Electric Motor Demonstrator Sets Two World Records in Two Hours After Five Years of Work
Dr. Codrin-Gruie (CG) Cantemir, a research scientist at Ohio State’s Center for Automotive Research was awarded a NASA research grant for his 10 MW Ring Motor concept. The grant sought innovative designs of electric machines that could meet the performance requirements necessary for use in hybrid electric propulsion systems for large transport aircraft such as a Boeing 737. NASA was looking for machines that were five times lighter and more efficient than any motor in production.
The 10 MW Ring Motor concept was conceived to meet these requirements. The concept includes a unique tuned coil principle and technology featuring variable cross section windings that are married to another original technology which achieves direct contact between coils and coolant. This enables stator magnetic field production at high frequencies while keeping specific high frequency losses in solid conductors at a minimum. This motor is an induction machine in reverse construction (out-runner) where the field generated by the internal stator induces currents in a squirrel cage located in the rotor. This interaction produces the torque and the power without the need of commonly used permanent magnets (PMs).
“Virtually only electric machines using high performance PMs are considered the favorite in the race for power density. PMs are producing the magnetic field for free – which is an obvious advantage - however the high-end PMs are a Chinese monopoly, are expensive, and in the case of a motor failure, the fire hazard may put a huge safety toll on an aircraft,” said Cantemir.
NASA encouraged Cantemir to explore using induction motors rather than PM machines because they are much safer and more robust. Recent tests have proven than even when handicapped by the lack of PMs, induction motors may fully compensate for power and torque shortcomings due to the novel technologies developed under this NASA grant.
In a recent preliminary test of the induction motor, Ohio State, in partnership with NASA, set two world records; the first for absolute normalized continuous power density and the second for continuous power density for an induction motor.
The previous world record for normalized continuous power density was set by Siemens of Germany at approximately 2.05 W/kg *rpm and used state-of-the-art Rare-Earth Neodymium Magnets and very expensive Cobalt-based magnetic material. The record set by the team is 2.6 W/kg*rpm using only American made ‘off-the-shelf’ cost-effective magnetic materials and no PMs.
One unexpected reason for the record is the use of aluminum rather than copper as the conductor material in the stator coils. The theoretical work conducted under this grant led to the conclusion that in certain situations, aluminum will behave better than copper. Hence, the tested motor does not use copper conductors at all, only aluminum. However, the ‘cherry on the cake’ as Cantemir says, is that his solution lacks the electric wire insulation. This is notable because electric insulation failure is the number one reason for failure in electric motors.
“As a result of these records, it is now very possible to reduce the cost per kilowatt in standard spinoff industrial applications,” said Cantemir. “It may be counterintuitive, but a power dense motor uses less materials and less tooling vs. conventional, hence the cost saving structure. Now, this motor is also backed by technologies, which can be fully robotized and easily conducive to mass production, so it is very possible that Ohio State and NASA just opened the door for a cost effective industrialization.”
The Technical Box
- Motor configuration: Out-runner, induction, 6 phases @ 30°, “2Y” or “6H”
- Electromagnetic materials: Silicon steel and aluminum (only) without PMs
- Motor weight: 97.5 kg (214.5 lbs) dry
- Motor designed rating(s): + 1MW / 2250 Nm @ 4000 rpm or 2000 Nm @ 5000 rpm
- Test goal: determination of heat rejection capability (in overcurrent and artificially decreased efficiency)
- Test speed: 1800 rpm - against a 4 pole/60 Hz motor/generator
- Test power: +460 KW / 1825 rpm @ 2430 Nm in overcurrent mode (load limited)
- Test result: +40 kW stator continuous heat rejection capability @ nominal temperature
- Power density as tested: 4.7 kW/kg @ 1825 rpm – not intended record for induction motors
- Normalized power density as tested: 2.6 W/kg*rpm – absolute record