JPH03293998A - Overvoltage detecting circuit for induction motor power supply device - Google Patents

Abstract

PURPOSE:To simlify the structure of the title circuit and reduce the price of the same by a method wherein an AC voltage at the connecting side of an induction motor for a PWN inverter is reduced by an insulating transformer and an overvoltage is detected by the secondary voltage of said insulating transformer. CONSTITUTION:In the driving device of an induction motor 5 driven by a PWM inverter 4, an insulating transformer 6 is provided at the induction motor 5 connecting side of the PWM inverter 4. The transformer 6 reduces the primary side voltage 102 of the degree of several hundred volt into the secondary side voltage 103 of the degree of several volt. A detecting voltage rectifier 7 rectifies a voltage signal 103 and outputs it into a comparator 8 as a voltage signal 104. The output signal 101 of the comparator 8 is 'Low' upon normal time. The output signal 101 is changed from 'Low' into 'High' by the generation of an overvoltage and stopping command is outputted. A control circuit is constituted of low-voltage instruments in such manner. whereby an insulating strength to ground can be reduced and the price of employing instruments becomes inexpensive.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention is often used for speed control of induction motors, and is directed to an induction motor power supply equipped with a PWM inverter in which voltage and frequency are controlled by modulating pulse width. The present invention relates to an overvoltage detection circuit that detects voltage and determines whether overvoltage occurs in order to protect the device from overvoltage that may occur when decelerating an induction motor.

[Conventional technology]

FIG. 2 is a wiring diagram schematically showing a power supply device that controls voltage and frequency to drive an induction motor. In this diagram,
An alternating current power supply 1 serving as a general power distribution system is rectified by a rectifier 2 to obtain direct current to form a direct current circuit 31. This DC circuit 3
1 is connected to a smoothing capacitor 3 to reduce the ripple current. Is this DC PW? An AC power source whose voltage value and frequency are variable is obtained by an I inverter 4, and an induction motor 5 is driven by this. When decelerating the induction motor 5, the frequency by the PWM inverter 4 is lowered, but at that time, the rotational speed of the currently rotating induction motor 5 becomes higher than the synchronous speed of the induction motor 5 determined by this frequency. In such a case, a generator action occurs in the induction motor 5, causing current to flow backwards to the PWM inverter 4. i. In a power supply device that performs rapid deceleration, such a phenomenon is actively utilized. A circuit in which a switching element and a braking resistor are connected in series is connected in parallel to the smoothing capacitor 3, and the induction motor 5 becomes a generator, and the regenerated power is consumed by the braking resistor, thereby enabling more rapid deceleration. There are some that are configured to do so, and others that have a power source configuration that allows so-called regenerative braking, in which this power is regenerated to the AC power supply l, to improve the efficiency of power consumption. However, the induction motor power supply device is generally operated at a constant speed and may be decelerated relatively slowly, and when there is a small possibility that the induction motor 5 will become a generator as described above, a braking circuit or configuration is required. Do not hire
However, even if the possibility is small, if for some reason the induction motor 5 is decelerated to the extent that it becomes a generator, the current becomes a charging current of the smoothing capacitor 3 and the DC circuit 31
The problem arises that the voltage increases, resulting in an overvoltage that threatens the insulation. In order to protect the power supply from damage due to such problems, appropriate measures such as detecting the voltage of the DC circuit 31 with the voltage divider 9 and shutting down the power supply as shown in the figure should be taken. Any possible configuration will be adopted.

Furthermore, even in the case of a power supply device that performs resistance braking or regenerative braking as described above, the switching element may not operate or the circuit that regenerates power from the DC circuit 31 to the AC power supply 1 may fail for some reason and the current is interrupted. Even in this case, the voltage of the DC circuit 31 as described above increases, so a protection device against overvoltage is essential even in a power supply device capable of resistive braking or regenerative braking.

In this figure, the voltage divider 9 detects the DC voltage of the DC circuit 31, and the detected voltage 100 is compared with the reference voltage 81 in the comparator 80, and when the detected voltage 100 is larger, an output to that effect is output. A signal 101 is output. .

The circuit voltage of the DC circuit 31 is about several hundred volts, whereas the driving voltage of the comparator 80 is about several volts, so the detection voltage 100 also needs to be about this voltage value. The voltage is stepped down to a voltage value suitable for the input signal of the comparator 80.

[Problem to be solved by the invention]

By the way, the voltage divider 9 is a high voltage DC circuit 31 of several hundred volts.
Since the voltage divider 9 is directly connected to the voltage divider 9, there is a possibility that the lead of the detection voltage 100, which originally has a value of only a few volts, may rise to a high voltage of several hundred volts. The comparator 80 is expensive because it needs to have insulation strength that can withstand a ground voltage of 100 volts, and the drive power source for the comparator 80 must also have a circuit with high ground insulation strength. There is also the problem that the size increases. Furthermore, since the voltage divider 9 is connected to the DC circuit 31 as the main circuit with a high voltage, the resistance loss is not small, and as the voltage divider 9 becomes larger, its temperature rise affects the surrounding circuit elements, resulting in poor reliability. There is also the problem that it becomes a factor that reduces sexual performance.

The present invention solves these problems and provides an overvoltage detection circuit for an induction motor power supply that does not require a voltage divider and in which the ground insulation strength of the comparator is as low as that of a normal electronic circuit. Take it as a challenge.

[Means to solve the problem]

In order to solve the above problems, the present invention provides a power supply device that rectifies an AC power source using a rectifier to obtain a DC power source, obtains an AC power from the DC power source using an I'l1M inverter, and drives an induction motor with the AC power source. In an overvoltage detection circuit for detecting overvoltage, an isolation transformer connected to the AC side voltage of the PWM inverter is compared with a detection voltage, which is a secondary voltage of this isolation transformer, and a reference voltage of a predetermined voltage value. and a comparator that outputs an electric signal to that effect when the detected voltage is large.

