Motor driving device and vacuum pump

A motor driving device for a vacuum pump comprising:

• an inverter for driving a motor, the inverter having a plurality of switching elements;
• a first arithmetic section for calculating a rotational speed and a magnetic pole electrical angle of a motor rotor based on information about motor phase voltage and information about motor phase current;
• a current command setting section for setting a d-axis current command and a q-axis current command in a rotating coordinate dq system based on a difference between the rotational speed and a target rotational speed;
• a driving command generating section for generating a sinusoidal wave driving command based on the d-axis current command, the q-axis current command, the rotational speed and the magnetic pole electrical angle;
• and
• a PWM signal generating section for generating a PWM control signal for controlling on/off of the plurality of switching elements based on the sinusoidal wave driving command,
• wherein, at the time of pump startup, when the rotational speed has a positive value indicating a positive rotational state, the current command setting section sets the q-axis current command of acceleration driving, and when the rotational speed has a negative value indicating a reverse rotational state, the current command setting section sets the q-axis current command of deceleration driving.