied for the input of the AND circuit 14, so that the S3 input circuit 5 outputs. As a result, the switch S3 is turned on and the induction motor is connected to the commercial power source. Immediately after the commercial power is turned on, a direct start-up current flows until the rotational speed reaches the synchronous speed.

  Next, the operation in the case where a short-circuit failure occurs on the induction motor IM side and the inverter output current becomes excessive will be described with reference to the time chart of FIG. When a short circuit occurs on the induction motor IM side when the inverter is stopped, the induced voltage of the induction motor IM is attenuated in a short time. Therefore, while there is an output with a short pulse width of the one-shot multi 10, the comparator 9 detects a decrease in the induction motor voltage, the input condition of the AND circuit 11 is satisfied, and the output becomes high level. Since the holding circuit 12 holds this high level, the output of the NOT circuit 13 is switched to a low level at this point. Then, even if the output of the timer 4 changes to a high level after a predetermined time, the input condition of the AND circuit 14 is not satisfied, so the switch S3 is not turned on, and a large short-circuit current from the commercial power supply to the induction motor is prevented. can do.

  FIG. 4 shows an example of the determination value of the induction motor terminal voltage abnormality. When the induction motor is driven by an inverter, the inverter controls the speed / voltage ratio (V / f) to be almost constant. Therefore, if the abnormality determination level is selected to be about half of the normal control voltage proportional to the speed in this way. Good.

  FIG. 5 shows another embodiment of the present invention. FIG. 5 differs from FIG. 1 in that the voltage sensor is connected to the induction motor side. In this way, even if the cause of the decrease in the induction motor terminal voltage is on the inverter side, if the determination is made after disconnecting the switch S2, the influence on the inverter side can be avoided, and the terminal voltage abnormality determination of the induction motor can be accurately performed it can. However, in the case of FIG. 1, since PT6 is on the inverter side, there is an advantage that the configuration of a normal inverter control system is sufficient.

The lineblock diagram of the induction motor drive power supply switching device by the 1st example. The time chart which shows the operation | movement of an induction motor drive power supply switching device. The time chart which shows operation | movement when there is a short circuit failure in an induction motor. Explanatory drawing which shows the example of the determination value of induction motor abnormality. The block diagram of the induction motor drive power switching device by the 2nd Example.

Explanation of symbols

  PS ... AC power source, IM ... induction motor, S1, S2, S3 ... switch, INV ... inverter, CT ... current detector, 1 ... overcurrent detector, 2 ... stop circuit, 3 ... S1, S2 off circuit, 4 ... Timer 5, S 3 input circuit 6 voltage sensor, 8 function generator, 9 comparator, 10 one-shot multi, 11 AND circuit, 12 holding circuit, 13 NOT circuit, 14 AND circuit.

Claims (2)

An AC power source, an inverter, an induction motor driven from the AC power source through the inverter, a failure detector for detecting a failure of the inverter, the inverter connected to the AC power source and the induction motor, and the inverter In a switching device for an induction motor drive power source comprising switch means for disconnecting the inverter in the case of failure, and connection means for directly connecting the AC power source and the induction motor when the inverter is disconnected,
When the fault detector detects an overcurrent of the inverter and detects a fault, a timer that responds after a certain period of time, a first AND circuit that uses the output of the timer as one input, and the switch means are switched off. A one-shot multi that outputs a pulse of a predetermined fixed time width after output of the switch-off circuit, a voltage sensor that detects a terminal voltage of the induction motor, a function generator that outputs an abnormality determination value of the terminal voltage, A comparator that compares the output of the function generator with the terminal voltage detected by the voltage sensor; and a second AND circuit that takes the logical product of the output of the one-shot multi, the output of the failure detector, and the output of the comparator A holding circuit for holding the output of the second AND circuit, a NOT circuit for inverting the output of the holding circuit, and an output of the NOT circuit as the other input of the first AND circuit An induction motor drive power supply switching device characterized in that it takes a logical product with the output of the timer, connects the connection means by the output of the first AND circuit, and connects the induction motor to the AC power supply. .   The one-shot multi outputs a pulse only for a short period in which the induced voltage of the induction motor at normal time remains sufficiently, and the function generator has a normal speed when the induction motor is controlled by the inverter− The induction motor drive power source switching device according to claim 1, wherein the switching device is set lower than a normal terminal voltage in accordance with voltage characteristics.

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