Fault-sensing and protecting apparatus for soft start circuit of inverter and method for the same

Fault-sensing and protecting apparatus for soft start circuit of inverter and method for the same are proposed. The soft start circuit of inverter includes a soft-starting resistor and a relay. The relay has a normally-open end connected to the soft-starting resistor in parallel. The fault-sensing and protecting apparatus for soft start circuit includes a central processing unit (CPU) and a photo coupler. The CPU is electrically connected to the inverter and an input end of the relay, respectively. The photo coupler includes a light emitting diode (LED) and a photo transistor. The LED is first electrically connected to a resistor in series and then electrically connected to the soft-starting resistor in parallel. The photo transistor is electrically connected to CPU. After the inverter starts to work, the CPU cuts out voltage applied to the inverter if the LED is lit. Therefore, the inverter can be prevented from damage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fault-sensing and protecting apparatus for soft start circuit, especially to a fault-sensing and protecting apparatus for soft start circuit used in an inverter.

2. Description of Prior Art

AC motor has been widely used as the mature of solid state inverter. The AC motor has the advantages of small rotor inertia, and excellent heat dissipation effect, and is extensively used in CNC machine and industrial robot as the progress of power semiconductor, microprocessor and control industry.

FIG. 1shows a block diagram for a prior art AC motor connected to a wall socket power. An AC power such as 110Vac or 220Vac is supplied from a wall socket power20to a converter30for AC to DC conversion. The converted DC power is then sent to an inverter40for DC to AC inversion. The inverter40is controlled by a central processing unit (CPU) to obtain an AC power with required frequency and level. The AC power with required frequency and level is used to drive a motor60.

The inverter uses a rectified and filtered DC power as input and therefore a soft start circuit with resistor is employed in the inverter for suppressing starting current. After starting, electrically-controllable switch unit such as relay, electromagnetic contact or SCR switch is used for bypassing the soft-start resistor. As shown inFIG. 2, the converter30is mainly composed of diode set32and filtering capacitor C. The wall socket power20is supplied to the inverter (not shown) after being rectified by the diode set32and filtered by the filtering capacitor C. When the voltage different between the filtering capacitor C and the wall socket power20is large, a charging current is considerable. A large starting current is generate to damage the diode set32, burn out fuse or turn off a fuse-less switch. Therefore, the soft-start resistor is added to suppress the starting current. More particularly, a soft-start resistor R is provided in parallel with the rectifying and filtering circuit. The soft-start resistor R consumes considerable power when the circuit starts. Therefore, switch unit such as relay, electromagnetic contact or SCR switch is used for bypassing the soft start resistor. The switch unit and the soft-start resistor R are referred to as soft-start circuit. As shown inFIG. 2, the CPU controls the relay72to close the switch end72A thereof (normally open end) for bypassing the soft start resistor R after the inverter starts.

However, the soft-start circuit malfunctions when printed circuit board (PCB) has line breaking problem, or the relay breakdowns. There is no protection circuit or malfunction detecting circuit for the soft-start circuit, and therefore the inverter has burning down risk after the soft-start circuit malfunctions. The soft-start resistor R has risk of burning down if the load is large (the load current is large). This happens when the soft-start resistor R is not bypassed. It is desirable to provide a protection circuit and malfunction detecting circuit for the soft-start circuit.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a fault-sensing and protecting apparatus for soft start circuit of inverter.

Accordingly, the present invention provides a fault-sensing and protecting apparatus for soft start circuit of inverter and method for the same. The soft start circuit of inverter includes a soft-start resistor and a relay. The relay has a normally-open end connected to the soft-start resistor in parallel. The fault-sensing and protecting apparatus for soft start circuit includes a central processing unit (CPU) and a photo coupler. The CPU is electrically connected to the inverter and an input end of the relay, respectively. The photo coupler includes a light emitting diode (LED) and a photo transistor. The LED is first electrically connected to a resistor in series and then electrically connected to the soft-start resistor in parallel. The photo transistor is electrically connected to CPU. After the inverter starts to work, the CPU cuts out voltage applied to the inverter if the LED is lit. Therefore, the inverter can be prevented from damage.

