Sewing machine motor control unit. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Electric motors

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Household sewing machines are often electrified by installing a MSH-2 commutator motor, powered by an AC mains of 220 V, 50 Hz. Control of this engine with a standard pedal is unreliable, in addition, it is not always possible to purchase it. In the proposed design, a self-made pedal is used, equipped with an optical position sensor, and a sharp press on it causes a forced acceleration of the engine. The speed set by the pedal does not change under the variable load on the motor shaft characteristic of sewing machines. It is possible to limit the maximum frequency, and the limit threshold can be adjusted during sewing.

The diagram of the control unit (without power units) is shown in fig. 1. The motor shaft speed sensor is an optocoupler with an open optical channel U1, the signals of which are amplified and formed by the transistor VT1 and the Schmitt trigger DD1.1.

(click to enlarge)

As shown in fig. 2, a small board is fixed on the motor housing 1 with a screw 2. The optocoupler 3 installed on it enters a specially drilled hole in the housing 1. The optical window of the optocoupler should be at a distance of 1 ... 2 mm from the fan impeller mounted on the shaft 5 4. A mask is applied to the surface of the impeller facing the optocoupler view A on detail 2). She is painted in black and white. You can also, by blackening the surface, stick strips of foil on it. By adjusting the position of the optocoupler relative to the impeller and selecting the value of the resistor R4, the maximum range of pulses on the collector of the transistor VT3 is achieved when the motor shaft rotates.

In total, there are 16 light sectors on the mask, as a result, 2.1 pulses are received at the input of the single vibrator DD16 per shaft revolution. In response to each of them, the single vibrator generates a pulse of fixed amplitude and duration, so the constant component of the voltage at the output of the single vibrator is proportional to the rotational speed. The constant component, amplified and filtered by the cascade on the op-amp DA4, serves as a feedback signal in the speed stabilization system. The steepness of the dependence of voltage on frequency is set by a tuning resistor R12.

The design of the pedal is shown in fig. 3. Its movable part 2 and fixed base 1 are connected by spring 3, which resists pressing Optocoupler 4 (U2 similar to U1, see Fig. 1) is placed on base 1. Depending on the distance from optocoupler 4 to reflector 5, mounted on movable part 2 , the amount of light emitted by the LED optocoupler 4, returned to the sensitive surface of its phototransistor, changes. As a result, the current of the phototransistor changes. The cascade on the DA1 chip converts current into voltage. The value of the resistor R7 is chosen so that the full stroke of the pedal corresponds to a change in the voltage at the output of DA1 from 0 to approximately -8 V.

Op-amp DA2 - a comparison element and an error signal amplifier of the stabilization system. Its inputs receive signals proportional to the speed of rotation and the position of the pedal, and the output voltage through the diode VD5 is applied to the input 3 of the op-amp DA3, which serves as a comparator.

Input 3 of the comparator is connected to a sawtooth voltage generator, consisting of a diode bridge VD1-VD4 and a cascade on a transistor VT2. The mains voltage reduced to 6 V is applied to the bridge. At the moments when the mains voltage passes through zero, when all the diodes of the bridge are closed, and the transistor VT2 is opened by the current flowing through the resistor R6, the capacitor C1 is charged almost to the supply voltage. In the rest of each half-cycle, the instantaneous value of the mains voltage is different from zero, therefore, the positive voltage rectified by the bridge , acting on the base of the transistor VT2, keeps the latter in the closed state. Capacitor C1 is discharged through resistor R10. By selecting the value of this resistor, they ensure that the voltage across the capacitor does not fall below approximately 0,2 V. Otherwise, the motor shaft will continue to rotate even when the pedal is released.

The decays of the pulses at the output of DA3 coincide with the moments when the mains voltage passes through zero, and the position of the fronts on the time axis depends on the voltage at the output of the op-amp DA2. Through the diode \ / D6 and the resistor R25, the pulses are fed to the base of the transistor VT4, in the collector circuit of which are the LED of the optothyristor U3.1 and the limiting resistor R28.

