Using Inverter with Incoming Power Bypass as UPS

I know this may be a little obvious, but can't you use a faster switching time relay or something?

Most UPSs switch over within 5 mS 16 mS is a tad too long, you will get a supply line dip when the switching time is over one cycle in length unless the supply is lightly loaded.

John.

Your problem is with the server's switching power supplies inadequate holding time for your application. As your inverter is not synchronized to anything, when you switch between your power sources, your load can experience sometimes extended period of voltage close to zero. When the server's cannot hold that long, it glitches.

Since you already have a complete DC to AC system in place, the sure and simple way is to add a rectifier (and battery charger) to it. When AC is available, it feeds your DC / AC converter and in addition charges the battery until it just floats. The rectifier needs to be voltage controlled and current limited (quite normal requirement in any case) to not to overcharge the battery. This setup has no switchover time as the DC/AC converter is always fed by the battery. There should be no problem with the power sharing with the engine driven generator, as that one is an inherently current limited AC generator with rectifier.

If you do not care to go this route, there is another, fully commercial, off-the-shelf solution:

http://www.Solaheviduty.com/products/powerconditioning/index.htm

Such a unit from the Sola company will protect from spikes, dropouts and brownouts. They are used widely, excellent, heavy and quite noisy.

I would have to agree that Leveles' solution seems to be the best. Why try to get things that switch faster when you don't have to worry about switching time with just a battery charging system for your inverter supply.

While not questioning the rational used to engineer the system as you described it, I would not have done it the way you did. I would have done as follows...

1) Used a standard UPS to feed the servers and radio gear.

2) Used a small manual transfer switch to feed UPS.

3) Connected the 12 volt to 120 volt inverter to one input on the transfer switch.

4) Connected the utility power connection point to the other transfer switch input.

IMHO... that is about the only way you are going to be able ensure that the system will keep the servers running.

The 16ms switch over time, from vehicle power to utility power, you are getting with your transfer switch is pretty good and I doubt you will be able to get it any better then that. That, the 16ms, is a long time for servers to stay running.

About the only other way that comes to mind how you might do this... is to have the utility power rectified to 12 volt DC then feed the inverter in parallel with the vehicle electrical system.

I am nearly convinced that the UPS is the way to go. Rather than use a transfer switch, I can still rely on the inverter's bypass switch. Put the UPS between the inverter and servers. Perhaps a mini-UPS will do the trick.

T

Tell me something if you please, what DC voltage are you doing this with?

Because... with a steady state of 3kW the current draw, at 12 volts nominal is 250 amps. What do you have in the way of batteries in this vehicle and what type of an alternator do you have in it? What is the peak demand of the equipment?

Probably 24 volt, military surplus radios. The truck waits for the inevitable invasion/collapse of law and order, and becomes the prize possession of whoever is able to steal it and live to tell...

Guest asked: Short of integrating a true UPS into the system, is there another approach to handling the switching time?

REPLY: Servers by themselves usually consume only 500W or less each. Install a 500W dedicated permanently ON sine wave inverter to handle only the server power supply. Let the rest of the eqipment run from the big inverter. Having suffered the same problem I inquired and was told the new high speed CPU are so sensitive to transients, that newer power supplies instantly initiate a controlled shut down at the slightest power line disturbance.

Getting one or two sine wave inverters ( without a transfer switch) to run continuously from the battery bank will solve your problem. You might also want to install a battery charger to top up the power consumed by the dedicated inverter. Its cheaper than a full UPS. BTW some of th enew power supplies are not happy with the poor quality from th echeap desk top UPS models.

Eureka,

As it turns out, there are two inverters in this system (paralled on 24V vehicle battery). What if I use one inverter in inverter mode only (disable bypass) and let the other inverter's charge current be the feed for the first inverter in supplying power for my servers. Provided, of course, there is enough charging current capacity to do so.

I also feel that some form of UPS is needed as many of the others do.....

Switching the AC, no matter how fast its done, will always bring problems, some you will see, some you will not see until much later. Switching the mains is simply a bad idea for computer equipment and the like!!

Isn't connecting a UPS up when he's already got an inverter running kind of silly? Just add a rectifier circuit to charge the batteries when he has a supply of power available and call it a day.

Not quite, you have to look at the basic circuit to see the subtle but important differences....no bridging of the mains to inverter for example......

No, Andy. The rectifier / battery / inverter is the most straightforward and no headache solution for UPS. But it is not the cheapest, and therein lies the rub. On the other hand it will run with no glitches as long as commercial AC or diesel is available.In this case 2/3 of the solution is already in place, so adding the 1/3 to it (rectifier and charger) is the logical one. It is nice solution in the sense, that it does not require any change in the engine driven charger. I strongly prefer a solution where custom modifications are not required. Those are trouble spots later. This one is straight thru.

My # 12 refers to #10

That is basically exactly what i would recommend, I was referring to installs where the mains would get switched....

I was referring in particular to this specific problem. I agree in another situation installing a UPS to provide power while switching sources is a good idea. But, basically all a UPS consists of is a battery with an inverter on it and a means to charge the batteries. I'm a firm believer in the KISS method. Murphy always seems to rear his ugly head when you try getting too cute.

You wrote (and I completely agree with it!):-

I'm a firm believer in the KISS method. Murphy always seems to rear his ugly head when you try getting too cute.

Good Post !!

Short, sweet and to the point. The classic KISS approach!

Hi Guest. As you are not logged-in your location is not known.

If your supply is 50 Hz you are losing a little less than a cycle; on 60 Hz losing 1 cycle.

Most equipments shut-down in this power-break normally except:

1. If it is running on rectified-Dc & have sufficient Back-up capacitor-Bank on DC supply.

2. An item running thruogh an input transformer; as it may have some energy-back-up in form of electromagnetic-storage-effect.

I think that your system have a contactor/ relay type transfer-system.

Had it be Electronic; through Thyristors the transfer time would have less than 2mS

as theturn-on & turn-off time in latest thyristors is in µS.

Such assembly is normally available with large UPS Manufacturers for Multi-module Redundent UPS-Systems; but do not know that they sell it as separate item.

Wish you best of luck.