Inverter for asynchronous motor. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Voltage converters, rectifiers, inverters

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The proposed inverter consists of a microcontroller, a protection unit against exceeding the permissible load current and powerful voltage switches on the IGBT. controlled by specialized driver chips.

Fig. 1

On fig. 1 shows the inverter circuit.

The clock frequency of the microcontroller is set by the external circuit R5R6C2. The tuned resistor R5 included in it can be set such that the frequency of the generated three-phase voltage corresponds to the required one. At the outputs of the RBO-RB5 microcontroller, control signals are generated for the nodes A1-A3 - powerful 300 V voltage switches. These nodes are identical and built according to the standard scheme. If desired, the three IR2110 microcircuits installed in them can be replaced with one - IR2130 At the output of the RB7 microcontroller, pulses are generated to set the current protection trigger to its initial state.

A three-phase voltage close to a sinusoidal shape is generated at the outputs of the KhTZ-XT5 inverter due to a software change in the ratio of the intervals of the open and closed states of the "upper" and "lower" arms of the switches A1-A3. In each phase, 36 pulses of variable duration are formed per period of the output voltage. Unfortunately, the limited resources of the microcontroller used do not allow more.

The load current sensor of the inverter for the protection unit against exceeding its permissible value is the resistor R10, included in the common negative power circuit of the switches A1-A3. If the voltage drop across this resistor exceeds 1,7 V, the logic level of the voltage at the output of the DA1 comparator changes, which “transfers” the trigger from the elements DD2.1, DD2.2 to a high level state at the output of the element DD2.2. This level, entering the nodes A1-A3, prohibits the operation of the driver microcircuits installed there, which leads to the immediate closing of all IGBTs and to the termination of the current in all three phases of the electric motor connected to the inverter. The trigger returns to its original state at the signal of the microcontroller. The protection threshold is set by a trimming resistor R1.

The 300 V voltage source is assembled according to the scheme proposed by E Muradkhanyan and E Piliposyan in the article "Adjustable rectifier for powering electric motors" ("Radio", 2006, No. 11, pp. 40-43), taking into account the amendment in "Radio", 2007, No. 6, p. 50. The source was supplemented with a mains filter. When operating the inverter, it is important to ensure the sequence of switching on the supply voltage. The first voltage of 220 V is applied to the transformer T1 (Fig. 1) and only then the voltage of 300 V is turned on.

The inverter was tested with a 1 kW asynchronous three-phase motor, the windings of which were connected in a delta. The shape of the current in the phases, checked with an oscilloscope connected through a current transformer, turned out to be almost sinusoidal. During the test, it was found that the starting torque on the motor shaft is insufficient, and the starting current is too high.

The fact that the output voltage of the 300 V source increases smoothly for about 3 s after it is turned on was used to eliminate these shortcomings by soft starting the motor. To do this, it is necessary to change the frequency of the three-phase voltage in proportion to the current value of the source voltage of 300 V. To implement this idea, the PIC16F84 microcontroller was replaced by the PIC16F676, which has a built-in ADC.

Fig. 2

The replacement scheme is shown in fig. 2.

An analysis of the current value of the source voltage of 16 V was introduced into the program of the PIC676F300 microcontroller. When it changes from 0 to 300 V, the frequency of the generated three-phase voltage increases from 12 to 50 Hz and then remains equal to the achieved value.

Programs for microcontrollers PIC16F84 and PIC 16F676 can be downloaded hence.

Author: A. Titov, Skhodnya, Moscow Region; Publication: radioradar.net

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