#9
  
This might be better in Home Improvement but the Furnace uses power in my shop and keeps me warm while woodworking. I rest my case. Here's the question.

I have a big furnace in my shop currently with a 1.0 gal./hr. or about 135k b.t.u./hr. nozzle. When it's firing it requires quite a bit of airflow which it has and heats the shop fast.. I also use a wood stove that uses that same airflow to heat the shop uniformly. Over the years I've had to replace the motor a few times. The starting caps and/or windings burned out.(It's a large squirrel cage fan.) For a while I had a two speed motor wired so I could lower the speed when just circulating air through the filters. I need to replace the motor again and I'm ready to wire in a relay so it will run on high speed only when the burner is running. I can handle that part. My question is if I replace a 1/3hp with a 3/4 hp 1725/1450 rpm (which is a Dayton salvage from another big furnace) will my operating cost change substantially given the "load" or "work" is the same? I know I should have put an amp meter on it before posting but it's a pain to push start it fast enough for the run windings to take over. Angry
Sign at N.E. Vocational School Cabinetmaking Shop 1976, "Free knowledge given daily... Bring your own container"
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#10
  Motor size vs. efficiency MstrCarpenter This might be better...
(09-05-2020, 11:27 PM)MstrCarpenter Wrote: This might be better in Home Improvement but the Furnace uses power in my shop and keeps me warm while woodworking. I rest my case. Here's the question.

I have a big furnace in my shop currently with a 1.0 gal./hr. or about 135k b.t.u./hr. nozzle. When it's firing it requires quite a bit of airflow which it has and heats the shop fast.. I also use a wood stove that uses that same airflow to heat the shop uniformly. Over the years I've had to replace the motor a few times. The starting caps and/or windings burned out.(It's a large squirrel cage fan.) For a while I had a two speed motor wired so I could lower the speed when just circulating air through the filters. I need to replace the motor again and I'm ready to wire in a relay so it will run on high speed only when the burner is running. I can handle that part. My question is if I replace a 1/3hp with a 3/4 hp 1725/1450 rpm (which is a Dayton salvage from another big furnace) will my operating cost change substantially given the "load" or "work" is the same? I know I should have put an amp meter on it before posting but it's a pain to push start it fast enough for the run windings to take over. Angry

 I would say the operating costs will be about the same or less.   The 1/3 hp motor sounds like it was staying too long on the starting winding trying to get blower up to speed causing a larger electrical load while coming to speed.   As you said the work being done is the same.  I doubt you will see a difference in your bill.
 Hopefully Tom will see this and comment on it.    I have a feeling if you can check the amp load on the 1/3 hp one you will find it to be over fla on one or both speeds.   Roly
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#11
  Motor size vs. efficiency MstrCarpenter This might be better...
The 1450 rpm is on 50Hz power (1500 rpm synchronous speed for a 4-pole motor), so you can ignore that.  Assuming it's a direct-drive blower, the operating speed will always be 1800 rpm for a 4-pole on 60Hz power (minus a few percent at full load).  So will the blower speed, of course.  So the only way to change air flow, and therefore power input to the blower wheel, is by choking off or opening up the air flow. 

Power input to the blower wheel will decrease with air flow reduction, which is a property of centrifugal blowers and pumps*, so that's one way to get the current draw under control.  And you absolutely do need to put a clamp-on ammeter on that blower and compare to the motor's nameplate FLA (full load amps) value.  

Belt-drive blowers used to come with an adjustable motor sheave, where you could screw the outer half in or out to change the pitch diameter and thereby change the blower wheel speed.  I'm not in HVAC, but I've always assumed that was to tune the motor to the load, since the duct work back pressure (and front pressure, from returns) is different for every installation, and an installer wouldn't want a new installation to overload the blower motor.

It sounds like your existing installation doesn't have enough static pressure to keep motor current under the rated value.  That's not surprising if there's no ductwork.  I salvaged the blower from my attic A/C system when it was replaced years ago, and when I ran it on the garage floor, it pulled something like twice the rated current.  It ran in my attic through an extensive duct network for about 5 years that I lived there, and who knows how long before that.  Obviously the flow resistance was adequate. Just an FYI.

Having said all that, I agree with Roly that as a general rule, the load is the load.  Working a motor at a low output does put it in a lower efficiency than it would be at rated load, which is normally the highest efficiency or close to it by design, but for intermittent home shop use, it's not likely to make a noticeable difference.  Large motors, like 500 hp and up, have extremely high efficiency (like 96% or more) and oddly enough, that doesn't change but a little across the output power range (1/4 to 5/4 output power).

Single-phase motors, however, are a funny animal.  Low power motors have terrible efficiency, and the lower the rated power, the lower the efficiency and power factor (the percentage of current that drives the load and makes heat, but it's always less than 100%).  So it's entirely possible, likely even, that a 3/4 hp motor operating at half power will have the same efficiency or even higher as a 1/3 hp motor running at full power.  

Just a caveat about the current draw, though - the 3/4 hp may draw more current even if it has higher efficiency, because power factor is likely to be lower when running under partial load.  Your electric meter does not register this extra current (reactive power) and only registers the 'real' portion of the current**.  So even though the current draw may be higher, your meter is not likely to spin any faster, and may even spin slower, than with the motor you're taking out.

