• Lesson Three: How the motor spins

  How the motor spins

  Next, it will be explained what makes the motor rotate for a three phase AC induction motor. In Figure 9 the stator slots are illustrated by the small circles and the black lines illustrates the coils in one phase of the stator winding.
   
  
  
   Figure 9. Stator windings and magnetic fields
   
  The first phase winding is divided into two coils using eight slots each so a total of sixteen. For each coil four are used for going in, marked U1 and four for coming out marked U2. Notice that the number of slots in the stator will always be a multiple of 6 because the 3-phase coils will always use one slot going in and one coming out.
  Now when voltage is connected a current starts running through the coils and this will create a magnetic field around the coil in the slot. The magnetic field is shown as green and blue striped lines in the picture above.
  Applying the right hand rule that was told about earlier will show that the direction of the magnetic field around the slots where the phase goes in will be clockwise and around the slots where it comes out it will be counter clockwise. This is how an electromagnet with a north and South Pole in the induction motor is created. As this motor only create one north and one South Pole it’s concluded that this is a two pole motor.
  The stator core and especially the part between the slots usually referred to as the stator teeth will act as the core of the electromagnet and help transferring the magnetic field onto the rotor. The stator core is used for two reasons. The first reason is to keep the winding coils in place. The second reason is to help conduct the magnetic field. Now, one phase only is not going to make a motor so the other two phases has to be added. Add the same amount of coils for phase 2 and phase 3 which are indicated with the colours green and red.
   
   
  
   
  Figure 10. Phase coils and magnetic fields
   
  These two phase coils have their coil ends marked V1-V2 and W1-W2. The coloured dots indicates the current in each phase. As the phases are fed with AC the currents direction varies all the time in a sinusoidal curve pattern.
  Now as the current changes direction all the time its power to create the magnetic field will vary according to the sinusoidal curve pattern.
  That means if phase 1 had the highest current value it will soon change direction and drop. Next in line will be phase 3 that reaches its peak current and the magnetic field will move onto phase 3 coils and then later to the phase 2 coils. This means that the magnetic field will start to rotate counter clock-vice in the stator.
  Switching the phases that are coming from the electrical network so that phase 2 becomes high after phase 1, the magnetic field will rotate the other way. Each phase will have its own coils and depending on the size of the motor the amount of slots used for one phase will vary. As the diameter of the stator core increases the number of stator slots also increases. The phase coils will then be repeated to fill all slots to meet the desired pole number.
  So with the use of AC we can get a rotating magnetic field in the stator. As we now have a rotating magnetic field projected onto the rotor from the stator. The rotor bars are currently standing still but because of the moving magnetic field, a current will be induced in the rotor bars. Now that we have a current in the rotor bars and the magnetic field from the stator, the rotor bars will be affected by Lorentz force that was explained in chapter 4.3 and start to spin in the same direction as the rotating magnetic field.
  The motor is called an asynchronous motor because the rotor doesn’t spin with the same speed as the rotating magnetic field. How much slower the rotor rotates compared to the rotating field is called the slip. With no load on the motor the slip is close to zero. However as we increase the load on the motor, the slip also increases and If the slip gets too big the motor stalls.
  
  It is quite complicate to explain this in text, but it was a lot easier to explain in PowerPoint since it was possible to use animated pictures and videos to demonstrate the rotating magnetic field and the spinning rotor. (LearnEngineering)
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