In a motor drive apparatus that performs feedback control on an output of a brushless motor according to armature currents flowing through armature windings of the motor, currents flowing through current detection resistors provided phase by phase to a motor drive inverter circuit are detected as armature currents in the respective phases. A duty ratio corresponding to a deviation between a target current and the detection current is calculated phase by phase and ON/OFF control is performed on switching elements of the respective phases forming the inverter circuit by PWM control. Owing to this configuration, phase voltages to be applied to the armature windings of the respective phases are changed and drive currents are flown through the armature windings of the respective phases. Hence, a motor output is controlled.
FIG. 14 is a view of a circuit configuration showing an example of a brushless-motor drive apparatus in the related art described, for example, in Patent Document 1.
As is shown in FIG. 14, the brushless-motor drive apparatus in the related art is configured in such a manner that upper switching elements 5uH, 5vH, and 5wH (hereinafter, also collectively denoted simply by 5H), armature windings 9, and lower switching elements 5uL, 5vL, and 5wL (hereinafter, also collectively denoted simply by 5L) are interconnected phase by phase and current detection resistors 6u, 6v, and 6w (hereinafter, also collectively denoted simply by 6) are disposed between the lower switching elements 5L and a ground. In order to detect currents flowing through the current detection resistors 6, a state of the switching elements is changed so that the lower switching elements 5L are ON by a drive control circuit 4 that is a PWM control means. In this instance, armature currents of the armature windings 9 of the motor pass through the lower switching elements 5L and the current detection resistors 6 and flow to the ground. The armature currents are measured by detecting potential differences across the current detection resistors 6 in this state in which currents flow through the current detection resistors 6.
When the lower switching elements 5L are ON, the upper switching elements 5H are OFF so that currents are prevented from flowing through the upper and lower switching elements of the same phases by flowing currents through the armature windings 9. In this manner, the armature currents can be detected only when the upper switching elements 5H are OFF and the lower switching elements 5L are ON.
Conversely, in a case where ON/OFF control is performed on the switching elements 5H and 5L with PWM control signals of the drive control circuit 4, there is a period during which no currents flow through the current detection resistors 6. Accordingly, there is a circumstance where currents cannot be detected properly. To overcome this inconvenience, it is necessary to limit duty ratios of the PWM control signals, that is, voltages to be applied to the armature windings 9. Hence, the motor has to be used by lowering a utilization ratio of a power supply voltage due to the duty ratios and a motor performance is limited.
Under these circumstances, Patent Document 1 describes as follows. That is, for a brushless motor formed of a phase U, a phase V, and a phase W, in a case where ON/OFF control is performed on switching elements of the respective phase U, phase V, and phase W according to instructions of the duty ratios from the PWM control means, it becomes difficult to detect a current in the phase U when the switching elements of the phase U are switched ON/OFF at a duty ratio lower than a predetermined value. Hence, a current Iu in the phase U is calculated in accordance with Equation (1) below from a detected current Iv in the phase V and a detected current Iw in the phase W:Iu=−(Iv+Iw)  (1).
Also, it is described that in a case where the switching elements of the other phase V or W are switched ON/OFF at a duty ratio lower than the predetermined value, a current is calculated .using the currents detected in the other phases in the same manner.