----- Original Message -----
From: Gene Fuller
To: tips@contesting.com
Sent: Monday, April 12, 2010 12:48 AM
Subject: Prop Pitch Motor Power Supply
AC or DC is the question. I have heard from several sources that the PPM's
should be run from DC or else the brushes would not last as long as they might.
I had run one for 30-40 years on AC, with small to medium sized antennas, befor
any sign s of a problem. In view of the recommendatioins I added a diode bridge
to give me some DC. At the same time I had the commutator turned and put in new
brushes. Much to my dissapointment it now appears that after only 6 or 7 years
(turning a fairly large, 7-60 MHz) LP) at least the brushes are gone. Over a
period of a few days the speed dropped to about 0.1 RPM and finally quit all
together (same CW or CCW). I haven't had a chance to get the motor down off the
tower yet.
I was recently talking with an acquaintance who is an engineer and seems to be
familiar with electric motors and he was quite emphatic that AC would generally
be preferable. The reason is that with DC you tend to draw an arc from sector
to sector of the commutator as the armateur rotates, whereas with AC the arc
tends to be self-extingushing as the wave passes through zero. I didn't think
to mention to him that the motor operates at about 9,000 RPM. Perhaps he is
right with motors that operate at lower speed relative to 60 Hz, but with the
PPM running such high RPM's and drawing less current when operating on DC, DC
really is the right way to go.
Any comments would be appreciated.
Gene's / W2LU
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