Maybe they just used a motor that can take the stall current forever? A
standard permanent magnet motor will typically have a stall current that is
around 3-4 times the nominal running current (although, for DC motors, there
is a lot of variabilty in design).. If you've designed the motor so that it
can dissipate 16 times the heat (I squared..) by using bigger wire, etc.,
then you're all set.
Another approach: If you drive the motor from a constant current source,
then, by definition, you're dissipating the same power all the time.
For AC motors, it depends on the type of motor.. Some motors can be stalled
indefinitely without damage, because the effective inductance of the
windings limits the current. Also, just as for DC motors, you can build an
AC motor that is overdesigned (thermally) enough to tolerate the stall
current. Many basic PSC (permanent split capacitor) motors (they are
essentially a two phase motor, with a capacitor to generate the second
phase), which is reversible, are designed for indefinite stall time. And,
of course, if you drive an AC motor from a current limited source, then that
would solve the problem as well.
----- Original Message -----
From: "Pete Smith" <n4zr@contesting.com>
To: <towertalk@contesting.com>
Sent: Tuesday, March 04, 2003 4:42 AM
Subject: [TowerTalk] DC motors in rotators
> I have a Yaesu G-800SA rotator turning a small tribander on a swinging
gate
> sidemount. The Yaesu has a locking rocker switch to control the direction
> of rotation. On more occasions than I care to remember I've overlooked
the
> fact that the switch had locked in one or the other direction, causing the
> sidemount to come up against the tower. Minutes to hours later, I would
> notice where the antenna was pointing and unlocked the switch, sure that I
> had burned out the motor. To my surprise, though, the rotator simply
backs
> off the stop when asked and continues working as if nothing had happened.
>
> I'm curious why this rotator seems so impervious to being run against an
> immovable stop. Are DC motors inherently protected against overheating or
> damage when stalled (for example, does the current through the motor
> somehow drop to a low level), or is there some other explanation, perhaps
> in the design of the controller itself??
>
> What would happen with an AC motor under similar circumstances?
>
> 73, Pete N4ZR
> The World HF Contest Station Database was updated 23 Feb 03.
> Are you current? www.pvrc.org/wcsd/wcsdsearch.htm
>
>
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
|