[GreenKeys] Hughes Printing Telegraph

[John Nagle]

On 7/19/2011 4:45 PM, greenkeys-request at mailman.qth.net wrote:
> Date: Tue, 19 Jul 2011 15:27:11 -0500 (CDT)
> From: Jim Haynes<jhhaynes at earthlink.net>
> Subject: Re: [GreenKeys] Hughes Printing Telegraph
> To: Rokumon Cat<rokumoncat at yahoo.com>
> Cc:GreenKeys at mailman.qth.net
> Message-ID:<alpine.LFD.2.02.1107191520100.4401 at Frances>
> Content-Type: text/plain; charset="iso-8859-1"
>
> On Tue, 19 Jul 2011, Rokumon Cat wrote:
>> >  ?
>> >  Someone correct me if I am wrong, but I think that if the motor is a
>    series wound motor, that if the load is removed, that it will 'run away'
>    and destroy itself if the load is removed while the power is applied.
>> >  ?
> That's a hazard with large motors.  With small motors it isn't a hazard
> because there are always enough losses to keep them from running away.
> I'm not sure just how big a motor has to be for that to be a problem.
> You can run the ordinary electric drill, or a Dremel tool, or a car
> starter motor with no load without danger.  When I was an E.E. student
> 50 years ago we had motors maybe 18 inches in diameter in the machinery
> lab and it was definitely a hazard with motors that big.

     DC motors have a maximum speed for a given voltage, because they
also act as generators.  When the voltage they would generate for
a given RPM equals the input voltage, they have reached no-load
speed (zero torque) and will not accelerate further.  Maximum
torque is at stall (zero speed).  The torque/speed relationship
is linear.  Maximum power is obtained at half of no load speed

See MIT's "http://lancet.mit.edu/motors/motors3.html" for the
theory.

    The commutator on the Hughes machine looks clean, but shows
wear marks, unlike the other parts of the machine.
Are the brushes wire or carbon?  Carbon brushes were first
used successfully in 1887, but earlier motors used wire
brushes, which are more subject to wear, don't lubricate
the commutator, and tend to need frequent adjustment.
If the motor isn't reaching full speed, that's the first place
to look.

				John Nagle