Generators and Power factor

Recently took delivery of the Wolf 800 Generator, at just under a 100 notes I thought it would come in handy for our all too common power cuts, not much output at 650 watts but will run the gas central heating and the tele!

Reasonabale little genny set it up runs fine does the job i want, however looking thru the instruction booklet I came accross the following table,

formatting link
and I am clueless to what it is going on about, maybe someone on this group can explain.

Lighting, Power Factor = 1 Load = ~650W

Power Tools Power Factor = 0.8 ~ 0.95 Load = 520W

Electric Motors Power Factor =0.4 ~ 0.75 Load = 220W

The first bit seems reasonable enough, use lighting up to its max output of

650W

The next two bits is where I am getting confused. Is there some difference between a power tool and an electic motor, after all I can't think of a power tool that dosn't have an electric motor! (maybe a Soldering Iron ?) So what does it all mean? Presumably the power factor is a percentage, so power tools 80 - 95% of 650Watts. does this mean that the (80% 0f 650=) 520W is a minimun, if it is the maximun then what does the 95% represent?

Anyway, I have plugged it all in and it does what I want, but I am curious as to what that table is all about, I have emailed the manufacturers but I won' hold my breath for their reply!

Another Question, can these types of generators ever damage TVs, Videos or computers? should I use a surge protector?

Reply to
Stuart
Loading thread data ...

Yes if you start them with the equipmet plugged in.

A good one may help but just unplug first.

I would suggest instead getting a battery driven UPS for the computer or TV and keep the generator for the lights and central heating.

Reply to
Mike

You will probably find in the small print it says an output of 650 VA rather than watts, of which more anon...

OK this one is easy enough. What they are saying is the load is a plain resistive one. For an incandescent bulb this is true (however it is not true for many fluorescent lights). When you are dealing with loads that look like a resistor, then you can treat AC circuits pretty much like you would a DC one. All the normal formula apply; so 650 VA (or volt amps) is exactly the same as 650W.

Now things get a bit more interesting. When you start talking about loads that have some capacitive or inductive elements as well as a resistive one, you no longer have a simple load, but a complex one. This is now more correctly called an impedance.

Inductors (i.e. things with windings) will exhibit no resistance (other than that of the wire from which they are made) when passing DC. Inductors like steady state current flow, but will "fight" changes of current (by releasing energy stored as a magnetic field in its coil). This will result in the alternating current flow to no longer be aligned with the alternating voltage. The current flow is now delayed (or phase shifted) by 90 degrees for a perfect inductor.

Capacitors show a similar effect, but in the opposite way, they will block DC, but pass AC ever more readily as the frequency rises. In a perfect capacitor the current waveform will lead the voltage one by 90 degrees.

So to arrive at the total impedance you need to do a sum which adds the capacitive and inductive reactance to the DC resistance. To complicate matters, the phase shift involved with the AC reactive components means the sum must be done in the complex number domain to allow for the magnitude and angle componets of the reactance to be taken into account (i.e. you are adding vectors and not scalars)

The result will usualy not be a simple numeric answer either, but a vector which has a phase angle component.

Yup, just like DC. Unless the lights are fluorescent in which case they will have a capacitive element to their load as well.

The power factor simply the ratio of "real" to "apparent" power. The apparent power is what your generator "sees" as the load. The "real" power is what is actually gets transferred into the load to do real work, rather than that used to overcome the reactive effects of the inductors and capacitors in the load.

There is a nice description here:-

formatting link
Anyway, I have plugged it all in and it does what I want, but I am curious

Its one of those simple questions where the answer starts to get quite complex (pun not intended) quite quickly!

Once it is up and running it ought to have a reasonably decent sine wave output so long as you are not loading it near the limit. You would need to be careful as loads switch in and our however as this could mess up the output of the genny briefly.

Reply to
John Rumm

Now that was a pretty fair - and accurate - description of power factor and its ramifications.

Wonder how many others will read the OP and jump straight in with *their* interpetation, instead of reading the replies first?

:-)

Reply to
Wanderer

It was...but didn't answer the question about the difference between power tools and electric motors...!

Reply to
Bob Eager

No, and although it all sounded very clever it failed to answer any of the questions! OK it did answer the one about lights but then again I answered that myself!

So, Power Tools - "0.8 - 0.95" If the 520Watts equates to 0.8 (80%) Is that a min or max, what is the 0.95?

And is this only power tools without electric motors? Such as an incredibly big soldering iron?

