Method of driving fan motor

Abstract

A fan motor is driven under a pulse width modulation (PWM) control. PWM signals to be supplied to a circuit for driving the fan motor are formed from carrier waves having a specified frequency and a duty ratio of the PWM control. Rotational speed of the fan motor is increased by increasing the duty ratio. In a low speed region, the fan motor is driven with a high carrier frequency to suppress driving noises of the fan motor, while it is driven with a low carrier frequency in a high speed region to reduce switching losses in semiconductor elements. The carrier frequency may be continuously decreased when the fan motor speed reaches the high speed region, or it may be stepwise decreased. The low carrier frequency used in the high speed region is set to such a frequency that guarantees a sufficient number of switching times at a maximum speed of the fan motor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an entire structure of an apparatus for driving a fan motor, as a first embodiment of the present invention;

FIG. 2 is a graph showing voltage patterns supplied to phase windings of the fan motor;

FIG. 3 is a graph showing levels of driving noises and wind noises relative to a rotational speed of the fan motor;

FIG. 4 is a graph showing a relation between a duty ratio in the PWM control and a rotational speed of the fan motor;

FIG. 5 is a graph showing a carrier frequency that is reduced stepwise when the duty ratio reaches a marginal duty ratio;

FIG. 6 is a graph showing an amount of heat generated in semiconductor elements, the amount of heat being compared between a conventional apparatus and an embodiment of the present invention;

FIG. 7 is a graph showing voltage patterns supplied to phase windings, the voltage being supplied with an electrical angle of 120°;

FIG. 8 is a circuit diagram showing an inverter in a driving circuit connected to phase windings of the fan motor;

FIG. 9 is a graph showing a carrier frequency that is reduced in three steps when the duty ratio reaches the marginal duty ratio;

FIG. 10 is a graph showing a carrier frequency that is continuously and linearly reduced when the duty ratio reaches the marginal duty ratio;

FIG. 11 is a block diagram showing an entire structure of an apparatus for driving a fan motor, as a second embodiment of the present invention;

FIG. 12 is a block diagram showing an entire structure of an apparatus for driving a fan motor, as a third embodiment of the present invention; and

FIG. 13 is a block diagram showing an entire structure of an apparatus for driving a fan motor, as a fourth embodiment of the present invention.

Claims

1. A method of driving a fan motor under PWM control, using semiconductor elements, the method comprising: generating a voltage representing a duty ratio of the PWM control;generating carrier waves having a specified carrier frequency;forming PWM signals based on the carrier waves and the voltage representing the duty ratio; andsupplying the PWM signals to the fan motor for driving a fan, wherein:rotational speed of the fan motor is controlled by changing the duty ratio of the PWM control; andthe specified carrier frequency is reduced to a predetermined minimum frequency when the rotational speed of the fan motor has reached a marginal speed, at which wind noises of the fan become predominant over driving noises of the fan motor, to thereby reduce switching losses in the semiconductor elements.

2. The method of driving a fan motor as in claim 1, wherein: the specified carrier frequency is stepwise reduced to the predetermined minimum frequency.

3. The method of driving a fan motor as in claim 1, wherein: the specified carrier frequency is linearly reduced to the predetermined minimum frequency.

4. The method of driving a fan motor as in claim 1, wherein: the duty ratio that realizes the marginal speed when a torque required to drive the fan motor is deviated to the highest level is set to a marginal duty ratio; andthe specified carrier frequency is reduced to the predetermined minimum frequency when the duty ratio reaches the marginal duty ratio.

5. The method of driving a fan motor as in claim 1, wherein: the minimum carrier frequency (fmin) is set according to the formula: fmin=Pn·Rmax·Nmin/10, where the fan motor is a three-phase motor, Pn is the number of pole pairs, Rmax is a maximum rotational speed of the fan motor per minute, and Nmin is the number of switching times performed in an electrical angle of 60°.