Blocking off inlet on squirrel cage blower

Blocking or partially blocking the Inlet to a Squirell Cage Blower gives the same effect as blocking the suction on a Vacuum Cleaner. The blowers design loses flow and requires less power to run in a Vacuum so it speeds up since the torque load decreases.

There are many applications of these blowers and I have had many run for years with no problem since they have motors sized to handle the load in every installation I have come across.

Thanks BudT, I built an indoor wood furnace before and used a direct drive blower from a mobile home on it for 10-15 years with no problem. So I will just disregard that information.

Thanks again, 03deuce

I recently built a unit heater for my outdoor wood boiler and I used a squirrel cage blower from a mobile home furnace its a direct drive model. The other day a friend of mine seen it and told me I would have to block off the inlet some or the motor would burn out before too long. So he went on to demonstrate he took a piece of cardboard and slid it along the side inlet and to my amazement the motor sped up and ran easier and there was a lot more air coming out of it. I know when the blower was in the mobile home the inlet wasn't blocked off. Has anyone ever heard of this or know if it should be blocked of a little?

Thanks 03deuce

This approach is more likely to overheat the motor - if the inlet is blocked, where does the cooling airflow come from?

I was unaware the Motor was in the Air Flow on his Blower setup. All the Direct Drive Squirrel cage Blowers I've seen have the motor outside th Blower Housing where there is no Blower air flowing. Even the ones on automobile heating and air conditiong systems.

And, I was under the impression the Electric Motor turned faster since the LOAD WAS REDUCED????? However, I'm not an Electrician so that information could be Bogus as John pointed out.

Good point Bud - it's another case of incomplete information. It can't be stressed enough: if you want a good, reasoned answer, you have to pose the whole question.

Right on both!

The motor must have an integral fan (inside the motor), separate from the squirrel cage, to cool the motor.

The squirrel cage can be considered a pump. Under partial vacuum it pumps less air, so does less work, and uses less power.

Vacuum cleaners, on the other hand, normally use the air they ar pumping for cooling, so blocking off the input WILL overheat the motor.

On residential and light commercial heat/AC the motor is in the air flow, located either inside the squirlcage if direct drive or on the fan housing in the return on belt drive. -- JHF

Theoretically, if you block the inlet of a blower (lesser area), it is pressumed that there would be less air to blow. The motor wouls exert more effort. Most blowers' design are maximized.

Blocking the entrance vs. the exit do radically different things. If you block the inlet the pressure inside the fan assembly will drop (lower air density) this will reduce the load on the motor, which can speed it up. If you block the exit, the pressure inside the fan assembly increases, you slow the motor down (by increasing the load).

Maybe the guy wants to sell you motors is all I can figure out.

I'm just speaking from my own logic. If the motor sped up by blocking part of the inlet, it did something to reduce air turbulence and therefore means it is under less of a load............... who cares why?

Less load means a cooler motor. How could a motor get hotter if you reduced load?

Many vacuum cleaners use the same air that is used for transporting dirt to cool the motor. If you block the inlet, you can easily hear that the motor speeds up, indicating less load (and quite measurable on an ammeter), but there is virtually no cooling. The motor power does drop, but not as much as the airflow does, so the motor temperature will rise.

The reason that the motor speeds up is the fact that there is less back pressure coming from the outlet of the fan due to the decrease in inlet volume of air. This in turn also creates a higher velocity of air at the outlet due to the higher fan speed making it seam as though there is more airflow through the fan. Compare the velocity verses the volume of air between a leaf blower and a shop air hose.

The reason for the lower back pressure is that the fan was designed to force air through an open duct system and not in to a closed burner box which creates the higher back pressure.

Experiment with this. Disconnect the fan from the burner box and run the fan. Covering the inlet should not affect the motor speed more than a few RPMs, and should not increase the air velocity, but will decrease the volume of air at the outlet.

As for the question of blocking the inlet or not depends on several things. You should adjust the air flow to create max efficiency of the boiler. Why not come up with an adjustable inlet and keep an eye on the motor temp. Should not take long to come up with a happy medium.

Good Luck.

Hello 03Deuce,

Depending upon the CFM or l/min. rating for your blower system you will have to be mindful of airflow restrictions. Please note that back pressure can cause damage to your blower motor if airflow restrictions as too great. Another factor to be mindful of is the maximum ambient air temperature rating for your blower system. If back pressure is introduced on a continuous basis, and hotter air backs up into your blower this could cause the blower motor to over-heat.

