# How to Determine Over-Amperage on Pulley Size Change?

I have a VPB-808 Fan (rated 4100 RPM) from Viron. I am currently trying to pull 1300 CFM at 7" SP, which needs ~2668 RPM (2.85 HP). I have a pulley drive motor (Baldor, EM3611T, 3HP, rated 1760 RPM, 4.2A). It is drawing about 2.8A with the following configurations. It is currently only blowing about 950 CFM.

I currently have pulley sizes of 1.75" (fan) vs 2.5" (motor).

It seems that if I increase my pulley size any bigger, I may cause the motor to be overamped. Would it cause the same over-amperage effect if I decrease the pulley size on the fan side (to 1.5")?

What would be the best course of action? Change the motor? I had the impression that I would have enough power as my motor is rated at 3HP.

AZFIREBALL
The ratio of RPM between the motor and fan needs to be 1.5. Therefore you need a 3 inch dia. pully on the motor and a 2 inch on the fan. 3/2=1.5.
Check the amps pulled by the motor with this combination to see if you are over driving the motor.

russ_watters
Homework Helper
Gold Member

The diameters of those pulleys seem to be too small for that task, not sure.
If so, the belt may be slipping, and the fan may be rotating at much less than the required 2670 rpm.
Do you have a tachometer to measure that?

Bigger pulleys are always easier on the belts, which consume less power in internal bending and heating.

AZFIREBALL
I agree the pulleys indicated in my answer are too small to transmit 3 hp reliably.

They were simply an example of two pulleys in the right ratio (1.5:1) to meet the RPM requirements.

For continuous or heavy usage, the pulleys need to be much larger and may even need to be running more than one belt (duel or triple) to reduce possible slippage and insure greater reliability.

I agree the pulleys indicated in my answer are too small to transmit 3 hp reliably.

They were simply an example of two pulleys in the right ratio (1.5:1) to meet the RPM requirements.

For continuous or heavy usage, the pulleys need to be much larger and may even need to be running more than one belt (duel or triple) to reduce possible slippage and insure greater reliability.
Thank you for the information.

If amp pulled by the motor is too high, would increasing the motor HP a good path forward? Is there a calculation to determine the amp draw before changing anything out? I wouldn't want to change the motor and it still will be overamped.

Homework Helper
Gold Member
The manufacturer is showing that that power is suitable for those parameters.
Therefore; either something is wasting energy, or those parameters are not the actual ones (higher total static pressure or airflow).

Without measuring anything to verify where on its performance curve your fan is actually working, you could reduce the amps to an acceptable value for the circuit by throttling down the supply flow with a manual damper (less fluid flow reduces amps for centrifugal machines, always).

russ_watters
The manufacturer is showing that that power is suitable for those parameters.
Therefore; either something is wasting energy, or those parameters are not the actual ones (higher total static pressure or airflow).

Without measuring anything to verify where on its performance curve your fan is actually working, you could reduce the amps to an acceptable value for the circuit by throttling down the supply flow with a manual damper (less fluid flow reduces amps for centrifugal machines, always).

I am having the exact problems. All my understanding and calculation conclude that the motor should be able to perform this task, but I am not sure how it may over amp the system. I will contact the manufacturer and get additional information.

AZFIREBALL
Check the fan bearings and other possible 'drags' on the blower if the measured amps are too high for the motor being used. The chart indicates 3 hp is enough to meet the specs. you specify.

Lnewqban
Mentor
Tell us about your drive. If, for example, you are wrapping a 1/2" V-belt around a 1.75" pulley, there is a lot of belt friction that adds to the motor power. Does both the fan and motor turn freely when the power is off and the belt removed?

Mentor
It is currently only blowing about 950 CFM.
At what static pressure? What kind of system is it connected to?

Tell us about your drive. If, for example, you are wrapping a 1/2" V-belt around a 1.75" pulley, there is a lot of belt friction that adds to the motor power. Does both the fan and motor turn freely when the power is off and the belt removed?
I made a mistake in my post, which may have caused some misunderstanding.

I was referring to the radius of the pulleys. I was able to find the specs of the pulleys. They are 5.25" (motor side) and 3.25" (fan side) with an "A" size belt. The ratio between the pulleys is definitely >1.5.

At what static pressure? What kind of system is it connected to?

It is currently connected to an exhaust system. The static pressure is 7"WC.

