Converter of single-phase mains voltage to three-phase with a frequency of 50-400 Hz. Encyclopedia of radio electronics and electrical engineering

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

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This converter is designed to power three-phase asynchronous electric motors up to 1000 W at 36 and 42 V at a nominal frequency of up to 400 Hz from a household power supply. Such motors are commonly used in industrial portable power tools.

Distinctive features of this device are relatively small dimensions and the ability to connect motors with different nominal frequencies, as well as changes within certain limits of the motor shaft speed by adjusting the frequency of the supply voltage. With the appropriate replacement of the transformer and other elements of power units, the converter can be adapted to power motors with a different rated voltage and higher power.

Fig. 1

The converter circuit is shown in fig. 1. On the logic elements DD1.1, DD1.2, DD1.4, a multivibrator is assembled, the oscillation frequency of which can be changed by a variable resistor R2 within 150 ... 1200 Hz. The frequency of the three-phase pulse sequence formed by the node on the microcircuits DD2, DD3 and the element DD1.3, and the output three-phase voltage is three times less - 50 ... 400 Hz. To switch to another frequency interval, you will have to change the capacitance of the capacitor C1.

The nodes A3.2-A3.4 are connected to the outputs of the elements DD1-DD3. forming the voltage of phases A, B and C, supplied to the electric motor through connector X1. Since these nodes are exactly the same, consider the circuit of only one of them - A1. His work is explained in Fig. 1 waveforms of signals at characteristic points.

An integrator is assembled on the op amp DA1, which converts rectangular pulses into a symmetrical sawtooth voltage. Transistors VT1, VT3, VT5, VT8 are open when the voltage at the output of the op-amp is higher than Unop1- At the output of the shaper, the voltage in this state is close to -20 V. When the output voltage of the op-amp is lower than Upor.2, transistors VT2, VT4, VT6, VT7 and the voltage at the output of the shaper becomes +20 V.

At intermediate (between Upor.1, and Upor2) voltage values ​​at the output of the op-amp, all transistors of the shaper are closed and phase wire A is disconnected from voltage sources of +20 V and -20 V. Since some time passes between closing one group of transistors and opening another, due to the difference in thresholds and the rate of change of voltage at the output of the integrator, the simultaneous opening of all transistors with the flow of "through" current through them is excluded.

Fig. 2

The converter power supply circuit is shown in fig. 2. It has a T1 transformer with an overall capacity of 800 VA. This allows you to power such three-phase power tools at a nominal frequency of 200 Hz from the converter, such as the IE-1025A drill, IE-3601B wrench, IE-2004B grinder, etc. Winding II of this transformer with a voltage of 30 V is designed for a current of 20 A, and winding III voltage 36 V - for a current of 0,5 ... 0,8 A. If the selected transformer does not have winding III, an alternating voltage of 36 V can be obtained from a separate low-power transformer.

A controlled rectifier based on diodes VD1, VD4 and optodistors U5, U1 is connected to the winding II of the transformer T2. With the help of a node on the transistor VT3, the switching on of the output voltages of +20 V and -20 V, which feed the powerful transistors of the converter, is delayed by 1 ... 2 s relative to the rest of the output voltages of the block. This is done so that the formation of a three-phase sequence of pulses has time to take on a stationary character before powerful nodes start working. Resistor R10 is designed to limit the starting current of the motor.

The remaining output voltages are obtained from the rectifier on the diode bridge VD2, operating from the winding III of the transformer T1. Pay attention to the voltage regulator for digital microcircuits. The 5 V required for this is formed by summing two voltages of different polarity obtained from stabilizers on transistors VT1 and VT2. The trimmer resistor R1 regulates these voltages, keeping their sum unchanged. This is necessary to achieve the symmetry of the sawtooth voltage generated by the integrators of nodes A1 - A3, relative to the upper and lower thresholds for opening transistors in these nodes. Transistors VT1 and VT2 are installed on heat sinks with an area of ​​at least 30 cm2 each.

The converter is assembled in a housing with dimensions of 350x210x180 mm. Inside the case there is a chassis on which the parts of the power supply are fixed - transformer T1, capacitors C7, C8 with resistors shunting them. Diodes VD3, VD4 and optodistors U1, U2 are installed on a common ribbed heat sink with dimensions of 110x80x30 mm.