On fig. 4 shows a diagram of the power part of the control unit, the numbering of its elements continues that started in fig. 1. Thyristor U3.2 in the diagonal of the VD8 bridge opens in each half-cycle with the beginning of the light pulse generated by the LED U3.1. The electric motor M1, included in the second diagonal of the VD8 bridge, receives mains voltage. The fact that the opening thyristor light pulse continues until the end of the half-cycle prevents premature (before the end of the half-cycle) closing of the thyristor due to short-term contact disturbances characteristic of collector motors in the brush assembly.

Let's return to fig. 1. In addition to the nodes discussed above, the device has a limiter for the average value of the voltage supplied to the motor. The limiter consists of a single vibrator DD2.2 and a transistor switch VT3. The decline of each control pulse (coinciding in time with the zero instantaneous value of the mains voltage) starts the DD2.2 single vibrator, the pulses of which open the transistor VT3. As a result, the transistor VT4, and with it the optothyristor U3, cannot open until the one-shot pulse ends. Due to this, the average voltage on the motor cannot exceed the value that depends on the position of the variable resistor R24.

Practice has shown that often, if the limit threshold is too low, the engine cannot start under load, although it works normally after preliminary acceleration. In connection with this circumstance, a unit for forced shutdown of the limiter is provided, assembled on the OS DA5. While the voltage at pin 6 of DA4, proportional to the rotational speed, is less than the threshold set by the trimmer R20 in absolute value, the voltage at the output of DA5 is negative, the diode VD7 is closed and the low logic level of the voltage at the input R of the single vibrator DD2.2 disables the latter, allowing the motor start confidently. With an increase in the rotational speed, the low level at the input R DD2.2 is replaced by a high one, allowing the operation of the single vibrator.

The unit can be powered from any stabilized source with output voltages of +9 and -9 V, capable of delivering a current of at least 100 mA in the positive voltage circuit and 30 mA in the negative circuit. An alternating voltage of 6 V is supplied to the diode bridge VD1-VD4 from a separate secondary winding of the mains transformer. If there is no such winding, you can use an additional step-down transformer that gives the desired voltage.

The block used fixed resistors MLT, variable R24 - SP-1; trimmers R12, R20 - SPO-0,15. Capacitors C1, C3, C5 - metal film, C7 - MBHC, oxide C2, C4, C6 - K50-35. Transistors KT502V can be replaced with KT502A, KT502D, KT502E, KT361B, KT361V, KT361G, and KT503V - with KT503A, KT503D, KT503E, KT315B, KT315V, KT315P. Instead of the K564AG1 microcircuit, its foreign analogue CD4098B is suitable, instead of KR140UD608 - K140UD6, K140UD7, KR140UD708. The KTs405B diode bridge can be replaced with KTs402A, KTs403A, KTs403B, KTs403V, ​​KD509A diodes - with KD503A, KD510A, KD518A.

An unloaded MSH-2 motor at a rated supply voltage can develop a very high speed (up to 20000 min-1). Therefore, it is desirable that during the adjustment of the control unit, the engine be mechanically loaded by the drive of the sewing machine running idle (without fabric and threads). For sewing machines of most types, the maximum rotational speed of the motor shaft under these conditions is approximately 3000 min-1, which corresponds to the pulse repetition rate of a single vibrator DD2.1800 Hz.

The duration of these pulses should be 0,8 ms. If, at the maximum speed of the motor shaft, the op-amp DA4 enters saturation, the duration must be reduced. It is corrected by selecting the value of the resistor R16. The pulse duration of the single vibrator DD2.2 should be changed using a variable resistor R24 ​​in the range of 2 ... 6 ms.

Pressing the pedal all the way and moving the engine of the tuned resistor R12 from the left (according to the diagram) to the right, set it to the position from which the engine speed decreases. The tuned resistor R20 is adjusted according to the most confident start of the engine under load.

If you have to adjust the control unit with an unloaded engine, the speed of the latter can be reduced to the required 3000 s-1 using a variable resistor R24, if necessary, temporarily changing the ratings of it and resistor R22.

Author: N.Shukov, Gomel, Belarus

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