Design & Technology
Inverter development, a miracle created by 3 years of hard work September 8th, 2009
By Kim, Wan Seung
R&D Strategy team
The battery in a hybrid vehicle provides energy to the electric motor, which serves as a secondary power source. Battery and electric motor have different voltages, so a device that converts voltage is necessary. That device is the inverter. The inverter converts direct current (DC) from a battery to 3-phase alternating current (AC) to control the electric motor’s torque. All of the inverter in Pride Hybrid was imported, but the inverter in Forte LPi Hybrid was developed solely with in-house technology.
Developed over a 3-year period, the inverter is superior to those found in vehicles by rival automakers. It is easy to mount and boasts superior driving efficiency and fast 0-to-100km acceleration time. Since most of the core parts were produced in-house, the inverter is cost competitive – resulting in additional savings for buyers.
The biggest challenge in developing the inverter was securing reliability. The core tasks were coming up with the mechanisms to protect against instantaneous overvoltage and a short circuit in the power module. During testing, the first sample fell apart in an instant. With this shaky start, engineers were dismayed and at a total loss. The inverter’s job is to detect a breakdown and carry out its protection function within 6㎲. However, the sample failed to do so.
Since the hybrid system couldn’t be developed without the inverter, the engineers couldn’t just give up. They modified the design seven times as they measured the waveform of each circuit. They measured more than 30,000 waveforms while creating the sample and addressed each problem one by one. They even gave up their summer break, and a miracle happened on the last day. The long-awaited waveform finally read properly on the gauge. The R&D team achieved hardware protection in 4㎲, which far surpassed the target of 6㎲. The waveform was the most perfect and beautiful sight they ever saw. After seeing the waveform, an employee of the partner firm co-developing the power module said, “It is a perfect waveform, just like you see in a textbook.” He added, “It’s near impossible to achieve this feat in just five months.” That gave everyone a major boost in confidence.
Although it was their first attempt, the R&D team didn’t give up and eventually completed a world-class inverter. Thanks to the experience, Kia now has inverter design technology that is on par with the best and secured the foundation for hybrid technology.