Power Circuit
Control Circuit
Circuit Operation
When the start button of the variable frequency starter is pressed its contacts a3-a4 close, the contactor coil C is energised which causes the main contacts on terminals 1-2, 3-4 and 5-6 to close, it’s auxiliary contact on terminals 13-14 also closes. The auxiliary contact maintains power to the contactor coil when the start button is released. Power is connected from the power supply to the transformer/rectifier unit which then supplies a dc voltage to the inverter unit. At start up the inverter module will create a minimal frequency ac output dependant upon the unit settings. The frequency outputs from the inverter unit to the motor terminals and the motor will start and accelerate to the full speed as dictated by the frequency output of the inverter unit. Once the motor has started the speed can be controlled remotely via the variable frequency inverter unit as required by the requirements of the control system.
To stop the motor the stop button is pressed its contacts a1-a2 open which de-energises the contactors, thus stopping the motor by removing its power supply and resetting the control circuit.
There are many variations on the transformer/rectifier units and inverter modules. Some transformer/rectifier units are 24 pulse systems others are 12 or 6 pulse, others do not have transformers. The best control of the frequency and therefore speed will be when the best quality dc is supplied to the inverter; it follows therefore that the higher the number of phases from transformer system the smoother the dc. It is also a fact that the higher the number of phases the higher the cost. The transformer/rectifier and inverter units will require their own control signals, these should be in addition to those shown in the control diagram and will be device specific.
A key feature of a contactor is that when the correct contactor for the motor and electrical system is selected it operates only when power is applied to its coil and even when subjected to through faults, the contacts still operate without welding. When power is removed from it’s coil the normally open contacts open and will have the appropriate isolating distance between the fixed and moving contacts, this is not true of electronic components. Electronic components can fail either open or closed circuit, they therefore do not meet the isolating distances required by regulations. It is also necessary to ensure that the protective device to the inverter unit will limit the fault level at the unit to it’s short circuit withstand level.