On several occasions over the last 10 years I have had customers with fan drive motor vibration issues that looked like a classic case of unbalanced. Instead we found ‘Lack of Horsepower’ as the cause. And all were powered through a Variable Frequency Drives (VFD). What I’m going to discuss in this column isn’t a common occurrence but it’s one that should be noted and remembered. You could be a maintenance hero if you recognize and correctly identify this issue; it’s all about Horsepower, or the lack thereof. And one more quick point before we get started, all discussions below are based on a VFD being programed to accommodate Pumps and Fans; Volts per Hz. Why? Because pumps and fans are centrifugal and torque requirements are exponentially related to loading, load here is not linier; maybe. VFD’s can also be programed for a constant torque mode. However, this mode is generally reserved for conveyors, compressors, anything with a high starting load, anything that is positive displacement, etc… If we were to use constant torque in this application, we would lose those efficiency gains that had us install the device to begin with. It’s all about profitability; remember.
Fan Details – On fans where I have seen this issue, all were belt driven (AMCA Arrangement 9), backward incline or radial blade centrifugal fans. And on the backward incline fans, the blade angle was fairly steep, approaching that of a simple radial blade (90°). And if one knows his fans, you should remember that several fan manufacturers (Twin City predominantly) at one time published a general caution on radial fans; ‘under certain conditions, the fan wheel break HP can exceed the motor HP’. WOW!!!! Can this be true? Yes it is true and the condition they described in their caution was at maximum flow, not minimum. Our VFD related issue is at the other end of the spectrum, low to medium flows. Say what?
As we increase the blade angle, providing more and more of a backward angle (trailing blade angle; from 90° toward 120°) we lengthen the fan curve to provide a greater flow variation over the same pressure range. Just by looking at the typical fan curves it is pretty easy to see that it takes a pretty good pressure swing to affect the flow through a radial fan. Whereas a backward incline fan, modulating pressure gives me a pretty good variation in flow.
In HVAC systems, we have two mechanisms that allow us to adjust flow; speed modulation and pressure modulation. Pretty simple, right? Maybe.
When we adjust fan speed, to determine changes in flow, we reference our affinity laws;
· speed changes cause proportional changes in flow,
· and changes in speed are proportional to HP cubed.
When we adjust fan pressure, to determine changes in flow, we need to reference our fan curve and our affinity laws;
· changes in fan pressure are proportional to HP squared,
· I need a specific fan curve to relate the pressure changes to flow changes,
· and changes in flow are proportional to HP cubed.
In them olden days, before the advent of Pulse Width Modulated Variable Frequency Drives (VFD, cheap speed control), we had fixed speed fans where we had to open and shut dampers to change flow, e.g. we modulated system pressure. Basically, when we modulate pressure across a fixed speed fan, we are changing the system curve; we are changing total pressure across the fan. To lower the flow we increase total pressure across the fan (shut a damper) and to increase flow we decrease total pressure across the fan (open a damper). The problem with this method, we lose efficiency the more we close or the more we open the control damper. When we use fan speed (VFD) to modulate flow, we are actually sliding the fan curve up and down the system curve. This gives us a greater range of flow control and we are managing efficiency by keeping the fan at its best efficiency point, most of the time anyway.
With a pressure modulated system, I don’t worry about total pressure causing a motor overload condition when reducing flow because of the difference between HP cubed as compared to HP squared; we can always make pressure as long as we sized our motor based on expected system flow. However, if I try to flow more through the system (increase flow) than what the motor can handle, we enter a condition of motor overload, not enough HP to push air through the system. And believe me, that motor will do everything it can to push air through the system. It’s going to maintain speed or die trying. This is true for both speed and pressure modulated systems.
In a pressure modulated system, when I reduced flow, when I shut the damper, I have excess HP because there was no speed change for the lower flows; less work, less power, no overload. And when I open the damper, I am loading the fan (need more power) because of the additional flow; more work, more power, more potential for overload. Pressure modulation is not very efficient but it is effective. Kind of like a sledge hammer in a jewelry shop.
And even if we install a VFD to control motor speed there is still motor slip or the motor would not run. If we increase motor load, increasing motor slip past 5%, we begin to overload our motor; windings heat up, the motor runs rough, we get high torque pulses as the rotor tries to catch the rotating field. We see this, especially the torque pulses, as a high vibration with all of the energy at the motors fundamental frequency, the X1. On a fixed speed installation, with a simple across the line motor starter, there are thermal overloads that protect the motor from this condition. On a VFD, the overload setting is a programed set point, based on the motor’s full load (and speed) Name Plate Rating. But, does this programed overload set point protect the motor when running at slower speeds? Maybe not!!!
Back to the fans - In reality, radial fans will give me a flat HP curve (not a lot of change in flow) and in backward incline fans, we will see more of a vertical HP curve (lots of flow differential). Remember, there is a relationship between speed, flow and HP.
