**I took some voltage readings on the field windings F1 and F2.
This is on the edge of my experience with electrical testing,
so you may need to coach me with getting the readings that you need.
Motor stopped:
From Ground to F1 = +107.9 - Ground to F2 = -107.8
Leads on F1 and F2 = 215.0
Motor @ 50 RPM :
From Ground to F1 = +107.5 - Ground to F2 = -107.4
Leads on F1 and F2 = 214.4
Motor stalled:
From Ground to F1 = +105.7 - Ground to F2 = -105.5
I have noticed if the main power disconnect switch is let on the machine
for a length of time, the motor will feel hot to the touch.
Is this normal? The F1 and F2 wires show voltage anytime the main power
disconnect switch is on.
It appears to me that this is the original motor supplied with the lathe.
**Earl Weaver
Email: [email protected] <mailto:[email protected]>
On 7/28/2021 9:47 AM, Gene Heskett wrote:
On Wednesday 28 July 2021 05:33:46 andy pugh wrote:
On Wed, 28 Jul 2021 at 00:46, Gene Heskett <[email protected]>
wrote:
Yes, this is an approx. 1984 machine.
That new? I would have guessed 1934, the year I was born. That means
of making a variable speed motor is ancient tech because its not
very efficient.
I think you are maybe thinking of the Ward Leonard set, as used on the
early Monarch 10EE, where an AC motor ran a DC generator which, in
turn ran a DC motor.
That is pretty old-tech, but a DC motor with solid state drive is
hardly old-school at all (only the use of field coils is slightly
outdated at that motor size)
Even with my broadcast history, big motors in the 10 to 25 HP range have
always been std 3 phase. I've only read about using the control of speed
via the field currant applied, and here I probably need to mention just
for the record that loss of field leaves the motor running on residual
magnetism in that old steel, and if unloaded, will spin up till it
explodes. If loaded, it will trip every breaker back to the fuses on the
pole.\
And having some experience with that, I would not bet more than 50 cents
that they would actually break the circuit, I've been witness on two
occasions when it did not, the ceramic tube containing that fuse had
blown so many times that it was metal plated on the inside and failed to
open the 7200 volt line, eventually throwing hot pieces into the grass
below the pole, causing fires we had to stomp out. We also had to
replace 4 750mcm cables from the substation cans on the pole, into and
thru the weather head, and on into the distribution bus inside the
building. That building, which preceded the NEC by a couple decades, had
no entrance breaker, and an open transmitter door by a curious new
operator crowbared a 4000 lb plate transformer, and a GE AK-225 breaker
feeding that transformer failed to open from old age, which led to the
7.5 amp fuse wires in the poles flag switches to try and open.
Thank $DIETY we had a spare transformer so we were only off the air
something under 3 days. Just one of the BTDT's over my nearly 40 years
in broadcast engineering.
An AC motor and a vfd should cut the energy bill in half
compared to that.
But will have less low-speed torque. Bear in mind that the lathe is
geared for 2500rpm and was probably running at 250rpm (steel part 8"
dia, carbide tooling)
I had assumed a change in the gearing to address that. Probablt with
stuff from Tractor Supply. At 2500 revs, throwing swarf, I'd want a
metal wall between that swarf and me. That stuff would be start a fire
hot.
A VFD would be down at 5Hz to achieve that.
Which I am doing on my ancient Sheldon. With torque enough to cut a 10
thou chip with a carbide tool. At 100 revs, the vfd is at 16HZ. Bring in
the backgear and I can turn the house around.
I think that a DC motor is appropriate here, but probably a PM servo
motor would be better.
I won't argue that. Both of my brushed PMDC motors are getting long in
the tooth and running fine on the OEM brushes. And don't have a minimum
speed other than enough to overcome friction. And the motors FLA torque
is available down to zero speed. Temp rise limits with time at low fan
speeds will eventually wave a hand at you of course. Generally the job
is done by then, but its a consideration not to be ignored.
Is this the motor originally supplied with the lathe?
I suspect it is, but Earl would have to answer that.
Cheers, Gene Heskett
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