Field weakening
How motors exceed their natural base speed by deliberately fighting their own magnets.
You'll learn
- Explain field weakening at engineering level
- Read the numbers that define the behaviour
- Apply the idea to a real drive unit
Overview
Field weakening is the control technique that lets a PMSM spin well beyond the speed its magnets would naturally allow, at the cost of some torque and efficiency.
In depth
Recall that back-EMF rises with speed and eventually limits the current — and therefore torque — the inverter can force through the windings. Field weakening deliberately injects a negative Id current, which creates a stator flux component that opposes and reduces the net air-gap flux from the magnets. Lower net flux means lower back-EMF at a given speed, freeing up voltage headroom to keep useful torque current, Iq, flowing.
| Region | Id | Iq | Behaviour |
|---|---|---|---|
| Below base speed | ≈0 | Up to max | Constant torque |
| Above base speed | Negative, increasing | Falling | Constant power, extended speed range |
Key takeaways
Remember this
- Field weakening injects negative Id to reduce net air-gap flux and back-EMF.
- This frees voltage headroom to sustain torque-producing Iq at higher speed.
- It always costs some efficiency and requires reliable control to avoid overvoltage faults.
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