Tula DMD Aims for More-Efficient E-Machines
Dynamic Motor Drive uses intermittent torque pulses to optimize the motor’s ‘sweet spot’ of efficiency.
The race is on among OEMs and suppliers to optimize the efficiency, performance and cost of electric-vehicle (EV) propulsion systems. Electric machines have moved to the front of the development class, with engineers working to squeeze gains out of devices that are already, in some cases, more than 90% efficient.
Tula Technologies is well known within the IC engine community for its Dynamic Skip Fire (DSF) control technology that enables rapid, precise “any time, any cylinder” cylinder deactivation (see SAE Tech Paper 2013-01-03599) for improved fuel economy and reduced NVH. The company’s latest innovation, Dynamic Motor Drive (DMD), uses control fundamentals proven in production DSF applications and extends them into e-machines.
“The control philosophies of DSF and DMD are very similar,” explained Tula CEO Scott Bailey, in an interview with SAE International. “Both IC engines and electric motors have a ‘sweet spot’ of efficiency in their motor maps. Our control philosophy is to find ways to operate e-machines at their peak efficiency for a greater percentage of the time.”
DMD uses the frequency of higher-energy torque pulses as close as possible to the motor’s sweet spot of efficiency to produce the desired average torque, explained John Fuerst, Tula senior VP, engineering. “We’re a controls company and DMD came naturally to us; the physics work the same way whether in an IC engine or an e-machine,” he said.
The single-digit (four or five points) efficiency gains available in e-motors “are worth a lot when you don’t have to pay much for them,” Fuerst said. His team’s analyses concluded that most e-motor losses occur in the core, in the inverter, or in the power electronics. Reducing core and inverter losses is done by pulsing the machine when the resistance is slightly lower, based upon the phase of the rotor or on the status of the magnetic field, he said, and the condition the power electronics at that moment.
“We’re looking at how the losses go up and down depending on the cycle of the e-machine and making sure we exercise or actuate it when those high efficiency opportunities are there,” Bailey noted. “Those losses eventually become heat, just like in an ICE.”
No radical hardware
The Tula executives claim the DMD strategy will deliver efficiency improvements and cooler running from any e-motor topology, but have found that the synchronous-reluctance (SyR) machine is ideally suited for DMD controls. For that type of motor in development, Tula developed its own rotor design that Bailey said has been optimized for DMD controls and is cost-neutral compared with non-DMD rotors. The development stator is an off-the-shelf item.
“The industry increasingly is seeing the [SyR] machine as a potential future high-value motor topology,” Bailey said. “One of the things we wanted to do with DMD was not develop it for today’s e-machines, such as rare-earth-metal, permanent-magnet machines that are super costly. We tried to think ahead to where the e-machine industry is going, while identifying a motor design that is amendable to the DMD strategy.” The design uses a pulse strategy that takes advantage of the electrical fields and uses rapid control for changing those fields.
“Looking at that rotor, you won’t see anything to suggest the cost will be different than a conventional rotor in a synchronous machine. We’re trying to stay away from radical hardware changes,” Bailey explained. He noted that ample intellectual-property “white space” still exists for new innovations in e-motor development. Tula Technologies was issued its first DMD patent in August 2020, and Bailey said there are “half-dozen more following on its heels.” While others are focused on improving motor architectures such as windings, for example, “There isn’t a tremendous focus on controls to gain two, three, four points more efficiency. That’s our focus,” he said.
As part owner of Tula and a flagship customer for combustion-engine DSF, General Motors will have opportunity to be a DMD-vanguard customer. Tula also is “deeply engaged” with Cummins as the diesel giant transitions into the electrification space. “There will be other players,” Bailey said. “We’re at the beginnings of this and we’re reaching out to the e-motor people first. The power electronics side of it will follow quickly. Our IP strategy allows us to be more open and to actively engage with more customers.”