Speed is lost, voltage is reduced, motor current is limited and HP is lost. As long as flow is decreased proportionally, there shouldn’t be an issue. Unless our fan characteristics; e.g. the HP require to drive the fan at various flows, is FLAT. Then we have a condition of motor overload. Why? Because with the motor operating at a lower speed and with limited current (due to the VFD) we can only produce so much HP. And if flow has not been reduced enough to match motor HP, we are in a condition of motor overload. And how can I tell? Motor slip has increased and if the slip is significant, we will see a high motor vibration. But what about my motor overload protection programed into the VFD, why doesn’t that trip the motor? The reason; when I program the overload set point based on motor full load performance (Name Plate Rating) and I have an overload condition at lower speeds, with limited voltage and current, I never reach the overload set point. Electrically the motor is perceived as ‘all good’ except it’s beating itself to death because of excessive slip. Oh My!!!!!
And no, unfortunately the ‘Slip Compensation’ function included as a programing feature of most VFD’s does not correct this issue. This feature does not work for process control (pressure or flow control), it was intended for process ‘speed’ control (conveyers, etc…).
And the fix; the easiest way correct this problem is to throttle down on a damper. Yes that’s correct; introduce some back pressure into the system. This forces the fan to run faster at all flows and the faster the speed, the more torque and HP is introduced. As an alternative, since the problem is at the bottom end of fan performance and if you never operate in this region, just add a note to your SOS that cautions the users about this condition and limit operation below certain speeds.
So why not just reprogram the VFD to Constant Torque mode? Isn’t this the correct setting for this type of issue? It could be the answer if I were to see excessive starting current and excessive load through a larger portion of the RPM rang. Since this issue is at the bottom end of system performance, under normal conditions, as filters load and the original facility HVAC balance is maintained, we probably wouldn’t notice the issue. So there is no reason to set the VFD up outside of the manufacturers recommendations; fans and pumps = volts per Hz, conveyors and compressors = constant torque. Following the manufacturer’s recommendations gives me the best return on my VFD investment. But if I change filters (a clean system) or maybe change HVAC configuration without checking the effect on my fan, the issue could sneak up and bite us in the buttocks. So how would you check for this condition? I use a strobe tachometer to check shaft speed relative to what the synchronous frequency should be. My acceptance here; Motor Slip shall not to exceed 5%.
And because you actually read this little white paper, you can break out your strobe tachometer to immediately identify excessive motor slip and then suggest closing down on a damper to correct the vibration issue; you get to be the Maintenance Hero. Gosh, fixing a X1 vibration indication without balancing the rotor; WOW!!!
Below are a couple of ‘Tales From the Crypt’ that show examples of this uncommon occurrence and more importantly what we did to correct the issue.
· Story 1 – 75 HP Belt Driven Overhung Fans, Richland Washington
At the Hanford nuclear reservation 242A Evaporator, the main exhaust ventilation system was being replaced by two new exhaust trains that included a pair of 75HP overhung fans. My mission was to provide vibration acceptance of the fans during commissioning. In this circumstance, the new fans and filtration systems were installed but not yet tied to the facility. This allowed testing without worrying about radiological contamination. Good idea; we thought!!!
The commissioning procedure had included closing down on the filter intakes to simulate HVAC system losses as if the filter trains were tied to the building. However, it was a missed step. When I first saw the fans operate, the VFD’s were set at 40 Hz or about 1200 motor RPM. A quick round of vibration data on the fans and motors showed no issues at 40 Hz. So we began testing. The test called out collecting vibration data on the motors and the fans from 20 Hz through 60 Hz from the VFD. Fan vibration acceptance was in accordance with AMCA 204 and since the AMCA does not cover motors, we used general criteria from the ‘outdated’ ISO 2372 standard for Class II equipment, not to exceed .18 IPS RMS; a solid C. We basically stated in our acceptance documentation that we knew the ISO was outdated. However, the .18 is representative of the other similar equipment at the facility. Remember, acceptance is not too high as to pose a safety concern and not too low as to be operationally limiting.
So we lower fan speed to 20 Hz and after flow has steadied out, the motor started bucking. I was getting .5 to .7 IPS RMS on the motor yet maybe .3 to .35 on the fan bearings. Then as we ramped up the speed, things got better (could meet the criteria) around 40 Hz. The spectrum analyzer I was using showed all of the vibration energy at X1. So what was the problem?
We secured from the testing and regrouped over several meetings. We sent the crews out to check simple items; nuts and bolts, belt alignment and VFD condition. Everything checked out. I even went back and completed a simple bump test to verify there was no resonance condition. And balance, bah humbug, since the motor smoothed out as RPM increased, there was no issue. Then, as I walked by the filter inlets, I noted that the dampers were wide open. And I thought to myself, “didn’t we have a step to throttle down on these to simulate building losses”, why were they open? And was this at least part of the reason for the high vibration?
So we repeated the test, this time with the dampers partially closed and gee whiz buckaroo, all of the vibration levels at all measurement points came within the criteria.
