Control circuit for single-phase AC motor for dryer

This application is a continuation of International Patent Application No. PCT/CN2010/070392 with an international filing date of Jan. 28, 2010, designating the United States, now pending, and further claims priority benefits to Chinese Patent Application No. 200920056315.2 filed on May 11, 2009. The contents of all of the aforementioned applications, including any intervening amendments thereto, are incorporated herein by reference.

1. Field of the Invention

The invention relates to a motor control circuit, and more particularly to a single-phase AC motor control circuit for a dryer.

2. Description of the Related Art

Nowadays, most motors for dryers employ mechanical centrifugal switches with contacts for starting or for switching control. The mechanical centrifugal switch is connected to a starting winding of a motor of a dryer. As the motor of the dryer is started, and rotation speed thereof is 70%-85% of synchronous speed, the mechanical centrifugal switch is switched on under the action of centrifugal force, switches off the starting circuit and operates. During on/off of the mechanical centrifugal switch, arc spark is generated, which affects work life thereof. In addition, the mechanical centrifugal switch must be disposed in the motor of the dryer, which features complex structure, makes it inconvenient to design overall structure of the motor, and increases size of the motor, and material cost of the motor. It is well-known that during a starting phase, the starting winding and the main winding require comparatively large current, if the starting windings, the main windings, and the heated strips of multiple dryers are simultaneously powered up during the starting phase, starting of the motor is to be affected, and a comparatively large workload is formed, which brings about certain impact on a power system, increases starting time, or even makes it impossible for normal starting, and features imperfect control.

In view of the above-described problem, it is an objective of the invention to provide a single-phase AC motor control circuit for a dryer that is capable of addressing the above-mentioned problems.

To achieve the above objectives, in accordance with one embodiment of the invention, provided is a single-phase AC motor control circuit for a dryer, comprising a starting control unit, a first drive circuit, a bidirectional triode thyristor BCR 1 , a second drive circuit, and a bidirectional triode thyristor BCR 2 , the bidirectional triode thyristor BCR 1 is serially connected to a motor starting winding, and a starting capacitor, and then to a utility power AC input, the starting control unit is connected to a control end of the bidirectional triode thyristor BCR 1 via the first drive circuit, the bidirectional triode thyristor BCR 2 is serially connected to an electrically heated strip, and then to the utility power AC input, the starting control unit is connected to a control end of the bidirectional triode thyristor BCR 2 via the second drive circuit, and the first drive circuit is interlocked with the second drive circuit, so that as the bidirectional triode thyristor BCR 1 is on, the bidirectional triode thyristor BCR 2 is off, and as the bidirectional triode thyristor BCR 1 is off, the bidirectional triode thyristor BCR 2 is on.

In a class of this embodiment, the starting control unit comprises a rectifying and voltage-stabilizing circuit, a detecting circuit, and a voltage comparison circuit, an input end of the rectifying and voltage-stabilizing circuit is connected to the utility power AC input, an output end of the rectifying and voltage-stabilizing circuit provides low-voltage DC power supply for the detecting circuit and the voltage comparison circuit, the detecting circuit detects an electric signal of the motor main winding, and inputs the electric signal to the voltage comparison circuit for processing, and an output end of the voltage comparison circuit is connected to an input end of each of the first drive circuit and the second drive circuit.

In a class of this embodiment, the voltage comparison circuit employs a hysteresis comparison circuit.

In a class of this embodiment, the first drive circuit and the second drive circuit are photoelectric coupling circuits.

Advantages of the invention over the prior art comprise: 1) it has an independent drive circuit for the electrically heated strip, a drive circuit for the motor starting winding and that for the electrically heated strip are interlocked with each other, as the heater strip is powered on, the starting winding is powered off, as the starting winding is powered on, the heater strip is powered off, the invention features perfect control and convenient use, and ensures normal operation of the motor of the dryer; 2) the invention can be installed on outer surface of the motor of the dryer, which simplifies overall design and assembly of thereof, reduces size thereof, and saves raw materials; 3) the bidirectional triode thyristor is used for replacing a conventional mechanical centrifugal switch, and no arc spark is generated during on/off thereof, which is safe and reliable, and thus the invention has long work life; 4) the hysteresis comparison circuit is capable of switching off an operating loop of the motor of the dryer as being higher than a current value of the main winding, and switching on the operating loop thereof as being lower than another current value of the main winding, which makes control comparatively accurate and perfect, and improves operating efficiency of the motor of the dryer.

FIG. 1 is a block diagram of a single-phase AC motor control circuit for a dryer of an exemplary embodiment of the invention;

FIG. 2 is a detailed block diagram of FIG. 1 ; and

FIG. 3 is a circuit diagram of a single-phase AC motor control circuit for a dryer of an exemplary of the invention.

Further description of the invention will be given below in conjunction with specific embodiments and accompanying drawings.

As shown in FIGS. 1 and 2 , a single-phase AC motor control circuit for a dryer comprises a starting control unit, a first drive circuit, a bidirectional triode thyristor BCR 1 , a second drive circuit, and a bidirectional triode thyristor BCR 2 .

