High-power versions of Hitachi SJ700 variable speed drive

Claimed to be a radical development of drives technology, the SJ700 is said to be the first off-the-shelf unit with integral easy-sequence programmable functionality (EzSQ) as standard.

The SJ700 is compact in size, which is an increasingly important feature as users and systems builders seek to reduce control panel size. It also incorporates a patented power switching technology to reduce dVdT, with the IGBT (insulated gate bipolar transistor) stack at the heart of the drive. High dVdT is associated with motor insulation breakdown by the increased terminal voltage of motors when used with AC drives.

Stuart Harvey of Silverteam, Hitachi's drives and automation company in the UK, explains: "Large drives tend to be installed and used for many years so can look very dated by the time they are replaced. The SJ700 resets the benchmark for small- and medium-sized drives, offering multipoint positioning and near-servo performance from a standard induction motor. The 160kW to 400kW SJ700 makes similar performance available right through the power range."

Responsiveness and accuracy

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

Stuart Harvey continues: "The built-in EzSQ functionality provides a very powerful controller that negates the need for an external unit such as a PLC 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.

"With lifts, for example, we can offer very smooth floor transitions with adjustable S ramps and brake release confirmation signals, while the superior torque and speed control make it particularly suited in hoist applications like cranes or theatre flying systems.

"You do not have to buy a separate PLC, so there are panel space savings, installation and commissioning time savings and an increase in reliability."

10-year design life

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.

"We tested all the leading drives with many different makes of motor. Voltage spikes leading to insulation breakdown were always a potential problem, particularly with cable runs over 50m, so we gave the designers the targets of reducing problems to negligible levels and making the drives usable with cables over 100m long 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 on sizes up to 22kW, integral EMI/RFI filters across the whole range, a full range of communication options, customisable display panel, intelligent energy saving, active frequency matching for restarting and flying load catching, regenerative braking under E-stop conditions and full RoHS compliance.