Deep Dive Into Kia’s Innovative Dual-Wye-and-Delta EV Motor Configurations

Grab a seat, class, it is time for a lecture from Dr. Paul Turnbull on the fundamentals of EV motor design and Kia’s groundbreaking solution to switching between so-called Wye and Delta configurations for those motors to optimize performance to match driving conditions.

Though he earned his PhD in Mechanical Engineering from the University of Michigan, Turnbull still proudly wears his undergraduate alma mater Michigan State “Spartans” tee shirt. The veteran of Visteon, Ford, and General Motors was in charge of electric machine design and validation at those companies.

Now he’s lead electric machine engineer for the industry technical analysis group Munro Live, where he’s dissected the electric motor used in the Kia EV9 battery-electric family-hauler SUV for a Munro Live YouTube video.

“There is some really innovative electric machine development going on,” Turnbull observes. He points out that the Hyundai Group has already innovated with the motors in its earlier E-GMP-platform products such as the Hyundai Ioniq 5.

That car, he reminds us, employs four connectors in its 3-phase electric motor rather than the usual three because that lets the car use its electric motor as a transformer to boost voltage from 400-volt charging stations to match its 800-volt battery pack.

“They’re using the inductance of the motor as a part of the boost converter in their charging system,” he said. This avoids the need for an additional dedicated power inverter in the car to let it use the far-more-common 400-volt chargers.

Nifty. But the EV9 goes further, with six connectors in its otherwise similar motor. “What they’ve done is they have both the beginning and the end of each of the three phases connected to the inverter,” Turnbull said. “That gives them full access to the winding configuration of the motor.”

Why do that? Because there are two possible configurations for the windings, the Y-shaped “Wye” configuration and the triangular “Delta” configuration. Each has its pros and cons and ordinarily engineers have to pick one and work around its weaknesses.

The EV9's Vitestco Technologies-supplied inverter contains two sets of switches that let the motor switch between configurations as needed. VITESCO TECHNOLOGIES

For the EV9, Kia lets the car switch between configurations as needed, so the motor runs in the optimum configuration for the conditions, maximizing power and efficiency.

Using both rather than either/or isn’t a new idea. It is just that while the concept has been around for decades, it has been impractical for use in vehicles until now. “Wye/Delta switches have existed in industry for a long time,” Turnbull noted.

“But typically, you have to turn off the machine, lock it out, and then flip the switch, and then turn the machine back on in order to get an effective switch from Wye to Delta,” he said. “When you switch from Wye to Delta, the personality of the motor completely changes.”

The “Y” or star-shaped configuration routes all power coming into the motor to a central point. This provides excellent low-speed torque, which is perfect for stop-and-go city driving.

The Delta configuration, on the other hand, routes power end-to-end between the three input points for the 3-phase motor. “This allows two different parallel paths for the current, so you end up with lower resistance on the winding and you get much higher power from the motor, about 1.7 times the power,” lectured Dr. Turnbull in his de facto online EV Motor 101 class.

Dr. Turnbull depicts the differences between Wye and Delta motor configurations. MUNRO LIVE

“With the Kia EV9, they’re able to optimize for city efficiency in the Wye connection and then, on the fly, this inverter is able to switch – with power running – to change it to a high-power configuration with high efficiency at highway speed.”

“Absolutely brilliant,” Turnbull crowed. “In the past, it has never been practical. Because in order to do it, you have to have, in the inverter, two completely separate sets of switches, and the switches are the most expensive part of the inverter.” But as production volumes of EV components rise, part costs are finally falling. “The volume of electric machines has gotten to the point now that EVs are more popular, where the cost of two sets of switches are no longer prohibitive,” he said.

Also, Kia can cost-justify the extra set of switches because the company is using them to provide the boost-circuit functionality seen in the Ioniq 5. “With all that functionality built in, it more than pays for the extra set of switches in the inverter,” Turnbull stated. “It’s the first that we’ve seen in the industry, groundbreaking innovation that Kia has put on the road for us.”

“It is something that I think a lot of the other automakers are going to now be looking at as an innovation they might try to incorporate as well,” he concluded. Class dismissed.