| 摘要 |
An alarm system comprises a blower driven by an electric motor, the blower being fitted to the premises to be protected and serving to blow air into the premises. With all doors and windows shut the blower supplies a steady flow of air, the flow rate depending on the nature of the blower and how fast air can leak out of the premises around doors and windows, through air vents etc. The current drawn by the motor is constant in these circumstances and is monitored. When entry is made to the premises, by opening a door or window, the air flow rate increases and the load on the motor changes. This is reflected in a change in the current drawn by the motor and the blower can blow air from the premises, air entering around doors and windows etc. |
