what is field weakning ?

[D@F]

hi all, i want to understand what is fiel weakning.
evry time i'm doing a search, i just found some equation , that doesn't realy explain to me with "word" or "draw" what it is ....

i made some search for my motor ( syncronous with powered rotor) i see that the field from the rotor have a big impact on the "time delay" between the curent and voltage . but i'm not sure if this probleme is the same when you use it as an alternator and as a motor .

[johu]

The "field" is something created by the rotor.
It either has

• Short circuit bars (asynchronous/induction motor) - in that case the field is proportional to stator current and speed difference of stator and rotor (slip). When the inverter runs out of voltage at higher speeds the stator current drops, thus the rotor current drops and thus the field is weakened intrinsically

• Permanent magnet - in that case the field has a constant magnitude. It can be weakened by using part of the stator magnetic field to oppose the permanent magnets field.

• A wound rotor - in that case the field is proportional to the current you feed into the rotor.

Why do we need field weakening in the first place? Well, when a rotor with a given field rotates inside the stator it generates a voltage in the stator - also called "back EMF". The voltage that your inverter puts into the stator must always be higher than the "back EMF". Otherwise no current flow is generated or current flows from the motor to the energy source (battery). Since your battery voltage is limited at some point you have to weaken the rotor magnetic field to be able to keep up.

No math used Does that make it clearer at all?

[D@F]

Yes i understand that.
But when i check all the graph attached i don’t realy understand what is the d and q axes, and so What is the Iq.

[nailgg]

Assume that the motor phase currents (a,b,c) are three vectors that are 120 degree apart from each other. After you apply the Clarke transformation on the three phase currents (a,b,c), you get Ialpha and Ibeta currents. Ialpha and Ibeta are two orthogonal vectors and their resultant is the same as the actual motor current vectors' resultant. But the Ialpha and Ibeta are rotating with the rotor flux speed, so you apply the Park transformation on Ialpha and Ibeta, and the output is Id and Iq. In the Park transformation, the reference frame is rotated with the rotor flux speed so that the vectors can be seen standstill w.r.t. the reference frame. In other words, Id and Iq are constant values in steady state. Id and Iq can be simply thought as the flux component (reactive power) and torque component (active power) of the motor, respectively.

In the field oriented control method, one sets the Id to the motors rated flux value. Id is there in order to create a magnetic field. The Id is (almost) linearly reduced to a value near zero beyond the base speed, that's what the field weakening is. The weaker the field is, the less back-emf motor produces, the more current your inverter can send to the motor.

Iq is simply the real power, it's the torque that the motor produces.

The slip control algorithm Johannes uses doesn't utilize the Id and Iq values in order to achieve the field weakening. He simply increases the phase voltage linearly up to 100% until the frequency hits fweak, then continues increasing the frequency by commanding slip, while keeping the full voltage. As you increase the frequency, the V/Hz ratio is reduced, so the field is weakened.

Here is a nice animation explaining the Clarke and Park transformations:

Source of the animation: https://www.switchcraft.org/learning/20 ... or-dummies

[D@F]

realy thanks for your answer.
i have now more information to understand.
so field weakening is made by sliping frequency over the omega B (on the fig5 in my previous post)
if i need more rpm at this torque, we enter in field weakening mode and then set frequency for somthing arround 1200rpm and then we have omegaC same torque at 660rpm ?

i'm wright ?

[nailgg]

Field weakening is made when you are spinning the motor higher than the base speed. It looks like you have a 8 pole 50 Hz motor so your base synchronous speed is 750 rpm. Normally, field weakening is achieved by increasing the frequency over omega C for your graph, not B. But on some particular applications, one might start weakening the field earlier, so it looks like this is the case for your graph.

Beyond the base speed, you reach the constant power region of the motor. Power is the product of torque and speed, so as your speed increases, the torque decreases since you are in the constant power region. So you cannot have the same torque at 1200 rpm with the one at 600 rpm (for this particular motor, of course). That's why the Iq curve starts declining after 750 rpm.

[D@F]

what do you think if i'm trying do field weakening by lowering the power on the rotor on my syncrounous motor?