Rotating transformers are mainly classified into three categories based on the functional relationship between output voltage and rotor angle.

1. Sine-cosine rotating transformer - The functional relationship between its output voltage and rotor angle is a sine or cosine function.

2. Linear rotating transformer - Its output voltage and rotor angle have a linear functional relationship. Linear rotating transformers are further divided into hidden-pole and salient-pole types according to the rotor structure.

3. Proportional rotating transformer - Its output voltage is proportional to the angle.

Multipolar rotating transformers are similar to multipolar self-synchronous motors, the main difference lies only in the number of phases of the windings. The accuracy of multipolar products is an order of magnitude higher than that of bipolar products.

A dual-channel rotating transformer combines two rotating transformers with different numbers of pole pairs. Usually, the one with fewer pole pairs is called the coarse machine, and the one with more pole pairs is called the fine machine. There are two structural forms: common magnetic circuit and separate magnetic circuit. The latter mechanically combines the coarse and fine machines into one unit, each winding having a separate core and separate magnetic circuits. The former has the coarse and fine machine windings embedded in the core simultaneously, with independent windings and a shared magnetic circuit. The above two rotating transformers form an electrically variable-speed dual-channel rotating transformer system. This is different from a mechanically variable-speed dual-channel rotating transformer system composed of two identical and independent rotating transformers and a reducer. Because the mechanically variable-speed dual-channel system cannot meet the requirements in the synchronous follow-up system, an electrically variable-speed dual-channel system must be used. This system not only improves the accuracy to the second level, but also has a simple and reliable structure.

A reluctance rotating transformer is a special form of multipolar rotating transformer. It uses the reluctance principle to achieve electrical signal conversion. The stator core has large and small teeth, with small teeth evenly distributed at the ends of the large teeth. A single-phase excitation winding and two-phase output windings are embedded in the large slots on the stator. The rotor core is composed of stacked laminations with evenly distributed small teeth, and the number of teeth is the number of pole pairs. When the excitation winding is energized, due to the change in air gap permeability with the rotor angle, the output voltage of the output winding changes periodically with the number of rotor teeth, playing the role of a multipolar form. It has a simple structure, small size, high accuracy, and no contact, greatly improving the reliability of the system, with an accuracy of seconds.

Working principle

The stator windings D1-D2 are connected to an AC power supply for excitation, and the rotor windings Z1-Z2 are connected to the load ZL. When the main shaft drives the rotor to rotate by an angle θ, the induced voltages generated in the various rotor windings are respectively:

In the formula, k is the effective turns ratio (transformation ratio) of one phase stator and rotor windings. If the rotor winding is used for excitation and the stator winding is used for output, the expression is the same (only the value of k is different). By using different wiring methods or different winding structures, output voltages with different functional relationships with the angle can be obtained. Different structures can also be used to produce special-purpose rotating transformers with ballistic functions, circular functions, and sawtooth wave functions.

Two identical sine-cosine rotating transformers can be used to form a single-channel angle measurement system. One rotating transformer is the transmitter, and the other is the control transformer. The transmitter is excited by an AC power supply. The accuracy of the rotating transformer is 6', and the accuracy of the single-channel system is no less than 6'. To improve the control accuracy of the system, a dual-channel angle measurement system (Figure 2) can be used. Four rotating transformers with the same structure are used. Two XZ1 and XZ2 form a coarse channel angle measurement system, and the other two XZ3 and XZ4 form a fine channel angle measurement system. XZ1 and XZ3, XZ2 and XZ4 are connected through a speed increaser with a speed-up ratio of i (i=15~30). When the main shaft drives the coarse channel XZ1 to rotate by an angle θ1, the fine channel XZ3 will rotate by an angle iθ1. XZ2 is coaxial with the load, and when its angle is θ2, the angle of XZ4 is iθ2. The output voltage of the coarse channel is Uc1=kUr sinδ, and the output voltage of the fine channel XZ4 is Uc2=kUrsiniδ, where δ=θ1-θ2. The output voltages of the two are processed by a coarse-fine converter and then amplified to drive the load. A dual-channel servo system can be formed using a dual-channel angle measurement system. When the error angle δ is small, the fine channel signal is used for control; when the error angle δ is large, the coarse channel signal is used for control. Therefore, the control accuracy of the system can reach 3″~7″. In order to reduce the nonlinear error caused by the gear backlash of the reducer, an electrically variable-speed dual-channel angle measurement system can be used, that is, a multipolar rotating transformer. It installs a single-pole and a multi-pole rotating transformer in one body and shares one shaft. A single-pole transformer is used to form a coarse channel system, and a multi-pole rotating transformer is used to form a fine channel system. This can improve accuracy and simplify the structure.