Resolver are commonly used as position and speed sensors in electric vehicles. For example, resolvers are used to measure the positions of the driving motor and generator, the position of the electric power steering wheel motor, and the angle of the gas valve. However, resolvers come in various forms, and reluctance resolvers are widely used in electric vehicles due to their good processability, large relative displacement, high reliability, and low cost.

Reluctance resolver: One phase excitation winding and two phase output winding are fixed in the stator slot, and the rotor magnetic pole shape is specially designed to make the air gap in a regular shape. When the rotor rotates, the change in air gap causes the signals of the two phase output windings to form a sine cosine relationship.

How to choose a reluctance resolver

Selection of poles: Generally, the same number of poles as the motor poles is selected, so that the measured electrical angle does not need to be converted to the electrical angle of the inverter coordinate transformation. Of course, as long as the measurement of electrical angle error meets the requirements, it is also possible for the poles of the two to be mismatched. If the motor has 4 poles and the rotary transformer has 2 poles, then when the rotary transformer rotates 90 degrees, the corresponding electrical angle of the motor is 180 degrees, and vice versa. Theoretically, the higher the number of poles, the smaller the electrical error

Selection of primary side transformation ratio: The current mainstream excitation signal amplitude is 7Vrms/4Vrms, and the feedback signal is finally input into the decoding chip for decoding. For cost reduction and simplicity, it is necessary to ensure that the selected transformation ratio ensures that the amplitude of the feedback signal is within the input range of the chip and close to its recommended value. In the field of new energy vehicles, the transformation ratio is 0.286.

7Vrms corresponds to 19.8Vpp, so at a transformation ratio of 0.286, the amplitude of the feedback signal corresponds to 5.66Vpp, which exceeds the recommended input range of the decoding chip. At this point, the requirements can be met by adjusting the amplitude of the excitation voltage (in fact, for personal applications, it is recommended that the excitation voltage be less than the recommended value in the manual. The benefits are as follows: low excitation voltage requires less driving current, making it easier to design circuits and improving reliability accordingly. In addition, low excitation voltage results in low residual voltage, which can reduce electrical errors to a certain extent. As for the benefits of high excitation voltage, it is high signal-to-noise ratio and ensures that the established magnetic field is strong enough. However, from the perspective of design testing and cost evaluation of adjusting feedback circuits, the disadvantages outweigh the advantages).

Size selection: Ultimately, the rotating rotor needs to be installed on the motor rotor, so the motor shaft diameter and the inner diameter of the rotating rotor need to match. The closer the size is, the smaller the eccentricity will be.

Matching of Electrical Parameters for Resolvers

Excitation frequency: Recommended frequency is 10kHz/16.5kHz. If the frequency is too low, the input impedance will be low and the required driving power will increase. Furthermore, with low frequency, there is less angular information carried at high speeds, which cannot meet the application requirements. Excessive frequency mainly increases the value of residual magnetic voltage, which has an impact on hardware decoding speed and bandwidth. It is not recommended to make changes, and the maximum frequency should not exceed 20kHz.

Electrical error: used to measure the degree to which the output electromotive force strictly conforms to the sine cosine relationship with the rotation angle, expressed in angular minutes and angular seconds.

Impedance: The input impedance is closely related to the excitation power, and the output impedance can be ignored (the receiving load of the rotary decoding chip is much larger than the output load of the rotary converter, and there is basically no power loss). The main component of input impedance is resistance, and the impedance angle is generally less than 10 °. The driving power can be approximately calculated directly by the square ratio of the effective value of the excitation voltage to the input impedance.

Phase shift and residual pressure: The former represents the deviation angle between the excitation signal and the feedback sine signal, while the latter represents the magnetization degree of the magnetic steel, mainly related to the material.