EV Engineering & Infrastructure

Part II: Inside the engineering of a magnet-free EV motor

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In this two-part series, we examine the engineering behind a rare-earth-free electric vehicle (EV) motor and the trade-offs involved in replacing permanent magnets. Part 1 looked at the supply-chain case for magnet-free motors, the wound-rotor architecture Vimag is developing, and the engineering problems that arise when permanent magnets are removed.

Part II turns to how Vimag’s proposed solution works in practice. It examines the rotating-transformer excitation system, the control strategy behind its “software-defined magnet,” and whether the design’s adjustable field can offset the added losses, cooling demands, and component complexity.

The stator is conventional. It carries three-phase windings and is driven by an inverter that creates a rotating magnetic field, as in most ac traction motors.

The rotor is where the design differs. Instead of permanent magnets, it carries field windings. To energize them without brushes, Vimag uses what its patent describes as a rotating transformer, also called a rotary transformer.

A rotary transformer operates on the same principle as a conventional transformer: magnetic induction across a gap. The difference is that one winding remains stationary while the other rotates with the shaft. High-frequency alternating current is applied to the stationary primary winding and couples across a small air gap into a secondary winding that rotates with the rotor. A rectifier mounted on the rotor then converts the induced ac into the direct current required by the field coils.

System diagram of the VMSM architecture, showing how a rotary transformer and rotor-mounted rectifier provide brushless excitation while software adjusts the wound-rotor magnetic field across different operating conditions.

This arrangement transfers power without physical contact, eliminating brushes, slip rings, and the associated wear surfaces. Vimag didn’t originate the underlying concept. Rotary-transformer excitation for synchronous machines has been studied for decades, and national laboratories, including Oak Ridge National Laboratory in the United States, have published research on polyphase rotary transformers for non-rare-earth traction motors. According to its patents, Vimag’s contribution is a specific implementation of that approach combined with its control system.

The control system is what turns the wound-rotor motor into what Vimag calls a “software-defined magnet.” Because current produces the rotor field rather than a fixed permanent magnet, the controller can adjust its strength while the motor is operating. At a standstill or low speed, where maximum torque is required, it can increase the field current. At higher speeds, it can reduce the field to keep voltage within operating limits. When the vehicle is coasting, the field can be switched off, reducing the losses associated with maintaining magnetic flux.

This adjustable field is a key technical advantage of the topology. It can support a wide constant-power speed range and improve field-weakening performance at higher speeds. The possibility of refining motor performance through over-the-air updates follows from the same principle: if the field profile and current references are controlled through software, changes to that software can alter how the motor operates.

The design still carries important trade-offs. Excitation current consumes energy and generates heat in the rotor, so cooling remains a significant requirement. Coordinating the stator current, rotor field current, rotor-position estimate, and excitation loop is also more complex than controlling a permanent magnet motor. The rotary transformer and rotating rectifier add components and cost that a permanent magnet rotor doesn’t require.

Whether the complete system can match the efficiency, size, and cost of a well-designed PMSM will depend on execution. Independent performance data will therefore be necessary to evaluate Vimag’s claims.

The engineering trade-offs

It’s worth considering where this type of motor performs well and where it doesn’t, because the answer isn’t “everywhere.”

First, efficiency depends on speed. Permanent magnet motors perform well in low-speed, stop-and-go driving because they produce torque without requiring rotor excitation current. That closely matches the duty cycle of a scooter in Indian city traffic, and it’s a less favorable operating range for a wound-rotor motor, which must supply excitation current continuously.

At highway speeds and during field-weakening operation, the balance can shift. A teardown of the externally excited motor used in the Nissan Ariya illustrated this distinction: the permanent magnet motor holds an advantage in urban stop-and-go conditions, while the externally excited motor can perform better at higher speeds because it avoids the losses associated with counteracting a fixed magnetic field.

That has a direct implication for Vimag’s focus on electric two- and three-wheelers. These vehicles spend much of their operating time at low to moderate speeds in dense traffic, where a wound-rotor motor may be less competitive on efficiency. The supply-chain case for the design is strong. The efficiency case is less certain and will depend on the effectiveness of the control and cooling systems. Measured efficiency maps will be needed before claims that the motor “matches or exceeds” permanent magnet designs can be properly evaluated.

Second, the cost comparison remains uncertain. The motor avoids expensive and price-volatile rare-earth magnets, which can reduce both material costs and supply risk. However, it also adds copper, a rotary transformer, a rotating rectifier, and more sophisticated power electronics and control.

Whether the overall bill of materials is lower will depend partly on rare-earth prices, which have become higher and less predictable since 2025. That volatility may be one of the strongest arguments for the design. Even if the magnet-free motor costs about the same on average, it reduces the risk of a supply interruption stopping production, which is the same risk behind Bajaj Auto’s warning.

Third, there’s the manufacturability argument, which is central for an Indian company. Copper, electrical steel, rectifiers, and controllers are materials and components India already produces or can support through its existing industrial base. A rotor filled with sintered rare-earth magnets remains tied to a more concentrated external supply chain.

Even if the motor ultimately proves comparable to a PMSM in performance and cost rather than clearly superior, the ability to manufacture it domestically could provide a strategic advantage that isn’t captured by an efficiency map.

