The "Fiery Eyes" of EV Main Drive Motors – Customization of Variable Reluctance Resolvers

Introduction: An Indispensable "Eye" for Motor Control

In the drive motor of new energy vehicles, there is an inconspicuous yet critically important component -the variable reluctance resolver (often shortened to "resolver"). It acts as the motor's "eye," constantly telling the motor controller: "Where is the rotor positioned now? What is the rotational speed?"

Without this "eye," permanent magnet synchronous motors cannot achieve precise commutation control, resulting in rough, jerky power output -or even complete failure to operate. Data show that the penetration rate of resolvers in the main drive motors of new energy passenger vehicles is as high as 78.6% -meaning nearly every new energy vehicle's main drive motor is equipped with one.

So, how does this tiny sensor actually work? Why does it need to be "customized"? And what key parameters should you pay attention to during customization?

1. What is a Variable Reluctance Resolver?

1.1 From Transformer to Resolver

Everyone is familiar with the "transformer" -it steps voltage up or down. A resolver can be thought of as an advanced version of a transformer: while a conventional transformer outputs a fixed voltage, a resolver outputs a voltage that varies with the rotor's rotation angle.

The variable reluctance resolver is the most widely used type. Its core structure is remarkably simple:

· Stator: Fixed in place, wound with an excitation winding (input) and sine/cosine output windings.

· Rotor: Made of stacked silicon steel laminations with teeth, containing no windings at all, and rotates with the motor shaft.

1.2 Working Principle: Air-Gap Changes Determine Signal Changes

When an AC excitation voltage is applied to the stator's excitation winding, an alternating magnetic field is generated in the air gap between the stator and the rotor. As the rotor turns, the magnetic reluctance of the air gap changes periodically because the rotor teeth (protrusions and recesses) alternately pass the stator teeth.

This variation causes the induced voltages in the two output windings (sine and cosine) to change in amplitude -one following a sine pattern, the other a cosine pattern.

By interpreting these two signals, the motor controller can accurately calculate the rotor's absolute angular position at any moment.

1.3 Why Has It Become the Top Choice for EV Main Drive Motors?

The variable reluctance resolver has come to "dominate" the new energy vehicle sector thanks to the following strengths:

· Extremely simple and reliable construction: No rotor windings, no brushes -no wear issues, and its theoretical service life can match that of the equipment itself.

· Exceptional environmental resilience: Operating temperature range from -50°C to +155°C, resistant to high temperatures, vibration, and oil contamination.

· Strong electromagnetic interference immunity: Based on electromagnetic induction principles, it is naturally immune to the harsh electromagnetic environment inside the motor.

· Absolute position output: Provides the current angle immediately upon power-up, with no need for a "homing" procedure.

2. Why Does the Main Drive Motor Require a "Customized" Resolver?

Some might ask: aren't resolvers standard off-the-shelf components? Why not just buy a ready-made one?

The reality is far from that simple. Each new energy vehicle main drive motor is designed with different power, speed, mounting space, and operating conditions -and these differences directly determine that the resolver must be "tailor-made."

2.1 Mounting Dimensions Vary Widely

Different motors come in different frame sizes, from 25 to 300, each requiring a resolver with matching dimensions. Mounting methods also differ -through-hole mounting, surface mounting, flange mounting -each corresponding to a different mechanical interface design.

2.2 Electrical Parameters Differ

Excitation voltage, excitation frequency, number of pole pairs, transformation ratio -all these electrical parameters must perfectly match the motor control system. Using a resolver with mismatched parameters can, at best, reduce measurement accuracy, and at worst, render the entire motor control system inoperable.

2.3 Accuracy Requirements Vary Significantly

The position accuracy required for a mainstream passenger vehicle's drive motor is completely different from that of a high-performance vehicle. The more pole pairs, the higher the accuracy -ranging from 2, 4, to 6 or more pole pairs, with accuracy improving from 25 arc minutes to 10 arc minutes or even higher.

2.4 Operating Conditions Are Not Uniform

Some motors operate in oil-cooled environments, requiring the resolver to be oil-compatible; others pursue extreme high speeds, necessitating support for 20,000 rpm or even higher; still others require automotive-grade AEC-Q certification -each condition imposes distinct design requirements on the resolver.

3. Resolver Customization: What Exactly Can Be Customized?

3.1 Mechanical Customization

· External dimensions: outer diameter, length, mounting hole positions, etc.

· Mounting method: through-hole, surface-mount, or flange type.

· Rotor shaft aperture: must precisely match the motor shaft diameter.

· Lead wire specifications: wire length, gauge, and connector type.

3.2 Electrical Customization

· Number of pole pairs: a key parameter determining measurement resolution; can be tailored to requirements.

· Excitation voltage: typically AC 7 Vrms, but adjustable per system needs.

· Excitation frequency: commonly 10 kHz, but other frequencies can be specified.

· Transformation ratio: determines the output signal amplitude, affecting the signal processing of the downstream decoding chip.

· Input/output impedance: must match the drive circuit.

3.3 Performance Customization

· Accuracy grade: from standard precision to high precision (e.g., ±4 arc minutes) available.

· Maximum rotational speed: standard 20,000 rpm; higher speeds can be customized.

· Operating temperature range: tailored to actual working conditions.

· Protection class: e.g., IP69 or higher.

3.4 Special Customizations

· Oil compatibility: special materials and sealing designs for oil-cooled motors.

· Eccentricity tolerance: patented winding schemes to compensate for mounting eccentricity errors.

· Integrated temperature sensing: optional NTC thermistor for combined position and temperature detection.

4. Customization Process: From Requirements to Delivery

A typical resolver customization process generally includes the following stages:

Step 1: Requirements Communication -The customer provides motor parameters, available space, performance targets, etc.

Step 2: Solution Design -The supplier develops the electromagnetic and mechanical designs based on the requirements.

Step 3: Simulation Verification -Finite element analysis (FEA) is used to optimise electromagnetic performance.

Step 4: Prototype Manufacturing -Prototype samples are produced and tested on test benches.

Step 5: Mass Production -After validation, the product moves into volume production.

Currently, leading domestic resolver suppliers have shortened the customisation delivery cycle to as little as 4-6 weeks. Some have already built fully automated production lines with annual capacities exceeding 1 million units.

5. Industry Trend: Domestic Substitution Is Gaining Momentum

For a long time, the high-end resolver market was dominated by foreign brands such as Japan's Tamagawa and MinebeaMitsumi. However, in recent years, Chinese resolver technology has made significant breakthroughs；At the same time, domestic resolver-to-digital converter (RDC) chips are also advancing rapidly. Xinchi Semiconductor's SC2121 and other products have achieved functional parity with internationally renowned models, and Fengchao Technology has also launched high-performance RDC chips. The full-chain domestic substitution -from sensor to decoding chip -is accelerating.

Conclusion

Though small in size, the variable reluctance resolver is an indispensable core sensor in the control system of new energy vehicle main drive motors. Its customisation directly affects the motor's control accuracy, operating efficiency, and reliability.

With the continued growth of the new energy vehicle industry and the deepening of domestic substitution, resolver customisation services will become even more mature and efficient. For motor engineers, understanding the working principles and customisation essentials of resolvers has already become a mandatory course.

After all, only when the motor is fitted with the right "eyes" can it see clearly and run steadily.