Speed ​​controller for three-phase asynchronous motors. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Electric motors

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I bring to the attention of readers a diagram (Fig. 1) and the design of a device that allows you to adjust the speed of a three-phase asynchronous motor (IM) in the range of 300 ... 8000 rpm (hereinafter - RFV). I am sure that it will be useful to many radio amateurs, as it gives three-phase asynchronous motors new quality indicators: power supply from a single-phase network with virtually no power loss, the ability to control the starting torque, increase efficiency, independence of the direction of rotation from the phase of the applied voltage, regulation in wide ranges of speed both at idle and under load, and most importantly, the ability to increase the maximum speed from 3000 to 6000 ... 10000 rpm.

(click to enlarge)

The main characteristics of RFV:

• Supply voltage ~ 220 V
• Power consumption, no more than 15 W (excluding motor power)
• Motor type three-phase asynchronous Fн=3000 rpm, Рн=120 W.

As you know, there are several ways to control the speed of the IM - by changing the supply voltage, the load on the shaft, using a special rotor winding with adjustable resistance, as well as frequency regulation, which is the most effective method, since it allows you to save the energy characteristics of the IM and apply the cheapest and most reliable HELL with a squirrel-cage rotor. Before considering the operation of the RFV, it is necessary to remind the reader of the main characteristics of IM.

1. Efficiency factor Efficiency = (Pv / Rp), where Pv is the mechanical power on the motor shaft, Pp is the electrical power consumed from the network. At idle, efficiency = 0, since Pv = 0. At the rated power on the shaft Rn, the efficiency has a maximum value (0,75 ... 0,95) for different engines.

2. The phase currents of the IM are shown in Fig.2.

3. Frequency of rotation of the stator magnetic field n1=(60Fp)/p (rpm), where Fp - frequency of the supply current, Hz; p is the number of pairs of stator poles. Thus, at a standard frequency Fп=50 Hz, the magnetic field, depending on the number of pairs of poles, rotates with a frequency (see table).

4. Slip S=(Fp-Fp)/Fp (%). The rotor speed .p is always less than the frequency Fp by the amount of slip S (2...6%), for example Fp=960; 1420; 2840 rpm The principle of operation of AM is based on the interaction of the rotating magnetic field of the stator with the currents that are induced by this field in the conductors of the rotor winding.

5. Torque М=Рв/О, where О is the angular speed of rotation of the rotor О=2πFв/60.

6. Overload capacity Kp \u1,5d Mkr / Mn \u2,5d XNUMX ... XNUMX, where Mkr is the critical moment; Mn - nominal moment.

7. Cosϕ=Iса/Iср=0,1...0,2 at rated speed, where Iса - active stator current, Iср - reactive stator current. An increase in the motor load is accompanied by an increase in only the active component of the stator and, consequently, an increase in cosϕ to 0,8...0,9. Hence, the role of engine loading is clear in order to improve the cosϕ of the supply network.

8. Starting current Ip - stator current when starting the IM, Ip/In=5 ... 7. Starting torque of the IM is not great. When starting, the IM must develop a torque exceeding the braking torque of the mechanism, otherwise it will not turn around. Mp/Mn=0,8...1,5.

The functional diagram of the RFC is shown in Fig.3.

The master oscillator is designed to change the frequency of the AM supply current. It changes the rotor speed. The Three-Phase Sequence Pulse Conditioner (PTS) converts a DC voltage into three square wave voltages that are 120° out of phase. The preamplifier matches the low-power outputs of the FIT with a powerful final stage, the task of which is to supply the phases of the AD with the current necessary in shape and frequency. The power supply unit generates voltages of +5, +9 and +300 V to power the RFV.

Figure 4 shows all the necessary waveforms.

On the elements DD1.1 ... DD1.3, a master oscillator is assembled - a multivibrator with a variable generation frequency within 30 ... 800 Hz. Change the frequency with a variable resistor R2. FIT consists of counter DD2, element "NAND" DD1.4 and four elements "XOR" DD3.1...DD3.4. Three identical pre-amplifiers are assembled on transistors VT2 ... VT13 (one for each phase of HELL).

Consider the principle of operation of one of them (the upper one according to the scheme). When a high level appears at the output of the DD3.2 element, the composite transistor VT2, VT5 opens. From the output of the element DD3.2, a high level is fed to the input of the optocoupler DD4, as a result of which a low level is set at its output, which closes the composite transistor VT8, VT11. The other two amplifiers work similarly, only with a phase difference of 120 °. For voltage decoupling, transistors VT2, VT5 and VT8, VT11 are powered from separate sources of +9 V, and transistors VT14 ... VT19 - from a source of +300 V. Diodes VD10, VD13, VD16, VD17

 serve for voltage decoupling and for more reliable locking of transistors VT14 and VT15.

One of the main conditions for the normal operation of transistors VT14 and VT15 is that they should not be open at the same time. To do this, the control voltage is supplied to the input of the composite transistor VT8, VT11 from the output of the optocoupler DD4, which provides some delay in its switching. When a high level appears at the input of the optocoupler DD4 through the elements R8, VD7, the composite transistor VT2, VT5 opens, and the transistor VT15 closes. At the same time, the charging of the capacitor C9 begins. 40 μs after the appearance of a high level at the input of optocoupler DD4, a low level appears at its output, the composite transistor VT8, VT11 closes, the transistor VT14 opens. The appearance of a low-level optocoupler DD4 at the input cannot instantly close the composite transistor VT2, VT5, since the discharge of the capacitor C9 through the circuit R9, the base, the emitter keeps this transistor open for 140 μs, and the transistor VT15 - closed. The turn-off delay time of the DD4 optocoupler is 100 μs, so the VT14 transistor closes before the VT15 transistor opens.

