Technical Stuff
HVAC 101
This section of our site is devoted to knowledge pertaining to the HVAC industry. Anyone who wants to contribute to this section is welcome to submit articles for our review and inclusion. Some of the things that we would like to include are: Sets of specifications for complete system installation (sort of a good, better, best). Indoor air quality procedures. Simple system troubleshooting.
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Click Here for Other Specifications
Motor Power Requirements & Operation Cost Formulas
These are the fan laws. So, don't listen to the hype put out by some of the manufacturers who claim they invented them and produce the only equipment that they apply to.
DEFINITIONS:
CFM = Cubic Feet Per Minute.
W = Watts = Electrical unit of power.
HP = Horsepower = Mechanical unit of power.
1 HP = 745 Watts = Conversion of electrical power to mechanical power.
Fan Laws:
CFM2 = CFM1 x RPM2 / RPM1 or
CFM is directly pegged to rpm.
If RPM is cut in half then CFM is also cut in half. If 1,040 RPM produces 1,200 CFM then 520 RPM will produce 600 CFM.
HP @ S2 = HP @ S1 x (RPM2 / RPM1)2 This demonstrates that the horsepower required to turn the fan is related to the square root of the speed change. Or if the fan’s speed is cut in half, then the amount of air delivered is also cut in half but the Horsepower required is only ¼ of the original Horsepower required. OR, a fully loaded ½ HP fan motor running at 1,040 RPM and producing 1,200 CFM will only require 1/8 HP to deliver 600 CFM at ½ speed of 520 RPM. This demonstrates that the power required to turn the fan reduces a lot faster than the reduction in CFM being delivered.
Watts2 = Watts1 x (RPM2/RPM1)3 or Watts2 = Watts1 (CFM2/CFM1)3 This demonstrates that the electrical power required to turn the fan drops by the cube of the speed change. Or drop the speed (RPM) in half and the Power (Watts) required is 1/8 the original power required. Our example: ½ HP fully loaded fan motor running at 1,040 RPM and delivering 1,200 CFM requires 745 watts/2 or 372.5 Watts. Cut this motor’s speed in half and you cut the air delivery in half but the Wattage required is 372.5/8 = 46.6 Watts.
Therefore, a fan motor must run 8 hours at half speed to use the same amount of electricity as it would running at full speed for 1 hour.
Now lets bump this up-against reality. Lets assume that our example fan is in an average furnace. On an average winter day it runs about 1/2 of the time. It therefore uses 372.5 watts / 2 = 186 watt-hours of electricity. Now we install a FanHandler that runs the fan full time. Now lets say there is a call for heat 3 times during that hour and the fan reaches top speed three times for 3 & 1/3 minutes each time (which it probably won’t) 10 minutes per hour = 1/6 hour x 372.5 watts = 62 watts and the other 50 minutes it uses 5/6 of 46.6 watts = 38.83 watts for a total of 38.8 + 46.6 = 100 watts per hour. For a savings of 86 watt-hours. And the home is comfortable, the air cleaner or filters are working full-time at much higher efficiency.
Monthly cost for the FanHandler equipped fan at $0.10 per KWH = 86 watt hours x 24 hours x 30 days per month = 61,920 watt-hours or 61.9 KWH x $0.10 = $6.19 per month. If you ran the full-speed fan round the clock, it would cost 372.5 watts X 24 hours = 8.9 KW = $0.89 per day X 30 days per month = $26.00 per month. If you ran the full-speed fan ½ the time it would cost $13.00 per month. You can use your electricity costs and horsepowers etc. to do the comparisons. It will prove that the FanHandler saves energy!!
Simply put, you can run the fan, with a good motor, for eight hours at half speed for the same cost as running it one hour at full speed.
There are some motors being installed in equipment today that are designed to meet a price criterion. These motors will not follow the fan laws and will growl and rumble at low speed. They will not follow the fan laws. They will not reduce amp draw with speed reduction, or the amp draw reduction will not be in line with the fan laws. That is why we recommend that when you run across one of these motors you install one of the high quality Baldor motors that we market along with the FanHandler controls.
When I was just a child, there was an old wives tale
that if you killed a snake, it would wiggle till sundown. Equally
stupid is the modern myth that you can't run a sleeve bearing motor
less than 500 rpm or the bearings will burn out. I thought that all
the years that the FanHandler has been around, that this would have
been totally rejected by now. Today, (10/25/2004) I heard from a
contractor that his factory rep. had told him the FanHandler wouldn't
work because blah, blah, blah. My answer was that the only thing that
that factory rep had going for him was that he could down five
martinis without falling on his face. Here is a little demo/experiment
that we conducted.
LET’S TALK ABOUT MOTOR BEARINGS
SHATTERING SOME OLD WIVES TALES - OR, THE SAME OLD STUFF JUST A SHINY NEW SHOVEL
HOW SLOW CAN YOU OPERATE A SLEEVE BEARING DIRECT DRIVE FAN MOTOR?
THE
GUY AT THE MOTOR SHOP SAYS YOU CAN’T RUN A SLEEVE BEARING FAN MOTOR
AT LESS THAN 500
RPM!!!!!
BEARINGS WONT LAST IF YOU RUN A MOTOR AT LESS THAN 500 RPM !
WRONG !!!!
The picture in the upper left is of our bench-demo to prove that a direct drive fan motor with sleeve bearings can be run at very low speeds for extended times. We often hear the comment: "I thought you couldn’t run a motor less than 500 RPM or the bearings would burn out." To prove you can, we have taken a 1050 RPM sleeve bearing motor out of an old Lennox furnace that had been operating for more than ten years and we installed a FanHandler on it and set the speed to 300 RPM. We didn’t do anything to the motor other than oil it when the demonstration started on 7-22-98. We discontinued this demonstration about May2004, when we moved to our new shop. It was still going strong. It ran all that time at 300 RPM and without oiling the bearings.
Sleeve bearing catalogs shows run times for every RPM and pressure load. The slower the shaft turns in a bearing, the longer it will last. The lighter the load the longer the bearing will last. Also, when a fan motor shuts off, the oil wicks back into the bearing so the next start is on a dryer bearing surface.
The rumors about motor speed and sleeve bearing failure started when air conditioner condensers were flat and vertical, winter winds would turn the fan blade and the motor shaft turned in cold and dry bearings. (Dry because when a sleeve bearing cools, it sucks lubricant back into the porous bearing material.) When spring came around, the condenser motor bearings were shot. We’ve been around since 1953. Over these years other companies have tried to copy the FanHandler circuit. (Anyone with a soldering iron can get into the business.) The design was pretty close, but then the accountants got hold of it and cut quality. These cheap copies caused even good motors to overheat. Overheating also kills bearings.
There are some marginally designed and constructed motors that do not run well at very slow speeds. These motors have fewer iron laminates and copper windings so the amperage is high because of very little counter e.m.f. produced in the coils. These motors rely on rotation speed rather than construction to produce counter e.m.f. If the motor manufacturer cut corners on the windings and iron laminates, chances are that they cut corners when it comes to lining-up the rotor with the stator. Then there are motors that have been banging on and off for years and the bearings are worn so the rotor is no longer centered in the stator.