Therefore, restricting airflow at the inlet does make sense if these factors were to be considered. I hope this is helpful.

Best Regards,

b3cds

A "squirrel cage" type fan is of the same type that is often called a centrifugal blower. It is counterintuitive but when the supply of a centrifugal blower is restricted, typically the load on the motor decreases. It depends on where you are at in the fan curve. If a blower motor combination was designed for lets say operation at 10 inches of water differential pressure and in a new application there is only 5 inches, the torque needs of the fan in the new application would go up, therefor slowing down the motor and possibly burning it up. Putting a restriction back in (cardboard) would speed the fan up. Of course you still have to consider cooling air.

I believe the reason for this is that less restriction allows more air to enter the blower and thus it labors harder to move it. It might labor hard enough to drop the RPM of the motor and flow less air. The prime point is; we are dealing with a centrifugal blower and not an axial flow fan.

This is hard to relate with a discussion only so I suggest the best way to get this point is go to any manufacturers sight that sells centrifugal blowers and includes their fan curves on their web sight. You can see from these curves what more or less restriction (in inches of water) does to the HP needs, fan RPM and to the CFM out.

restricting the inlet, or outlet, changed the velocity vector for the better (higher airflow or more air coming out of it and ran easier or lower input current/vibration?), perhaps initial design could be improved

In the first 17 responses, no one has addressed the kind of motor. Since the blower came from a mobile home, it might well be a low voltage DC motor, in which case it is probably a brush type motor. DC motors have very different speed/load curves than similar power AC motors. If it is low voltage, then of course you have some kind of power supply, which could greatly affect the perforemance.

As usual, more details on the question will probably lead to more useful answers. A photo or two of the installation could help too!

To dkwarner : In the first 17 responses, no one has addressed the kind of motor. Since the blower came from a mobile home, it might well be a low voltage DC motor,

The thread topic itself mentions its squirrel cage motor, so this is not a low voltage DC motor.

If it increases speed then the slip has reduced so it must reduce current and heat.

Vacuum cleaners are universal series motors, blocking the inlet increases the speed a large amount and reduces current drain to very low levels, but as mentioned before they also use the airflow to cool the windings.

NO, the thread topic does NOT say squirrel cage motor! It (and the post itself) says squirrel cage blower.

Cars and trucks commonly use squirrel cage blowers with brushed DC motors. I would not be at all surprised if some of the newer ones use brushless DC motors. Those would still be low voltage DC powered. I haven't happened to work on moblie homes, so I can't say for sure, but I suspect they have similar blowers. Otherwise they would only be able to turn the furnace on when they were plugged in or running their own AC generators.

My comment and question remain valid.

The OP said mobile home, not motor home.

A mobile home is built at a factory as a whole unit, or two halves of a double wide, then transported to a sales lot. A customer buys it and the dealer delivers it to the customer's location. The mobile home then is hooked up to water, sewer, gas and/or AC electric service, and remains there until it is moved to another location, or carried away as scrap when it has come to the end of its useful life.

A motor home, aka an RV, has a motor and wheels like a truck or van, and you can drive it. Just like any automotive vehicle, it has a DC electrical system.

Don't confuse the two. I know the difference because the house next door is a mobile home that was moved in to replace one that had worn out. Its electical meter is mounted on a pole that I see evertime I step out front. My street has several more, and it used to have even more, but some were carried away as scrap.

As a ham radio operator, I have seen several hams bring out their motor homes to Field Day events on the last full weekend of June.

Yes, you're probably right! I was indeed thinking of a home with permanent wheels - a motor home or RV. I haven't personally dealt with either, 'tho I've seen plenty of both.

Thanks for pointing that out.

I would check 2 things: the amps the motor is drawing vs. the name plate amps. If it is high, the motor won't last long. Reduce it by lowering air flow. The mobile home setup has almost no restriction with room temperature air over the motor, but a lot of static on the supply side. So you may need to reduce the air flow to get down to the name plate amps or see if the motor is a muli-speed motor and tap to a lower speed.

Second, as mentioned, the return air was room temperature, so make sure the in-coming air is not too hot for the motor. -- JHF

Your friend was quite right.

See site < http://mysite.verizon.net/restkgfn/airpolcontrol > page: "Fan Motor Burning Mystery"

Any questions?