Mentor
It is currently connected to an exhaust system. The static pressure is 7"WC.
Is that measured or designed? It's suspiciously specific. But if it is accurate it means your system is more restrictive than expected. Static pressure should be lower if the airflow is lower unless the pressure is regulated by dampers, which is uncommon.

erobz and berkeman
Gold Member
Yeah... did you try to calculate a system curve, or did you just say by fiat "its 7" WC at 1300 CFM?

Because if you are measuring 7" WC now at 950 CFM , at 1300 CFM the system is going to require ( assuming negligible static elevation head )

$$H \approx \frac {7} {950^2} Q^2 = \frac{7}{950^2}1300^2 \approx 13.1 \rm{ WC}$$

according to the fan data sheet that clock in at about ##5 \, \rm{Hp} @ 3443 \, \rm{RPM}##

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russ_watters
Is that measured or designed? It's suspiciously specific. But if it is accurate it means your system is more restrictive than expected. Static pressure should be lower if the airflow is lower unless the pressure is regulated by dampers, which is uncommon.
Yeah... did you try to calculate a system curve, or did you just say by fiat "its 7" WC at 1300 CFM?

Because if you are measuring 7" WC now at 950 CFM , at 1300 CFM the system is going to require ( assuming negligible static elevation head )

$$H \approx \frac {7} {950^2} Q^2 = \frac{7}{950^2}1300^2 \approx 13.1 \rm{ WC}$$

according to the fan data sheet that clock in at about ##5 \, \rm{Hp} @ 3443 \, \rm{RPM}##

The 7"WC is based on design (calculated based on the pressure drop calculation across the blower inlet). I am planning to measure the actual static pressure once I get a chance to. All the dampers are open at the moment.

russ_watters and Lnewqban
Gold Member
The 7"WC is based on design (calculated based on the pressure drop calculation across the blower inlet). I am planning to measure the actual static pressure once I get a chance to. All the dampers are open at the moment.
Yeah, put a small petcock valve into the duct a few diameters ( I don't know what the industry standard is) downstream of the discharge, so you can test the static pressure. It's good to make this a permanent feature for future troubleshooting purposes. If you have significant duct work on the intake you will need to place a tap there as well. The static pressure provided by the fan is the difference between those measurements (across the fan).

Gold Member
You can also make a rudimentary plot of the fan curve from the data sheets at ##\approx## 2660 RPM. If you are actually spinning 2660 RPM, and running 950 CFM then the static pressure should be between 7" WC - 8"WC according to the data sheets. Closer to 8" WC.

Just replace the 7 with the 8

$$H(1300) \approx \frac{8}{950^2}Q^2 = \frac{8}{950^2}1300^2 = 15 \, \rm{in WC}$$

So you are going to have to spin that fan significantly faster with a higher Hp motor if you truly want 1300 CFM

Looks like about 6 Hp @ 3600 RPM

A 7.5 Hp motor is probably where you are going to want to be.

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Gold Member
Don't take this personally, but the model you were using for the system curve fell significantly short of reality. Pending confirmation that the fan is operating properly of course.

russ_watters
Gold Member
One of my projects 54" Diameter FRP Fan, mounted on a 100+ year old roof with a 5000 lbf concrete inertia base for a HCL scrubber system that was significantly underperforming at the time ( nonone likes breathing HCL fumes) . The fan I specked had a 200 Hp motor, everyone thought I was a fool ( a.k.a. young engineer) which was completely justified.

I did take some licks on that one ( kind of ). The electrical engineers specified the starter, it was a 16 hr day on install day. Everything finally got hooked up around 10 pm. Everyone held their breath and somewhat timidly flipped the switch ( it was the largest motor in the mill ). Seemed to start fine, relief!... then the breaker kicked...and again...ok let's change the wiring on the motor one of the electricians devised. And ...the breaker still popped. That was a depressing moment...I thought, welp... I'm getting the axe ( everyone else joked - but I could see by the color of their face that it was a real possibility). Luckily I had finished early in shutdown week. The next day we determined it was a significantly undersized started. We needed one rated for high inertia loads. That solved the problem.

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Lnewqban
Mentor
Similarly, I'd caution against using the fan curve to estimate static pressure on a system already believed to be performing unexpectedly. It needs to be measured. There are system effects (bad duct configurations) that can reduce the efficiency of fans.

Gold Member
Similarly, I'd caution against using the fan curve to estimate static pressure on a system already believed to be performing unexpectedly. It needs to be measured. There are system effects (bad duct configurations) that can reduce the efficiency of fans.
I was thinking along the lines of concomitant methods. They already have the fan operating. I agree, much better to physically test.