Diodes VD22 ... VD23 protect transistors VT14, VT15 from voltage increase when switching an inductive load - IM windings, as well as for closing the winding currents at times when the voltage changes polarity (when switching transistors VT14, VT15). For example, after closing the transistors VT14 and VT17, the current passes for some time in the same direction - from phase A to phase B, closing through the VD24 diode, the power supply, VD23, until it drops to zero.

Consider the principle of operation of the final stage using the example of phases A and B. When transistors VT14 and VT17 are opened, a positive potential is applied to the beginning of phase A, and a negative one to its end. After they are closed, transistors VT15 and VT16 open, and now, on the contrary, a positive potential is applied to the end of phase A, and a negative one to the beginning. Thus, phases A, B and C are supplied with alternating voltages of a rectangular shape with a phase shift of 120 ° (see Fig. 4). The frequency of the AM supply voltage is determined by the switching frequency of these transistors. Due to the alternate opening of the transistors, the current passes in series through the circuits of the stator windings AB-AC-BCVA-CA-CB-AB, which creates a rotating magnetic field.

The forms of phase currents are shown in fig. 5.

The circuit for constructing the terminal stage described above is a three-phase bridge [1]. Its advantage is that there are no third harmonic components in the phase current curves.

To power the low-voltage stages, a stabilizer VD1, VT1, VD6 is used, which allows you to get +5 V to power the DD1 ... DD3 microcircuits, as well as +9 V to power the preamplifiers (VT2 ... VT7). Each upper pair of preamplifiers is powered by its own rectifier: VT8, VT11 - from VD3, VT9, VT12 - from VD4, VT10, VT13 - from VD5.

The final stages are powered by a full-wave rectifier and an LC filter (VD2, L1, C3, C7) +300 V. The capacitances of capacitors C3 and C7 are selected based on the power of the AD, the larger the capacitance, the better, but not less than 20 μF with the inductance of the inductor L1 0,1 H.

In RFV, fixed resistors such as MLT, OMLT, VS can be used. Capacitor C1 - any ceramic or metal-paper; C2 ... C8 - any oxide. Inductor L1 can be excluded, but it will be necessary to increase the capacitance of each of the capacitors C3 and C7 to 50 microfarads. Chip DD1 type K155LA3, DD2 - K155IE4, DD3 K155LP5. Optocouplers DD4...DD6 - AOT165A1. Others can be used, in which the turn-on delay time is not more than 100 µs, and the insulation voltage is not less than 400 V.

The main requirement for transistors is a high and approximately the same gain for all (at least 50). Transistors VT2 ... VT4, VT8 ... VT10 type KT315A, they can be replaced by KT315, KT312, KT3102 with any letter indices. Transistors VT1, VT5 ... VT7, VT11 ... VT13 type KT817 or KT815 with any letter index. Transistors VT14 ... VT19 - KT834A or KT834B. To replace them, you can use powerful high-voltage transistors with a gain of at least 50. Since the output transistors operate in switching mode, it is necessary to install them on radiators with an area of ​​10 cm2 each. However, when using motors over 200W, larger heatsinks will be required.

Bridge rectifiers VD1,VD3...VD5 - KTS405A. Rectifier VD2 - KTS409A. With an AM power of more than 300 W, instead of the KTs409A bridge rectifier, it is necessary to use a bridge of single diodes designed for a reverse voltage of more than 400 V and the corresponding current. Zener diode VD6 - KS156A. Diodes VD7 ... VD21 - KD209A.

Diodes VD22 ... VD27 any, designed for a current of at least 5 A and a reverse voltage of at least 400 V, for example KD226V or KD226G.

Transformer - any power of at least 15 W, having four separate secondary windings of 8 V each.

When setting up the device, first turn off +300 V and check the presence of all oscillograms at the indicated points (see Fig. 4). If necessary, by selecting the capacitor C1 or resistor R2, the frequency change on the collector of the transistor VT5 is achieved within 5 ... 130 Hz. Then, when the AD is off, instead of +300 V, a voltage of +100 ... 150 V is supplied from an external source, the collector and emitter of the transistor VT11, the collector and emitter of the transistor VT5 are closed (to close the transistors VT14 and VT15 for a long time) and the current in the collector circuit is measured transistor VT14, which should be no more than a few μA - the leakage current of transistors VT14 and VT15. Next, the collectors and emitters of the above transistors are opened and the maximum generation frequency is set by resistor R2.

By selecting the capacitance of the capacitor C9 upwards, they achieve the minimum current in the collector circuit of the transistor VT14, which in the ideal case is equal to the leakage current of the transistors VT14 and VT15. In this way, the remaining two terminal amplifiers are adjusted. Next, they connect to the RFV output (to the X7 socket) AD, the windings of which are connected by a star. Instead of +300 V, voltage is supplied from an external source in the range of +100 ... 150 V. The IM should begin to rotate. If it is necessary to change the direction of rotation, any phases of the IM are interchanged.

If the terminal transistors operate in the correct mode, then they remain slightly warm for a long time, otherwise the resistances of resistors R18, R20, R22, R23 ... R25 are selected.

References:

1. Radin V.I. Electronic machines: Asynchronous machines. -M.: Higher. school, 1988.
2. Kravchik A.E. Selection and application of asynchronous motors. Moscow: Energoatomizdat, 1987.
3. Lopukhina E.M. Asynchronous executive micromotors for automation systems. -M.: Higher. school, 1988.

Author: A. Dubrovsky

See other articles Section Electric motors.

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