[Effect]

In the configuration of this invention, an isolation transformer is connected to the AC side voltage of the Plul inverter, and at the same time, a detection voltage is obtained by stepping down the high AC side voltage of several hundred volts to a low voltage suitable as the input voltage of the comparator. Since the secondary side circuit is insulated from the primary side circuit, the ground insulation strength of the comparator and the like connected to the secondary side can be significantly reduced. PWM
The AC side voltage of the inverter consists of a large number of pulse groups, the peak value of which matches the voltage value of the DC circuit, and the detected voltage is similar to this AC side voltage, so its peak value is the same as that of the conventional DC circuit. In the same way as obtaining the detection voltage by dividing the voltage using a voltage divider, a value proportional to the voltage value of the DC circuit can be obtained. Therefore, by inputting this detected voltage into a comparator and comparing it with a reference voltage of a predetermined voltage value, it is possible to perform a determination similar to the conventional one.

〔Example〕

The present invention will be explained below based on examples. FIG. 1 is a schematic wiring diagram of an induction motor power supply device showing an embodiment of the present invention, and the same components as in FIG. 2 are given the same reference numerals and detailed explanations will be omitted. In this figure, an isolation transformer 6 is connected to the AC side, which is the main circuit to which the induction motor 5 of the PWM inverter 4 is connected, and the AC side voltage 102 of the PWM inverter 4, which is a high voltage of several hundred volts, is detected by a comparator. The voltage is stepped down to a low voltage AC voltage 103 of about several volts suitable for 8. At the same time, the circuits on the primary side and the secondary side are insulated so that a detection voltage rectifier 7' and a comparator 8, which will be described later, connected to the secondary side do not require the same ground insulation strength as the main circuit. i! The pressure signal 102 is PW
Since it is the AC side voltage of the M inverter, it consists of a group of pulses having a high voltage peak value of several hundred volts whose polarity is reversed at predetermined time intervals, as is well known. This is converted into a low-voltage voltage signal 103 with a peak value of several volts using an isolation transformer, and a detection voltage rectifier 7 consisting of a full-wave rectifier converts negative pulses into positive pulses, resulting in a voltage in which all pulse groups become positive pulses. This signal 104 is used as an input signal to the comparator 8. If the detection voltage rectifier 7 is omitted, when the peak value of the pulse is negative, the comparator 8
Since this cannot be detected, a detection delay of up to half a cycle occurs.To avoid this, a detection voltage rectifier 7 is provided.

If this degree of detection delay is not a problem, the detection voltage rectifier 7 can be omitted.

The output signal 101 of the comparison n8 is a signal that becomes low when the voltage signal 104 as an input signal is smaller than the reference voltage 81, and becomes high when the voltage signal 104 is larger than the reference voltage 81. Naturally, it is output when the power supply device is operating normally. Signal 101 remains low. Output signal 101
A stop command is issued when the signal changes from low to Ilgh. 1, the pressure signal 104 consists of a group of pulses as described above, so the peak value of each pulse is equal to the reference voltage 81.
Even if it becomes larger, the reference voltage 81 is larger in the period between the pulses, so the output signal 101 eventually becomes a group of pulses similar to the voltage signal 104. As mentioned above, since the stop command is issued by the pulse that first exceeds the reference voltage 81, the waveform of the subsequent output signal is irrelevant, and there is no problem even if the pulse train is composed of such a pulse train.

Note that as the isolation transformer 6, an instrument transformer used for high-voltage AC voltage measurement can be used; There are some points that are different from those used in Instead, you can use what is available.

(Effects of the Invention) As described above, this invention steps down the AC side voltage of the PH inverter from a high voltage of several hundred volts to a low voltage of several volts using an isolation transformer to obtain a detection voltage, and at the same time, the secondary side circuit By being insulated from the primary circuit, the ground insulation strength of comparators, etc. connected to the secondary side, which conventionally needed to withstand high voltages of several hundred volts, is now required to withstand low voltages of several volts, significantly reducing the strength. This has the effect of simplifying the structure and reducing the cost.

The peak value of the detection voltage, which is the secondary voltage of the isolation transformer, is proportional to the voltage value of the DC circuit, as in the conventional case. Therefore, by inputting this detected voltage into a comparator and comparing it with a reference voltage of a predetermined voltage value, it is possible to perform a determination similar to the conventional one. In addition, as is well known, isolation transformers are highly efficient electrical appliances, so they do not cause large resistance losses like voltage dividers do, and they do not cause problems such as heating up surrounding components. This also has the effect of avoiding a decline in sexual performance.

[Brief explanation of drawings]

FIG. 1 is a schematic wiring diagram of an induction motor power supply device showing an embodiment of the present invention, and FIG. 2 is a schematic wiring diagram of a conventional induction motor power supply device. l... AC power supply, 2... Rectifier, 3... Smoothing capacitor, 31... DC circuit, 4... PwM inverter, 5... Induction motor, 6... Isolation transformer, 7.
...Detection voltage rectifier, 8.80...Comparator, Zth

Claims (1)

[Claims] 1) Rectifying AC power with a rectifier to obtain DC power;
An overvoltage detection circuit for detecting an overvoltage of a power supply device that obtains an alternating current from the direct current power source by a PWM inverter and drives an induction motor with the alternating current, comprising: an isolation transformer connected to the alternating current voltage of the PWM inverter; An induction motor comprising a comparator that compares a detected voltage, which is a secondary voltage of an isolation transformer, with a reference voltage of a predetermined voltage value, and outputs an electric signal to that effect when the detected voltage is large. Overvoltage detection circuit for power supply equipment.