On Tue, 06 Sep 2005 15:21:20 GMT, Ignoramus25850
wrote:

I have been playing with using relays of various kinds, as well as
contactors. I already built 2 phase converters, and a remote switch,
for instance. I have a few 90A and 75A contactors and solid state
relays and mag starter buttons and whatnot.

Here's what I have been thinking about. I have a 7 kW Onan DJE
generator that I have for emergencies. I want it to power my entire
house in emergencies (I know that I cannot run AC and some other
devices while under generator power).

It is expensive and painful to install a transfer switch. I want to
make something easier and cheaper to power the house in case of
emergencies, while at the same time preventing backfeeding.

I can, instead of a transfer switch, build a system that includes a
contactor, a mag starter, and a relay that only closes the main
generator contactor when the main circuit breaker is in the OFF
position.

It would work something like this. There will be a mechanical device
and a switch such that the switch could be closed only when the main
breaker is off. When the main breaker is on, the switch could not be
closed.

There will be a DC circuit, powered by a 9V battery, that would be a
signal input to a solid state crydom relay. The power contacts of the
relay would be in series with the power from generator. When the relay
is closed (only when the mains breaker is open), and a START button is
pressed on the start/stop switch (like ones used for mag starters),
the main contactor would close. The STOP button interrupts input to
the contactor, causing it to open. Turning the little switch near the
main breaker off would also interrupt the circuit, opening the
contactor.

This seems to be a very fool proof system. I can build it in 30
minutes, except that I would need more time to fabricate a mechanical
switch opener/closer.

I would like to ask that those who can visualize what I am describing,
to comment on this plan. Thanks

i
Sounds like a good idea - BUT. Can you ensure the generator contactor
can NOT stay closed when the switch is shut off to turn the main
disconnect on? You need a failsafe system that makes it IMPOSSIBLE for
the generator to "lock on" - whether by killing the generator before
being able to reconnect to mains or whatever. That is why virtually
all transfer switches employ what boils down to a mechanical DPDT
knife switch.

"Ignoramus25850" wrote in message
news
On Tue, 06 Sep 2005 12:04:31 -0400,
wrote:
On Tue, 06 Sep 2005 15:21:20 GMT, Ignoramus25850
wrote:

I have been playing with using relays of various kinds, as well as
contactors. I already built 2 phase converters, and a remote switch,
for instance. I have a few 90A and 75A contactors and solid state
relays and mag starter buttons and whatnot.

Here's what I have been thinking about. I have a 7 kW Onan DJE
generator that I have for emergencies. I want it to power my entire
house in emergencies (I know that I cannot run AC and some other
devices while under generator power).

It is expensive and painful to install a transfer switch. I want to
make something easier and cheaper to power the house in case of
emergencies, while at the same time preventing backfeeding.

I can, instead of a transfer switch, build a system that includes a
contactor, a mag starter, and a relay that only closes the main
generator contactor when the main circuit breaker is in the OFF
position.

It would work something like this. There will be a mechanical device
and a switch such that the switch could be closed only when the main
breaker is off. When the main breaker is on, the switch could not be
closed.

There will be a DC circuit, powered by a 9V battery, that would be a
signal input to a solid state crydom relay. The power contacts of the
relay would be in series with the power from generator. When the relay
is closed (only when the mains breaker is open), and a START button is
pressed on the start/stop switch (like ones used for mag starters),
the main contactor would close. The STOP button interrupts input to
the contactor, causing it to open. Turning the little switch near the
main breaker off would also interrupt the circuit, opening the
contactor.

This seems to be a very fool proof system. I can build it in 30
minutes, except that I would need more time to fabricate a mechanical
switch opener/closer.

I would like to ask that those who can visualize what I am describing,
to comment on this plan. Thanks

i
Sounds like a good idea - BUT. Can you ensure the generator contactor
can NOT stay closed when the switch is shut off to turn the main
disconnect on? You need a failsafe system that makes it IMPOSSIBLE for
the generator to "lock on" - whether by killing the generator before
being able to reconnect to mains or whatever. That is why virtually
all transfer switches employ what boils down to a mechanical DPDT
knife switch.


I am open to suggestions here. I would think that that's what
contactors are made for, to open and close reliably, when operated at
or below the rated power. Otherwise they would be very unsafe for all
kinds of machines that they control.

i


Contactors can and will fail closed! Not a good idea for a transfer switch!
I could not count the number of failed closed contactors I have replaced in
the last six years!!
Greg

On Tue, 6 Sep 2005 17:54:59 -0500, "Greg O"
wrote:



Contactors can and will fail closed! Not a good idea for a transfer switch!
I could not count the number of failed closed contactors I have replaced in
the last six years!!
Greg

I suppose you COULD use a DPDT contactor - but then why not just use
the old standby knife switch. The contactor needs power to activate
it.

And if the contactor sticks in the line position you can't use the
genset. If it sticks in the genset position, you cannot connect to the
grid. - so you are back to manually operating it anyway. Back to the
knife switch.