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Full Transcript:

Going to look at a topic now of when you connect a VFD to a motor, and you’re running your fan at various speeds the relationship with your horsepower and your torque based on the different speeds you’re operating.

So, we’re going to draw a graph here, and on the Y axis we’re going to have the percent of horsepower and percent of torque. So up here, we’ve got 100 percent, so we’re looking at 50 percent right here and down to zero percent right here. And then on the Xaxis, we’re going to be looking at the speed of the motor, but that speed is often referred to in frequency. In the United States, we have 60cycle frequency most commonly, so we’re going to look at a 60cycle frequency application. Now in a smaller motor, can occasionally run over that 60cycle frequency up to about 120 hertz (or cycles), and so down here we’ve got the speed as listed in frequency. So I’m going to write this as motor frequency, and out here on the far right, we’re going to go 120 Hz. That means in the middle, we’re targeting 60 Hz, and back here, we’re at 0 Hz at the origin.

So, the first graph we’re going to draw here is the one that most pertains to centrifugal fans. And the reason we’re going to start here is that centrifugal fans are variable torque machines. And what I mean when I say that is when you start the fan up, it starts in a resting phase. So you’ve got this heavy wheel on a shaft often, with bearings, and you’re starting that thing from rest, and you’re getting it going. That’s when the fan requires the most torque output from the motor. Because it’s got to get it started from rest to moving. But once you get a fan wheel rotating, that initial torque isn’t needed anymore because the fan is already rotating, so you don’t need as much push to keep it going. It’s free spinning on a shaft and bearings requires a lot less torque.

So what we care about more than torque is horsepower availability, because you always need a certain horsepower to drive this fan to be moving the type of air through the type of pressure that you’re trying to move it through, so we’re going to look at horsepower first. So up to 60 Hz, at 60 Hz, we are going to achieve 100 percent horsepower availability in the motor. But up until that point, we’re going to do a straight line from the origin up to 100 percent. So say you have a 100 horsepower motor. At 60 Hz, you have 100 horsepower available. And past 60 Hz, you continue to have 100 horsepower available. As high as the motor is able to run in frequency past 60 Hz, you have the full nameplate horsepower available. But notice at less than 60 Hz, you don’t have the full motor horsepower available that’s on the nameplate. And it is proportional to your speed. So at this 50 percent mark right here, we’re running at 30 Hz, and we have 50 percent of the motor horsepower available.

This is really important to understand. Because if you’re taking an 1800 RPM motor, and at 1800 RPM you have 100 horsepower available, but you now reduce that to 30 Hz, so you running at approximately 900 RPM, you only have 50 percent of the available horsepower, which means on 100 horsepower motor at 30 Hz, you would have 50 horsepower available. So this relationship is crucial to understand if you’re running a fan at less than the frequency that is available to go into the motor when it’s running across the line, which again in the U.S. is 60 Hz.

So this is the horsepower (going to write that right here) his is the graph showing the relationship between horsepower and frequency and how much percentage of your motor nameplate horsepower is available based on the frequency you’re operating.

OK the second curve we’re going to write on here is the torque available. Again, what we care about with industrial fans is the torque at startup. So we want the most torque we can when this thing is just past zero Hz and you’re firing it up. Good news, your torque starts at 100 percent. So again, from zero Hz to 60 Hz, we have 100 percent of the available torque from the motor, all the way out here to 60 Hz. That’s when torque starts to tail off. So from here on a smaller motor that can go out to 120 Hz, we’re going to come down to 50% from this point, and this relationship is a curve that looks like this. (I missed my dot a little bit there.) So we’re coming out and from 60 Hz to 120 Hz, we’re dipping and we’re coming out and we’re at 50 percent of available torque at 120 Hz.

Again, this matters more if your application really needs the torque. Let’s say, like a conveyor belt application, where you’ve always got this stretch from that belt on your motor and you always need that torque to be driving it, this is definitely something to consider. But with centrifugal fans, it’s rarely important to have maximum torque past your 60 Hz operating frequency, it’s not nearly as big a deal.

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