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Three-level drive ushers in `next generation of VSDs`

01 July, 2002

Three-level drive ushers in `next generation of VSDs`

Yaskawa`s "three level" variable speed drive technology, first shown in Europe at the SPS/IPC/Drives exhibition in Germany last November, has now reached the UK. The Varispeed G7 drive, which the company is hailing as the next generation of VSD technology, and a more significant development than sensorless flux vector technology, eliminates the damaging micro surges that are caused by reflected voltage waves and doubles the open-loop speed control range from 100:1 to 200:1.

Instead of the usual six output transistors, the new design uses 12 (see diagram below). The voltage of the main circuit is divided by capacitors and a neutral point (0V) is introduced to give the PWM voltage three levels of potential, instead of the usual two.

The effect, according to Yaskawa, is to halve the voltage waveform dv/dt, and thus the output voltage, preventing potentially damaging reflected voltage waves and motor bearing corrosion. It also allows cable runs of up to 300m and avoids the need for extra chokes. Bearing currents are kept below 0.3A, thus preventing corrosion.

The new drive also uses a new control method and a power circuit that stabilises the electrical potential at 0, simplifying the circuitry and producing a compact design. Each phase creates a PWM pulse with a different switching frequency, reducing noise levels by 5dB.

The control strategy adds a second "observer" to the one used in conventional sensorless flux vector drives, comparing the magnetic flux with a flux vector model. The result of using two independent observers is to double the speed range, according to Yaskawa. It also delivers torque control comparable to a DC drive, producing 150% torque at 0.3Hz and allowing rapid recovery from impact loads at low speeds.

Yaskawa also asserts that the G7 is "the most motor-friendly inverter drive ever", with the line voltage to the first coil being 35% below the safe working voltage of 690V, and the phase-to-phase voltage being 22% below the safe working voltage of the motor`s insulation, and 55% below the earth voltage potential.

Yaskawa concedes that this new technology will not be cheap. With a pricetag about 50% higher than a conventional inverter, the G7 is the most expensive drive the company has ever produced. But it claims that for problem installations, the high initial price is justified by the lower running costs. Mark Butters, Yaskawa`s UK sales manager, predicts that within two years the new technology could fall to the same price as conventional drives.

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