hooking generator to UPS?

Do not buy from the Guardian guy. Not because his unit will not work but because he is an idiot. In general, UPS are VERY tolerant of bad power on the input. We have tested many UPS in the past 5 years and while every one has its quirks, they are all pretty tolerant of what could be called bad power in this country.

Charles Perry P.E.

On Wed, 30 Jan 2008 19:23:53 -0800 (PST) snipped-for-privacy@gmail.com wrote: | Hello Group, | | I am looking for a permanently installed generator to hook to my | natural gas. Had an electrician out to the house who sells Guardians. | He says that Guardians make dirty power (no inverter) that is not good | for powering uninterruptable power supplies and other sensitive | devices like microwaves and garage doors. He disclaims the operation | of these devices on his quotes. | | I am looking for a 13-16kW generator to provide back up for a small | computer closet with multiple servers, switches, and other rack- | mounted, always-on computer equipment. The equipment is connected to | APC UPS units. The generator will also provide power for select | circuits inluding a 4 ton air conditioner, freezer, refrigerator, | household lights, and a saltwater fishtank with pumps and compact | flourescent lighting. | | There is no three phase at the house. | | All computer equipment is 120V on dedicated 30A and dedicated 20A | circuits with isolated grounds. | | 1) What are my options for permanently installed generators that | provide clean power? I noticed Honda makes portable ones, but I didn't | see a permanent one.

Larger is better for cleaner, in general. Professional models instead of home models are better.

| 2) Are there UPS systems (APC or otherwise) that tolerate dirty power | produced by the Guardians?

Most will. But get the type of UPS that is "double conversion continuous online" with sine wave output. This type converts AC to DC, parallels to the battery under a control circuit, and converts the DC back to AC. If you have anything in the closet that is sensitive to dirty power, this is the way to clean it up.

A UPS for the motors, however, raises a different issue. The UPS will need to be able to deliver the motor starting current (LRA). To do this with an ordinary UPS you would need to oversize the UPS big time. Find out what the LRA (locked rotor amps) is for all your motors. If you can ensure only one starts at a time, pick the largest LRA and add that to the load calculations. Otherwise add them all. Treat that as the load you need to power for both UPS and generator. Fish lives are at stake!

What the guy may have been thinking of is properties that only get mains supply for a few hours a day and run off a big battery bank+inverter+charger system when mains is unavailable. These typically have integrated very high current chargers that can recharge the bank in a couple of hours when a mains supply is available.

The chargers built in these units have a design which produce most of the charging current during the peaks of the ac input waverform. *Small* gensets tend to have a distorted waveform with "flattened" peaks at a lower voltage than nominal, especially under load. Which has a *drastic* effect on charging performance.

Looking at the instruction manual for such units often shows a manufacturer's recommendation of using an over-size genset, because of this. eg for a 2.5kW inverter, recommending that at least a 7VKA genset be used to achieve specified charge performance(and, of course, to power the load).

I have also found that some very cheap UPS, with just a few hundred watts output - behave very poorly when supplied from a small, eg 1200va, generator. They tend to cycle constantly between supply and battery - as the load drags the supply voltage down significantly.

None of this is, of course, relevant to the OP - a 13kVA genset isn't likely to exhebit these problems.. However, if the salesguy usually sells much smaller units, it may explain his position.

I can only echo the suggestion that was already made about using a UPS arrangement were the battery is always on line. You use a battery charger to charge the batteries but you size it so that the entire running load of the computer equipment is within the output of the charger. You attach a sine wave inverter to the batteries to actually power your computer loads. The inverter can be sized for starting any of the loads it would actually carry so that when it starts those loads it will borough the additional current for the starting load from the batteries. You size the battery bank to run as long as you need the inverter to run to insure an orderly shut down or to allow time to deal with a failure to start on your generator. You then size your generator to start the largest motor load and run all of the loads other than your largest motor load including the battery charger. In your case the largest motor load will probably be the motor compressor for your air conditioner. The restart delay that is built in to all air conditioner controls to keep them from trying to start against full head pressure will be all the load shedding that your generator will need.

use a power conditioner, sine cleaner, between the gen and the ups ac input.

most are just over-spec'ed 1:1 isolation transformers that will smooth most destructive glitches out.

over rate the UPS battery bank to absorb unexpected genset downtimes.