By now, you should be familiar with the Hollisterian reduction in available horsepower that occurs when you select a design point at an asynchronous motor speed. And you know that if you ever do need to design such a case, you will also need to oversize your motor by a factor equal to the motor design RPM divided by the actual operating RPM. Thus, if you are laying out an 1800 RPM motor to operate at design at 1500 RPM (let's ignore the fact an 1800 RPM motor is actually 1775 RPM, for now), you will need to select a motor nameplate HP that exceeds the brake HP of the design point by a factor of 1.2 (1800/1500). And because you are such a careful engineer, you've thrown in a bypass around the VFD in order to allow fan operation even if the VFD burns out.
So you are ready to go, right? What else could possibly trip you up now?
This is where the Florentine effect can get you.
The Florentine Effect
Let's go back to that 13.5 HP selection at 1500 RPM we discussed in the last post. Turns out, that's pretty close to a 77% width 30" fan operating at 14,000 cfm and 4" of static. When I select an EPQN fan using the Twin City Fan software, I get a brake horsepower of 13.22 HP. You know that in order to account for the derate due to the RPM that you need to pick a motor that can provide a nominal HP at least 1.2X this brake, or 15.9 BHP. You select the next larger size, or the 20 HP motor. That's a full 50% bigger than the design brake, and at least 25% bigger than you need, when you account for Hollisterian effects.
So you install the fan, and everything works just fine. In fact, you might go several years before any trouble raises its head. But then, suddenly, one day it does.
Let's say the VFD serving the fan burns out, or is taken out of service for a short while. The owner, wanting to preserve function of his fan system, even if he has to operate it at constant speed, does the obvious thing and flips the fan to bypass...
Suddenly the fan kicks on, starts roaring, and then the motor burns out. A follow up inspection might even find that some of the duct fittings have blown apart. What happened?
Well, let's think about what happened. The fan normally operated at 1500 CFM at peak design. The bypass, which is essentially a standard motor starter wired in parallel to the VFD, kicked the motor on at the line power frequency of 60 HZ, or 1800 rpm. This means that you weren't supplying air at 14,000 CFM at 4", but something greater. Following the fan laws, you would ride the system curve up to about 17,000 CFM at 5.5"! And that is assuming that you are operating a constant volume system or a VAV system at peak cooling load. If you were in heating on a VAV system with the boxes choked back to their minimum flows, you might develop even higher pressures!
Let's look at what happened:
(click to see larger image)
The above is a fan curve plot of the two operating conditions of your fan: 1500 RPM and 1800 RPM. The HP curves are plotted for both cases also. They share a common system curve (this assumes an unchanging duct system--a bad assumption for a VAV system). I've highlighted the resulting flow rates and brake horsepowers at the two resultant operating points where the system curve intersects the fan curve.
The first thing that should jump out is that the brake horsepower for this fan operating at 1800 RPM jumps up to 22.9 HP*--even greater than the 20 HP that you picked to protect from the Hollisterian effect! The other thing you should notice is that if this is, in fact, a variable volume system, the system curve shown at design is not the system curve that would be seen by the fan unless the system was at full cooling. If the boxes neck back at part load or in a heating condition, the system curve shifts to the left, pushing the intersection between the fan curve and the system curve closer to the fan curve peak. This greatly increases the amount of static pressure the fan can develop at this higher speed--up to about 9" in the case shown here.
*this can be calculated by the fan law formula:bhp2=bhp1(rpm2/rpm1)3
Thus in a VAV system, this can be a double whammy, kicking out your motor and damaging your duct system.
How to avoid this problem? Well, you could size the motor for the even larger size demanded by the Florentine effect--but that would still leave you with the possible problem of overpressurization of the ductwork in bypass. Probably the first thing to consider is whether or not you really need a bypass, anyway. With today's more reliable VFD's, putting in a bypass is far less of a necessity. Many drives can function for the life of the equipment with no failures at all. If redundancy is absolutely necessary, consider providing a second, parallel VFD instead of a standard starter. VFD prices have come down considerably since the parallel-starter bypass concept was developed. This is no longer the cost-prohibitive strategy that it was at one time.
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