|
|
[email protected] wrote:
|>
|> | On Mar 3, 2:03?am,
[email protected] wrote:
|> |> So, when this low voltage condition exists, why not just go ahead and operate
|> |> the AC to DC conversion at a level right at the current limit, and obtain the
|> |> remaining power from the battery? ?By drawing less from the battery, and the
|> |> most that is still safe from what can be gotten from the low utility voltage,
|> |> the battery can last longer than it otherwise would. ?Is it too complex or
|> |> costly to make an AC to DC conversion (basically apower supply) section that
|> |> would operate on these lower voltages? ?
|> |
|> | Yes. UPSes are designed as cheap as possible. Some have seen how
|> | cheap. Replacing a UPS is sometimes less expensive than replacing its
|> | battery. Why make it more complex and more expensive?
|> |
|> | Second, UPS battery is charged by a power supply equivalent to a
|> | wall wart. To accomplish what you have suggested means that battery
|> | charging power supply must be larger: increased costs. Or it must
|> | have an even larger operating input voltage - again increased costs.
|> | Just another reason why it is not done.
|>
|> How much larger does it get just to extend its lower voltage range?
|> How much larger does it get to make that extended lower voltage come
|> with a constant current level at a certain current limit?
|
|
| That depends on how much you want to supplement the batteries, and if
| it is even possible to draw the additional current from the AC line in a
| brownout condition. Every thing in the input stage has to be larger,
| including the circuit supplying power to the UPS & computers. The
| switching supply will need larger electrolytics, heavier rectifiers,
| more switching transistors, and a bigger switching transformer for the
| charger. You will need to add fans, or use huge heatsink. Some UPS
| use the same transformer to charge the batteries and for the output
| transformer, so you would need a complete new charging circuit.
So you are questioning the ability to draw current when the voltage is
at half level? If you can't, then that means there is a systematic
impedance change. Do you have in mine where that change is? Or are you
just trying to be difficult.
Not everything has to be larger.
If it is a limited power device, then the current could double for half
the voltage. Components dealing with the incresaed current will need to
be larger. Once the device has the voltage at a fixed working level to
derive other voltages, then part is just the same size.
If it is a limited current device, then very little will need to be
increased in size. There will have to be added the circuitry to limit
the current when it reaches a certain level.
| High current supplies like that can catch on fire. The Northern
| Telecom PBAX telephone system we had at Microdyne caught fire and smoked
| damaged 80,000 square feet of office, engineering, production and
| storage space. It took over three months to repair the offices near the
| fire, and you could still smell the smoke a year later. Our insurance
| paid for all the repairs, then sued the OEM. The damage came close to
| putting us out of business. Luckily, the phone room was in the
| engineering department, and some of the people could work from home, but
| it dropped productivity quite a bit. If any part of that system was
| home brewed, we would have had to pay for the repairs ourselves. The
| system was in a locked room, and caught fire late one night. if someone
| who was driving through that industrial park hadn't noticed the orange
| glow, we would have lost all the offices, both stockrooms and all of the
| engineering department.
If we are dealing with a constant power device, then a 1000 watt model
will have current levels at half voltage similar to a 2000 watt model at
nominal voltage. Are you saying that the 2000 watt model is more of a
fire risk than the 1000 watt model ... AND is so much more of a risk
that such current levels should be avoided?
If we are dealing with a constant current device, then "high current" is
a constant.
|> | Third, AC utilities providing voltages at that low level is a
|> | specification violation. It rarely happens. If detected, utility
|> | typically cuts all power off due to defects that might cause such low
|> | voltages. IOW a more complex UPS circuit and the larger battery
|> | charger would rarely prosper from such a condition. Better is to get
|> | you to buy a larger (more expensive) UPS - and keep this UPS as cheap
|> | as possible.
|>
|> Utility equipment is also done on the cheap. I've seen the half voltage
|> scenario ssveral times in different places. In one instance it persisted
|> for over 4 hours. In theory, a phase loss detector in a transformer should
|> signal an upstream cutoff. It doesn't always happen.
|
|
| Phase loss detection is the customer's problem, and is solved by using
| a PLM and either drive an alarm, or use it to shut down equipment that
| is easily damaged. There was a company in Leesburg, Florida making
| them. They were small enough to hold in your hand and made to mount
| with typical industrial relays and timer modules. if you lose a phase,
| but have some large 3 Phase AC motors running at the time, they will
| continue to run, but draw the additional current from the other phases,
| causing the motor to run hotter. Look up 'Rotary Phase Converter'
If a phase loss results in nominal voltage on 2 phases and zero on a
third phase, then that would be a phase loss to the customer. However,
if the utility wires things in such a way that a phase loss upstream
results in half voltage to SINGLE PHASE customers for an extended period
of time, then that is a utility problem.
|> This is one reason (there are others, too) I'm wanting to run my computers
|> directly on 240 volts. If there's a deep brownout, they can still run in
|> that condition. The impediments to doing this include finding the right
|> surge protection (easy for entrance protectors, hard for point of use) and
|> the fact that existing UPSes defeat the ability to operate over the wide
|> voltage range (why I raised this issue to begin with).
|
|
| Take a careful look at the power supplies you are considering,
| because a lot have a dead band between the two voltage ranges. All they
| do is sense the input and select 120 or 240 range to keep the costs
| down I see them specified as 90-130 & 180-260, leaving a dead band
| between 130-180 volts where operation isn't guaranteed.
