Patent ID: 9722525
Date: 2017-08-01
CPC Classifications: B25F,H02H,H02K,H02P

Claim:
1. A protective redundancy circuit for a power tool having an electric motor powered by an alternating current (AC) signal, comprising: a motor control module interfaced with a motor switch coupled in series with the motor to control the switching operation of the motor switch such that the switching operation of the motor switch is correlated to the waveform of the AC signal, the motor switch being operable to be closed by the motor control module within an AC half cycle and opened at a subsequent zero-crossing of the AC current, wherein, where motor control module is intended to operate the motor switch at full conduction, within each AC voltage half cycle, the motor control module introduces a delay period after a zero-crossing of the AC voltage half cycle and closes the motor switch after the end of the delay period to allow the motor switch to open at a current zero-crossing from a previous current half cycle within the delay period before the motor control module closes the motor switch; and a protective control module that measures the voltage across the motor switch during the delay period and before the motor control module closes the switch, and disables the power tool when the voltage across the motor switch during the delay period fails to meet a predetermined criterion.