text: 1. Field of the Invention
  The present invention relates to a blower system for vehicle air-conditioners, which is capable of discriminating whether or not a blower is in a locked state preventing it from rotation.
  2. Prior Art
  The blowers or motors employed in vehicle air-conditioner blower systems often lock up and become incapable of rotation even when the required driving current is supplied thereto. This happens, for example, when an object is drawn in through the air intake. To prevent the motor and/or the associated peripheral circuitry from being damaged at such times, it is necessary to cut off the power to the motor as soon as it locks. For this, the conventional vehicle air-conditioner blower system is provided with a lock discrimination circuit for discriminating whether or not the motor is in a locked state. For example, Japanese Patent Application Public Disclosure No. Hei 3-195389 teaches a lock discrimination circuit arranged to discriminate whether or not the motor is in a locked state based on the rotation condition of the rotor of the motor and to stop the supply of power to the motor when the locked state is discriminated.
  The conventional blower system with this lock discrimination circuit is not altogether satisfactory, however, especially when the system is set in the low-motor-speed operation mode. This is because the long time required for the motor to start in this operation mode makes it necessary to set a long discriminating time period so as to avoid erroneously discriminating that the motor is locked. More specifically, if a short period is set for the discrimination in the low-motor-speed operation mode, the lock discrimination circuit may conclude that the motor has locked before it has been able to get started.
  On the other hand, an attempt to avoid this problem by setting a long period for the discrimination encounters a problem when the motor locks up while running at high speed in the high-motor-speed operation mode, because the semiconductors and other components for controlling the current flowing through the motor coil are likely to be damaged by the large amount of power that is apt to be consumed in the driving circuit before the current supply to the motor is cut off.