714/ 536-6310

Alex

If the blower is located on the outside of the blower housing, you can reduce the airflow to whatever is best for your combustion process as long as your amperage is within the specifications of the motor. You would be better to adjust the adjustable pulley to obtain the proper air flow, rather than blocking the blower.

If the motor is in the airstream, block the discharge of the blower to create the necessary resistance making sure the amp draw is not too high. If you block too much air, the motor will overheat and not last very long. Never block the inlet to the blower.

A squirrel cage/motor like will try to spin at the same speed no matter how much air it is allowed to have, if you give it to much air the amp draw will go beyond what it is rated for (burning up the motor) and you may experience a lot of "slippage" (motor not spinning fast enough). It was okay in the furnace because there was enough resistance in the ductwork to keep the volume of air down to a manageable amount.

What you should do is get something a little more secure then cardboard, mount it, and take amp draw readings... if it is too high, cover up the sides a little more, it doesn't really matter if the amp draw is on the low side but for maximum air movement you want to adjust it 'till you are at maximum amp draw.

1. A squirrel cage fan does NOT necessarily have a squirrel cage motor!

2. Even if it does hnave a squirrel cage motor, I can't believe any reputable manufacturer would sell a fan that can't run indefinitely with no ducting. I've had just such a unit cooling my attic on hot days for many years.

3. "if you give it to much air the amp draw will go beyond..." I assume you mean too much air, meaning excessive input air. Fully open to the atmosphere with no input ducting should not be excessive. Truly excessive input (more than one atmosphere pressure at the input) would reduce the load on the fan, and thereby reduce the current, not increase it. If the excessive input had sufficient pressure, the incoming air would spin the squirrel cage and the motor would actually become a generator (although not a very efficient one), so the current could actually drop below zero and reverse!

I thought I'd add to this discussion with some empirical results from my own experience.

I'm using a centrifugal fan I salvaged from a residential HVAC air handler for whole house ventilation. Its mounted on a wall in cupola at the top of our living space and blows outside through self-opening/closing louvers. I've constructed a box that houses the fan on the inside that serves to 1) restrict inlet flow; 2) attenuate noise; 3) hide the fan. The motor is a 4-speed FASCO D701 rated for a max of 1/2 HP at 1071 RPM and 115v. Full load amperage ratings range from 3.3 to 7.7 depending upon speed chosen.

Observations: (Tests were run without the interior box housing, basically in free air)

If you run without any inlet restriction at any selected motor speed the current draw is about twice the motor's rating for that speed (e.g. on low speed, I measured almost 7 amps where rating is 3.3 amps). The motor gets very hot despite the fact that its mounted inside the fan housing such that air entering the fan motor passes over it. I suspect if I ran it long enough like this the motor's thermal protection would trip.

If I restrict inlet airflow, the fan speed increases, apparent airflow increases (I didn't measure but you can tell), current draw decreases, and the motor stays cool. Gadget Guy and others here are right. I just wish I had some way to measure RPM to see how that relates to everything else.

WoW! That is really interesting! Restricting the input increases air flow?! This motor must be operating with a great deal of electrical slip!

My first thought is that the unit was designed to draw its air through a filter, which restricted the airflow, so the fan operated in a significant partial vacuum, reducing the air resistance.

I just tried lubricating the bearings of my attic fan, and was not successful in place, so I plan on bringing it down to clean and lubricate in a couple of weeks. When I do, I'll make some measurements. Mine is a 50-year-old belt-driven centrifugal fan with a single speed induction motor, so it will undoubtedly react somewhat differently than yours...

In the past, I have measured speeds simply by connecting a battery and phototransistor to the input of my oscilloscope, and aiming the phototransistor at wheel spokes or blades (use sunlight or DC illumination, to avoid powerline ripple). My 'scope has a digital indication of frequency, so once correctly adjusted, the speed can be calculated from the frequency readout.

All Residential backward blade blower wheels are engineered to operate within specific static pressures.
If removed from ductwork they will overload due to lack of static resistance.
You can take a piece of ply-board & move it down over the air discharge & the amp draw will keep lowering as will the airflow volume delivered.
Most residential blowers' were rated no more than 0.50" of water column static resistance, (ESP, or External Static Pressure. They also have a minimum static point where if they go below it, they will begin to overload.
Many will bog-down if removed from ductwork; newer blower wheels are engineered to work against higher static pressures.
Axial fan blades do not unload against resistance like backward blade squirrel cage blower wheels. - udarrell