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20/02/2004
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Tom Shelley reports on developments that do much to reduce mains harmonics from power equipment and improve ease of use
A range of power inverters has a man machine interface closely modelled on a popular range of mobile phones and uses swing chokes to reduce harmonics fed back into the mains supply by about 25%.
The mobile phone interface concept could just as well be applied to any other piece of equipment, and while swing chokes are hardly a new idea, it is the first time they have been applied to inverters, allowing the developers to apply for a patent for this particular application.
ABB's latest inverter drives come in two flavours, standard and HVAC.
Both are controlled by eight buttons, a red one labelled "Stop" in the case of standard drives or "Off" in the case of HVAC drives, with a green button labelled "Start" on the standard drives and "Hand" for the HVAC drives. Above are two scroll up and scroll down buttons, with two small buttons for local/remote, and "?" (help) with two soft keys just below the screen whose function at any moment is indicated on the screen. In appearance and use, the arrangement functions exactly like that on a Nokia phone.
However, the more significant innovations are inside, both as regards 'wizard' type assistants for setting up the drive for different types of application, and in the use of swing chokes to reduce harmonics fed back into the mains supply.
Swing chokes are conventional wound chokes with a soft iron core, except that they have a stepped gap in the central section of the core. At low currents, magnetic flux passes through the iron across the narrow part of the gap. As current increases, the core on each side of the narrow gap saturates, reducing permeability nearly to one, effectively making it part of the wider air gap across the rest of the width of the core element. Because the effective air gap is increased, the inductance and impedance of the choke decreases. The choke parameters are selected so that the effective voltage drop across the choke is similar regardless of whether the load current is at maximum or less so. It is thus able to smooth out harmonic ripples at both high and medium load currents.
Other design refinements include the provision of 14 macros for particular fan applications in the HVAC version, and provision of macros for industrial applications in the standard version, including speed control, PID and hand/auto implementations. An integrated braking chopper is included in the standard version and a fireman's override in the HVAC version. The latter has a dedicated input which when activated will ignore all alarms or other commands and run the drive at pre-set speed until destruction.
Drives are available over the range 0.75 to 355kW, 200 and 400V. Modbus is standard, plus two other common building field buses for the HVAC version, with a wide range of field buses available as optional add ons. The standard drive can be run under sensorless vector control if desired.
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* Human machine interface has been deliberately made similar to that of the world's most popular range of mobile phones
* Swing chokes substantially reduce mains harmonics, especially under conditions of partial load
* Built-in software greatly assists setting up and diagnostics
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Author
Tom Shelley
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