*2.1. Structure*

A three-dimensional view of the proposed HTR-ASD is shown in Figure 2. It is composed of a homopolar-type rotor, a squirrel-cage rotor and a non-rotary shell with an excitation winding. The squirrel-cage core is formed by laminated silicon steel sheets and embedded with a squirrel cage whose middle part is composed of non-ferromagnetic material. The homopolar-type rotor is fabricated with high-strength solid steel. The shell and the excitation winding are the non-rotary parts of the machine.

**Figure 2.** Three-dimensional simplified view of an HTR-ASD.

As Figure 3 shows, the homopolar-type rotor is connected to the prime motor and the squirrel-cage rotor is connected to the load. When the HTR-ASD is working, the torque transmitted by the HTR-ASD is determined by the slip of the homopolar-type rotor and the squirrel-cage rotor, which can be expressed by:

$$s = \frac{n\_{HTR} - n\_{\rm SCR}}{n\_{HTR}} \tag{1}$$

where *nHTR* is the speed of the homopolar-type rotor and *nSCR* is the speed of the squirrelcage rotor. Therefore, when the load torque is certain, the slip can be changed by adjusting the dc excitation current, so that the speed can be adjusted.

**Figure 3.** The working platform of the HTR-ASD.

## *2.2. Operation Principle*

The excitation winding provides the flux for the HTR-ASD. The flux is closed through the shell, the air gap between the shell and the squirrel-cage rotor, the squirrel-cage rotor core, the air gap between the squirrel-cage and the HTR, and the HTR, as shown in Figure 4. Additionally, Figure 5 shows that the ac component of air gap flux density is generated due to the different air gap permeances corresponding to the rotor teeth and slots. When the HTR rotates, a synchronous rotating magnetic field is generated in space. The alternating component of the magnetic field generates a corresponding current in the squirrel-cage rotor. The current interacts with the rotating magnetic field in the air gap to generate electromagnetic torque.

**Figure 4.** The main flux path in no-load operation.

**Figure 5.** Air gap flux density distributions at different positions of the HTR-ASD.
