Permanent magnet motors exhibit a characteristic torque–speed envelope that defines the feasible operating region. At low speeds, current limits bound the available torque. Beyond the base speed, the motor becomes voltage‑limited and torque must decrease to maintain a constant power output.
Regions
- Constant torque region: Up to base speed, the stator current limit constrains torque. Torque remains approximately constant.
- Field‑weakening region: Above base speed, voltage saturation requires reducing the back‑EMF by injecting negative d‑current. Torque falls off roughly inversely with speed.
- Overspeed: Eventually the required voltage exceeds the inverter limit even with full field‑weakening. Operation here is discouraged.
Pitfall
Field‑weakening incurs additional copper and iron losses. Careful thermal analysis is required to avoid overheating during sustained overspeed operation.
Takeaways
- Understanding the torque–speed envelope is essential for sizing drives.
- Base speed marks the transition from current‑limited to voltage‑limited operation.
- Field‑weakening allows extended speed but reduces torque and efficiency.