creased. The speed of the conventional induction motor decreased to 1735 rpm at the load of 2.6 Nm. 2.6 Nm corresponds
to the 63% of the rated torque of 0.75 kW motor.
The same stators were used for both motor, while the aluminum bars and the short ring of the conventional motor were
replaced with HTS tapes in HTS motor. Rating of the conventional motor was 0.75 kW and torques of both motor were measured by using electrodynamometer.
To provide the same operating condition, air gap of both motors were 1 mm. Although the length of air gap, 1 mm, is quite
large for conventional motors, we gave the same air gap to both
motors for comparison of the characteristics.
Starting torque of the HTS motor reached more than two
times of that of the conventional motor as is shown in Fig. 7. This
large starting torque guarantees the fast acceleration of the HTS
induction motor during the starting. When the slip reaches about
0.7, developed torque decreases sharply in HTS motor. This is
because the resistance in the rotor circuit becomes smaller as
the speed builds up. As the speed approaches the synchronous
speed, the resistance in the rotor circuit vanishes completely and
the speed reaches synchronous speed.
Comparison of the mechanical power and slip curve near zero
slip is shown in Fig. 8. Up to 470 W, slip of the HTS motor was