On Thu, 17 Feb 2005 11:02:05 -0400, Terry Spragg
wrote:

No, Gary, we don't. Do you?

There is no difference between shutting off your A/C either at it's
control panel, breaker box, or by shooting up the distribution
transformer down the street. Non. Except for the legalities, of course.


By not turning off your air conditioner systems before disconnecting
your shore power cable or by only shutting down your main breaker this
is what happens:

The fields in the air conditioner motors suddenly collapse when power
is removed. The collapsing fields will generate a high voltage spike
throughout your boats mains system. Any other equipment that is turned
on at that time will receive those high spikes of voltage. Everything
that is turned on is connected to the air conditioner motors. You can
guess what can happen with voltage spikes fed to some types of
equipment.

If you shut down the AC systems first you avoid any kick back voltage
spikes being fed to other equipment. The fields just collapse and no
voltage spikes go anywhere.

And yes an ac motor is no different than a DC motor when it comes to
generating emf. A collapsing field is a collapsing field. If it was
originally powered by dc or ac you still get the same effect. Both
have voltage disconnected and the field collapses in the same manor.
Other than if you happen to switch the ac motor off at zero crossing.
Any transformer connected to the line will also do the same. Those old
battery chargers with the large transformer in them can also cause the
same problems. Although motors are the worst culprits as they usually
operate at higher currents and generate more kick back.

When the power company shuts down large areas it does not cause as
many problems because there are usually many loads still on line that
tend to absorb most of the kick backs from motors on line. But still
there can be some damage done by this.

Regards
Gary

Gary Schafer wrote in
:

The fields in the air conditioner motors suddenly collapse when power
is removed. The collapsing fields will generate a high voltage spike
throughout your boats mains system. Any other equipment that is turned
on at that time will receive those high spikes of voltage. Everything
that is turned on is connected to the air conditioner motors. You can
guess what can happen with voltage spikes fed to some types of
equipment.

If you shut down the AC systems first you avoid any kick back voltage
spikes being fed to other equipment. The fields just collapse and no
voltage spikes go anywhere.


Wow! All this time I thought Tesla's multiphase AC power system had
collapsing fields 120 times per second, not when I unplugged the shore
power cable!....

Which one of the boater electrical expert books at Waste Marine did you get
all this bull**** from, anyways, Gary?

AC systems don't work like DC systems.....(c;

On Thu, 17 Feb 2005 20:37:46 GMT, Larry W4CSC wrote:

Gary Schafer wrote in
:

The fields in the air conditioner motors suddenly collapse when power
is removed. The collapsing fields will generate a high voltage spike
throughout your boats mains system. Any other equipment that is turned
on at that time will receive those high spikes of voltage. Everything
that is turned on is connected to the air conditioner motors. You can
guess what can happen with voltage spikes fed to some types of
equipment.

If you shut down the AC systems first you avoid any kick back voltage
spikes being fed to other equipment. The fields just collapse and no
voltage spikes go anywhere.


Wow! All this time I thought Tesla's multiphase AC power system had
collapsing fields 120 times per second, not when I unplugged the shore
power cable!....

Which one of the boater electrical expert books at Waste Marine did you get
all this bull**** from, anyways, Gary?

AC systems don't work like DC systems.....(c;


I know you slept through the basic AC theory class but the DC one too!
:)
As I am sure you missed the lab part too,
You can try for yourself. Take a little 12 volt ac motor or
transformer and connect its 12 volt winding to a 12 volt ac power
source. Hold on to each lead of the motor or transformer bare wires if
you dare. Touch them and remove them from the 12 volt ac source. I bet
you let go rather quickly. :)

Better yet hook your oscilloscope to the winding instead of your
fingers. Look closely as you disconnect the winding from the power
source. Turn the gain down on the scope so you can see the high
voltage spike before it goes off the screen.

You keep disconnecting the shore cable without turning things off
first and you wonder why your mov's are popping! Those things do wear
out you know, or do you?

Regards
Gary

"Gary Schafer" wrote in message
...
You can try for yourself. Take a little 12 volt ac motor or
transformer and connect its 12 volt winding to a 12 volt ac power
source. Hold on to each lead of the motor or transformer bare wires if
you dare. Touch them and remove them from the 12 volt ac source. I bet
you let go rather quickly. :)

Better yet hook your oscilloscope to the winding instead of your
fingers. Look closely as you disconnect the winding from the power
source. Turn the gain down on the scope so you can see the high
voltage spike before it goes off the screen.

