PURPOSE: To shorten the cooling time of a refrigerator easily by making the number of revolutions of a motor variable through connecting a three-phase power supply with a synchronous motor for very low temperature refrigerator via three-phase frequency converter and phase-number conversion transformer.

CONSTITUTION: When a frequency setter 15 is connected with a frequency converter 13 connected with a three-phase power supply 12, the three-phase AC of said power supply 12 is converted into that with a desired frequency and a phase delayed by 120° through said frequency converter 13. The converted three-phase AC is inputted to a phase-number conversion transformer 14 with a Scott connection to generate two single phase powers with phase deviation of 90° each other. One sides of said two single phase powers are connected in common with each other to make a three-phase AC with a phase delayed by 90° to drive a synchronous motor for a very low temperature refrigerator 10. Thus, it is possible to make the number of revolutions of said synchronous motor 11 easily variable and to shorten the cooling time of said very low temperature refrigerator.