I think Rohn 45 will fit certainly fit Kurt's small and likely medium
prop pitch, but better ask him. There are some parts he would like to
have more of but plenty of PP otherwise.
One piece of hard learned advice: if you are going to roll your own
rotator don't use any sort of VFD or PWM amplifier anywhere near your
antennas. IGBT and MOSFETs are now REALLY fast and the circuit
engineers squeeze the most VA of power per cubic inch of amplifier and
heat sink by generating nsec risetime of the PWM pulses.
I'm not a fan of the RAK grub screw mast clamping (also doesn't fit 3"
masts). The K7LXC, KF7P, and K7NV clamps are all superior IMO.
Grant KZ1W
On 2/28/2019 13:54 PM, jimlux wrote:
On 2/28/19 12:41 PM, Patrick Greenlee wrote:
Re-purposed prop pitch motors are definite contenders in the
strength/robust department but fitting in a small space???
On 2/28/2019 1:58 PM, Chuck wrote:
On 2/28/2019 1:22 PM, RVZ via TowerTalk wrote:
Pretty hard to find a better, more reliable rotor than the BIG RAK.
Chuck KØTVD
www.dxham.net
Please advise the rotor that will handle the biggest windload and is
likely to be the most reliable that will fit a standard Rohn 45
rotor plate?
What will the ham world do when the last prop pitch motor dies??
Some years ago I was pricing big DC (and AC) motors with suitable
gearboxes and they are *really* expensive.
The BIGRAK as an example, is 4000 in-lb torque at something like 0.5
RPM, but as noted elsewhere prop-pitch tend to be somewhat higher torque.
The PP has a reduction gear of something like 7000:1 or 9000:1 - so the
basic DC motor is running at 5,000-10,000 RPM or thereabouts. It's
probably something like a 1/3 HP motor (240W @ 24V is 10A...not having
run one myself, I don't know if this is a typical current)
1/3 HP at 1 RPM is about 28,000 inch pounds.
(1700 foot pounds)
Typical AC electric motors have speeds around 1800 RPM, so an 1800:1
reduction is needed. Getting that in one step is unlikely, so figure
you need two gearboxes with 30-40:1 reduction.
So, a $500 3/4HP C-face TEFC electric motor, and a pair of $1000 speed
reducers will set you back about $3000. Plus maybe $500 for a decent
Variable Speed Drive.
At least, you're now using something that is a standard industrial
product and if you need to replace it, you can order the parts anywhere.
Will it fit inside the tower?
A 56C size motor is 6.5" in diameter. I don't think that will fit
inside Rohn 25 (barely).
(There are better solutions out there. using, for instance, a
worm&pinion reducer, but I didn't happen to have my catalogs handy).
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