JPS6124898B2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to an overcurrent protection circuit.

In a power circuit that supplies power to a load, overcurrent protection means are usually provided to protect circuit elements on the power supply side and the load side. In particular, in the case of a power circuit in which power is supplied to the load via a smoothing filter consisting of a smoothing reactor and a smoothing capacitor, short circuits in the load or in circuit parts or circuit elements between the smoothing capacitor and the load may occur. must be prevented since it can lead to burnout of the expensive circuit elements used in such circuits.

An example of a conventional overcurrent protection circuit for such a power circuit is shown in FIG. 1 is a controllable DC power supply (for example, the output from a thyristor rectifier connected to an AC power supply), 2 is a smoothing filter,
3 is a smoothing reactor which is a component of the smoothing filter, 4 is a smoothing capacitor which is also a component of the smoothing filter, 5 is a switch element, 6 is a load, 7
8 is a DC current transformer, 8 is a DC current calculation detector, and 9 is a comparator. The current transformer 7 is constructed of a high-speed moving type, and when the load side is short-circuited, the current transformer 7 detects the short-circuit current, and the detected signal is sent to the DC current calculation detector 8 for comparison in the comparator 9. This signal is compared with a preset value in the comparator 9, and when the output signal of the detector 8 exceeds that value, a switch cutoff command is generated and the switch element 5 , and protects the switch element 5 as well as other circuit elements.

However, the circuit of FIG. 1 has the drawback of being expensive because it uses a high-speed DC current transformer and a current calculation detector.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an inexpensive overcurrent protection circuit that can protect circuit elements in the event of a short circuit on the load side.

In the present invention, an AC current transformer, which is a relatively inexpensive current transformer, is inserted into a branch line including a smoothing capacitor, and a comparison means compares the output signal of this current transformer with a signal at a preset level. When the output signal of the current transformer exceeds this level, a control signal is generated, and a switch means provided between the smoothing filter and the load is turned off by this control signal. As the switching means, a conversion device such as an inverter that has its own switching function can be used, or a dedicated switching element can be used.As a comparison means, various types can be used depending on the type of switching means used at that time. A circuit configuration having the following can be used.

FIG. 2 shows a specific example of the overcurrent protection circuit according to the present invention, which uses an inverter as the switching means. In addition, in FIG. 2, reference numbers 1,
3, 4 and 6 indicate the same components as in FIG. 1, 10 is an AC current transformer inserted in the branch containing the smoothing capacitor, 11 is comparison means, 16
indicates an inverter, and the comparison means includes a resistor 12 that converts the output signal of the current transformer 10 into a voltage, and a voltage appearing across this resistor with an overcurrent setting voltage preset by an overcurrent setting device 14. It consists of a comparator 13 that generates an inverter cutoff command when the resistor voltage exceeds the overcurrent setting voltage, and an inverter control circuit 15 that receives this inverter cutoff command and performs the actual cutoff operation of the inverter 16. . In this circuit, in a steady state, the current of the capacitor 4 detected by the current transformer 10 is a relatively small alternating current that does not include a direct current component, so an inexpensive alternating current transformer is sufficient as the current transformer 10. There is no saturation. On the other hand, most of the sudden current that occurs when an inverter branch is shorted or a load is shorted is supplied from the capacitor 4, resulting in a relatively large current (enough to saturate the current transformer 10). By monitoring the current of the inverter 16, the occurrence of an overcurrent condition can be detected and the inverter 16 can be shut off to protect the circuit elements, in particular the inverter 16.

FIG. 3 shows another specific example of the overcurrent protection circuit according to the present invention, in which a chopper is used as the switching means. In addition, in FIG. 3, reference numbers 1,
3, 4, 6, 10, 12, 13 and 14 indicate the same components as in FIGS. 1 and 2, 17 is a chopper, 18 is a chopper control circuit, and 19 is a diode. The chopper control circuit 18 normally issues on/off commands in accordance with another control signal, but
Output signal from comparator 13 (chopper cutoff command)
When it receives a signal, it forcibly issues a chopping block activation signal.

Although FIGS. 2 and 3 show a case where a conversion device such as an inverter having a switching function itself is used as the switching means, the present invention is not limited to this, and it is also possible to use a dedicated switching element as the switching means. The key point is to insert an AC current transformer, which is a relatively inexpensive current transformer, into the branch line containing the smoothing capacitor, and use the output signal of this current transformer to transform the smoothing filter and the load. This is to protect against overcurrent by turning off the switch means between the two.

As is known from the above description, the present invention achieves protection from overcurrent of components of a power circuit that supplies power to a load via a smoothing filter with an extremely inexpensive structure.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of an overcurrent protection circuit according to the prior art, FIG. 2 is a diagram showing a specific example of an overcurrent protection circuit according to the present invention, and FIG. 3 is a diagram showing another example of an overcurrent protection circuit according to the present invention. FIG. 3 is a diagram showing a specific example. 1: Pulsating DC power supply, 2: Smoothing filter, 3: Smoothing reactor, 4: Smoothing capacitor, 5: Switch element, 6: Load, 7: DC current transformer, 8: DC current calculation detector, 9: Comparison 10: AC current transformer, 11: comparison means, 12: resistor, 13: comparator, 14: overcurrent setting device, 15: inverter control circuit, 16: inverter, 17: chopper, 1
8: Chopper control circuit, 19: Diode.

Claims (1)

[Claims] 1. The output from the pulsating DC power supply is smoothed by a smoothing filter consisting of a smoothing reactor and a smoothing capacitor, and the output from this smoothing filter is supplied to a load via a converter such as a chopper or an inverter. This is an overcurrent protection circuit for a power circuit configured to compare an AC current transformer inserted in a branch of the smoothing capacitor and the output signal of this AC current transformer with a preset level. a comparison means for generating a control signal when the output signal of the AC current transformer exceeds the level with respect to the polarity corresponding to the discharge direction of the current transformer; and a comparison means connected between the smoothing filter and the load, and controlling from the comparison means An overcurrent protection circuit comprising: switch means that switches to an off state in response to a signal to cut off the supply of power from the output of the smoothing filter. 2. The overcurrent protection circuit according to claim 1, wherein a conversion device interposed between a smoothing filter and a load is used as the switching means.