Field weakening: Achieving higher speeds than the battery voltage would normally allow
Each permanent magnet motor has a "natural" maximum speed, determined by the battery voltage. Field weakening, often referred to by its English name, allows you to exceed this limit without changing the battery or the motor. This is a powerful setting, popular in racing, but it comes with a cost and risks that need to be understood.
The essentials
- A motor with permanent magnets generates a voltage as it rotates, and this voltage eventually matches the battery voltage, which limits the motor's speed.
- Demagnetization involves injecting a current that weakens the effect of the magnets, which pushes this limit higher.
- This current does not produce torque in a surface-mounted permanent magnet motor; instead, it heats up the motor and controller, and drains the battery more quickly.
- In the event of a sudden power loss at high speed during regenerative braking, the voltage generated by the motor can exceed the controller's maximum voltage rating.
Why does a motor reach its maximum speed and stop accelerating?
A permanent magnet motor also functions as a generator: when rotating, it produces a voltage, known as the back electromotive force, which is proportional to its speed. To supply current to the motor, the controller must apply a voltage that is higher than the voltage it produces.
As the speed increases, the voltage produced by the motor approaches the voltage of the battery. The controller can no longer push current, and therefore torque: the speed reaches a limit. This limit is calculated based on the motor's speed constant, Kv, and the battery voltage. See our guide on Kv, Kt, and motor poles.
The principle of flux removal
In vector control, the motor current is decomposed into two components: the q-current, which produces torque, and the d-current, which is aligned with the magnets. Normally, the d-current is kept at zero.
To induce demagnetization, the controller injects a negative current. This creates a field that opposes the magnetic field, which reduces the voltage that the motor produces at a given speed. The controller can then reintroduce current q, and therefore torque, allowing the motor to exceed its previous performance limit.
The term is misleading: the magnets are not weakened. It is their effect, as perceived by the windings, that is being compensated for, as long as the current 'd' is flowing.
What it costs
- Current without torque: In a surface-mounted permanent magnet motor, the "d" current does not generate torque. Instead, it adds to the total current, which increases the heat generated in the motor and the controller.
- Regarding performance: At high speeds, especially when operating in "no load" mode, an increasing amount of energy is lost as heat. This results in reduced range.
- Shared Current Limit: The total motor current remains capped. The more current is allocated to regenerative braking, the less is available for torque.
Internal permanent magnet (IPM) motors handle demagnetization better because the current also contributes to the torque. This is one of the reasons for their success in the automotive industry: see our guide on IPM and SPM motors.
The primary risk: sudden power loss at high speed
During regenerative braking, the motor spins faster than its natural speed: the voltage it would produce without the regenerative current exceeds the battery voltage. If the controller suddenly cuts power (due to a fault, protection, or power loss), the regenerative current disappears, and this voltage suddenly appears across the motor's terminals.
In this situation, the motor acts like a generator, sending current back to the battery through the controller's bridge. This can result in abrupt braking and a voltage that may exceed the limits of the transistors or the battery. That's why we maintain a safety margin in voltage, and why the current limiting is carefully adjusted.
Workshop tip
The demagnetization process should be adjusted in small increments, carefully monitoring the motor and controller temperatures, as well as the battery voltage. This procedure is best performed in a controlled environment, as a loss of traction or sudden braking at high speed can have serious consequences.
Desoldering or excessive voltage?
To go faster, we have two options: either derate the motor, or increase the battery voltage. Increasing the voltage raises the power ceiling without adding extra current, so without overheating or loss of efficiency, but it requires a battery, a controller, and often a BMS that are compatible. Derating, on the other hand, only requires an adjustment, but it comes at the cost of heat and reduced range. On a racing machine, we often combine the two. For batteries, see our electric scooter batteries.
Frequently Asked Questions
By how much does defluxing increase the speed?
The performance gain depends on the motor, and the voltage and current you are willing to use. The improvement is real, but it comes at the cost of increased heat; it's best measured on the actual machine by monitoring temperatures, rather than making assumptions beforehand.
Does flux removal damage the magnets?
Within reasonable limits, no: the opposing field is temporary. However, a very strong current flowing through a motor that is already very hot can contribute to demagnetizing the magnets. This is why it is important to monitor the motor's temperature.
Do all controllers support regenerative braking?
No. It requires vector control and a dedicated function for that purpose. Adjustable controllers like the VESC offer this, with a setting for the maximum current draw.