Convert blower motor for ductless ops

[zz28zz]

I got a couple of air handler blower motors after replacing the home a/c sys. Wired one up to run on high speed. Connected it thru a amp meter to 120V. Motor is rated at 7.2A on high speed. My amp meter is reading 10A and every 20 secs or so, it would hiccup and pull ~11A for a half sec or so. Figured it was shot, then A/C guy says it needs duct work attached for it to run right. Started covering the outlet with a metal plate to create the back press it needs. Sure enough amperage dropped to 6A after covering abt 1/4 of the opening. Left it running for a few hours and it sounds happy. A/C  guy also mentioned he had ran one with no duct-work and it burned up after a few hours.

Started thinking abt the current and voltage peaks being out of phase. I'm assuming since a motor is inductive, if I increase the load on the motor (by partially blocking the outlet), the inductance will increase. The run capacitor is sized with a certain load assumed on the motor. If I run the fan with no blockage of the air, there's less load, so there's less inductance and now the run cap is too big.

Am I on the right track here? Will motor be happier with less capacitance when run with no ductwork?

[amyk]

What you've described is very unusual for an induction motor; they're usually happiest to run at no load with minimum current draw, so I suspect the effect here may be due to something else, e.g. maybe the bearings are worn enough that the rotor is not directly centered in the stator (which will cause excessive current draw) and it needs the air pressure from the fan to push it into the correct place when running.

[james_s]

It's possible that you have a bad bearing that is ok when run with some load on it but with the output wide open the motor spins up faster and/or loading on the shaft changes and allows it to float around in the worn bearing. Can you try removing the blower wheel and running the motor with no load?

[Zero999]

Started thinking abt the current and voltage peaks being out of phase. I'm assuming since a motor is inductive, if I increase the load on the motor (by partially blocking the outlet), the inductance will increase. The run capacitor is sized with a certain load assumed on the motor. If I run the fan with no blockage of the air, there's less load, so there's less inductance and now the run cap is too big.

Well no, in the case of a fan blocking the outlet reduces the load on the motor. This the same for a vacuum cleaner. Blocking the tube will reduce the air flow, so there is less force acting on the rotor, thus reducing the current consumption: notice how the motor turns faster? It's because there's less load on it to slow it down. Of course there is a limit, because it fit's blocked too much, the motor might not be able to adequately cool itself and in the case of a vacuum cleaner, with a universal motor running at light loads can damage the bearings from overspeed, but it's plausible that allowing full air flow could cause overheating.

[Simon]

As zero999 said. It is a common misconception that is even held by my work collegues in engine cooling and air conditioning that blocking a fan makes it work harder, it does not.

The work a fan does is to rotate the impellor (flywheel) and move air, if you take the air movement out by blocking either end it will reduce the work load. It's a fan not a piston compressor. Fans follow fan laws. The flow is directly proportional to the speed and the pressure follows a square law. So if you double a fans speed it will move twice the air and be capable of 4 times the pressure, this means that the power required will be 8 times (2x4). This means that slowing the fan down only slightly will greatly reduce power consumption. For example running at 80% speed will drop the power draw to 1/2. Speeding fans up is a bad way to increase air flow because of this, it is more efficient to use a bigger fan and run at the same speed.

Pushing air through a duct also has laws. If you push air through a duct there will be pressure drop, if you want to double the flow you will get 4x the pressure drop but happily if you double the speed of your fan it produces 4x the pressure. So I would guess that the motor was given and impellor that was too much load for it in free air safe in the knowledge that once put into the AHU it would be restricted anyway so move less air because of the pressure drop in the pipework.

What sort of motor is it? I have played with a brushless automotive blower that runs at a constant speed no matter the load. Holding it down on the bench so that air cannot flow it only uses 2A, but as soon as I lift it off it uses 16A as it now has work to do other than turn a plastic wheel.

[Kalin]

Zero999 is correct. It is very common for fans I work on (10-30hp) to block the inlet partially when starting up to reduce the load. We can even check if an inlet grate or filter is plugging g by monitoring the current draw and when it drops we know the airflow if being restricted. The highest current draw is wide open with no restriction

Sent from my SM-G965W using Tapatalk

[Simon]

I once worked on a project with 4x 350W fans in a vehicle air con system. The customer turned round at the last minute and declared that we only had half the power budget. So we reduced the maximum allowed speed to 86%. in theory it would have been 80% but because it was already in the AHU with minimal ductwork and a heat exchanger to blow air over there was already enough pressure drop that meant we needed to run 6% faster to do the same flow. I know that once installed with the customers ductwork it would have drawn less than 1/2 the power anyway at full speed but it was not worth having the argument with them or my boss.

[jmelson]

Centrifugal blowers draw maximum mechainical power with maximum airflow (no restriction) and minimum power when blocked -- the air just spins round and round.  This can be easily heard as the motor slows down under load.

And, yes, with no restriction, it will fry the motor.

