Field weakening: Extension of the engine speed range
What is field weakening?
Field weakening is a technique that extends the operating speed range of a permanent magnet motor beyond its rated speed. This involves gradually reducing the effective magnetic field of the rotor—at the expense of available torque, additional speed is gained.
Why is this important?
Every permanent magnet motor has a natural speed limit, which is determined by its back EMF constant and the supply voltage. Above this speed, the generated back EMF corresponds to the bus voltage, and no further current can be fed into the motor.
Field weakening raises this limit and enables applications such as:
- In-wheel motors in electric vehicles that require both high torque at low speeds and high speeds
- Spindle drives that require a wide speed range without gear changes
- Fast traverse movements in automation
- Gearless direct drives where the mechanical speed range must be maximized
How does it work?
In a PMSM, the permanent magnets of the rotor generate a constant magnetic field. In normal FOC operation, the direct current component (Id) is kept at zero to maximize torque efficiency.
Field weakening specifically injects a negative Id component—a magnetic field that partially counteracts the rotor magnet. This reduces the effective flux, lowers the back EMF at a given speed, and allows the motor to continue accelerating.
Three parameters control the behavior:
- Starting speed — the speed at which field weakening begins
- Final speed — the speed at which maximum field weakening is achieved
- Percentage — the degree of field reduction
The reduction is applied linearly between the start and end speeds. Values above 25% can lead to instability.
Important: Field weakening is an expert feature. Incorrect settings can cause:
- Demagnetization of the motor (permanent damage)
- Overheating due to increased losses
- Dangerous overvoltage if the drive is switched off at high speed (back EMF can exceed the bus voltage)
The back EMF voltage must never exceed 80 V peak. Speed or position control is recommended — torque control with field weakening can lead to uncontrolled acceleration.
How does SOMANET implement this?
On SOMANET drives, field weakening is configured via the Torque Controller Object (0x2010, sub-indices 4–7) — either via OBLAC Drives or directly via the Object Dictionary. The function is available on all SOMANET platforms that support PMSM motors.
In view of the risks, Synapticon recommends conducting thorough tests with activated security functions (SLS, SMS) before putting the product into productive use.
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