On 2012-01-07, Wild_Bill wrote:
Good recommendations, as usual DoN. I thought about commenting on low
voltage testing, but left it out considering that Bob was getting good info.
Thanks!
Just connecting a low AC voltage to any winding should provide
useful/noteworthy results in testing induction motors with multiple
windings, since the separate windings share the same ferrous frame/core.. in
much the same way of connecting a low AC voltage to a transformer will
always cause some detectable output on any good windings around the core.
Yes -- but having the rotor rotating at some semi-reasonable
speed will carry induced magnetic poles around with the rotor, so the
behavior will be a bit closer to what you would see with a full three
phase applied, so it may make identifying things easier.
But since there turn out to be two isolated groups of
connections (either Delta or Wye -- who cares if all you have is two
wires to work with), the suggestion of someone that the higher
resistance ones are likely to be a built-in cooling fan makes a *lot* of
sense for a motor rated to cover that wide a range of speeds. That fan
really wants to be fed its own three phase -- from *before* the VFD, but
he does not have that AFIK.
The noted results information may take some further head
scratching/pondering, but will lead to reasonable conclusions.. even without
spinning the rotor.
But a little more information when spinning it.
For unmarked leads, one definitely should use some arbitrary markings to
retain any sanity while trying the testing.. paint colors, temporary letters
or any markings are better than trying to keep the leads sorted while
testing.
Absolutely. Ideally, some short lengths of a copper tubing
which will just slip over the wire's insulation, with numbers stamped
in, and then glued in place with tar so it *stays* with the wire. :-)
For diagnostic work, colored tapes will do -- and a notepad. :-)
I'll add that for some DIY types, it may be possible to use a low setting on
a variable speed drill to spin motor rotors or armatures (or in generators),
and a precise coupler isn't needed.. the coupler can be a piece of
automotive heater hose with a plug and a bolt in one end, or similar
improvised gizmo.
Yes -- but that will likely need someone to hold the drill motor
steady. With the lathe (assuming one big enough for the motor), the
motor is supported by its bearings and the chuck. And perhaps a couple
of boards to keep the foot of the motor from bashing the bed. :-)
There are a lot of testing procedures for motors, often covered in better
motor books which will generally also include descriptions of different
types of winding schemes and standardized lead numbering patterns.
Other types of 3-phase motors with more than 6 leads are often much more
difficult to decipher if there are none of the original wire numbers to be
found on the leads. Opening the case may reveal the wire numbers inside, but
otherwise most folks would need to have a motor shop sort out the lead
numbers.
Opening the case could show a separately powered internal fan,
intended to keep the air flowing while the motor is running at 3 RPM. :-)
It could also allow the two thin wires to be traced back to
something -- I suspect a thermal switch buried in the windings so it
knows when the windings are getting hot. But it could be a thermistor.
A NC switch would make more sense to switch off the command voltage
source at the VFD, and stop the motor when it is showing signs of
overheating. It should reset when it cools off, so be sure to wire
things so that one set of contacts re-closing won't start the motor in
the middle of the night. :-)
Enjoy,
DoN.
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