Inverter delivers near-servo performance from induction motors

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Hitachi claims that its new SJ700 variable speed drive (VSD) represents a dramatic step change in the state of the art for inverter technology, it being the first off-the-shelf unit with integral easy sequence programmable functionality (EzSQ) as standard.

It also incorporates a new patented power switching technology to reduce dVdT, with the IGBT (insulated gate bipolar transistor) stack at the heart of the drive, which is associated with motor insulation breakdown by the increased terminal voltage of motors when used with AC Inverters.

Stuart Harvey of Silverteam, Hitachi's Drives and Automation Company in the UK, says: "The SJ700 resets the benchmark for drives. We can now look at multipoint positioning and near-servo performance from a standard induction motor. People have been promising this for about five years, but the SJ700 is the first to be able to offer such performance from standard equipment."

The SJ700 is a sensorless vector drive, with 150 per cent torque at zero speed and almost total immunity to motor constants problems. It is said to set new levels of responsiveness and accuracy and includes enhanced autotuning for easier set-up. An optional feedback card can be used to close the vector loop for even finer control.

No need for a PLC

Harvey adds: "The built-in EzSQ functionality provides a very powerful controller that negates the need for an external unit in even the most demanding of applications. Its communication with both the external system and the drive's internal circuitry is naturally very fast and very secure. This enhances operational performance all round."

Hitachi believes the SJ700 should also set new benchmarks with the hoist or lift industries; because of the new technologies incorporated, the company can offer very smooth floor transitions with adjustable S ramps and brake release confirmation signals, together with direct-to-floor landing with the feedback card option. Also, because of its superior torque and speed control, it is particularly suitable for hoist applications such as cranes or theatre flying systems.

In addition, the SJ700 offers notable benefits for applications where high performance is not critical. Harvey says: "There are savings because you do not have to buy a separate PLC, as well as panel space savings, installation and commissioning time savings, and an increase in reliability."

Hitachi has designed the SJ700 for a working life of at least 10 years. As Harvey puts it: "Every detail has been looked at to design out possible failure modes. The patented switching is just one example of this.

Next-generation technologies

"We tested all the leading drives with many different makes of motor. Voltage spikes leading to insulation breakdown was always a potential problem, particularly with cable runs over 50m, so we gave the designers the target of reducing problems to negligible levels and to over 100m cables without line reactors. To achieve this they could not refine existing techniques, so they had to develop next-generation technologies."

Other features of the SJ700 include brake transistors to 22kW, integral EMI/RFI (C3) filters across the whole range of 0.75-400kW, a full range of communications options, customisable display panel, intelligent energy saving, active frequency matching for restarting and flying load catching , regenerative braking under emergency stop conditions, and full RoHS compliance.

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