Need Details About Backup Generators?

Question

I am trying to get more information about the internals of a backup
generator. Specifically those in the 3kw to 6kw range driven by gas
engines. I have questions like:

(1) What protections do they have against overload? I have been using DC power supplies for 35 years and they all have current foldback that comes into effect when overloaded. Do any generator manufacturers include a similar function?

(2) If the unit has the 240 volt plug, assuming the two hot wires, the
neutral and the ground, how well do they handle load unbalance? For example, if all the load is on one of the legs will that cause any problem.

(3) How do they implement the effect of a center tapped transformer so they "look like" the incoming power line? I have looked over a few in stores and I don't see any transformer.

(4) How do they control the amplitude and frequency of the output?

(5) How close is the output to a sine wave?

Thanks in Advance,

Pete

Answer 1

1. Circuit breakers. There is sometimes a dual breaker which will shut off both lines, followed by individual lower rated breakers for the 120V outlets.
2. Should work fine with full load up the the breaker maximum current on one leg only.
3. Two separate windings with a common center tap acting as neutral. Sometimes this is bonded to the generator chassis.
4. Frequency is usually determined by a mechanical engine speed controller. 3600RPM for 60Hz. Some generators with brushless alternator will supply slightly over 120V at no load, and drop slightly as load is applied. Those with brushes to control field current will regulate the output voltage.
5. Brushless type will supply a perfect sine wave. Brush types will have a slight amount of noise on the signal. I've used both types and had no problems with either.

Some manufacturers, particularly Honda use an inverter type AC output. I don't know how they create 240VAC nor how each leg is fed. Engine speed will vary according to load applied. The inverter will determine output voltage and frequency. They are not pure sine wave outputs, but I can't say how close it approximates or how many steps the sine wave consists of.

Have a look at my generator page and FAQ for other info...

http://members.rennlist.org/warren/generator.html
http://members.rennlist.org/warren/genfaq.html

Answer 2

The details on other answers are pretty good advice. The only thing I would add is.
1) At 80% efficiency, it takes about 1.65 hp to produce 1kw of electricity. There fore a true 18 hp engine will supply about 10.75 kW, but keep in mind, some manufacturers exaggerate the Hp rating on the engine.
2) Do not use "surge watts" to rate the generator. This is a marketing driven rating that has to do with inductive load, power factor, and motor starting. It has nothing to do with the actual usable power you can get from the unit.
3) Permanent magnet or variable speed generators (Like the small Honda's) can have a simulated sine wave that is pretty dirty, and the do not do very well starting motor loads like A/C or sump pumps.
4) If you overload your engine driven generator, the worst thing that can happen is that the engine loses speed, and the lights brown out. Most generators will be limited by the power the engine can produce. No one will give you a larger engine than they have to.
5) Brush-less alternators may have less noise, but brush type are more reliable and do better starting motors without burning out capacitors.
6) Buy a name brand, from a brand name supplier, IE Home depot, Lowes, etc. That way if you have problems, they can help.
I would buy a Generac, especially one with a Generac engine, and not the cheap Briggs engine.

Answer 3

Simple answers,

#1 Circuit Breakers.

#2 Again they will handle up to what the circuit breaker will handle on that leg.

#3 You can think of the generator as a transformer. Instead of having a primary winding and a secondary winding , the primary winding is replaced with the armature.

#4 The amplitude will vary to some degree with load and the frequency is controlled by the speed of the engine.

#5 This is by far the place where your generator will shine, it is a sine wave machine. It makes a sine wave that looks like the one from your electric provider.

A word of caution, be very careful when sizing a generator. If a 5kw does not have at least a 18 hp engine driving it, don't buy it.

Try to buy one of the top names, I would suggest in this order, Onan, Kohler, Generac.

Another rule of thumb, the generator should cost about $500 per kw.

Answer 4

We have a backup portable generator. We already had our generator. To avoid having cords running all over our home, we opted to pay an electrician to install a special panel box - beside our main elec. panel box. When the power goes off, we move the generator into place (I recommend the optional "trolley wheels offered with generators). We run the gen. plug thru a window & plug into the special panel box . The spec. panel box "overrides" the regular elec.panel box, to prevent electricity from reversing. If the elec. reverses, going out into your normal lines, it is a danger for anyone working on the lines. The new box prevents that. We have all our creature comforts - well pump, lights, tv, etc. Depending on your gen., you may have to choose to leave some high-use items off. Down side: It is LOUD! And, you have to go out, in the weather, to check it, refuel it, etc. If you haven't purchased a gen. yet, you might consider the newer, whole house "automatic" generators. When the power goes off, there's a 5 second delay, and then you are right back with full power! They are expensive ($4,000 or so), but make you pretty independent! My elderly parents have one, and it helps them live, independently, in a rural location. Call your elec. company for the names of local sales/installation. Best to you.

Answer 5

Hay Pete,, Ihave a 6000 watt back up for my house and it is just great , It is plug in my panel out side where the power comes in ,all that i do is turn off main power , and plug in the generator .TALK TO A QUALIFIED PERSON THIS ITS NOT CHEAP IT SAVED MY BUT A COUPLE OF TIMES GOOD LUCK.