Jon

[amyk]

Well no, in the case of a fan blocking the outlet reduces the load on the motor. This the same for a vacuum cleaner. Blocking the tube will reduce the air flow, so there is less force acting on the rotor, thus reducing the current consumption: notice how the motor turns faster? It's because there's less load on it to slow it down. Of course there is a limit, because it fit's blocked too much, the motor might not be able to adequately cool itself and in the case of a vacuum cleaner, with a universal motor running at light loads can damage the bearings from overspeed, but it's plausible that allowing full air flow could cause overheating.

Isn't it more the fact that blocking the input of a vacuum will lower the pressure (thus air density) inside because it's pumping air out, and that's what reduces the load?

[Simon]

Isn't it more the fact that blocking the input of a vacuum will lower the pressure (thus air density) inside because it's pumping air out, and that's what reduces the load?

Same difference, no air = no air pressure.

[zz28zz]

Thx for all the replies. I knew blocking the inlet would decrease the load but assumed blocking the outlet would increase it but the more I think about it, I think you guys are right. Looks like I'll be keeping the restriction plate on the outlet for now.

My original thought was to mount the blower above my boat to help dry it out after an outing. That's why I wanted it unrestricted, so it would blow air over a wider area. With the restriction plate, the airflow is somewhat concentrated. I may play with various grills and see what I come up with. Humm, wonder if I could accomplish the same thing by partially restricting the inlets? That way I could keep the wide air distribution pattern.

My understanding of A/C induction motors is that the speed is determined by number of connected poles inside the motor and the frequency of the line voltage. Different speeds are determined by which wire is connected since the different wires are connected to different number of poles. The motor I'm using has 4 different speeds so I have 3 wires left un-connected.
 

[james_s]

That is interesting, I had a niggling feeling in the back of my mind that blocking the output may in fact reduce the load but it's non-intuitive and I wasn't certain enough to mention it. It makes sense though when you think about the air just swirling around in a blower.

Does the same apply to a centrifugal water pump? What about a centrifugal compressor like that of a turbocharger?

[ejeffrey]

For a normal blower, the static pressure is so tiny compared to absolute pressure that it doesn't make a difference which side you block.

For a centrifugal water pump you always want to restrict the outlet never the inlet otherwise the static pressure can drop low enough to cause cavitation.  Keep in mind that many pumps are cooled by the working fluid.  Even if blocking the flow causes the overall power consumption to decrease the rotor might still overheat.

Centrifugal compressors only work properly with flow, otherwise they can oscillate.  That is why turbochargers have bypass valves.

[Simon]

if you block the outlet you will have more turbolence than if you block the inlet.

[james_s]

I wasn't suggesting blocking a water pump or turbo compressor as a practical application, only questioning the effect of doing so on the mechanical source.

Older turbochargers lack a compressor bypass valve. The one in my '84 Volvo is like this and you can hear the odd pulsing flutter if you suddenly close the throttle while under high boost. Adding a bypass valve is something I've been meaning to do for years.

[amyk]

Older turbochargers lack a compressor bypass valve. The one in my '84 Volvo is like this and you can hear the odd pulsing flutter if you suddenly close the throttle while under high boost. Adding a bypass valve is something I've been meaning to do for years.

This sound?

Apparently it's very desirable in parts of Australia...

[james_s]

Yeah more or less, it sounds vaguely like a turkey and it makes me cringe whenever it happens because I picture the shockwaves slamming into the compressor blades.

[zz28zz]

Update: Spent some time this evening experimenting with different airflow blockage configurations.
The blower has the motor mounted to one side so fabricating blockage panels wasn't attempted on that side.
Tried different amounts of blockage on the other side ranging from abt 30% to 100% blockage. Amp draw didn't drop very much, even at 100% blockage (one side only). As I approached 100% blockage, I could feel blower wheel vibration increasing. Aborted any further attempts at blocking inlet.

Used alum strips 1.75" wide across the outlet for blockers. Moving them around while watching ammeter. Ultimately the best combination of airflow VS amperage wound up being a little over 60% blockage using 3 of the alum strips. One placed across the top, bottom and middle of outlet.  This dropped the current to 6.5 amps (vs 10+ Amps unblocked). Air distribution pattern is better with this config as opposed to having blockers all at top or bottom of outlet. Ran it for abt 1 hr and checked motor/wire temps. Everything felt cool. Next step is to run it for ~1 week continuously sitting in the middle of a concrete floor to make sure it will be OK if left on overnight suspended above the boat.

Numbers below:

Outlet Height= 8.5"
Outlet Width= 10.75"
Blocker strip width = 1.75"
# of blockers= 3
Total outlet area= 91.375 sq"
Single blocker area= 18.81 sq"
Total blocker area (3 blockers)= 56.43 sq"
% of blockage= ~61%

[james_s]

It's probably not necessary to run it for a week, I suspect you'll find that within an hour the temperature of the motor will have reached equilibrium and won't really change much. If it has a rated amperage anywhere you could go by that, or calculate it out from the horsepower rating, IIRC motors like that are usually around 80% efficient.