Every AC machine, be it synchronous or induction, uses a Rotating magnetic field(RMF). The rotating magnetic field is a resultant magnetic field, produced by supplied current passing via windings, this windings and current produce RMF.
In this Article, i will explain to you in detail about the Rotating magnetic field, and its production, such as DC RMF, 2-Phase RMF, and 3-Phase RMF, etc. Before starting this, we will discuss the variation in the number of poles in short.
In an actual motor there can be 2 to14 poles. So for a 60 Hertz power supply magnetic field will rotate at 75.36 rad/sec. Rotating magnetic field can make the rotor rotate for induction as well as a synchronous motor. Speed of rotation of the rotor is a main function of synchronous speed. The Speed of rotation of a magnetic field is known as synchronous speed. So it is quite important to select a motor with a suitable number of poles, which will match rotational speed requirements.
Now i am going to explain to you about production of RMF, first is DC RMF.
To Switch the coil ON and OFF by changing their current direction to generate RMF, this production was done by Walter Bailey, he used DC currents and this arrangement had electromagnets and commutator rings as shown in the Fig:2 below.
When the DC current passes, it creates a magnetic field with a specific orientation as shown in Fig:3A. You can easily deduce this orientation by right hand thumb rule. If you add a second electromagnet which is oppositely wound, magnetic field lines get connected between the opposite poles(Fig:3B left side image). Again I add one more pair diagonally opposite to the first pair, it will produce a resultant magnetic field line, you can see in the Fig: 3B at right side of image.
To make the RMF rotating lets de-energized the first pair only(Fig:4 left side image). Observe the RMF position denoted by arrow. After that, here I will try to energize both pairs again, and reverse the current through one coil as shown(Fig:4 - right side image), the same thing repeats as expected, their resultant magnetic fields have changed it’s position and are seen to rotate at an angle.
Now considering all four cases. In each case, the field lines have the same magnitude but different angles. The resultant magnetic field rotates in a step size of 45 degrees(Fig:5). This design shows early Rotating Magnetic Field production techniques. Now lets moving to the 2-Phase RMF.
One of the genius minds, Nikola Tesla came with his own idea of 2-phase RMF. He used an AC generator, designed by himself, to produce alternating currents(AC) and supplied to his design. It was a clever modification of Walter Bailey's RMF. In this modifications were that he removed the commutators which were notoriously tedious to operate, and supplied alternating currents to 2-coils with an angular difference of 90 degrees between them. This means that the field produced by one coil is shifted in position in reference to the other. Now I will explain to you how it works in the next section.
Let’s consider the starting point, the current from coil A is a small positive value, whereas the current from coil B is a larger negative value(Fig:7A). Individual magnetic fields produced by both the coils separately as shown here(Fig:7B). If we add both fields together we will get this resultant magnetic field.
Did you know what happens as the currents vary, we will understand here in simple words. In this instance, coil A carries a positive current of the highest magnitude, and the current in coil B is zero. At this moment, the magnetic field produced by coil A is the only field in operation(Fig:8A). Finally, both the coil currents are positive and are at equal magnitude creating a resultant field as shown in(Fig:8B).
Here you can see Nikola Tesla’s 2 phase machine design produces a rotating magnetic field(Fig:9). With no rotating parts, this machine produces RMF.
Later on, Nikola Tesla’s 2 phase RMF was increasingly popular due to the practical viability of his designs and the efficiency of the AC polyphase generators needed for these motors. Now let me explain to you about 3-phase RMF production.
You all know that Nikola Tesla’s 2-phase RMF was more popular, but because some drawbacks were faced such as supplying alternating currents to 2-coils with an angular difference of 90 degrees between them and the currents that flow in the phases got displaced from each other by 90º. That’s why it was replaced with a 3-phase RMF designed by Mikhail Dobrovolski(Fig: 10A).
Now we will discuss how those drawbacks were overcome? In 3-Phase RMF there is 3 Phase supply to three phase winding. A 3-phase current supply will vary with the time. Now we need to find out how the resultant magnetic field varies due to the current variation.
Let’s freeze at this instant. The magnetic fields produced by the individual coils are illustrated here. This can be easily deduced using the thumb rule(Fig:11A). Now, let’s combine all these magnetic field lines together to get the resultant magnetic field. In short, this is the shape of the RMF at this instant as shown in the Fig:11B below.
Now let’s vary the currents for a small time interval and freeze the scene. Let’s trace the resultant path again. Remember that the field density is higher for conductors with larger currents. Comparing both instances, the resultant field has clearly rotated. Calculating for all other current instances, the magnetic field is seen to rotate one revolution for one cycle of current as shown in the Fig:12B. That’s how 3 phase RMF actually rotates.
So, to round up our discussion on RMF, can you guess who is the winner of RMF’s system and why? You guessed right, 3-phase RMF is the winner of the RMF system. But to know the reason why it is, you can check the FEA simulation below.
Let’s use the modern technique of finite element analysis(FEA) to settle the vote. As you can see in the Fig:13 below the 2-Phase arrangement that gives a dip oscillation in the rotating magnetic field and in the 3 phase design these oscillations are effectively reduced.
This article concludes the journey through the history of rotating magnetic fields and their production by great inventors. It is amazing to tell you that RMF has some properties, which we have mentioned in the next part of this article. I wanted you to go through this, because when you learn RMF you should know the properties of RMF.
I hope you understood and enjoyed this explanation of RMF and its production.