The variable frequency module will consist of a rectifier unit which feeds dc into the variable frequency generator. Although variable frequency is used to start the motor it can also be used to control the motor speed over the long term, however additional equipment may be required if variable speed is required.
The voltage rating of the variable frequency module must be equal to or greater than the electricity supply voltage.
The variable frequency module must have type 2 coordination when used in combination with the selected fuses, contactors and overloads.
The variable frequency module must have a minimum current rating equal to or greater than the motor full load current.
The variable frequency module must have a short time withstand (Icw) in excess of the fuse time-current characteristic.
The over riding consideration is the type 2 coordination as this will give the minimum variable frequency module rating, the motor full load current, starting method and the projected frequency of operation should then be taken into account to determine the module size to be used.
The variable frequency module must be of the type that does not utilise the protective earth conductor, or other earth connection as a circuit conductor, nor shall the protective conductor be used as a reference point for the generation of the waveform, unless an isolating transformer is incorporated in such manner that the supply phases are galvanically isolated from the variable frequency module. The maximum earth leakage from the variable frequency module and any RFI or EMC filters forming part of the starter power circuit shall not exceed one ten thousandth of the nominal current rating of the variable frequency module. For example if the rated current is 65A then the maximum permitted leakage current is 6.5mA per phase to earth.
The waveform quality generated by the variable frequency generator will have an effect on the motor, the closer the voltage wave is to a sine wave the better, since that indicates the least level of harmonics. Harmonics will lead to increased heating within the windings of the motor and may reduce the motor life. The greater the level of harmonics the greater the heating effect on the windings.
It should be noted that electronic components can fail either open or closed circuit, and do not meet the isolating distances required by regulations. It is also necessary to ensure that the protective device to the variable frequency module will limit the fault level to the variable frequency module fault withstand level at the system fault level where the variable frequency module is installed.
It is also necessary to be mindful of the need to keep the heat gain within the starter from the variable frequency module to a minimum. As with all electronic devices the variable frequency module does not operate at 100% efficiency, those losses are reflected as heat gains within the starter.
By using a variable frequency module that is in series with the contactor and one that is self powered the standard control system can be employed. Where the module requires external inputs for control these should be derived in addition to the contactor control circuits. Under no circumstances should the stop or emergency stop circuits be routed via the soft start module these circuits must act directly on the contactor.
The variable frequency module must have variable settings for the max current and ramp up time, where the max current level is measured at the input to the rectifier unit.