Speed control device for sewing machines

An electric sewing machine has an A.C. driving motor controlled by a triac; a controlling circuit for the triac includes a thyristor powered by a lower-voltage battery via an adjustable RC-circuit; the controlling circuit is electrically insulated from the A.C. power-supply circuit by an optoelectric coupler, the optical part of which is connected to the A.C. circuit and the electric part of which controls the gate of the thyristor in synchronism with the A.C. voltage; the thyristor in the controlling circuit is coupled to the triac by means of an insulating pulse transformer.

BRIEF DESCRIPTION OF THE INVENTION 
The present invention relates to a speed control device for an electric 
motor, particularly for a drive motor of a sewing machine, in which the 
power supply of the machine motor and the control part thereof are 
electrically separated by means of an easily insulating member such as a 
pulse transformer, and a battery is used as a power source of the control 
part so that the control part is electrically insulated from the motor 
power supply. Thus the insulation of a pedal controller is simplified to 
prevent dangers such as electric shock of the machine operator. 
A conventional device for speed control of the sewing machine motor is as 
shown in FIG. 1 of the attached drawing. According to the conventional 
device, the machine motor M receives AC power source V and is phase 
controlled by a bidirectional three-terminal thyristor TRIAC. The 
full-wave rectifier bridge circuit D-D receives AC power source V and 
constitutes, together with the Zenor diode ZD, a trapezoidal wave voltage 
source for controlling the trigger current of the thyristor TRIAC. The 
N-gate thyristor PUT receives at its anode the charged voltage of the 
capacitor C to be charged with the trapezoidal wave-voltage source via the 
variable resistor R2 for controlling ignition phase of the anode, and is 
operated to discharge the charged voltage of the capacitor at the side of 
the primary coil of the pulse transformer PT. The switch S is operated in 
association with the resistance control of the variable resistor R2 and is 
opened at the maximum value of said resistor and is closed in the other 
condition. 
The gate of the thyristor PUT receives the trapezoidal wave voltage divided 
by the resistors R3, R4. The secondary coil of the pulse transformer PT 
constitutes a gate trigger circuit of the bidirectional thyristor TRIAC. 
If the switch S is closed and as the value of variable resistor R2 is 
reduced, the anode voltage of N-gate thyristor PUT has a point exceeding 
the gate voltage as the voltage of the capacitor increases in a constant 
voltage period of each of the trapezoidal waves. N-gate thyristor PUT is 
ignited at the phase of this point, and the voltage of the capacitor C is 
abruptly discharged via the pulse transformer PT, and the bidirectional 
thyristor TRIAC is thereby ignited. Once the capacitor C discharges, it 
starts charging instantly, and sometimes repeatedly discharges several 
times in the same trapezoidal wave due to the operation of the N-gate 
thyristor PUT. Then the subsequent discharge gives no influence to the 
bidirectional thyristor TRIAC, which has already been ignited. In the 
vicinity at the termination of the wave after the constant voltage period 
of the same trapezoidal wave, the anode voltage exceeding the gate voltage 
ignites, in the same manner, the N-gate thyristor PUT to cause the 
capacitor C to discharge, thereby to make in agreement with the rising 
point of the subsequent trapezoidal wave and a starting point of the 
charging of the capacitor C. The output of the pulse transformer PT by the 
discharge at this time is set so as not to newly ignite the bidirectional 
thyristor TRIAC. On the other hand, if the value of the variable resistor 
R2 is reduced, the rising curve of the charged voltage of the capacitor C 
is made steep to ignite the N-gate thyristor PUT at a more advanced phase. 
Since the ignited phase of the bidirectional thyristor coincides with said 
advanced phase, the motor M is controlled at a large conductive angle and 
the rotation of the motor is increased. 
The present invention has been provided to eliminate the defects and 
disadvantages of the prior art. It is a primary object of the invention to 
dispose an insulating treatment at the controlling part of the device for 
securing the safety or the operator precluding the danger of electric 
shock. 
The other features and advantages of the invention will be appareant from 
the following description of the invention in reference to the preferred 
embodiment as shown in the attached drawing.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention will be explained in reference to FIG. 2 of the 
attached drawing, in which the elements indicated by the same letters as 
in FIG. 1 have the same objects and functions. Therefore, the explanation 
of these elements is not repeated, and only the different parts will be 
discussed. In FIG. 2, PC is a photocoupler including a light emitting 
diode LED and a phototransistor PTr. The light emitting diode LED receives 
the power source voltage V which is rectified by the intermediate full 
wave rectifying circuit of diodes D1-D1, and the phototransistor PTr opens 
and closes a battery voltage E relative to the gate of N-gate thyristor 
PUT. This gate receives the battery voltage E divided by the resistors R3, 
R4. The phototransistor PTr is so set as to saturate the collector current 
with a quantity of light more than a certain level. Thus the trapezoidal 
wave current flows through the resistors R3, R4 in both sides of the power 
source V, and the gate voltage of N-gate thyristor PUT becomes a 
trapezoidal wave in synchronism with the power source V as is in FIG. 1. 
R5 is a protective resistor. S1 is a switch for opening and closing the 
battery source E. The switch S1 is opened at the maximum value of the 
variable resistor R2 and is closed at the other conditions of the variable 
resistor R2. The variable resistor R2 is controlled by operation of the 
pedal controller (not shown) so as to control the inclination of the 
rising curve of the charged voltage of the capacitor C which is charged 
with the battery voltage E. The control circuit in FIG. 2 trigers, by 
means of the DC voltage E, the bidirectional thyristor TRIAC receiving AC 
source at its anode. Since the trigger current is controlled in 
synchronism with the AC source V, the same control effect can be obtained 
as is in FIG. 1. Since the electric potentials of the resistor R2 and the 
switch S in FIG. 1 receive the AC source V (e.g., 100 V), a countermeasure 
for the electric shock is required, and the insulation of the casing of 
the controller generally requires a technique of high quality. On the 
other hand, since the resistor R2 and the switch S1 shown in FIG. 2 are 
insulated by the pulse transformer TR and receive only a small amount of 
voltage (e.g., 6 or 7 voltage) from the battery source E only, there is no 
danger of electric shock and the insulation may be so simplified. The 
control of the circuit in FIG. 2 is effected by the bidirectional 
thyristor TRIAC. Instead, the control may be effected by a reverse 
blocking thyristor. In this case a diode is provided in series to the 
gate, and the lighting emitting diode LED is connected to the resistor R 
and to the cathode of the reverse blocking thyristor. 
As mentioned above, the present invention employs the battery power source 
for ignition of the speed control circuit for the machine motor using the 
commercial AC electric source, thereby to simplify the insulation 
treatment for the control part of the device precluding the dnager of the 
electric shock.