Selection Requirements for Resolvers

Overall selection order: structure type → number of pole pairs → accuracy → electrical parameters (matching the decoder chip) → mechanical installation → environmental conditions → reliability and certification.

(一）. Choosing the Type of Structure

1. Brushed resolver: Brushes and slip rings make contact, low cost; brushes wear out, limited lifespan, suitable for short-term use or low-lifespan requirements, almost never used in modern servo or new energy motors.

2. Brushless resolver: Non-contact, no wear, long lifespan, high reliability, preferred choice, divided into two types:

• Wound resolver: High accuracy, low electrical error; average vibration resistance, relatively high cost, suitable for high-precision servo, military, and aviation.

• Reluctance (variable reluctance) resolver: Rotor has no winding, sturdy structure, strong vibration and impact resistance, compact, widely used in new energy vehicle drive motors; accuracy slightly lower than wound type.

3. Framed / Frameless (separate parts)

• Frameless (stator and rotor separate): Embedded inside the motor, saves space, mainstream in the motor industry; requires precise coaxial alignment for installation.

• Framed integrated: Comes with outer casing and bearings, standalone component, easy to install, used for external angle measurement.

（二）Pole-zero (the most critical matching factor)

Key point: Try to match the resolver pole pairs with the motor pole pairs. This way, the controller doesn’t need to convert between mechanical and electrical angles, which helps avoid angle misalignment, low-speed jitter, and sudden speed changes.

• Each full turn of the motor makes the resolver output P electrical cycles; the more pole pairs, the higher the angle

• resolution. Common setups are 1, 2, or 4 pole pairs; new energy motors often use 4.

• If the pole pairs don’t match, the software has to convert to electrical angles, so make sure the decoding chip supports it.

（三）Accuracy Indicators

1. Electrical Error: The core accuracy metric, measured in arcminutes (′) or arcseconds (″), indicates the deviation between the actual output and the ideal sine/cosine.

• Standard reluctance resolvers: ±5′ to ±60′

• Wound single-pole: ±3′ to ±15′

• Multi-pole precision resolvers: can reach tens of arcseconds; dual-channel coarse-fine setups can reach arcsecond level.

2. Zero Voltage: Residual output at zero position; the smaller, the better, as it can cause zero point shift.

3. Actual System Accuracy = Resolver body accuracy + R/D decoder chip error + installation eccentricity error.

（四）ectrical Parameters (Must match R/D decoder chip)

1. Excitation Voltage (Rated Excitation): 4Vrms, 7Vrms, 10Vrms, etc., must match the decoder chip's excitation output.

2. Excitation Frequency: 400Hz, 1kHz, 5kHz, 10kHz; industrial servo and new energy applications usually use 5kHz-10kHz. Higher frequency means faster response, but cable parasitic capacitance has a bigger effect.

3. Transformation Ratio (Turns Ratio): Output voltage / Excitation voltage, determines SIN/COS output amplitude; output amplitude should be within the decoder chip's optimal input range. Common value in new energy vehicles is 0.286.

4. Input Impedance: Impedance on the excitation side, should match the driving capability; if impedance is too low, it overloads the excitation chip.

5. Phase Shift: Phase difference between output and excitation, generally 3-12° electrical; too large increases decoding errors.

6. Output: Two differential analog signals, SIN and COS; for long distances, differential output is preferred for better interference resistance.

（五）Mechanical Selection

1. Rotor inner diameter (shaft diameter): Matches the motor shaft, pay attention to tolerances, and ensure coaxiality; any eccentricity can introduce large angular errors.

2. Stator outer diameter and thickness: Must fit within the motor's internal installation space.

3. Maximum allowable speed: Higher than the motor's maximum operating speed, with a safety margin; for high speeds, pay special attention to rotor dynamic balancing.

4. Mounting method: Shoulder positioning and screw fixing; for frameless resolvers, strictly control the stator‑rotor air gap (usually a fraction of a millimeter), as uneven gaps directly reduce accuracy.

5. Leads/terminals: Lead length and temperature rating; for vehicles, choose high-temperature cables.

（六）Environmental and Operating Conditions

• Operating Temperature: Industrial standard −40℃~125℃; Automotive −40℃~155℃; Military can go up to −55~200℃.

• Vibration and Shock: For vehicles and construction machinery, check the G values; magnetoresistive anti-vibration is better than wound type.

• Protection Level IP: Built-in motors usually don’t need a high IP; external use requires an IP rating.

•  Electromagnetic Environment: High-power motors in strong EMC scenarios should prioritize differential signals and shielded cables.