Question

In an electric motor, what physical phenomenon occurs when the motor speed exceeds the base speed, requiring a negative d-axis current to reduce magnetic flux?

Accepted Answer

The physical phenomenon that occurs when a motor exceeds its base speed is called field weakening. Every electric motor generates a back electromotive force, which is a voltage produced by the motor acting as a generator as it rotates. This back electromotive force opposes the supply voltage provided by the motor controller. As the motor speed increases, the back electromotive force also increases until it approaches the limit of the supply voltage, at which point the motor cannot spin any faster because there is no remaining voltage available to increase the current or torque. To exceed this base speed, the magnetic flux in the air gap must be reduced to lower the back electromotive force, allowing the motor to spin faster with the same supply voltage. Field weakening is achieved by injecting a negative current into the d-axis of the motor. The d-axis current creates a magnetic field that opposes the magnetic field generated by the permanent magnets or the field windings. This magnetic opposition cancels out a portion of the total flux, effectively weakening the motor&#x27;s internal magnetic strength. While this reduction in flux allows for higher rotational speeds, it also reduces the torque capability of the motor, meaning the motor can spin faster but with less twisting force.

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In an electric motor, what physical phenomenon occurs when the motor speed exceeds the base speed, requiring a negative d-axis current to reduce magnetic flux?