Moreover, the CPU can first send a signal to open-circuit the normally open end of the relay and then sends a short time current to the soft start resistor. When the LED is not lighted, the CPU can identify that the relay has permanent short circuit.

DETAILED DESCRIPTION OF THE INVENTION

With reference toFIG. 3, the fault-sensing and protecting apparatus for soft start circuit according to the present invention comprises a CPU (not labeled, which is the same as numeral50inFIG. 1), a photo coupler and a serial resistor R1. The fault-sensing and protecting apparatus is used for a soft start circuit of an inverter as shown inFIG. 2, wherein the fault-sensing and protecting apparatus protects the whole inverter system and senses fault in the relay of the soft start circuit. The components similar to or the same to those ofFIG. 2use the same numeral for clarity.

The photo coupler74comprises a light emitting diode (LED)74A and a photo diode74B, wherein the anode of the LED74A is connected to a node A at one end of the soft-start resistor R, and the cathode of the LED74A is connected to a node B at the other end of the soft-start resistor R through the serial resistor R1. The emitter of the photo diode74B is connected to ground and the collector of the photo diode74B is connected to the CPU, and also connected to a 5V power source through a resistor R2.

An electrically-controllable switch unit72employs, for example, a relay, a magnetic contact (MC), a silicon controlled rectifier (SCR), or a triggering bidirectional thyristor (TRIAC). One port of the electrically-controllable switch unit72provides normally open end or normally close end, while the other end of the electrically-controllable switch unit72is controlled by a controller, a CPU or other circuit to short-circuit (turn on) the normally open end or break off (turn off) the normally close end. Hereinafter the electrically-controllable switch unit72is exemplified as a relay, it should be noted the electrically-controllable switch unit72can employ other kinds of switches. When the wall-socket power supplies to the inverter system, the normally open end of the relay72is connected to the soft-start resistor R and keeps in normally open state such that most of the start current flows through the soft-start resistor R and the start current is limited. After a predetermined time, the CPU controls the input end72B of the relay to short-circuit the normally open end72A. Therefore, the current is prevented from flowing through the soft-start resistor R and power consumption can be prevented.

The fault-sensing and protecting apparatus for soft start circuit according to the present invention can be used to detect error for the relay in the soft start circuit. Generally, the relay error includes malfunction and permanent short circuit due to high current. The fault-sensing and protecting apparatus according to the present invention can be used to detect these two types of relay errors.

When the relay72shown inFIG. 3malfunctions, all of load current flows through the soft-start resistor R. According to Ohm law, voltage difference across the soft-start resistor R increases. The voltage difference across the soft-start resistor R will trigger the photo coupler74after subtracting voltage drop across the serial resistor R1. The photo coupler74informs the CPU for warning or stop outputting power, thus protecting the soft-start circuit. For example, the CPU triggers a beeper for alarm, or triggers a flashing light for warning. Those ways of warning are well known art and not detailed here. More particularly, the LED74A in the photo coupler74emits light when the voltage across the soft-start resistor R is excessive. The impedance of the photo diode74B decreases after it is lighted by the LED74A. The voltage at node4(collector voltage) of the photo coupler74also decreases. Therefore, the CPU can sense that the relay72shown inFIG. 3has malfunction. The CPU can send alarm or stop function of the inverter system.

If the normally open end72A of the relay72has permanent short circuit due to high current, CPU can sense the permanent short circuit by following steps. The CPU first sends a signal to turn off the relay72, namely, to break off the relay72. When the relay72has permanent short circuit, the relay72cannot be turned off by the signal. Afterward, the CPU sends a short-time current through the soft-start resistor R, wherein the short-time current is sufficiently large to produce a voltage difference across the soft-start resistor R. The voltage difference across the soft-start resistor R is sufficient to trigger the photo coupler74. For example, the short-time current is a current of 10-50 mA with 0.3 to 0.5 mS duration. If the relay72has permanent short circuit, the short-time current flows through the normally open end72A of the relay72and the photo coupler74is not triggered. Therefore, the CPU identifies that the relay72has permanent short circuit. This test can be conducted before the operation of the inverter system and the CPU can identify whether the relay72has permanent short circuit.