Last week I provided just one example of how variable speed, constant pressure systems aren’t always about constant pressure. Specifically, I showed how a variable speed controller can extend system life. My example was an older system that was getting heavy usage (and cycling) with an irrigation system.
Here’s a slightly different example. I’ve never given a seminar or presentation on constant pressure systems without someone asking, “How much do they cost?” Although I can usually provide a ballpark list price and refer them back to the distributor for their pricing, my answer is “it depends.” The reason is that although there is additional cost associated with a variable speed drive, there can also be some significant savings that offset this cost.
The example cited most often is being able to use a small tank. However, an overlooked, sometimes far more significant savings can be obtained because of the smaller cable required. Here’s why … Regardless if the input is single- or three- phase, most VFDs generate a three-phase output voltage (Franklin Electric’s MonoDrive and SubDrive2W are notable exceptions). So, we use a three-phase motor, and for the same horsepower, the current (amp) carrying requirements are smaller for a three-phase motor than for a single-phase. Therefore, in many installations, we can go with a smaller gauge of drop cable if it’s three-phase.
Here’s an example: Let’s say you’ve got a 3 horsepower system with a total cable run of 500 feet. From the single-phase cable charts on page 11 of the AIM Manual, #8 is only good for 470 feet. So, to ensure adequate voltage to a single-phase 3 horsepower motor, you’re going to need #6, which is good for 750 feet.
However, it we use a SubDrive 150 and a 3-phase motor, we use the three-phase charts on page 16 of the AIM Manual. In this case, #10 is good for 620’. So, by adding a VFD into the system, we’ve gone from 500 feet of #6 cable to 500 feet of #10 cable.
Now, with the price of copper, and therefore drop cable, these days, your savings on 500 feet of #10 versus #6 cable will be very significant, probably in the hundreds of dollars. In some cases, you may even save money with a VFD.
This is just one example. But, the point is that whenever you bidding a job, it’s a good idea to run the scenario above. You may be surprised at how little the difference is between the system cost of a conventional system and a variable speed system. And, once again, you’ll have all the benefits of constant pressure.