So despite other types of 3-phase motors with magnets like Brushless DC or Permanent magnet synchronous motors, there is no physical magnet used in the stator of ACIMs, and therefore the stator field intensity is adjustable by controlling the amount of current we inject into the stator (known as magnetizing current), and this phenomenon, gives an extra degree of freedom in control of AC induction motors with respect to permanent magnet counter parts in which the field intensity (flux) is constant and depends on the strength of the magnets used.
There are two main types of AC induction motors based on the formation of their rotor :
1. Squirrel cage type rotor
2. AC induction motors with wound type rotor
The first category of the AC motors however, are more popular among the industry as they are more cost effective and reliable.
AC induction motors are normally controlled with devices called “Variable Frequency Drives” known as VFDs, in which they apply the rotating field on the stator in a controlled manner so the rotor can follow and reach the desired mechanical speed.
There are lots of ways that an AC induction motor controllers can fix the speed of the rotor on desired speed, but the most advanced and reliable ones are normally coming from the Field Oriented Control or Vector Control enabled models. In Field Oriented Control (FOC) enabled versions, the Torque and the stator field ( flux ) are controlled separately, and this gives a huge advantage over normal VFDs as they offer much more reliable and robust control, specially in applications that the mechanical load on the shaft of the motor can vary time to time.
SOLO is using FOC for controlling AC induction motors with a methodology that can be seen below in the diagram. In simple words, SOLO tries to control the speed of ACIM motor using two different control parameters:
1. Controlling speed with regulation of Torque using quadrature current (Iq)
2. Regulation of magnetizing current to control the intensity of stator’s field (flux) using direct current (Id)
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