Confused about Magnetic Polarity Direction in a Rotating Magnetic Field

Thread Starter


Joined Jun 28, 2018
I'm learning about rotating magnetic fields that one would find in an induction motor. I believe I understand for the most part how the process works. However, I'm kind of stuck on one part. I'm confused as to which way (from N to S) the magnetic field points. I feel like the conventions used to tell us the direction of the magnetic field are a little bit arbitrary or ambiguous. Referring to the first image (a page from one of my textbooks) below, I've drawn a couple of right hand rules below the 3-phase sine graph.

The right hand on the left is the one I'm most familiar with and I know it as Maxwell's Right Hand Corkscrew Rule - point your thumb in the direction of the current and the direction of the magnetic field circling the current will be in the direction that your fingers curl. I've actually never liked this rule because the convention is to have the lines of a magnetic field leave the North pole and enter the South pole. But if the the field is circular around the current, where do the North/South poles end or begin? I've read that this type of circular magnetic field doesn't have a definitive North and South pole, but I've seen it time and time again in examples where a North and South pole are designated and it always confuses me.

The second right hand drawing on the right is actually from another textbook and I've included the page from that textbook below the first image. It confirms Maxwell's Right Hand Corkscrew rule but it also says that if you have a coil of wire with current running through it, point your fingers in the direction of the current and your thumb will now indicate the direction of the magnetic field from North to South.

Okay, so far this generally makes sense, and I've drawn the second hand showing the fingers of the hand in the direction of the current instead of the coil of the second image.

But this is where I start to get confused. Referring back to the first page below the process of how a rotating magnetic field is described. Now I'm only focusing on current A at instant one. This corresponds to only the very left of the 3-phase sine wave graph and Figure 8a on the left below the graph. Now the corresponding text of the first image says that positive currents are flowing from line to neutral and you can see the direction of the current travelling through the stator in figure 8a.

I've always assumed that when current A is at a positive max (at instant 1) that that would correspond to the North pole of the stator pole pair. And it seems like this direction of polarity would also agree with the two right hand rules I've drawn below. But if you look at figure 8a, it appears that the bottom pole is designated as the North pole and the top the South. Do I have something wrong or is there an error in one of the two textbooks? I feel like the first page is wrong. The big arrow in figure 8a should be pointing the other way.