The bidirectional triode thyristor BCR 1 is serially connected to a motor starting winding, and a starting capacitor, and then to a utility power AC input.

The starting control unit is connected to a control end of the bidirectional triode thyristor BCR 1 via the first drive circuit.

The bidirectional triode thyristor BCR 2 is serially connected to an electrically heated strip, and then to the utility power AC input.

The starting control unit is connected to a control end of the bidirectional triode thyristor BCR 2 via the second drive circuit.

The first drive circuit is interlocked with the second drive circuit, so that as the bidirectional triode thyristor BCR 1 is on, the bidirectional triode thyristor BCR 2 is off, and as the bidirectional triode thyristor BCR 1 is off, the bidirectional triode thyristor BCR 2 is on.

The starting control unit comprises a rectifying and voltage-stabilizing circuit, a detecting circuit, and a voltage comparison circuit, an input end of the rectifying and voltage-stabilizing circuit is connected to the utility power AC input, an output end of the rectifying and voltage-stabilizing circuit provides low-voltage DC power supply for the detecting circuit and the voltage comparison circuit, the detecting circuit detects an electric signal of the motor main winding, and inputs the electric signal to the voltage comparison circuit for processing, and an output end of the voltage comparison circuit is connected to an input end of each of the first drive circuit and the second drive circuit.

As shown in FIG. 3 , the rectifying and voltage-stabilizing circuit comprises a diode D 1 , a resistor R 1 , a fuse VR, a stabilivolt tube ZD 1 , and capacitors C 1 and C 2 . The detecting circuit comprises resistors R 2 and R 3 , diode D 2 , and a capacitor C 3 , and operates to detect an electric signal of the motor main winding RX. The electric signal is rectified via the diode D 2 , and then output via the capacitor C 3 . The voltage comparison circuit is a hysteresis comparison circuit, and comprises a pre-comparator IC 1 , resistors R 4 -R 7 , a capacitor C 4 , and a post-comparator IC 2 .

The first drive circuit comprises a photoelectric coupler U 1 , and resistors R 10 and R 11 . A resistor R 8 is serially connected between an output end of the post-comparator IC 2 and that of the rectifying and voltage-stabilizing circuit, a pin 1 of the photoelectric coupler U 1 is connected to the output end of the post-comparator IC 2 , and a pin 2 of the photoelectric coupler U 1 is connected to the output end of the rectifying and voltage-stabilizing circuit. The second drive circuit comprises a photoelectric coupler U 2 , and resistors R 13 and R 14 , a pin 1 of the photoelectric coupler U 2 is connected to the ground, and a pin 2 thereof is connected to an output end of the post-comparator IC 2 .

Operation principle of the invention is: as power is on, the motor main winding is energized and operates, the detecting circuit detects a current signal of the motor main winding RX is high enough, and inputs the signal to the input end of the hysteresis comparison circuit, the output end of the hysteresis comparison circuit outputs low level. Namely, as input of the pin 1 of the photoelectric coupler U 1 is low level, input of the pin 2 of the photoelectric coupler U 2 is low level, the photoelectric coupler U 1 is on, the photoelectric coupler U 2 is off, the photoelectric coupler U 1 triggers and controls the bidirectional triode thyristor BCR 1 to be connected to the starting winding R 0 , and the starting capacitor C 0 . At this time, the drive circuit for the electrically heated strip does not operate, and the motor starting winding starts the motor for operation. As rotating speed of the motor is 70-85% of synchronous speed, the detecting circuits detects that a current signal of the motor main winding RX is comparatively low, the output end of the hysteresis comparison circuit outputs high level, namely the input of the pin 1 of the photoelectric coupler U 1 is high level, and the input of the photoelectric coupler U 2 is also high level. The photoelectric coupler U 1 is off, the photoelectric coupler U 2 is on, the photoelectric coupler U 2 triggers and controls the bidirectional triode thyristor BCR 2 to be connected to the electrically heated strip RL. At this time, the drive circuit for the motor starting winding stops operation, and the drive circuit for the electrically heated strip controls the motor to operate. Reference and comparison voltage of the hysteresis comparison circuit is used to implement automatic adjustment. As rotating speed of the motor is reduced to a certain value, for example, 40% of the synchronous speed, the detecting circuit detects the current signal of the motor main winding RX is high enough, the output end of the hysteresis comparison circuit is low level, namely as the input of the pin 1 of the photoelectric coupler U 1 is low level, the input of the pin 2 of the photoelectric coupler U 2 is also low level. The photoelectric coupler U 1 is on, the photoelectric coupler U 2 is off, and the photoelectric coupler U 1 again triggers and controls the bidirectional triode thyristor BCR 1 to be connected to the starting winding R 0 , and the starting capacitor C 0 . At this time, the drive circuit of the electrically heated strip stops operation, the motor starting winding restarts the motor, and thus a circulation process is formed.

While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.