So use the bigger motor and don't worry about it.  And sorry for the diatribe, but I'm quarantined in my travel van in my driveway after coming back from another country on 'essential' business, and have absolutely nothing better to do.  Though I could do some work I suppose, but this is more fun, and it is Labor Day weekend after all.  I'm also re-watching the whole Breaking Bad series at the same time.   Yes

* Impulse fans and pumps, like a table fan or boat prop, work the opposite way; increase the static pressure, and power input increases.
** Watts (W) is 'real' power, and volt-amps (VA) is 'real' plus 'reactive' power, or apparent power. A simple ammeter reading multiplied by voltage is the apparent power. A Kill-A-Watt unit will display watts and volt-amps, along with power factor (VA x pf = W).  VA is always equal (pure resistive load) or higher than (any reactive load, like motors, capacitors, transformers) W.  The reactive component is out of phase with voltage, and is just sort of bounching back and forth between the load and the utility.  Sometimes called 'magnetizing current' when talking about motors and transformers, it's a bit like pendulum where it's trading elevation and speed, back and forth, but not actually doing anything, even if it's a heavy pendulum that could take your head off if you got in the way (like a kid on a swing).  Only commercial meters, and I guess so-called 'smart' meters, register VA, but only commercial users get charged for excessively low power factor.  Not home owners.  Unless that's changing these days with smart meters.   Raised
Tom

“This place smells like that odd combo of flop sweat, hopelessness, aaaand feet.”







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#12
  Motor size vs. efficiency MstrCarpenter This might be better...
A big Thank-You goes out to Roly and Tom. I've learned a lot from you two. I do know about the drastic drop in amps when you totally block the intake of a vacuum system like dust collection. Learned most of it right here. So now I'm beginning to realize that the early demise of those motors was probably due to user error. There are two 25" x 25" x 2" filters right in front of the 4' x approx. 18" squirrel cage fan that resides right under the heat exchanger. It's a wood shop so decent filters get clogged up pretty quick. That makes the burner shut off at the high limit before the shop gets warm. My solution? Slide the filters apart creating a much easier path. I only considered closing them back up when I was creating dust. I can't say I won't do it again, so I think I'll be better off going with the larger motor. Two speeds are nice for my application so hopefully I won't be looking for another motor.

Tom, The fan is belt driven and still has a variable pitch sheath. I'd guess the reduction is about 1:3. I'll have to look at the motor again because I know I saw two speeds but only recall seeing 60hz. I do have other three speed blower motors but they are in the 1/4 to 1/2 hp. range. I have very little ductwork. My friend is a tin-knocker and as quick way of getting it functional, he made a 48" x 30" to 48" x 12" sweep 90*that's just below the ceiling and then put a pair of 12" x 24" 45* on aimed at opposite corners of the 24' x 48' shop. It has worked so well for me that I don't see any reason to ever put it all in the ceiling. I guess that's the long version of "there's very little restriction in the ducting."

Thanks again for the insight, stay safe and healthy.
Sign at N.E. Vocational School Cabinetmaking Shop 1976, "Free knowledge given daily... Bring your own container"
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#13
  Motor size vs. efficiency MstrCarpenter This might be better...
An adjustable sheave is a good thing here.  According to the Affinity Laws (aka Fan Laws or Pump Laws), for a given fan, air flow is proportional to fan speed, sound pressure is proportional to the square of the fan speed, and input power is proportional to the cube of the fan speed.  So a small reduction in blower speed can yield a large reduction in input power and by extension, motor current.  That makes it easy to tune the blower to prevent motor overload.

But by all means, use the larger motor.  But I’d still strongly recommend checking the motor current before letting it run long.
Tom

“This place smells like that odd combo of flop sweat, hopelessness, aaaand feet.”







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#14
  RE: Motor size vs. efficiency TDKPE An adjustable sheave...
(09-07-2020, 06:34 AM)TDKPE Wrote: An adjustable sheave is a good thing here.  According to the Affinity Laws (aka Fan Laws or Pump Laws), for a given fan, air flow is proportional to fan speed, sound pressure is proportional to the square of the fan speed, and input power is proportional to the cube of the fan speed.  So a small reduction in blower speed can yield a large reduction in input power and by extension, motor current.  That makes it easy to tune the blower to prevent motor overload.

But by all means, use the larger motor.  But I’d still strongly recommend checking the motor current before letting it run long.

Tom,  since most furnace blower motors are PSC type the speed changes are by changing the amount of rotor slip.  I would think this will also lower the available HP at other than highest speed but I am not sure if the if the load reduction from lower cfm would be enough to compensate if the motor was overloaded before ?   Something else to think about in your driveway.   Roly
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#15
  RE: Motor size vs. efficiency Roly [quote='TDKPE' pid='...
(09-07-2020, 10:32 AM)Roly Wrote: Tom,  since most furnace blower motors are PSC type the speed changes are by changing the amount of rotor slip.  I would think this will also lower the available HP at other than highest speed but I am not sure if the if the load reduction from lower cfm would be enough to compensate if the motor was overloaded before ?

His is belt-driven, which was usually just a cap-start 4-pole motor.  Or a three-phase in commercial installations.  Direct-drives are something like 4-speed, and as you said, speed reduction is via increased slip.  I'm not in HVAC, so it's entirely possible multispeed motors were also used with belt drives, but I've never seen one.  

Therefore, it never happened.   Laugh Big Grin 

But if he want's to reduce motor load, he can simply baffle the inlet or outlet to cut the power down, and/or reduce the motor sheave diameter.  But I think he said he'd already fried his motor, so another one is in the works or in his spare motor inventory.

(09-07-2020, 10:32 AM)Roly Wrote: Something else to think about in your driveway.

Hey, Bob Ross is kickin' it right now on PBS.  Yellow ochre and happy little clouds.   Cool
Tom

“This place smells like that odd combo of flop sweat, hopelessness, aaaand feet.”







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