The correct answer is C. picked up about 50 to 70 per cent of rated speed.
A centrifugal switch is a switch that is used to disconnect the starting winding of an electric motor when the motor has reached a certain speed. The switch is typically located on the motor shaft and is activated by centrifugal force. When the motor starts, the centrifugal force causes the switch to open, disconnecting the starting winding. This reduces the current draw of the motor and allows it to reach its full speed more quickly.
The centrifugal switch is typically set to open at a speed of about 50 to 70 per cent of the motor’s rated speed. This is because the starting winding is only needed to help the motor start up. Once the motor is up to speed, the starting winding can be disconnected without affecting the motor’s operation.
The other options are incorrect because they do not represent the typical speed at which a centrifugal switch is set to open. Option A is incorrect because it is too low. Option B is incorrect because it is too high. Option D is incorrect because it is not a typical speed at which a centrifugal switch is set to open.