DRIVING FORWARD VARIABLE SPEED TECHNOLOGY
December 2006

Many of the key challenges of vector controlled drives technology have been overcome by an intelligent sensorless development incorporated into the new Hitachi iSLV range, recipient of the 2006 Frost & Sullivan Award for Excellence in Technology.

According to Philip Larkin of Hitachi’s UK drives and automation company, Silverteam: “iSLV has advanced microprocessor technology to develop a processor and algorithm that enable significantly faster calculation of the motor’s rotor position. Significantly it can calculate the algorithm within the time frame of the carrier frequency pulses, rather than the sine wave cycle."

Sensorless vector control involves the dynamic calculation of rotor position using accurate measurements of the three-phase voltages and currents, along with those of the induction motor’s electrical characteristics. The microprocessor that lies at the core of any sensorless vector drive typically takes time to perform these calculations. This is because each iteration of the drive’s algorithms requires a set amount of time as the equations are repeated with updated data.

“In practice this reduces performance in the field. iSLV’s ability to perform the calculation faster than the carrier frequency pulses overcomes this issue completely and ensure uncompromised performance.”

With iSLV the current measurement is taken from the DC (direct current) bus via a shunt, rather than using current transformers on each phase of the supply as is usual with sensorless vector. The readings are completely accurate, whereas those from a transformer are time-based averages of only the lower sine wave frequencies. Other sensorlss vector methodologies exisit, but are slowed down by the fact that they are not based on direct measurement of the fundamental PWM waveform.

"With a knowledge of which output insulated gate bipolar transistors (IGBTs) within the drive are switched on at any given moment, it is possible to deduce each motor phase current directly from the DC bus current," says Larkin. "This allows an evaluation of the instantaneous current flowing in each motor phase, without the problem of delays typically caused by current transducers."

Thus, the iSLV drive enables motor characteristics to be measured continuously during normal operation, rather than as a separate process. Moreover, motor characteristics can be constantly updated to reflect changing external conditions such as temperature, resulting in improved torque speed performance and stability. The iSLV technology also allows the elimination of the autotuning step, which many drive users frequently perceive as an irritant.

"Intelligent sensorless vector technology will enhance the utility drives, improve performance and ease of use, so will benefit end users greatly and advance vector drive technology into its market by a significant step."

The Frost & Sullivan award, presented in America, recognises the potential of iSLV technology to have significant impact across several market sectors.