Patent ID: 8803462

Claim:
A motor drive device that is used for a drive circuit for an electric motor, the drive circuit including a direct current power supply that supplies electric power to the electric motor and a switching element that switches on and off an electric power supply path between the electric motor and the direct current power supply, the motor drive device using the switching element to control supply of the electric power to the electric motor, the motor drive device comprising: a first contact point potential acquisition means that acquires a contact point potential between the electric motor and the switching element when the switching element is switched from on to off and the contact point potential changes in conjunction with a termination of the supply of the electric power to the electric motor; a second contact point potential acquisition means that acquires the contact point potential while the electric power is being supplied to the electric motor when the switching element has been switched on; a first determination means that determines, based on the contact point potential that has been acquired by the first contact point potential acquisition means, whether a mode of change in the contact point potential that is associated with the termination of the supply of the electric power to the electric motor is a mode when the electric motor and the switching element are normal or a mode when one of the electric motor and the switching element has failed; a second determination means that determines whether the contact point potential that has been acquired by the second contact point potential acquisition means is equal to or larger than a predetermined first threshold potential; and a failure diagnosis means that determines that the electric motor and the switching element are normal if it is determined by the first determination means that the mode of the change in the contact point potential is the mode when the electric motor and the switching element are normal, determines that the electric motor has failed if it is determined by the first determination means that the mode of the change in the contact point potential is the mode when one of the electric motor and the switching element has failed and if it is also determined by the second determination means that the contact point potential is equal to or larger than the predetermined first threshold potential, and determines that the switching element has failed if it is determined by the first determination means that the mode of the change in the contact point potential is the mode when one of the electric motor and the switching element has failed and if it is also determined by the second determination means that the contact point potential is less than the predetermined first threshold potential.