Rotary transformers are commonly used as position and speed sensors in electric vehicles. For example, position sensors for driving electric motors and generators, position sensors for electric power steering wheel motors, and angle measurement for gas valves all use rotary transformers. However, rotary transformers come in various forms, and reluctance rotary transformers are widely used in electric vehicles due to their good process ability, large relative displacement, high reliability, and low cost.

Variable 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 to form a sine cosine relationship. After reading it, did you look confused. Let's first review the most basic theories of "electricity and magnetism", hoping to clarify the basic working principle of the reluctance type rotary transformer.

1. The basic relationship between magnetism and electricity

1) Ampere's rule ("electromagnetism"): A spiral tube carrying an electric current generates a magnetic field.

Faraday's law of electromagnetic induction ("magnetoelectricity"): Changes in magnetic flux generate induced electromotive force.

2. Magnetic resistance effect

When the charge carriers of metals or semiconductors move in a magnetic field, the Lorentz force generated by the changes in the electromagnetic field produces the magnetoresistance effect. After reading it, I was a bit confused again.

To better understand, let's imagine an experiment as shown in Figure 3. A U-shaped silicon steel sheet 1 is wound with a spiral wire and a small silicon steel sheet 2 is prescribed at the U-shaped opening. When a current is applied to the spiral wire, silicon steel sheet 2 and silicon steel sheet 1 are attracted together, as shown in Figure 4, which is the result of the magnetic resistance effect.

Magnetic field lines (i.e. magnetic induction lines) always take the path with the smallest magnetic resistance (maximum magnetic permeability) and select the circuit with the smallest path. Due to the higher magnetic resistance of air compared to silicon steel, magnetic field lines will preferentially travel inside the silicon steel sheet, as shown in Figure 3. Since magnetic field lines always choose the minimum loop path, the magnetic field lines at silicon steel sheet 2 will require straightening. Silicon steel sheet 1 and silicon steel sheet 2 are like being attracted together under the tension of magnetic field lines, as shown in Figure 4.

3. Transformer

The transformer has two sets of spiral coils. Primary spiral coil 1 and secondary spiral coil 2, as shown in Figure 5. When AC current V1 is applied to primary spiral coil 1, according to Ampere's law, primary spiral coil 1 generates magnetic induction. The magnetic induction wire passes through the iron core and passes through secondary coil 2. According to Faraday's law of electromagnetic induction, secondary spiral coil 2 generates induced electromotive force V2. The ratio of turns of two sets of spiral coils is equal to the voltage ratio.

4. Variable reluctance resolver

The schematic diagram of the working principle of the reluctance type rotary transformer is shown in Figure 7.

The speed sensor "magneto resistive rotary transformer" adopts a single-phase excitation winding and two-phase output winding fixed in the stator slot. 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 regular cosine relationship, as shown in Figure 8.