Driving device, driving system, and method of driving electric motor

A driving device comprising:

• a voltage controller configured to cause an electric power converter to apply a drive voltage to an electric motor by controlling an operation of the electric power converter, the electric power converter being configured to convert input electric power into A/C electric power having desired voltage and frequency and configured to supply the converted electric power to the electric motor;
• a parameter setter configured to set at least one of a rotation speed of the electric motor and a parameter related to the rotation speed as speed information;
• and
• a phase adjuster configured to periodically calculate an index based on a current flowing in the electric motor and the speed information set in the parameter setter and to adjust a phase of the drive voltage applied to the electric motor by the voltage controller in such a manner that the calculated index becomes smaller every time the index is calculated, the index depending on both the current flowing in the electric motor and the rotation speed of the electric motor and being different from the phase of the drive voltage,
• the phase adjuster further configured to judge whether the phase of the drive voltage applied to the electric motor should be advanced or delayed based on the calculated index and to advance or delay the phase of the drive voltage by a predetermined phase shift amount based on a result of the judgement.