Gold Member
Because if you are measuring 7" WC now at 950 CFM , at 1300 CFM the system is going to require ( assuming negligible static elevation head )

$$H \approx \frac {7} {950^2} Q^2 = \frac{7}{950^2}1300^2 \approx 13.1 \rm{ WC}$$

according to the fan data sheet that clock in at about ##5 \, \rm{Hp} @ 3443 \, \rm{RPM}##

You can look at it the other way, and assume the fan design condition is a correct case. In that case, a measured 950CFM would correspond to about 3.7 inches water. In both cases, there is a problem with the static pressure and/or the flow measurement. I am going to assume that the field amperage measurement is likely correct, since it was probably taken by a device used in other locations (so probably accurate), and it will be the one that sets the upper limit as a constraint. If the amperage/HP of the motor reading is incorrect, then my advice is not helpful.

Sheave (pulley) changes may be more inexpensive than installing new power monitors and accurate air flow devices. In this situation, with questionable flow and pressure, I would use the current fan design case (2668 RPM at measured 2.8A), and, assuming a power factor of 1 at constant voltage (be careful--talk to your site EE about this), calculate a new RPM corresponding to something close to 90% of motor rated power. When I do this, I get a speed required to achieve the new load as ~2,950 RPM using affinity laws (check my arithmetic).

After this change, get new pressure, flow and amperage measurements. The change should help to indicate which measurement(s) is/are incorrect. If it is still not clear, and you need more data, push the load closer to the maximum. You should get about 1090 CFM@3050RPM (est Full load), or 1140CFM@3200RPM (full load +.15 service factor). Even with constraints on the system that are unknown or unmeasured, if the system behaves as expected, you know that a larger motor is a solution (maybe not the best one) until the flow, pressure, and power are more certain, and system issues that deviate from design expectations can be identified.

erobz, Lnewqban and berkeman
Homework Helper
Gold Member
Similarly, I'd caution against using the fan curve to estimate static pressure on a system already believed to be performing unexpectedly. It needs to be measured. There are system effects (bad duct configurations) that can reduce the efficiency of fans.
That ^^^

Exactly knowing the actual curve of the fed system is key.
Very commonly found in commercial and industrial operating systems.

Just blinding installing stronger muscle to push harder may solve the current situation today, but at the expense of higher investment and operational costs for years to come.

A pitot tube and a tachometer in expert hands can save a ton of money.
Contacting a commisioning service could help, if tests and accurate measurement are not feasible in-house.

Gold Member
You can look at it the other way, and assume the fan design condition is a correct case. In that case, a measured 950CFM would correspond to about 3.7 inches water
The fan or the motor would be the issue in that case.

And actually, I was basing that off of measured 7 inWC @ 950 CFM in the present system. So it can't be reversed like that.

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Gold Member
Just blinding installing stronger muscle to push harder may solve the current situation today, but at the expense of higher investment and operational costs for years to come.

I agree.

Thank you everyone for the information.

This is the first time I work on an exhaust system, so I have not considered a lot of the information given on here. They were really helpful.

Just to give a bit of history: The blowers were previously designed to flow 1500 CFM at 4"WC (based on nameplate). When the blowers were abandoned in place, they were underperforming. Then we decided to change the motor. Measurements made sense, until recently when we tied it in to our new exhaust system. We consulted a few places and they all agreed that these fans will work on the new system (theoretically), with the current setup. But now the fans are not performing right.

I need to find out the root cause before proposing to get a new motor, or else my boss will be on my ass.

russ_watters and Lnewqban
Gold Member
Measurements made sense, until recently when we tied it in to our new exhaust system.
^^
What is the total system layout? Thats kind of big thing not to mention until this point.

Homework Helper
Gold Member
Thank you everyone for the information.

This is the first time I work on an exhaust system, so I have not considered a lot of the information given on here. They were really helpful.

Just to give a bit of history: The blowers were previously designed to flow 1500 CFM at 4"WC (based on nameplate). When the blowers were abandoned in place, they were underperforming. Then we decided to change the motor. Measurements made sense, until recently when we tied it in to our new exhaust system. We consulted a few places and they all agreed that these fans will work on the new system (theoretically), with the current setup. But now the fans are not performing right.

