 In this video, we're going to be discussing how frequency is affected by RPM or rotations per minute and the number of poles in an alternator. Now, before we get into it, if you don't understand how a sine wave is formed in an alternator, you need to go back and watch the video on sine wave analysis and also watch the video on how a sine wave is formed, and there's a big description of how this sine wave comes to be. Now, when we're talking about frequency, it's important that we are understanding that frequency is measured in cycles per second. So what we have here is a cycle, and a cycle is from where a wave form begins to where it starts to repeat itself, and if it kept on going to go up and it would go down and away we go and we get another wave form happening. So when we're talking about frequency, we're talking about how many of these complete cycles occur in one second because frequency is measured in cycles per second or in hertz. Now, two things affect the frequency in an alternator, and first off, we're going to discuss the rotational speed. Now as we look at this, here's our two-pole alternator. I've got a south pole, and I've got a north pole. I've got an armature, which is basically the conductor that goes through there. And as I have the north pole set up in the south pole, I'm going to have invisible lines of flux, so magnetic lines of flux are going to be flowing from north to south. Now as this conductor starts to rotate around, it will cut the conductors and create a wave form. So for the first 180 degrees, as A goes up and rotates all the way here, it starts at this point, goes up, goes down. Then as A kind of shifts around, it goes the opposite direction underneath and back up. It goes up in the opposite direction and down again. So what we see is in a full 180 degrees of mechanical rotation, we create one complete sine wave. Now if I timed that and had this happen, so let's say that this side here, A goes all the way around in 360 degrees, mechanical rotation, it does that in one second. We create one sine wave in one second, therefore we get one hertz or one cycle per second. So you can see as we start to speed this up, let's say we sped this up and we did this, it spun 10 times in one second. So if this side A spun 360 degrees and it did that 10 times in one second, we would get 10 of these wave forms happening and then we would say we have 10 cycles per second or 10 hertz. Now the thing with alternators is that they are not measured in seconds, they're measured in minutes, rotations per minute. So what we need to do is we need to convert the rotations per minute that will come on the alternator into rotations per second because remember we need to figure out what it is per second, not per minute. Now if I have an alternator spinning at 1800 rpm, like I do right here, we need to convert rpm rotations per minute to rotations per second. Well last time I checked there are 60 seconds in a minute, so all we have to do is go 1800 divided by 60 means that this alternator, if it was spinning at 1800 rpm is spinning at a clip rate of 30 rotations per second. If it's a two pole alternator like this, it is going to create 30 of these sine waves because this is going to spin 30 times per second, 30 sine waves you're going to get 30 cycles per second or 30 hertz. The second factor that affects frequency is the number of poles. Now if you look here, I got my two pole alternator and we know that we went from north to south, from south to north and it created a full sine wave. What happens here is if we look at the four pole alternator, well we end up doing that in half the amount of distance. Over here it took a full 360 degrees to go from north to south, south to north. If I look here, if I start here, I can go north to south, south to north and I do that in 180 degrees. So in 180 degrees with more poles, I end up creating one full sine wave. If I go a full 360 degree rotation, I go north to south, south to north, north to south, south to north, I create two full sine waves. So here we go, if I go north to south, that would be this hump on the sine wave, south to north would be the bottom side, north to south, I go up and over and south to north again underneath and I get two full sine waves. So with a four pole alternator, 360 mechanical revolutions gives me two complete sine waves. So we can start seeing that if we add more poles in here, we end up with more sine waves which gives us more frequency. As is often the case when we discuss relationships and characteristics, there is always going to be a formula. So we can take everything we've learned, we now know that if we increase the speed of an alternator, it's directly proportional to the frequency. If the speed goes up, we get more sine waves created in the same amount of time. If we increase the number of poles, it's directly proportional. So therefore we're going to turn this all into a formula. Here you have it. This is our formula for frequency F, which is frequency in hertz or cycles per second. P is the number of poles. We divide that by two because there's always two poles. There's always going to be a north pole and a south pole. You know that if you have a two pole alternator, you're going to create one full sine wave. So that's why we divide that by two. Then we have N, which is rotations per minute, or it's the rotational speed. And again, we have to divide that by 60 because we want to take that RPM and kick it down to RPS or rotations per second. So P over two times N over 60 gives us frequency, or we can combine this whole thing together because we also like to make life easier in ourselves and it looks like this. Frequency is equal to P times N over 120. Two times 60 gives you 120. P times N gives you PN. And there you have it. That is the formula for frequency for an alternator. Poles are directly proportional. Speed is directly proportional. And then we have this constant of 120 below. And that is how the rotational speed and the number of poles affect the frequency of an alternator.