Field weakening and top speed
How a motor keeps spinning faster once it has run out of voltage.
- Explain field weakening and top speed at engineering level
- Read the numbers that define the behaviour
- Apply the idea to a real drive system
Overview
Every motor reaches a speed where its back-EMF equals the available DC-link voltage. Beyond that point the inverter cannot push any more current in unless it changes strategy — which is exactly what field weakening does.
In depth
Back-EMF rises linearly with rotor speed. Once it reaches the inverter's available voltage, no further current can flow without intervention.
Field weakening deliberately injects a negative Id current, which opposes the permanent magnet's own field. This reduces the effective flux linkage and, with it, the back-EMF at any given speed — buying headroom for more speed at the cost of some torque and added copper loss.
- Maximum, current-limitedAvailable torqueFalls with speed
- Rises with speedAvailable powerRoughly constant
- Near zeroId commandNegative
- BestEfficiencyReduced
The flattening of the power curve and drop in torque you feel at motorway speed in an EV is field weakening doing its job, not the motor running out of ability.
Key takeaways
- Base speed is where back-EMF first reaches the DC-link voltage limit.
- Field weakening trades torque and efficiency for extended top speed.
- The characteristic constant-power region above base speed comes directly from this trade-off.
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