This old dishwasher motor
appears to be well within it’s rated load
for this size of impeller, and I want to do
some experiments with different shapes
of impeller blades. I’m laying out arcs for
the blower vanes, and I’m using calipers to consistently space
the centers of the arcs. This will be the hub. And hopefully when I
glue this on there, I won’t end up
with a rotor that wobbles. I’ll use a drill to center it and now just gravity clamp it
in place. That’s pretty good. That runs nice and round.
But, I won’t spin this up yet because these are just freshly
glued and they might fly off. And, it looks like it’s
a little bit out of balance. This one’s too heavy. I didn’t want to whittle
away too much of the rotor so I added a counter-weight. Now I have to design
the spiral for the housing. The arc is gonna go
from this radius to this radius as it goes around
so if you go from here, and then we go to this radius,
and this radius, and then this radius. And, so I’m gonna set my compass to about halfway between
these two radii here. And, then I need to find
a point here, where it joins these two up. Bring it in to halfway
between this radius and this radius,
maybe about this much. And, once again, I got to find
a spot for the compass that meets those. I kept hitting
the post of the bandsaw. My 16 inch bandsaw
isn’t big enough, but my big 20 inch bandsaw
is in my other shop set up as a saw mill right now. Making the wall
for this spiral housing has always been a lot of work and so this time I’m trying
something different again. I’ll put these pieces on and I’ll cut them out afterwards Now I just have to wait
for the glue to dry and then I can cut it out. I need to work out the geometry
of these mounting holes. And I’ve already measured them
and they’re all equal distance of 111 millimeters, so I know
this is an equilateral triangle, so to get the radius
from the shaft to the hole, I can take this distance 111 divided by two, and that
gives me this distance here. And to get this distance,
I divide it by 30 degrees cosign because I know that angle
in here is 30 degrees, so that comes up to
64 millimeters, and let’s set
the calipers to that and that looks like it’s right. Unfortunately I already drilled
out this hole, so I had to make that plug to put a compass
in the middle of it. And, one of the holes
will go here and to go 120 degrees
to that one, and next hole goes here.
And, just to check Uh oh, that doesn’t
quite work out. So, I’m gonna come from here
out like this. When I looked at footage of
this rotor spinning, I noticed some of the fins seemed to
be not quite on straight specifically this one.
So, I was able to pry that off and put it back in the
right place, and that made this counter-weight
almost unnecessary. It’s tempting to spin
this rotor up as it is, but there’s so much
centrifugal force on these fins that they might break off,
so I need to glue this on. And, every time I’ve
glued one of these on, I’ve always gotten it
off-center a little bit but I had a cool idea
for centering this. Just take the off-cut,
put that on the drill, and that serves as a guide for
where I need to put this part. My goal with these experiments
is to test an impeller that’s typically found
in the dust collector, which has got
straight fins like this, against one that has
the fins forward-facing, and backwards-facing.
And to actually make them face backwards, I’m just
gonna reverse the motor so that instead of this way
it goes that way, and I’ll flip around the
housing, and then I have a backwards-facing impeller
without having to make a new one First test is with
a straight fin impeller. Now, let’s switch that over to
the forward-facing impeller. So, what’s interesting is this
impeller only ran at 2000 RPM versus 3000 RPM for this,
but the wind speed coming out was the same, so for the same
RPM, this blows a lot harder, but it also needs more power. The next step is to switch to
a backwards-facing impeller, and for that I have
to flip the housing and I also have to
reverse the motor. The way this motor’s wired
internally, I can’t reverse it electrically, but
I can reverse it just by flipping
around the stater. 143 watts.
3215 RPM. This is noise level
straight impeller. This is noise level
backwards-turned impeller. The numbers are less straight
forward than I was hoping because when I had the fins
forward-facing the motor was way overloaded and it was
only running at 1984 RPM. That’s with the rotor
going like this. So, at that point, the
efficiency of the motor drops dramatically.
With the straight fins, the motor’s running at 3052 RPM,
still slightly overloaded. Interestingly enough, the air
speed coming out of the blower was the same for both,
even though for the forward-facing fins, it was only
running at two thirds the speed. So, that tells me if you’re
limited by RPM, but not by power you can actually get
more air flow like this. Although in terms of static
pressure at the lower RPM, that would a lot less.
With the backwards-curved fins, the power consumption
was a lot less, so just 74 watts over idle
versus 213 watts over idle, so almost down to a third.
The air speed was less, even though it was running
at a higher RPM. So, you basically would need
a bigger blower to get the same performance
running it this way. But, the static pressure
that it was able to put out was greater with the
backwards-curved fins. So, it would make sense,
in terms of efficiency, to run the rotor
backwards-curved and just make it bigger, because
it’s so much more efficient that you can run a larger rotor
with the same size motor. Now, for the forward-curved,
this was real inefficient, but the motor is also
running very inefficiently, so the lower efficiency
going this way is probably not as dramatic
as this would suggest. So, now I can understand
why sometimes you see forward-curved fins, and
sometimes backwards-curved fins. The forward-curved fins,
for the same RPM and the same blower size,
will give you more air flow, but probably less efficiency.
And, the backward-curved ones, that’s best for having a larger
rotor with a small motor, and it also gives you the
best static pressure. And, of course, the straight
fins is a nice complement between the two of them,
and it’s easiest to build. And, it seems intuitive
that forward-facing would give you higher air flow,
so I put a marble in here and turn this rotor, and it just whips it out like I
was playing a game of Jai Lai. Whereas going backwards,
if I put this marble in again, and I turn it
the other way around, it comes out much more slowly.