Variable frequency drives have been around long enough that they have pretty well become a standard for drive applications.
VFDs allow us to save energy and to control the processes that use rotating machinery.
During the selection stage of the equipment such as a motor driven fan or pump, the system of the fan or pump, the VFD, and the motor are all considered.
So let’s use a new fan as an example. Proper fan selection requires a knowledge of the air flow range you intend to operate at. Generally a designer won’t have just one point but will have multiple rating points. And while you try to maximize the efficiency at each point generally you have to pick the predominant rating point and maximize that.
As mentioned even though VFD’s have been around a long time we are still not using them to their full capability, so let’s look at an example where we might be able to do that.
Lets say a plant is considering installing a new boiler. Right now it requires 50,000 pph
during the week and 25,000 pph on weekends.
And because everyone looks to the future we will use a possible expansion scenario of 100,000 pph.
Now let’s say in order to generate this we need forced draft fan air flows of 20,000 CFM for weekend duty… 40,000 CFM for weekday duty… And 80,000 CFM for future duty.
Pretty well the first thing we need to do is select the motor whether it’s 1200 RPM, 1800 RPM or 3600 RPM. I.e. a 2, 4, or 6 pole motor.
I’m mentioning the number of poles because there is a huge jump in speed between the 4 and 2 pole motors i.e. between 1800 RPM and 3600 RPM.
If there was such a thing as a 3 pole motor we would have a 2400 RPM motor. And we would have another option. But we don’t and that becomes a large gap for synchronous speeds.
So engineers and operators have a decision to make when they’re faced with the above scenario and what we see frequently in design is the motor selected is an 1800 RPM. Perhaps using the argument that the 3600 RPM applications appeared to be too fast. Many designers would use synchronous speed for the future rating (labeled as the test block) and then run below speed for all the other rating points.
So choosing an 1800 rpm motor we will hit the highest rating point at 1800 RPM and then by slowing down we could hit the rest .. 40,000 CFM at 900 RPM and for 20,000 a speed of 450 RPM.
Using this strategy which is very common we have allowed certain limitations to encroach upon our design even though those limitations no longer exist with a VFD.
Secondly we are treating the future condition as a reality when in fact it is at best a hedge. The future expansion may not take place and therefore the larger steam flow not be required.
This is where we can start to use more capability of the variable frequency drive.
So in this particular scenario since the future condition is more than “iffy” we could select the operating speed for that condition at say 2400 RPM…or overspeed of 90 Hz knowing that again it is likely that it might never run at this condition.
If we were to use the overspeed capability then the new conditions RPMs will be as follows: 2400 RPM for the highest point, 1200 RPM for the second point and 600 RPM for the third point. The motor will be much not having to run below 50% as frequently and therefore will be operating in a better range for its efficiency and reliability.
The fan will be better sized at the main operating point and if you have to hit the future point it can still deliver.
There seems to be a prevalent biased towards over-speeding… Motors can be oversped otherwise VFD manufacturers would not allow the frequency to go over 60 Hz.
However of course, it is a good practice to always check with your motor supplier whenever talking about a VFD either for overspeeding or underspeeding. As underspeeding below 25% can cause heat buildup due to the slow speed of the motor cooling fan.
So in summary, using the full capability of the VFD will give additional options options to select rotating equipment if one considers over-speeding as an option and not to simply use the VFD for slowing down equipment.
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