Stitching Machine Motor Speed Control
Motor operated stitching machines have a series of carbon buttons in an enclosure operated by foot pedal. As the pressure on the foot pedal is increased or decreased by the foot, these buttons come close or move farther away, their resistance changes and hence the speed of the motor. Even though crude, it seemed to be okay, until my wife complained.
She said that the speed is little too fast even at the minimum pressure on the foot pedal, particularly for some repair work or at embroidery. Most of the housewives also like a little more control over speed of the machine. So here you have it.
The circuit is shown in Schematic 30. This is a standard triac speed control circuit much similar to domestic fan control circuit. Contrary to other triac circuits, you will find that an additional component known as diac is used in this circuit.
Triacsfire more symmetrically when used along with diac in AC power control applications. The diac is a bidirectional trigger diode which does not conduct (except for a small leakage current) until the break over voltage is reached. Its function is designed specifically to trigger a triac or SCR.
In the beginning triac Ql is not conducting; C1 is charged through variable resistor R3. This charge is coupled to Diac through R1, R3. When trigger level of the diac is reached (about 36 V), D1 fires and Triac TR1 is switched on. R3 and C1 combination sets the firing point of the triac from zero crossing along with Rl and R2. LI and C3 combination acts radio frequency filter for the radio interference caused by triac firing.
Entire circuit operates on mains. Care must be exercised when mounting components on normal Veroboard is risky. Remove alternate tracks and mount components. Triac should be mounted on a small heat sink as the triac tends to get hot particularly at lower speeds. All capacitors are polyester or polycarbonate rated at 600V or more. Rl is a preset for minimum speed control. Adjust this according to your requirement. R2 is the linear variable resistor like the volume control in the radios. Use the one with plastic shaft. L1 is a radio interference choke. Take 28 gage winding wire and make 10 turns on a 6 mm former. It can be wound on a round capacitor for C3 and even one end can be soldered to it also.
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There is an obvious error in the Stitching Machine Motor Speed Control diagram: The diac D1 should be placed between the triac gate and the capacitor C2.
Rolfi