So I went back and reviewed the fan curves and got with Allen Bradley on their VFD. What we found was with wide open dampers (low total pressure across the fan), below 30 Hz to the motor, the fan wheel break HP required was about 10% greater than what the motor could provide. WOW!!! And as we spooled up the fan, the motor finally caught up with the fan as far as the HP to drive it.
I used this anecdote to describe the condition; Basically, we were trying to accelerate our Ferrari in 6th gear from 20 to 100 MPH. In 6th gear, it bucks and jerks and stalls, I can’t make enough power to push it smoothly until I get to around 40, then the engine rev’s are high enough to make enough power to accelerate like a banshee.
When we completed final acceptance testing of the fans after the hot tie-in, we left provisions in our procedure to allow adjustment of the building balance dampers if the condition re-appeared. Luckily, there were enough losses from the building to keep total pressure across the fan high enough for everything to work well.
Lesson Learned; if I were to have included a check of motor slip at the bottom end of the RPM range, we would have caught this issue a heck of a lot sooner. Using a strobe tachometer and inspecting for this condition is now part of my ‘low hanging fruit’ checks. Mounting bolt tightness, electrical (balanced current/voltage between phases, VFD) checks, inspecting the structure (resilient mounts), cracks, precision alignment and motor slip are cheap and easy and rule out the most common issues that cause vibration.
· Story 2 – 50 HP Belt Driven Overhung Fans, Asan Korea
At a customer’s facility in Asan Korea, they have over 40 similar manufacturing lines, each with a very high temperature processes at the front end. Each pair of the lines has 3, 50 HP AMCA arrangement 4 fans that provided cooling (and backup) for their respective high temperature process. All of these fans are powered through VFD’s and generally run at 50 to 60% of the motor Name Plate speed. These systems had been installed and operational for approximately 10 years. My mission for this customer was providing PdM and CBM training.
During my initial classroom sessions, I generally ask many, many questions to get a feel for what the engineers and technicians know, or don’t know. Plus I have them bring whatever monitoring equipment they normally use for the collection of PdM data. One of the engineers showed up with an older SKF CMVA vibration analyzer. After the initial training session he shared with me a spectrum of a motor with vibration amplitudes substantially higher than similar equipment. It happened to be on one of the 50 HP cooling fans. From his data, he kept showing the high X1 peak and telling me the motor needed to be balanced; could I show them how to complete a trim balance using his analyzer.
In these cases I never drop an opportunity to include additional training; in the field on their equipment is premium ‘hands on’ time. However, I am always skeptical on why there is a need to balance equipment without an underlying reason. So I asked the engineer; Is this a new motor? No. Have you done any work on the motor? No. Have you checked the mounting nuts and bolts? No. Have you checked belt alignment? No. Have you verified the VFD is in good working order? Deer in the headlights, No. Do you have a strobe tachometer? Yes.
O.K., so after the inquisition, I hope you (the reader) know what I’m about to suggest; let’s take this fan out of service, get the backup on line and go after the low hangin’ fruit; check nuts and bolts, belts, alignment and electrical issues before we get into a balance. Checking a VFD for correct operation was new to them and actually worthy of yet another article to come (future writing). And last but not least in our new repertory of low hangin’ fruit checks; check the motor slip with a strobe tachometer.
So as we discuss the upcoming activities with their operations supervisor, and as we were discussing the VFD checks, the need for the strobe tachometer came up. The operations supervisor asked “why are we using a strobe tachometer?” My reply, “motor slip caused by an overloaded motor at lower speeds”. Then I went on to explain the motor speed, motor HP and how fan wheel break HP can exceed motor HP at reduced speeds. And when I stated damper positioning can cause the issue, it came up that on this particular fan, 2 years prior, the fan outlet damper actuator was removed and the damper had been locked wide open. It was originally only 80% open. Plus they had to slow the fan to establish the same flow (approximately 40% speed). And from the vibration technician, the high vibration on this unit was discovered soon after. WOW!!! Could it be? A low HP condition due to excessive flow? Yep, and the root cause; introduction of error!!!
As an exercise in how to address all vibration problems as they came up, we went through all of the basic checks, nuts and bolts, electrical, VFD, alignment, etc…, and what did we find? 7% slip on the motor at a motor speed of 720 RPM (40% speed) and all of the system dampers wide open, the motor was overloaded yet the current was not excessive, no overload trip. So then we started to throttle the fan inlet damper (vortex damper) and ramping up fan speed to maintain system flow. Once we got to about 25% closed on the inlet damper and made our final speed adjustments (final speed 1000 RPM, 55% speed), motor slip was around 2% and overall vibration decreased from 10 mm/s RMS to less than 2 mm/s RMS, a very significant improvement. WOW!!! Correcting a X1 indication and no balancing.
Two lessons from these stories; motor slip on VFD’s at lower speed is a tell tail for motor loading issues and If I see excessive load combine with previous work and then a jump in a CBM indication, you can be sure at least 99% of the time you have found the cause your issue.
Maintenance, what a Concept
MMJennings
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