I need to find out the root cause before proposing to get a new motor, or else my boss will be on my ass.
https://greenheck-cms-prod.azureedg...tion-articles/fa118-03.pdf?sfvrsn=e3d52f92_13

https://greenheck-cms-prod.azureedg...articles/fanselections.pdf?sfvrsn=56002ffa_13

https://greenheck-cms-prod.azureedg...n-articles/perf_basics.pdf?sfvrsn=6df9b7ac_13

^^
What is the total system layout? Thats kind of big thing not to mention until this point.

We have a tool that requires 1300 CFM of exhaust that was previously tied to another exhaust system. They were running into problem with that system, so they decided to move that tool's exhaust trunk to the fans (the fans in this thread, one of them is a spare) that are abandoned in place. The system that was previously tied to these fans are no longer in operation and the tie-in trunk was capped for the tie in of the new tool.

The tool itself has about 11 exhaust lines, all tied into an 8" trunk. This trunk has about 200 ft of piping and 12 elbows. Then it's expanded to a 12" trunk (about 20 ft with 4 elbows). Then it's reduced to the inlet of the fans.
The discharge side is pretty much just a straight 15ft 12" line.

Gold Member
We have a tool that requires 1300 CFM of exhaust that was previously tied to another exhaust system. They were running into problem with that system, so they decided to move that tool's exhaust trunk to the fans (the fans in this thread, one of them is a spare) that are abandoned in place. The system that was previously tied to these fans are no longer in operation and the tie-in trunk was capped for the tie in of the new tool.

The tool itself has about 11 exhaust lines, all tied into an 8" trunk. This trunk has about 200 ft of piping and 12 elbows. Then it's expanded to a 12" trunk (about 20 ft with 4 elbows). Then it's reduced to the inlet of the fans.
The discharge side is pretty much just a straight 15ft 12" line.
Ok, so its completely sealed off from any other ductwork and fans. False alarm.

Homework Helper
Gold Member
We have a tool that requires 1300 CFM of exhaust that was previously tied to another exhaust system. They were running into problem with that system, so they decided to move that tool's exhaust trunk to the fans (the fans in this thread, one of them is a spare) that are abandoned in place. The system that was previously tied to these fans are no longer in operation and the tie-in trunk was capped for the tie in of the new tool.

The tool itself has about 11 exhaust lines, all tied into an 8" trunk. This trunk has about 200 ft of piping and 12 elbows. Then it's expanded to a 12" trunk (about 20 ft with 4 elbows). Then it's reduced to the inlet of the fans.
The discharge side is pretty much just a straight 15ft 12" line.

Consider that the poor fan is working in a sucking-from-many-points situation, rather than pushing air, which is its best performance configuration and design assumption.

It can only create certain lower-than-atmospheric pressure in its inlet; the rest of the moving-air-work is done by the atmosphere pushing air through many inlets, duct branches and changes of direction and toward that unique low pressure zone.

Multi-brach exhaust duct systems need to be tested and balanced, as well as carefully sealed from undesired leaks.
Otherwise, your piece of equipment could be “feeling” only a fraction of the exhaust efforts of your motor and fan.

With enough upstream restriction, the fan could be operating to a minimum flow (like having a close upstream damper), way off the desired operational point.

https://www.spiralmfg.com/designing-efficient-dust-collection-system/

You may be able to find good guidance in ASHRAE manuals, if available.

Gold Member
Likely not possible, but... move the fan(s) as close to the machine as possible so they push the air out thru that 220 feet of plumbing. (Better yet, put the fan directly above, on the roof?)

The present configuration is like trying to drink a thick milkshake thru a 6ft. straw with a dozen bends and kinks in it; definitely non-productive.

Likely not possible, but... move the fan(s) as close to the machine as possible so they push the air out thru that 220 feet of plumbing. (Better yet, put the fan directly above, on the roof?)

The present configuration is like trying to drink a thick milkshake thru a 6ft. straw with a dozen bends and kinks in it; definitely non-productive.
That configuration is nearly impossible, as how far the fan was built from the tool.

The fan or the motor would be the issue in that case.

And actually, I was basing that off of measured 7 inWC @ 950 CFM in the present system. So it can't be reversed like that.
The actual static pressure measured is ~5.5 inWC @ 950 CFM. So using the equation you provided, we may need an increased motor for 10 inWC @ 1300 CFM.

Gold Member
The actual static pressure measured is ~5.5 inWC @ 950 CFM. So using the equation you provided, we may need an increased motor for 10 inWC @ 1300 CFM.
Can you determine if that operating point is on your fan curve @ 2668 RPM?

russ_watters