Motor drive unit and electric power steering system

A motor drive unit that drives a motor including a three-phase motor supply terminal, the motor drive unit comprising:

• a case that is at least partially electrically conductive;
• a circuit board disposed inside the case;
• a power connector to which an external direct-current power source is connected;
• and
• a case connection portion connecting the circuit board and the case;
• wherein
• the power connector includes a positive terminal and a negative terminal, and includes a positive line portion connected to the positive terminal and a substrate positive connection portion of the circuit board, and a negative line portion connected to the negative terminal and a substrate negative connection portion of the circuit board;
• the circuit board includes a power circuit portion and a control circuit portion;
• the power circuit portion includes:
• a common mode filter connected to the substrate positive connection portion and the substrate negative connection portion;
• a normal mode filter connected to the common mode filter;
• and
• an inverter circuit connected to the normal mode filter;
• an inverter output terminal converted by the inverter circuit into a three-phase power supply to output the drive signal and the motor supply terminal are connected;
• the common mode filter includes:
• a first capacitor connected to the substrate positive connection portion;
• and
• a second capacitor connected in series with the first capacitor and connected to the substrate negative connection portion;
• the case connection portion includes:
• an electrically conductive surrounding portion that surrounds at least any one of at least a portion of the positive line portion and at least a portion of the negative line portion;
• and
• a case connection line connected to a point between the first capacitor and the second capacitor, and the electrically conductive surrounding portion of the case;
• and
• the case connection line and the electrically conductive surrounding portion are connected to each other.