It often happens than new technologies enable new forms of automation — or makes them more practical. One new electric-motor design recognized by this year’s R&D 100 Awards could prove one such technology.
Lisa Eitel | Executive editor
Most electric vehicle or EV motors use permanent-magnet synchronous motors or PMSMs with the magnets on their rotors, and these rare-earth materials are imported to the U.S. — 98% from China. That could prove a liability given today’s political upheaval, trade standoffs, supply-chain risks, and commodity-price fluctuations trending upward.
While eliminating permanent magnets, wound-rotor synchronous motors or WRSMs conventionally use brushes or slip-ring systems for an electromagnetic solution to deliver rotor excitation current. The drawback is these electromagnetic elements can be bulky, inefficient at high speeds, and in need of occasional maintenance. That said, all WRSMs have several advantages over PMSMs — particularly in applications for which efficiency, controllability, and sustainability are priorities.
| WRSMs with brushes or slip rings | Permanent magnet synchronous motors | WRSMs with contactless rotary transformers |
| Brushes and slip rings wear over time, requiring replacement | No mechanical wear of electric-transfer elements | No mechanical wear of electric-transfer elements (for longer motor life) |
| Commutators incur friction losses | High efficiency due to direct permanent-magnet excitation sans contact losses | No friction losses for efficiency closer to PMSM levels. |
| Brushes and slip rings can cause sparking | No risk of electrical arcing | No risk of electrical arcing (for inherently safe operation in high-voltage environments) |
| Brushes and slip rings may destabilize at high rpm | Usually offer superior stability at elevated rpm | Smooth and stable operation at high rpm |
| Brush systems generate EMI | Naturally low electrical noise so useful in sensitive systems | Cleaner transfer to reduce electrical noise |
| Environmental factors (dust, vibration, moisture) degrade brushes | Generally reliable but magnet strength can degrade should the motor be exposed to high temperatures or other extreme conditions | Very robust in harsh settings |
| Cost effective | Rare-earth magnets can be costly and are subject to supply-chain interruptions | Conventional materials are cost effective |
WRSMs enable precise control of the rotor field via external excitation which in turn makes for unbeatable field weakening at high speeds to satisfy EV and industrial-machinery applications requiring broad speed ranges. WRSMs can also adjust excitation for optimal efficiency and torque control across varying loads. In contrast, PMSMs suffer from constant magnet flux than can cause inefficiencies at high speeds and limited flexibility in torque control.
Rotary-transformer excitation from Oak Ridge National Laboratory (ORNL) makes for a wound-rotor motor that better competes against permanent-magnet rotors.
Now, researchers at Oak Ridge National Laboratory (ORNL) have designed a rotary-transformer–based wireless excitation system to help WRS motors better compete against PMSMs. This design leverages 17 years of ORNL expertise in wireless power transfer, 82 years of ORNL electromagnetics expertise, and 40 years of power electronic converter expertise. How does it work? Well, the system has a stationary (primary) side and a spinning (rotor) side. On the stationary side, ORNL’s high-frequency power inverter operates as a high-frequency resonance inverter that powers the primary coil through a resonant tuning network. The primary coil generates a magnetic field that is linked to the spinning rotor–side coil inducing voltage. This voltage is rectified on the receiver coil and rectifier integrated assembly while output dc current is applied to the rotor windings.
The design’s advantages abound. The rotor current is accurately estimated sans sensing or communication devices. The rotor side uses an integrated secondary coil with a rectifier on the same printed circuit board. The simple rotor-side architecture eliminates resonant tuning components on the rotor side as well as capacitors. The PCB uses a trace-stranding and transpositioning system that can mimic Litz wire with low losses. A dc-measurement method can integrate into the stationary side to read the current on the rotary transformer. A position sensor is semi-integrated into the rotary transformer so there’s no need for an expensive resolver, either.
A polyphase version of the rotary transformer eliminates housing-assembly eddy-current losses that are usually the primary cause of PMSM losses.
R&D World | rdworldonline.com
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