The paper proposes a nonlinear adaptive backstepping speed controller which can be used in both the constant-torque region and the field-weakening region for an interior-permanent-magnet synchronous motor. By using the proposed method, the adjustable speed range can be extended to 1.6 times that of the base speed. In addition, the d-axis current is suitably adjusted to increase the output torque of the interior-permanent-magnet synchronous motor. A DSP-based fully digital speed-control system is implemented. The DSP is used to execute the current and the nonlinear speed-loop control algorithms. The implemented system has satisfactory performance, including good transient responses, good load-disturbance-rejection responses, and good tracking ability. Several experimental results validate the theoretical analysis.