battery sizing.

[shawnbebout]

I am going to try and make a UPS. I have researched battery sizes and think I found what I need, but I would like someone to tell me if I am right, or if I need a clue. I need to power a 10amp/120volt unit for 3-5 minutes. I will run a deep cycle or ups battery through a invertor to convert voltage. Does (amps) times (hours) apply? 10amps x .333 equals 3.33ah???

 

[MikeHalloran]

Uh, no.

Provding 10 amps at 120 volts, the line voltage, completely neglecting conversion losses, requires 100 amps at 12 volts, a typical UPS battery voltage.
Five minutes is 1/12 of an hour.

100 amps x 1/12 of an hour =~ 8 ampere-hours

A 1500VA UPS having an 8.5 ah battery is just a tick under $200 at Walmart.com. You might want a somewhat larger one, which costs more, but nowhere near what your labor will cost to roll your own.

Did I mention that when your prototype circuit turns out to be not exactly right, as they all do, all that will remain is a pile of smoking, burned, possibly still burning, useless junk that used to be expensive components? Experimenting with power electronics gets really expensive really fast, even if you don't get hurt.

Buy a UPS. How big and expensive depends partly on the holdup time you need, and partly on how well your load will tolerate nonideal waveforms.



Mike Halloran
Pembroke Pines, FL, USA

 

[shawnbebout]

I was thinking of making a EPS system for the blowers on Large inflatable slides. I figured powering a 10 amp blower for 3 to 5 minutes, just long enough to get the kids down safely. I did a internet search for EPS and found they already make them. I figured I could make a simple one with a battery charger, invertor, battery and contactor. It looks like you can buy one that

1800 WATT INVERTER WITH 3600 WATT PEAK POWER
provides up to 10 hours of uninterrupted backup power for a small office and up to 1.5 hours of runtime for essential home appliances and small electronics during a power outage

and it only costs $450. It only takes 800 watts to run my blower (start up is more) so that would be overkill.

Thanks for your time and effort
shawn

 

[MikeHalloran]

Uh, no.

A UPS of anywhere near that nominal size will trip its breaker instantly when you try to start your motor.

A while back, I was working with a 120V/1HP high pressure blower, probably similar to yours. It drew something like 47 amps during the first second, and stayed above 20 amps for ten seconds or so.

A gasoline- powered generator that could start your motor, I'm guessing >3200 watts, would probably weigh less than a UPS that could start it.


Mike Halloran
Pembroke Pines, FL, USA

 

[ScottyUK]

Buy an engine-driven blower to run alongside the electric one. Honda sell them, no doubt the bottom-feeder manufacturers do too.


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If we learn from our mistakes I'm getting a great education!

 

[shawnbebout]

Our motors are "low draw" motors. I think they use a capacitor to help with the starting. Here is the info I got from the manufacture when I went to buy a generator.


> > For example: A BP-1-H.O uses 6.9 running amps.
> > Therefore 6.9 x 115V (volts) = 793 watts to run
> > continuously.
> > But on start-up, it will use up to 17 amps (17 x 115V
> =
> > 1955 watts) to get it
> > started, which is just under a 2000 watts

So the draw on these motors aren't as bad as a normal motor. They are designed to be a low draw, because we rent these to households. And who knows what else is on the circuit, that we plug the motor into.
It takes a inflatable blower about 20 seconds to come up to full power. So the normal startup draw is not there. So the one I was looking at is a tad undersized, so I would find one good for 2000 watt surge.

 

[shawnbebout]

http://www.boatandrvaccessories.com/POWERSOURCE1800.htm

It says there is a auto transfer switch built in. So your motor would be running off shore power, then when shore power drops out, the backup takes over. It would be a micro second for switch over. When they use these things, it doesn't even give a computer time to crash. The motor would already be running, so there would be no start up draw.