Brief analysis of the basic working principle of the resistive rotary transformer

Resolvers are often used in position and speed sensors in electric vehicles. For example, the position sensors of drive motors and generators, the position sensors of electric power steering motors, and the angle measurement of gas valves all use resolvers. However, resolvers come in a variety of forms, and reluctance resolvers are widely used in electric vehicles because of their good processability, large relative displacement, high reliability, and low cost.

Reluctance rotary transformer: One-phase excitation winding and two-phase output winding are fixed in the stator slots. The rotor pole shape is specially designed to make the air gap in a sinusoidal shape. When the rotor rotates, the change in the air gap makes the two-phase output winding signals in a sine-cosine relationship. After reading this, are you confused?

Below we will first review the most basic theories of "electricity generating magnetism" and "magnetism generating electricity", and then introduce the basic working principle of the reluctance rotating transformer.

1. Basic relationship between magnetism and electricity

1). Ampere's law ("electromagnetism"): The spiral tube carries electric current, which generates a magnetic field.

Figure 1.

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

Figure 2.

2. Magnetoresistance effect

"When the carriers of metals or semiconductors move in a magnetic field, the magnetoresistance effect is produced due to the Lorentz force generated by the change of the electromagnetic field." After reading it, I was a little confused again.

To better understand it, we do 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 placed in the U-shaped opening. When current is passed through the spiral wire, the silicon steel 2 and the silicon steel sheet 1 are attracted together, as shown in Figure 4. This is the result of the magnetoresistance effect.

Magnetic lines of force (i.e. magnetic induction lines) always take the path with the smallest magnetic resistance (maximum magnetic permeability) and choose the loop with the smallest path. Since the magnetic resistance of air is greater than that of silicon steel, the magnetic lines of force will preferentially take the inside of the silicon steel sheet, as shown in Figure 3. Since the magnetic lines of force always choose the path with the smallest loop, the magnetic lines of force at silicon steel sheet 2 will require straightening, and silicon steel sheets 1 and 2 are like being attracted together under the pulling force of the magnetic lines of force, as shown in Figure 4.

Figure 3

Figure 4

3. Transformer

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

Figure 5.

Figure 6.

4. Reluctance rotary transformer

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

The speed sensor "reluctance rotary transformer" uses a one-phase excitation winding and a two-phase output winding fixed in the stator slot. The rotor pole shape is specially designed to make the air gap in a sine shape. When the rotor rotates, the change in the air gap makes the two-phase output winding signals have a sine-cosine relationship, as shown in Figure 8.

Figure 8: Schematic diagram of a resolver speed sensor

Figure 9. Calculation formula for resolver speed sensor

Figure 10. Speed ​​sensor rotation signal output waveform