# What is the “field-weakening” operation?

I'm studying brushless and induction motors, in my studying material "field-weakening operation" and "field-weakening region" are often cited without an explanation. As far as I understood is a method to weaken the rotor flux by controllig stator currents according with the physical voltage and current constraints, but i don't understand what is the final goal.

• What does google tell you about these things - have you tried googling? – Andy aka Jul 11 '17 at 9:45
• Yes, i have found little stuff and pretty confusing. – Matteo Caruso Jul 11 '17 at 9:48
• Maybe provide a link to the more useful pages? I'm suggesting this because although Brian has made an answer that sounds perfectly plausible (and probably right), you might be better informed with an explanation on what you found to be confusing. – Andy aka Jul 11 '17 at 9:55
• shodhganga.inflibnet.ac.in/bitstream/10603/93562/12/… – Matteo Caruso Jul 11 '17 at 10:14
• Here's another document: theses.lib.vt.edu/theses/available/etd-031899-212402/… – Matteo Caruso Jul 11 '17 at 13:26

... I don't understand what is the final goal.

The final goal is to increase the operating speed of the motor without needing to increase the voltage or current required from the power supply. That is to provide increasing speed with constant power. Since mechanical power is speed multiplied by torque, constant power with increasing speed means reducing torque at increasing speed.

Most loads require the same torque or more torque to operate at higher speeds. However some of the torque used drive a vehicle is used to accelerate inertia and climb hills. Engine driven vehicles have gear boxes that are shifted to progressively higher gear ratios as the speed increases. That is constant power operation. At higher speeds, the rate of acceleration and the ability to climb hills is limited. With electrically driven vehicles, the use of gear shifting can be avoided if the motor can be operated at constant power in the upper part of the speed range. It might be possible to operate over the same speed range without sacrificing high-speed torque, but that would require expensive power capability that only be used a small percentage of the operating time.

Another use for a constant power drive is driving the center of a spool that is accumulating paper or some other material with a constant surface speed and tension. Since power is surface speed multiplied by tension, the required power remains constant with speed change. An empty spool has a small radius, so the shaft speed is high and the torque is low. As the material accumulates, the radius increases and the shaft speed is reduced to maintain constant surface speed.

Consider that the back-EMF is basically the product of motor speed and field strength. So as you weaken the field, the back EMF reduces, increasing the motor current and the power dissipated in the motor. The motor will attempt to run faster to restore the back EMF.

If it's lightly loaded, this results in a speed increase.

If it isn't, there is a danger of stalling, and destroying the motor.

• How do i weaken the field? Controlling the d-component of the current? – Matteo Caruso Jul 11 '17 at 10:05
• And in a PM brushless machine? – Matteo Caruso Jul 11 '17 at 10:16
• The "P" in "PM" means Permanent. That normally makes field weakening impractical. – Brian Drummond Jul 11 '17 at 10:36
• This is the core of the problem, becouse as far as i understood it is possible to "weaken" the field just injecting a negative d-component of the current so that produces a field opposite to the one of the permanent magnet, i think actually weakening only the airgap field. – Matteo Caruso Jul 11 '17 at 13:24
• @Matteo that's exactly right. – pericynthion Jul 11 '17 at 21:01