Potential applications

Vimag’s target list extends beyond passenger cars, and that range helps clarify where the technology may be most practical.

In electric two- and three-wheelers, which are the company’s stated near-term focus, the main appeal is supply-chain security and domestic manufacturability rather than maximum efficiency. India’s two- and three-wheeler market is large, price-sensitive, and directly exposed to the effects of Chinese magnet export controls. A motor that removes that dependency could still be practical for the market, even if it carries a modest efficiency penalty in city driving.

The company says it’s testing the technology with a top-five two-wheeler manufacturer, the EV division of an established Indian brand, and a major three-wheeler powertrain supplier. These partners haven’t been identified, which is common during pilot-stage development but means the relationships aren’t yet confirmed commercial programs.

In passenger vehicles, the topology may be a more natural fit because cars spend more time at the higher speeds where an adjustable-field motor can be competitive. This helps explain why major automakers investing in externally excited motors have generally focused on passenger vehicles rather than scooters.

BMW has used externally excited motors across several vehicle generations, with drive units reported in the 140 to 360 kW range and efficiencies above 95%. Renault and Nissan have also pursued magnet-free designs. Vimag has referenced work with a passenger vehicle manufacturer and a European Tier 1 supplier, which follows that broader industry pattern.

Beyond vehicles, the company identifies industrial machinery and HVAC as potential markets. This is a logical extension because the adjustable-field, magnet-free design may suit applications where a wide speed range, demanding thermal conditions, and operation in sealed or harsh environments matter as much as peak power density.

Industrial drives are also generally less constrained by weight and package size than automotive traction systems, which reduces some of the topology’s disadvantages. HVAC equipment often operates continuously and can benefit from efficient part-load performance, which adjustable excitation may help provide.

The common thread is that the VMSM may be most useful where performance across a broad operating range and reduced rare-earth dependence matter more than achieving the highest possible power density in the smallest package. That still represents a meaningful range of potential applications.

How Vimag compares with other motor approaches

Vimag is one of several companies working to reduce or eliminate rare-earth materials in electric motors. Comparing its design with the main alternatives helps clarify what is distinctive about its approach.

The oldest magnet-free option is the induction motor, which Tesla used in its early vehicles and still uses in some drive units. It requires neither permanent magnets nor a separate rotor excitation supply, but it can be less efficient at low speed and standstill and more difficult to control efficiently in urban driving conditions.

Switched-reluctance motors provide another magnet-free option. They’re mechanically simple and durable, but they’ve historically been associated with higher noise levels and torque ripple. Neither approach is the same as Vimag’s wound-rotor design.

Other companies are retaining the permanent magnet architecture while replacing rare-earth magnets with alternative materials. Niron Magnetics in the United States, backed by General Motors and Stellantis, is developing iron-nitride magnets and expanding production capacity. Ola Electric in India has received government approval for in-house ferrite motors for its scooters. Conifer in the United States is developing axial-flux motors optimized around lower-cost ferrite magnets.

These approaches preserve the relative simplicity of a permanent magnet rotor but accept some performance limitations associated with weaker magnetic materials.

Vimag takes a different approach. It retains the wound rotor, removes permanent magnets entirely, and relies on rotary-transformer excitation combined with control software. Its closest comparisons are therefore the externally excited motors used by BMW, Renault, Nissan, and General Motors, along with rotary-transformer research from organizations such as Oak Ridge National Laboratory.

Vimag isn’t introducing an entirely new motor topology. It’s applying an established design, already used by global automakers, with its own excitation and control approach. The goal is to make that architecture practical for domestic production in a cost-sensitive market.

What is verified and what remains a claim

For a technical readership, it’s important to distinguish between what’s established and what still rests on the company’s own statements.

The underlying topology is real and well understood. Wound-rotor and externally excited synchronous motors are already in production from major automakers and do eliminate rare-earth permanent magnets. Rotary-transformer excitation for brushless field supply is also a decades-old technique supported by published research. Vimag’s five Indian patents and its $5 million funding round are on the record.

What hasn’t yet been independently verified is the motor’s performance. The claims that it matches or exceeds conventional permanent magnet motors, improves range, reduces battery demand, and does so in the two- and three-wheeler duty cycle where the topology may be least advantaged are based on statements from the company.

There’s no independent dynamometer data, published efficiency map, or third-party teardown comparable to what’s available for the Nissan Ariya motor. The company’s testing partners haven’t been identified, which is common during pilot-stage development but means those relationships aren’t yet confirmed commercial programs.

The performance claims deserve the closest scrutiny because excitation losses and rotor cooling are among the factors that distinguish a strong wound-rotor implementation from a less effective one. Those are also the areas where Vimag hasn’t yet published detailed results.

None of this makes the technology less worth watching. A domestically manufacturable, rare-earth-free traction motor could address a significant supply-chain challenge for India’s EV industry, and the underlying topology is established. The open question is execution: whether Vimag’s excitation and control system can manage the efficiency and cooling penalties inherent in the design, and whether it can do so at a cost and scale that a price-sensitive market will accept.

Those answers will come from measured data and production deployments, not patents and press releases. When that evidence becomes available, it will be worth examining.

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