SAFETY RULES ARE YOUR BEST TOOLS

Recently, the Menzshed in Halswell, Christchurch, purchased a new bandsaw. The saw worked well, but had one alarming feature. When it was turned off, the blade continued running for approximately 20 seconds. During this time it was almost silent, and that was seen as a real danger to both the operator and anybody else nearby.

The motor on the saw was a 2hp singlephase AC induction motor (Image 1). Single-phase induction motors generally have one or more starting capacitors — these are the tubular objects mounted on the motor. There was clearly a need to bring the motor to a halt quickly. All sorts of electrically and mechanically operated brakes were considered, and just as quickly rejected. A quick internet search found a method of braking called ‘DC injection’ (Image 2).

That was seen as a real danger