The remaining parts of the power supply are mounted on a fiberglass board measuring 140x60 mm. On a similar board with dimensions of 140x110 mm there are parts of the converter itself, with the exception of powerful field-effect transistors, placed on a separate board of the same dimensions. Each of these transistors is equipped with a separate finned heat sink measuring 40x30x10 mm. Places of thermal contact of transistors with heat sinks are smeared with heat-conducting paste.

On the front panel of the housing there is a switch SA1, fuse holders FU1 and FU2, a three-phase voltage frequency regulator - a variable resistor R2 (see Fig. 1) and an X1 connector - a standard socket for connecting power tools. The peculiarity of this socket is that the plug can be connected to it in two ways, providing a different order of phase sequence and, consequently, different directions of rotation of the motor shaft. The pins of the plug are 20x6,5x1,5 mm. The main requirement for the connector is a permissible current of at least 25 A per phase.

Domestic microcircuits used in the converter can be replaced with similar imported ones: K155LAZ - 7400, K155IE4 - 7492, K155LP5 - 7486, KR140UD708 - tsA741 or NE5534. In the power supply, instead of diodes D243A, you can install D231A, and instead of optodistors TO125-12,5 - TO132-25. The diode bridge KTs402G is replaced by KTs405G. The remaining diodes and zener diodes are suitable domestic or imported ones.

Capacitor C1 (see Fig. 1) - film K73-17, the rest - ceramic K10-17. Suitable, of course, and similar imported capacitors.

Resistor R10 in the power supply is made of a piece of nichrome wire with a diameter of 1,5 mm and a length of 120 ... 150 mm, twisted into a spiral with an outer diameter of 10 mm. At the ends of the spiral, tinned petals for soldering wires are fixed with M4 screws and nuts. Resistors R11, R12 in the same block - PEV-7,5 or imported with a rated power of at least 5 watts. Trimmer resistor R1 is an imported analog of SPZ-19.

Capacitors C1, C2 of this block are film K73-17. Oxide capacitors: C4 - tantalum K53-18; C5, C6 - TEARO SE series; C7, C8 - K50-18; the rest are from JAMICON. Capacitors K50-18 can be replaced by K50-37, KEA-I-10 of Bulgarian production or capacitors of the DIN41250 standard, produced in the GDR.

The converter is connected to the network with a three-wire cable with a ground wire (PE) connected to the device case, its metal chassis and to the magnetic circuit of the T1 transformer.

When adjusting the manufactured converter, first of all, supply voltage is applied to the DD1-DD3 microcircuits (see Fig. 1) and make sure that there is a three-phase pulse sequence at the outputs of the DD3.2-DD3.4 elements. Variable resistor R2 set the maximum pulse frequency.

Then the supply voltage (+12 V and -12 V) is applied to the DA1 op amp at node A1 and to similar op amps at nodes A2 and A3. Observing triangular pulses at the outputs of the op-amp with an oscilloscope, a trimmer resistor R1 (see Fig. 2) achieves their maximum symmetry relative to the common wire. The non-identity of the waveform at the outputs of three op-amps can be eliminated by selecting, within a small range of capacitance, the capacitor C3 (see Fig. 1) and the corresponding capacitors in nodes A2 and A3.

With a decrease in the frequency of the master oscillator, the triangular pulses, due to the transition of the op-amp to the limiting mode, take the form of a trapezoid, but this does not affect the operation of the converter in any way, since the rate of voltage change in the intervals between the thresholds remains the same.

Before connecting the collectors of transistors VT5 and VT6 with the gate circuits of field-effect transistors VT7 and VT8, it is necessary to temporarily connect them to the mentioned collectors through the resistive circuit shown in fig. 3, oscilloscope input. The shape of the pulses observed in this way should be inverse, shown in the lowest oscillogram in Fig. 1. If necessary, change the duration of the pause between pulses, select resistor R6. Its significant reduction can be achieved by replacing the diodes VD1 and VD2 (simultaneously!) with jumpers.

After checking and adjusting the nodes A2 and A3 in the same way and removing the temporary connections, you can apply signals to the gates of the field-effect transistors, as shown in the diagram in Fig. 1, make sure that the shape of the signals on the sockets of the socket X1 corresponds to the required one and proceed to practical work with the converter.

Author: V. Kostitsyn, Biysk, Altai Territory; Publication: radioradar.net

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