Two Technical Routes for Resolvers: What Really Sets Variable Reluctance And Wound-Rotor Resolvers Apart?

In applications such as new energy vehicle electric drive systems, industrial servo systems, and aerospace, where precise rotor position sensing is required, the resolver is an indispensable key component. Its job is simple: to tell the controller in real time "where the rotor is now." But there is more than one technical route to achieve this goal.

Today's mainstream resolver products fall into two major categories—wound-rotor resolvers and variable reluctance resolvers. Both have "resolver" in their names and produce similar output signals, but their internal structures, operating principles, and application scenarios differ greatly. Understanding these differences is crucial for selection and application.

One Rotor Has Windings, the Other Does Not

The most fundamental difference lies in the rotor.

A wound-rotor resolver has windings on both the stator and the rotor. The rotor winding serves as the primary side and receives the excitation voltage; the stator winding serves as the secondary side and generates an induced voltage through electromagnetic coupling between the stator and rotor. The degree of electromagnetic coupling varies with the rotor angle, outputting signals that have a sine/cosine relationship with the angle. Simply put, a wound-rotor resolver relies on "coil-to-coil" transformer coupling.

A variable reluctance resolver is completely different: both the excitation winding and the output windings are wound on the stator core, and the rotor is merely a toothed laminated silicon steel component with no windings, leads, or slip rings. As the rotor turns, the teeth and slots alternately change the air-gap permeance, thereby modulating the mutual inductance between the stator windings and inducing sine/cosine signals that vary with the angle in the output windings. It relies not on coil coupling, but on "reluctance variation."

This structural difference is like a seed that determines almost all subsequent performance characteristics.

Accuracy: Wound-Rotor Still Has an Edge, but the Gap Is Narrowing

Wound-rotor resolvers have long led in accuracy. Because both the stator and rotor use sinusoidal distributed windings, the magnetic field distribution is more regular, the higher harmonic content in the air-gap permeance is low, and the output signal has better sinusoidal quality. At the same size, a wound-rotor resolver is usually more accurate than a variable reluctance resolver. Dual-channel wound-rotor products can even achieve arcsecond-level accuracy (within 5″), meeting the demanding requirements of radar, artillery, high-precision turntables, and similar applications.

Accuracy was once the weak point of variable reluctance resolvers. The salient-pole shape of the rotor and the winding structure introduce relatively large higher harmonics, and early products often achieved only "graded" accuracy. In recent years, however, through design methods such as segmented sinusoidal-parameter windings, magnetic wedge optimization, and PCB connections, the signal error of variable reluctance resolvers has been reduced from ±15 arcminutes to within ±5 arcminutes, with harmonic amplitude reduced by about 67%. In scenarios such as new energy vehicle electric drives, where accuracy only needs to be "good enough," variable reluctance resolvers are fully capable. Some domestic variable reluctance resolver products have reached a maximum accuracy of 2 arcminutes, entering the industrial-grade level.

Reliability: Variable Reluctance "Wins at the Starting Line" Structurally

If accuracy is the traditional strength of wound-rotor resolvers, then reliability is the inherent advantage of variable reluctance resolvers.

The rotor of a wound-rotor resolver has windings and solder joints. Under the centrifugal force of high-speed rotation, the rotor coils face the risk of wire breakage and solder-joint loosening. This is a structural hazard that cannot be completely eliminated by process improvements. For electric drive motors whose speeds often reach tens of thousands of rpm, this risk cannot be ignored.

The rotor of a variable reluctance resolver is a solid silicon steel sheet with no windings, solder joints, or leads, fundamentally avoiding the above failure modes. This makes it particularly suitable for ultra-high-speed scenarios—some variable reluctance models support speeds up to 160,000 rpm. At the same time, because there are no wearing parts such as brushes or gratings, its theoretical service life is extremely long, making it suitable for unattended equipment that runs continuously for long periods and reducing downtime maintenance costs.

In terms of environmental resistance, variable reluctance resolvers also perform excellently. Taking the variable reluctance products that SDM focuses on as an example, they can withstand a wide temperature range of -40°C to 155°C, 30g vibration, and 100g shock, with excellent oil resistance and high-temperature resistance, and a maximum application speed of 30,000 rpm. These characteristics allow them to be directly integrated inside electric drive systems and adapt to harsh conditions such as immersion in cooling oil.

Cost and Manufacturing: Wound-Rotor Is "Subtracting," Variable Reluctance Is "Adding"

Wound-rotor resolvers are more complex to manufacture. Dual windings on the stator and rotor mean more winding processes, more complex process parameters, and greater reliance on manual labor. Although production automation upgrades have continued in recent years, and some products have moved from fully manual to semi-automatic winding, reducing unit costs to some extent, overall manufacturing efficiency is still limited by the dual-winding structure.

The structural advantages of variable reluctance resolvers translate into manufacturing advantages here. With no rotor winding, the stator winding method is similar to that of common motor stators and can be processed using equipment such as internal winding machines, making processing simple and assembly efficient. With one less rotor winding, the corresponding winding, soldering, and inspection processes are eliminated, leaving more room for cost control in large-scale production.

In terms of end price, domestic variable reluctance resolvers can be priced at just over ten yuan, while the price of comparable Japanese products is usually 2–3 times that. This cost advantage is especially critical in price-sensitive fields such as new energy vehicles.

Application Divide: Each Occupies Its Own Territory

The application scenarios of the two have already formed a relatively clear division of labor.

Wound-rotor resolvers mainly serve industrial machinery (CNC machine tools, injection molding machines, textile machinery), aerospace, robotics, the nuclear industry, and other fields, as well as high-precision servo control for defense equipment such as radar and missiles. In these scenarios, accuracy requirements often override everything else, and sensitivity to cost is relatively low.

Variable reluctance resolvers, on the other hand, have found their biggest stage in new energy vehicle electric drive systems. In the motor control systems of mainstream electric models such as the Toyota Prius, Nissan Leaf, and Tesla Model 3, variable reluctance resolvers are used in practice. The large-volume demand from domestic new energy vehicle makers such as BYD is also mainly met by variable reluctance resolvers. Rail transit traction systems are likewise an important application area for variable reluctance resolvers.

Market data also confirms this trend. In 2025, global variable reluctance resolver market sales reached 2.234 billion yuan and are expected to grow to 6.181 billion yuan by 2032, with a compound annual growth rate of about 15.9%. Meanwhile, traditional wound-rotor resolvers still hold about 65% of the market share, but the annual growth rate of non-contact resolvers has reached 38%, with growth mainly driven by new energy vehicles and industrial automation.

A Table to See the Core Differences at a Glance

The two technical routes are not in a relationship of one replacing the other; rather, each serves a different quadrant of demand. Wound-rotor resolvers remain irreplaceable in high-precision, absolute-positioning applications; variable reluctance resolvers have opened up their own space in the directions of reliability, high speed, and large-volume cost control. For the rapidly iterating new energy vehicle and industrial automation industries, the approach represented by variable reluctance resolvers—"subtracting in structure, adding in reliability"—precisely fits the current core demand for components with long life, maintenance-free operation, and low cost.