The starter control circuit can be divided into five parts, the power circuit control circuit, the pilot contactor control circuit, the fault circuit, the auxiliary circuits and the SCADA circuits.
The power control circuit will depend on the starting method and must include all the necessary ancillary components concerned with the starting of the motor. Its operation must be governed by the status of the relevant pilot contactor, once the start sequence has been initiated by the pilot contactor the power circuit effects the required starting method and then keeps the motor running until the pilot contactor de-energises which in turn de-energises the power control circuit to stop the motor.
The pilot contactor circuit incorporates, within the starter, the pilot contactor(s), the start alarm contactor, the start alarm timer, the manual and automatic start relays. Although the pilot contactor and start alarm contactor are designated as contactors they are in fact relays of the same type as the manual and automatic relays thus maintaining the principal of component commonality. External to the starter at each control panel should be located the local and remote relays which are also of the same type as the relays in the starter again for component commonality. The local and remote relays because they form part of the control system should be powered from the pilot contactor circuit.
There are a number of constituent parts to the pilot contactor circuit, the stop section, the inhibit interlocks, the permissive interlocks, local control station start, manual start and auto start controls.
The stop section should include contacts from all the emergency stop buttons that are fitted, including the local control station stop button, the fault relay and where fitted the local isolator early break late make contact. The operation of any of these devices should stop the motor, the resetting of the device should not cause the motor to start, it must only allow the motor to be started.
The inhibit interlock circuit must prevent the motor from starting and stop it if it is running, however if the motor is in automatic mode when the inhibit is removed the motor will restart, in manual mode the motor may be started by the pressing of the start button when the inhibit is removed.
The permissive interlock circuit must prevent a motor from starting if it is not running, if the motor is running then the withdrawal of the permissive interlock will not stop the motor.
The fault circuit has a normally energised relay with a normally open contact in the pilot contactor stop section, thus when the fault relay de-energises all the control relays and the pilot contactor(s) de-energise this stops the motor. The pilot contactor(s) and control relays cannot be energised until the fault relay is energised by the removal of the fault or the reset of the fault activating device and then by pressing the reset button on the starter which re-energises the fault relay which in turn enables operation of the control push buttons.