That is why equipment is required to have a snubber network. This will
absorb most of the spike down to a safe level for other equipment on the
mains.

Meindert

"Larry W4CSC" wrote in message
...
Gary Schafer wrote in
:

Wow! All this time I thought Tesla's multiphase AC power system had
collapsing fields 120 times per second, not when I unplugged the shore
power cable!....

There's a huge difference timing here. The AC power system goes from maximum
voltage to 0 over a period of 8.3msec, which is slow compared to breaking
the circuit instantaneously.

Meindert

"Gary Schafer" wrote in message
...

By not turning off your air conditioner systems before disconnecting
your shore power cable or by only shutting down your main breaker this
is what happens:

The fields in the air conditioner motors suddenly collapse when power
is removed. The collapsing fields will generate a high voltage spike
throughout your boats mains system. Any other equipment that is turned
on at that time will receive those high spikes of voltage. Everything
that is turned on is connected to the air conditioner motors. You can
guess what can happen with voltage spikes fed to some types of
equipment.

A few arguments against this: if you use the switch on the A/C unit itself,
you disconnect the motor coil completely. It is virtually unloaded and will
therefore produce a high inductanc voltage.

However, if other equipment is connected while unplugging the shore power,
much of the spike will be dampened by this other equipment. Furthermore,
equipment that is commercially sold, is required by regulations to be able
to withstand spikes up to a certain level, just to prevent this kind of
potential damage caused by other connected (or suddenly disconnected)
equipment.


If you shut down the AC systems first you avoid any kick back voltage
spikes being fed to other equipment. The fields just collapse and no
voltage spikes go anywhere.

Well, the voltage spike will at least produce an arc across the switch,
which in turn delivers the spike onto your power system.

Meindert

On Thu, 17 Feb 2005 22:34:26 +0100, "Meindert Sprang"
wrote:

"Gary Schafer" wrote in message
.. .

By not turning off your air conditioner systems before disconnecting
your shore power cable or by only shutting down your main breaker this
is what happens:

The fields in the air conditioner motors suddenly collapse when power
is removed. The collapsing fields will generate a high voltage spike
throughout your boats mains system. Any other equipment that is turned
on at that time will receive those high spikes of voltage. Everything
that is turned on is connected to the air conditioner motors. You can
guess what can happen with voltage spikes fed to some types of
equipment.

A few arguments against this: if you use the switch on the A/C unit itself,
you disconnect the motor coil completely. It is virtually unloaded and will
therefore produce a high inductanc voltage.

That is how it is designed to operate. The high voltage not being
connected to anything else will then be harmless.

However, if other equipment is connected while unplugging the shore power,
much of the spike will be dampened by this other equipment. Furthermore,
equipment that is commercially sold, is required by regulations to be able
to withstand spikes up to a certain level, just to prevent this kind of
potential damage caused by other connected (or suddenly disconnected)
equipment.

It may or may not be dampened sufficiently. It all depends on what
type and how many other things happen to be turned on.

Different types of equipment may be able to withstand certian amounts
of overvoltage spikes that are normally seen on the line. But this
situation is far from a normal intended operation.



If you shut down the AC systems first you avoid any kick back voltage
spikes being fed to other equipment. The fields just collapse and no
voltage spikes go anywhere.

Well, the voltage spike will at least produce an arc across the switch,
which in turn delivers the spike onto your power system.

It may produce an arc but the kick back energy will not be coupled
into other equipment that is not connected across the source at the
time (the motor coils). Any thing as a result of the arc will be
dampened by the whole mains supply system that is still connected.

You may get away with just unplugging the whole system for a long time
with no problems. But chances are sooner or later it will cause
problems for something.
Always turning things off before disconnecting mains power is the best
bet.

Regards
Gary

Meindert

Gary Schafer wrote:

On Thu, 17 Feb 2005 22:34:26 +0100, "Meindert Sprang"
wrote:



"You may get away with just unplugging the whole system for a long time
with no problems. But chances are sooner or later it will cause
problems for something.
Always turning things off before disconnecting mains power is the best
bet."




"Sooner or later it will cause problems" but 70 VAC will cause problems sooner
THAN later.