JP2576145B2 - Transistor inverter for three-phase AC motor - Google Patents

Description

The present invention relates to a transistor inverter that obtains a three-phase AC voltage from a DC voltage source via a bridge circuit of six transistor switches and supplies the three-phase AC voltage to a three-phase AC motor.

[Conventional technology]

In this type of inverter, a sharp voltage change occurs between the collector and the emitter of the transistor when the transistor switch is opened and closed. As a result, electromagnetic noise is generated, which adversely affects the surroundings. Further, the switching loss of the transistor and the rising voltage are large, and therefore, an element having a large capacity and a large withstand capacity is required as each transistor. To solve these problems, an RC snubber circuit or an RCD snubber circuit is connected in parallel to each transistor.

[Problems to be solved by the invention]

However, a snubber loss that cannot be ignored in the conventional snubber circuit as described above has occurred.

An object of the present invention is to provide a transistor inverter for a three-phase AC motor in which snubber loss is significantly reduced.

[Means for solving the problem]

According to the present invention, there is provided a transistor inverter which obtains a three-phase AC voltage from a DC voltage source through a bridge circuit of six transistor switches each having a diode connected in parallel, and supplies the voltage to a three-phase AC motor. A snubber capacitor is directly connected in parallel with each transistor switch, each transistor switch opens and closes only once per AC cycle, and one pair of transistor switches connected in series is always turned off after one of them is turned off. This is achieved by controlling the other snubber capacitor belonging to the relevant transistor switch pair to be turned on after the completion of the charging / discharging process.

[Action]

According to the present invention, it has been considered that it is not permissible to simply connect a lossless snubber, that is, a capacitor directly in parallel with each element in the conventional transistor inverter.
It is possible to simply connect capacitors directly to all six switches, taking into account the fact that opening and closing only once, and setting the time for turning off the transistor switch pairs of the vertically opposed arms together to be sufficiently large, etc. It is because I found something. The present invention is suitable for an induction motor in which the phase relationship between voltage and load current is relatively fixed.

〔Example〕

FIG. 1 shows an embodiment of a transistor inverter according to the present invention.

In FIG. 1, reference numeral 1 denotes a DC voltage source, which may be a variable DC voltage source. Reference numeral 2 denotes a load, particularly a three-phase induction motor. 31 to 36 are six transistors connected in a three-phase bridge connection, each of which is connected to one diode 41 to 46 in parallel. One snubber capacitor 51 to 56 is directly connected in parallel to each transistor. Reference numeral 6 denotes a control circuit for controlling the opening and closing of each of the transistors 41 to 46.

FIG. 2 shows ON / OFF command signals for the transistors 31 to 36 provided from the control circuit 6 and transistors 32, 34, 36
Voltage (collector-emitter voltage) V ₃₂ , V ₃₄ , V ₃₆
When a voltage V _UV between the output line, (an electrical angle of 360 °) 1 cycle of alternating current in the time chart of the phase current i _U is worth is shown.

Since the general operation of a transistor inverter is well known, it is omitted and only the points important for the present invention will be described.

Transistor 31 until the time t ₁ of FIG. 2 is turned off, the transistor 32 is turned on, the current i _U is flowing to the negative electrode of the power source 1 through the transistor 32 from the motor terminal U. When the transistor 32 at time t ₁ is turned off, the voltage of the transistor 32 is jumping in a moment, but is about to exceed the potential of the power supply 1, it is clamped by the diode 41, the current
i _U flows instantaneously to the positive electrode of the power supply 1. Conventionally, a high whisker-like surge voltage is generated in the transistor 32 at this moment, and the withstand voltage of the element is threatened. However, in the present invention has a capacitor 51, the current i _U capacitor 52 at the moment the transistor 32 is turned off at time t ₁ catch. With a slight delay, the capacitor 51 receives half of that. As a result inter-electrode voltage V ₃₂ of the transistor 32 in inclined a slow determined by the capacitance and the current i _u of capacitors 51 and 52 is increased,
FIG. 2 shows that the power supply voltage _VDC is reached after the charging / discharging time _Tc . Current i _u capacitor 5 when V ₃₂ reaches V _DC
Commutation from 1,52 to the diode 41 smoothly occurs, and almost no surge voltage occurs. Although the set time t _s is the switch 31 is turned on at the elapse time t _2, the time and current i _U flows in 41, because the forward voltage drop of the diode is small, on-duty of the switch 31 is quite It is minor.
If so performs the inverse basic operation described up to now point t ₇ ~t _8, the switch may by performing one time a very minor responsibilities on and off one cycle 360 °.

Capacitor 51 to having this proper charging and discharging time t _c
By connecting 56 and setting the set time t _{s at} which both pairs of transistors in the series are turned off to be greater than t _{c, a} new good transistor inverter is obtained.

The snubber capacitor only needs to have an effect of suppressing a sudden change in voltage when the switch is turned off.
The connection described above has an electrical effect substantially equivalent to that of 51 to 56, and various other connections can be considered.

In order to obtain a variable voltage variable frequency inverter, the input DC voltage is preferably variable by a chopper or the like because the square wave inverter itself cannot change the output voltage.

〔The invention's effect〕

According to the present invention, since a snubber capacitor having no loss is realized, the duty of turning on and off the element is drastically reduced, and the switching loss is drastically reduced. As a result, the efficiency of the inverter section is 95%
Some have risen to 98%.

Further, in some cases, the rate of change dv / dt of the ground voltage generated in the three wires to the motor is reduced to several hundredths, and in some cases, the level at which electromagnetic noise cannot be measured, that is, there is no example.

Further, the output waveform discharge time t _c is sufficiently large becomes trapezoidal wave from square wave, more efficiency of the motor to approach a sine wave, the motor noise by reducing harmonics, there is a reduction of vibration.

Further, depending on the type of the motor, the present invention can be applied to an inverter for a two-phase or four-phase polyphase motor.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing one embodiment of the present invention, and FIG. 2 is a time chart for explaining the operation of the embodiment of FIG. Third
FIG. 10 is a circuit diagram showing another embodiment of the present invention. 1. DC power supply, 2. 3-phase motor, 31 to 36 .. Transistor switch, 41 to 46 .. Diode, 51 to 59 .. Snubber capacitor, 6. Control circuit.

Claims (1)

(57) [Claims] 1. A transistor inverter in which six transistor switches each having a diode connected in anti-parallel are bridge-connected, a three-phase AC voltage is obtained from a DC power supply via the bridge connection circuit, and supplied to a three-phase AC motor. A snubber capacitor is directly connected in parallel with each transistor switch.
Transistor switch pairs that open and close only once and are in series with each other
3. A transistor inverter for a three-phase AC motor, wherein one of the transistors is always turned off and then the other is turned on after a time when the charging / discharging process of the snubber capacitor belonging to the transistor pair is completed.