Many are that way. Many more are full range. Full range is becoming
more and more common. Maybe you should take the careful look.
|> A battery charger (in a UPS or as part of a separate component system)
|> that can be set to specific power or current limits can also be useful
|> in using small generators to supplement the battery in a total loss of
|> power.
|
|
| if you buy a small generator, get one with an accurate governor, or
| that is a DC/inverter package like some of the Hondas.
And regulator.
|> | Fourth, if a low voltage condition exists, unstable AC power is best
|> | disconnected from equipment anyway. Maintaining voltage too low can
|> | be harmful to electric motors and to the distribution system.
|> | Therefore, when that defect is detected, reliability says better is to
|> | disconnect.
|>
|> For anything that increases the current, and is not rated for that increase
|> in current. That's why my idea of making the power supply component have a
|> current limit and just scale down its power intake, stretching the battery
|> run time.
|
|
| You can't limit it very much and have anything useful.
With limited current, you can have half power at half voltage. That may
only be half as useful, but it can be useful. For example, if the UPS
is fully loaded, half the power needed to run the loads can be drawn
from AC and the other half from the battery ... instead of all from the
battery.
|> | Fifth, a UPS is for saving data. Making the UPS more complex for a
|> | rare type of failure is not useful. If a UPS needed to operate
|> | longer, then the UPS is too small anyway. Better is to get you to buy
|> | the more expensive UPS with a bigger battery.
|>
|> What the UPS is for depends on the scenario. In many cases I have seen,
|> especially for ISPs and web services, it's to ride out power outages.
|> The sizing is done by CFOs. Batteries are the biggest part of the cost.
|
|
| If the CFO is so cheap that he is willing to use home brew backup
| power, its time to find another job. The costs to get a custom system
| through UL approval will buy several lifetimes worth of batteries. Do
| you have a PE willing to sign off on your modifications? Will you be
| able to find fire insurance for the business that will pay a claim after
| a fire?
The CFO sets the price limit. It's called a budget. Engineering can
sometimes wiggle this some. But not always. Or not generally by a
great deal.
So, where batteries are the substantial cost, and there is the real
possibility of getting half the voltage for some reason, then a UPS
system that works this way and provide more run time on that fixed
battery capacity.
There is a valid market issue. Is there a market for such a device?
Probably not. Can it be designed? I think it can. The components have
already been done.
|> | Those UPSes are designed to sell mostly on price - as cheap as
|> | possible. Why do anything to make its price higher? Five different
|> | reasons why the suggestion is not useful.
|>
|> An increase in charger cost, that can provide longer batter run time than
|> an the increase in battery capacity of the same cost, provides more run
|> time for the dollar.
|
|
| Not if it isn't legal to use. If the fire inspector sees a homebrew
| UPS during any visit, he is allowed to have the power to the building
| turned off till it is removed. You still don't see the big picture,
| Phil. Its one thing to homebrew a power supply to use at home, but for
| a business, it is a huge liability.
If it is an engineered custom design, this is not true. Businesses have
various kinds of systems custom designed for them all the time.
|> This is why I often have the desire to just build a UPS from separate
|> components. I'm not an electrical engineer. At best I am an electrical
|> hobbyist.
|
|
| Do the math, Phil. The extra hardware costs, plus the testing &
| added liability costs quickly wipe out any possible savings. An
| properly sized generator with an automatic transfer switch is cheaper
| than getting a custom UPS approved for commercial use. if the generator
| is propane powered, it won't have a problem with the fuel going bad
| while in storage.
If YOU have done the math on the hardware component costs, then YOU have
done a design.
|> | Michael is a technician. He knows what a technician knows - what to
|> | do. He often does not know why - what an engineer learns. Therefore
|> | he has a repeat habit of attacking others because he does not know
|> | why. Attacking others is trick often used to mask the posters
|> | technical naivety. Unfortunately it works because others see the
|> | insults and then believe the insulter. Rather than learn the facts,
|> | many instead believe the first one to post insults. It is also how
|> | Rush Limbaugh operates.
|>
|> I've known many technicians (and engineers). I've also listened to a few
|> radio talk show hosts, and watched equivalent ones on TV (such as Glen Beck).
|> Most manage to remain respectable. There are exceptions like Michael and
|> Rush.
|
|
| Why? Because I tell you something is flawed, and you never listen?
It's more a case of not believing you because you have a track record of
saying some of the dumbest things I've heard online.
| I have worked in electronics my whole life, including manufacturing of
| power supplies. I also qualified new suppliers for the few switching
| power supplies we bought, and dequalifed multiple vendors when the
| shipped product didn't match the build quality of the test samples. One
| of the TV stations I worked at had 500 KW diesel generators for backup
| power, and the only UPS was for the video automation computer. A few
| battery powered emergency lights were mounted around the walls, but if
| the three generators didn't start, two UHF TV stations, five 50 KW FM
| stations, trunked business radios and multiple government agencies were
| without power for their equipment.
You've told us about some of your experiences, before.
|> I've learned years ago that when I see someone insulting someone else online,
|> and not just on Usenet, that it more likely means the insulter is upset at
|> something. I've also learned that it can be hard to find out just what that
|> is because so many cases are merely a misunderstanding somewhere.
|
|
| The only way to get some people's attention is to resort to insults
| and you are know for hurling your share of insults, as well.
I go to extra effort to avoid using insults. For a few people like you,
that gets to be very hard to do, and sometimes it just has to happen.
But check your own posts. You'll see insults generally begin with your
first followup post in the original thread.
