Background Art A conventional circuit for driving a commercial fan is generally designed such that the fan is driven at a constant speed under a certain condition using an induction motor.

Therefore, if a certain condition such as a high temperature or humidity is met, electric power is applied to the induction motor mounted with a fan to cause the induction motor to rotate such that ventilation can be produced.

In general, in a case where a 2-or 3-phase induction motor is employed, the rotation of the induction motor can be caused by using a capacitor or changing a magnetic circuit installed in the induction motor.

At this time, in a case where an external power source has a constant frequency and voltage, it exhibits a negative output-to-load characteristic. Thus, if a load pressure in a discharge port is increased, load imposed on a fan is also increased. Therefore, torque exerted on the motor is also increased, and thus, the rotating speed of a rotor is decreased. Accordingly, a volume of air to be discharged is decreased, and consequently, a volume of air to be ventilated is also decreased.

A low-priced ventilation system is manufactured without any countermeasure to overcome the above phenomenon, whereas a high-priced ventilation system solves the above problem by including a ventilating fan control circuit (U. S. Patent Application

Publication US2001/0033147) to which a feedback control circuit for the pressure measurement is added.

Here, according to a conventional method applied to the high-priced ventilation system (U. S. Patent Application Publication US2001/0033147), an induction motor is rotated by generating a desired voltage waveform through pulse width modulation (PWM) using a DC voltage as electric power to control the 3-phase induction motor. At this time, an amount of load imposed on a fan by measuring a DC link current, and thus, the induction motor is controlled to increase the rotating speed thereof so that a constant volume of air can be discharged.

In a case where the conventional ventilation system as described above uses a method of measuring a pressure of air in the discharge port or measuring a phase current or DC link current, however, the total cost of the system will be certainly increased due to a pressure sensor for measuring the air pressure or a microprocessor for processing signals from the pressure sensor and creating commands for controlling the motor. Therefore, there is a problem in that the conventional method is not suitable to the home ventilation system or the small-capacity and low-priced ventilation system for use in office.

Disclosure of Invention The present invention is conceived to solve the aforementioned problem in the prior art. An object of the present invention is to provide a fan driving circuit having a positive output-to-load characteristic for allowing a constant volume of air to be discharged by means of a plurality of ventilation fans used in an identical vent or fans for ventilating/exhaust pipes in which load is changed, by causing a load condition of the driving circuit to be changed and thus the rotating speed of an induction motor to be increased or decreased without using any other expensive devices even when the load in the ventilating/exhaust pipes has been changed.

According to an aspect of the present invention, there is provided a fan driving circuit having a positive output-to-load characteristic, comprising a 3-phase induction motor for driving a fan, a power unit for applying electric power to the 3-phase induction motor, a capacitor C2 which is connected with the 3-phase induction motor to produce a

phase difference between two phases, and an impedance unit which is connected in series with the 3-phase induction motor to allow a constant discharge volume of air to be produced even though load imposed on the fan is changed.

Brief Description of Drawings Fig. 1 is a circuit diagram of a fan driving circuit having a positive output-to-load characteristic according to an embodiment of the present invention; Fig. 2 is a graph illustrating load curves and operating points of the fan driving circuit having a positive output-to-load characteristic according to the present invention; and Fig. 3 is a circuit diagram of a fan driving circuit having a positive output-to-load characteristic according to another embodiment of the present invention.

Best Mode for Carrying out the Invention Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

Fig. 1 is a circuit diagram illustrating an embodiment of a fan driving circuit having a positive output-to-load characteristic according to the present invention. The fan driving circuit comprises a 3-phase induction motor 20 for driving a fan 10, a power unit 30 for applying electric power to the 3-phase induction motor 20, a capacitor C2 40 which is connected with the 3-phase induction motor 20 to produce a phase difference between two phases, and an impedance unit 50 which is connected in series with the 3-phase induction motor to allow a constant discharge volume of air to be produced even though load imposed on the fan 10 is changed.

Fig. 2 is a graph illustrating load curves and operating points of the fan driving circuit having a positive output-to-load characteristic according to the present invention.

