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Choosing a DC motor for a DIY Centrifugal Blower

  1. Nov 6, 2016 #1
    I burned out my scavenged motor today and now I think I want to apply a little more thought to moving the project forward.

    I'm working on a centrifugal blower that can suck air through a charcoal filter so I can keep the air clean around my soldering area. I chose a tuna can for the housing although my impeller (pictured) barely fits. I think I will need a larger housing.

    The motor I was using came from a hair dryer. I don't know the ratings but it seemed well behaved at 13.8v, drawing 65 mA when things settled down. The shaft was roughly the same size as my 5/64" drill bit, so I used that to drill the hole. The rotor wasn't perfectly true but it was close.

    Anyway, I glued the blades to my impeller and mounted it on the motor with some hot glue. I let it sit for 15 minutes or so for peace of mind that the glue solidified. Then I fired it up. Fire is correct...the impeller ran and I was impressed with the amount of air it pushed amazed me. Then I noticed smoke coming from the motor body. Then the fire came. I killed power, watched the tiny flames die, and then pulled the housing apart to attempt to let the parts cool.

    I was more amazed that, an hour later, the motor still turned when I reassembled it. It ran slower and I could see blue sparks issuing from the area where the commutator is. Being a cautious sorceror, I threw the motor in the trash. Maybe I could've salvaged the wire for an RFC or something.

    Anyway, I'm hoping to buy a new motor to replace the failed one. I'm looking for ideas for what kind of motor to buy. I see lots of options for RPM. Somehow I don't think that rotating at 15K RPM is going to work. Maybe I should look in the 1000-2000 range. I'm asking here because I think someone else has been down this road before and I'm going to try to find him/her.

    Another option is to build a smaller impeller with lighter material and more clearance between the edges of the tuna can. I like this impeller's strength and air movement but I'm not married to it.

    Ed Impeller.jpg
  2. jcsd
  3. Nov 8, 2016 #2
    Balance your impeller. You can do it the same way propellers are balanced for model airplanes.

    Motor current demand depends on how much work the motor is doing, but you also want the motor to operate close to it's most efficient rpm. You need to match the motor's Kv rating to the rpm you plan to spin the impeller. That's probably why the hair dry motor fried. Use a Kv value that puts Kv times operating voltage around 25% greater than target rpm.

    With DC power you have the option of brushed or brushless motors. You can use an off the shelf speed controller if you want to use brushless. That will make it easy to vary fan speed by supplying a signal to the ESC using a servo tester. Otherwise you can use a brushed motor with a PWM driver to vary speed. Another option would be to just use an EDF assembly (electric ducted fan). A shop like HobbyKing stocks that stuff for low cost.

    Small fans typically have to spin a lot more than a couple thousand rpm. Probably more like 10k rpm would be required. Otherwise if you want to operate at low rpm you'll need a motor with an appropriate Kv value otherwise it will just burn up again.
  4. Nov 8, 2016 #3

    jim hardy

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    I always say learn from your predecessors.

    Try a search on "Fan Laws" for a feel as to speed of a fan versus pressure differential it makes.

    Then take apart one of those battery powered "Handy Vac" car cleaner vacuums and study how they did it. I see lots of them in thrift stores.

    Beware of home brewed rotors and high speed motors. Centrifugal force is awesome and will embed plastic in walls and spectators.
    When i was 13 i put a bronze fan blade on a vacuum cleaner motor that unbeknown to us kids had more than enough power to overspeed it. One piece went nearly a half inch into the solid oak floor, another sliced through a spruce dresser like a sword and the rest we used pliers to pull out of the plaster walls .

    It's a wonder anybody survives their teen years.

    old jim
  5. Feb 23, 2017 #4
    Here is my favourite and pocket friendly 12 Blade 64mm EDF.

    It sounds muchh much better have a look at the video below

  6. Feb 23, 2017 #5

    jim hardy

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    Hmmm. Hair dryers are 120VAC aren't they? So you had a universal motor i think ? They'll run on DC.

    Now, you reported 13.8 volts and 0.065 amps which is only 0.9 watt, not enough to burn up much of anything.
    So i'm thinking that was running the motor by itself with no blades attached.
    When you affix blades and run it , it should slow down.and the current should go up dramatically. Did you make any current measurement while pumping air?

    The sparking and slowdown sounds like fried windings.
    My guess is that the motor does not move enough cooling air through itself to carry away the heat produced in its windings.
    That might be due to reduced speed, or due to your repackaging job with the tuna can no longer forcing air through the motor.

    I'd find the heater blower fan motor from an automobile.
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