On 2013-08-21, Bob La Londe wrote:
"PrecisionmachinisT" wrote in message
...
"DoN. Nichols" wrote in message
...
On 2013-08-20, Pete Keillor wrote:
On Mon, 19 Aug 2013 18:02:08 -0500, "Lloyd E. Sponenburgh"
lloydspinsidemindspring.com wrote:
[ ... ]
If the squeal is coming from the motor itself, the reactor(s) will help
smooth the 'simulated sine wave' coming out of the inverter drive.
I think that the squeal is likely from loose laminations in the
stator of the motor -- and likely just right to be resonant at the
default frequency of the VFD -- so a change in frequency would likely
make a big difference.
[ ... ]
A replacement motor will likely not show the same problem --
even if the same brand. But it might be the most expensive approach.
My impression has always been that the PWM frequency was simply too low on
first-generation inverters; and that while anything built within the past
couple decades might well still create some annoyong electrical noise,
acoustic emmissions will be at a frequency substantionally above the human
hearing threshold
I don' know what the total range of human hearing is, but when I was younger
I could hear or be aware of frequencies upto about 20kz.
Well ... the usual figure given is 20 Hz to 20 KHz.
Depending on the
sound generator somewhere between 19.5 and 20.5 I would notice it go
substantially silent. Now as I have gotten older I have noticed that I
can't hear much above about 17.5.
I have had poor high-frequency hearing at least since it was
discovered in high school. In particular, things pretty much drop off
about 2.6 KHz -- though I remember playing with headphones and a signal
generator and discovering that I could hear a little just above 20 KHz,
though I don't know whether the system was generating sub-harmonics.
I did a lot of things at a kid which could have damaged my
hearing.
But I also remember, shortly after the high-frequency loss was
discovered, sitting in my room with a pair of Koss Pro-4A headphones
(pretty good sealing around the ear), and with a frequency sweep test
record (LP for those who remember older technology). As the frequency
was increasing, I was turning up the gain to try to keep hearing it. A
friend, sitting about 10 feet away, screamed "How can you stand it!"
about the time I was losing it totally.
Anyway, the range of human varies with age, and does so noticeably with men
in the high frequency range.
While I am a man, I don't think that I am in the "high frequency
range" in any sense -- but my hearing in the high frequency range was
lost quite early, as I mentioned above. :-)
Also, its possible that some people have a
wider range of hearing atleast at the peak level of their physical
development. All of that being said, who knows what odd frequencies can be
generated by a VFD.
The makers know the range of the clock frequencies. Whether
they will be *audible* depends on whatever they are driving, because
there is nothing in the VFD itself capable of acting as a transducer
from electrical signals to audio signals.
The *motor*, however, can quite easily do this, depending on how
well it is made. The stator is made of a stack of laminations of a thin
metal selected for its magnetic properties, which forms a number of
poles around which is wound many turns of enameled wire. As the current
increases, it generates a magnetic field. If there is a gap between any
two of the laminations, it will attract the two pieces together closing
the gap, and making a "tick" as they touch. When the current (and
field) decrease (this is AC after all), they separate again, and are set
for the next cycle. A really loose lamination could cause a buzz at
normal line frequency. That does not carry very far. But if the VFD is
running at a mid audio frequency -- say 400 Hz up to 12,000 Hz, you will
get a quite audible sound.
And it is possible that the free length of the lamination might
be just right to resonate at or near the frequency of the VFD -- which
is why shifting the frequency can make a big difference.
The wires can move too -- but usually the proportion of current
required and the short lengths and heavy gauge of the wire means that
they will not move enough to generate significant sound in the typical
motor.
Power transformers (once commonly used in electronic equipment)
have the same construction as the motor stators, but typically a wooden
wedge used to be driven in between the coil and the center pole of the
laminations, thus minimizing the freedom to vibrate. (Really good ones
in the early days were put in a metal can and the can was then filled
with melted tar, so no chance of lamination vibration there. But later
and cheaper ones have produced enough noise at 60 Hz so I could hear
them.
*Most* motors are well enough made (stator dipped in a coating
which hardens, both to hold the windings in place, and to keep the
laminations together).
Hmmmm... I'm not sure any of that makes a point, but its some nice anecdotal
information to consider. LOL.
And a bit more information added, showing how a motor can
generate sound -- and why it is more audible with a VFD than with the
power company's 60 Hz (or 50 Hz in the UK).
Enjoy,
DoN.
--
Remove oil spill source from e-mail
Email: | (KV4PH) Voice (all times): (703) 938-4564
(too) near Washington D.C. |
http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---