In Fig. 2, operating points 100 and 200, which are located on lower and higher voltages A and B according to a load curve K, are shown, respectively, in cases where the voltages A and B are applied to the 3-phase induction motor 20 (refer to Fig. 1) according to load applied to the fan.

Fig. 3 is a circuit diagram of a fan driving circuit having a positive output-to-load characteristic according to another embodiment of the present invention. In this fan driving circuit, a resistor R2 60 is connected in series with the impedance unit 50 and a switch 70 is connected in parallel with the resistor R2 60 to interrupt a current flowing along the resistor R2.

Hereinafter, the operation and advantage of the present invention will be explained with reference to Figs. 1 to 3.

To create a positive speed-to-load characteristic curve of the ventilation system mounted with the fan 10 serving as load of the 3-phase induction motor 20, the impedance unit 50, which includes a capacitor Cl 51 and a resistor R1 52 as load in series, is added to a conventional driving circuit of the 3-phase induction motor 20 including a capacitor.

Therefore, constant amplitude of an alternating voltage is applied to the 3-phase induction motor 20 due to voltage distribution of an alternating voltage with constant amplitude and frequency between the 3-phase induction motor 20 including the capacitor C2 40 and the impedance unit 50 serving as load in series therewith.

At this time, the fan driving circuit operates at equilibrium points in the torque and rotating speed due to the characteristic curve shown in Fig. 2. In Fig. 2, a curve'K'is a load curve of the fan, a curve'A'is an output characteristic curve of the 3-phase induction motor 20 when the lower voltage is applied thereto, and a curve'B'is an output characteristic curve of the motor when the higher voltage is applied thereto.

The operating principle will be discussed in detail with reference to Fig. 2. If the load in the fan is increased while the 3-phase induction motor 20 is rotating at a constant frequency at an operating point 100, slip in the 3-phase induction motor 20 is increased.

Therefore, equivalent impedance of the 3-phase induction motor 20 is reduced, whereby the current is increased.

At this time, the rotating speed of the motor is rapidly increased due to generating torque proportional to the square of current, and thus, the equivalent impedance of the 3- phase induction motor is again increased. Therefore, the voltage distributed according to the constant load in series is also increased (refer to a curve'B'), and the motor is rotated at a new operating point 200.

That is, under the load characteristic of the motor and fan shown in Fig. 2, as the load in the fan is increased, an applied voltage is increased. Thus, the motor is in a stable state at the operating point where the rotating speed of the motor is higher.

On the other hand, if the load in the fan is decreased, equivalent resistance is instantly increased and the current is simultaneously decreased. Thus, the rotating speed is rapidly reduced and the equivalent impedance is also reduced, and the lower voltage is consequently applied to the 3-phase induction motor 20 (refer to a curve'A').

Accordingly, the motor stably operates at a new operating point 100.

That is, the load imposed on the fan is continuously changed, and the operating points can also be maintained between the maximum speed-to-torque operating point and the minimum speed-to-torque operating point.

In the meantime, since the switch 70 and resistor R2 60, which are connected in parallel with each other, are added to the fan driving circuit as shown in Fig. 3, the serial resistance in load can be increased and thus the 3-phase induction motor 20 can be kept at a lower rotating speed. At this time, if the current flowing into the resistor R2 60 is cut off by turning on the switch 70, the voltage distributed to the 3-phase induction motor 20 is rapidly increased such that the rotating speed of the fan 10 can be rapidly increased. This may become another embodiment of the present invention.

Industrial Applicability According to the present invention so configured, there is an advantage in that a discharge volume of air can be kept almost constant without using any expensive control devices such as microprocessor and pressure sensor, even in the places where a ventilation system having a common vent for the higher or lower floors is used like an apartment house, or where the load imposed on the ventilating fan is changed, e. g. in a case where a single vent is used for the plurality of ventilating fans.

Explanation of Reference numerals for Designating Main Components in the Drawings 10: Fan 20: 3-phase induction motor 30: Power unit 40: Capacitor C2 50: Impedance unit 51 : Capacitor C1 52: Resistor Rl 60: Resistor R2 70: Switch