Just out of interest I connected my 750watt jig saw and it worked just fine, even under load! It also works the water pump at the low setting which is

40watts, so what exactly is "Electric Motors" (0.4 - 0.75) 220W represent.
Reply to
Stuart

A max of between 0.8 and 0.95, depending on the power factor of the actual load being used. These will vary.

That's pretty well a resistive load. I've never heard of a soldering iron being called a 'power tool' although I guess that one could argue that technically it is.

The maximum safe load without possible damage to the generator. You can connect more for various reasons; the power factor may be a bit 'better' in some cases, and most things will stand an overload for a while without melting, burning out or catching fire!

I'm not sure of what they're getting at with the 'power tools' or 'electric motors' bit...except that perhaps power tools uses motors with a better power factor.

Sometimes I wish I'd studied proper electrical engineering!

Reply to
Bob Eager

Most power tools use a series-wound universal motor, and this type of motor can have a high power factor. But an induction motor (such as in a fridge, or blower motor in a c/h boiler) can have an awful power factor.

You've sussed it. Real Power, in Watts = Volt*Amps*PF.

Possibly, but I don't think I would chance it.... too much to lose just for a bit of rubbishy telly during a power cut.

Might be safer to use a UPS and let the generator float-charge the battery.

Reply to
Tony Williams

no offence, but you were robbed... I bought one (a rebranded version) for £43+vat at Makro

no idea, but I "tried" to use an angle grinder (500w motor) hooked up to the genie and it failed miserably, however, a 1300watt pressure washer worked fine... weird

LJ

Reply to
in2minds

Let me have a go at a simplified version of John Rumm's explanation.

For any AC device, the voltage and current are not constant - but vary sinusoidally (as in a sine wave) 50 times per second.

When the load is purely resistive - as in a normal light bulb, or soldering iron - the current is exactly in phase with the voltage - i.e. the phase angle is zero.

When the load is reactive - as in electric motors etc. with capacitance and/or inductance - there is a phase shift so that the current is no longer in phase with the voltage, but leads it or lags behind it by a phase angle, which varies from device to device.

In all cases, the power consumed is calculated by multiplying the RMS (Root Mean Square) Voltage by the RMS Current by the Power Factor - where the Power Factor is the cosine of the phase angle.

Your generator is apparently capable of delivering 650 watts for resistive loads (where the phase angle is zero, and the power factor is 1). So, at

240v, it can deliver a current of 650/240 = 2.7 amps.

But it can *still* only deliver 2.7 amps when reactive loads are used. So if you have a device with a power factor of 0.4 (phase angle 66 degrees), you only get a power output of 240 x 2.7 x 0.4 = 260 watts rather than the original 650.

As someone else said, the generator has an output of 650 VA rather than 650 watts. This is a measure of volts x amps (ignoring phase angle) and only equates to 650 watts for purely resistive loads.

HTH.

Reply to
Set Square

It's the difference with some electric motors have carbon brushes and some which use inductive electric motors.

The "Electric Motor" part of the table is motors without carbon brushes (Inductive motors).

The "Power Tools" part of the table is for electric motors with carbon brushes fitted (Reactive motors).

Reply to
BigWallop

Or even more simplified. There are two different types of Electric Motor, Inductive, which doesn't use carbon brushes to make it work, and Power Tool Electric Motors that do use carbon brushes to make it work.

Inductive loads have a huge variation in the voltage and current they draw from a gennie, and cause the output to swing positive and negative a lot more, especially on start up, than an electric motor that uses carbon brushes to send the voltage and current through the winding coils.

The table seperates them into two catagories of "Electic Motors" and "Power Tools" and that's the only difference.

Simple.

Reply to
BigWallop

Starting current versus running current.

A very high efficiency motor can draw HUGE startup currents quite out of proportion to its actual running current, and its also instructivce to see whether its input or output ower being quoted.

A cheap electriv motor may lose up to 40% of the power - or more. A good one can be > 95% efficient.

Reply to
The Natural Philosopher

Ah, thanks...thought it might be something like that...!

Reply to
Bob Eager

It may be simple, but it totally fails to address the question about power factor! Nowhere have you made any reference to phase angle between volts and current - which is fundamental to the whole thing.

Startup current vs running current is an entirely different issue. Whilst it needs to be taken into consideration, it has nothing to do with the question which the OP asked - which was why the Power Factor was different for different types of device.

Reply to
Set Square

They're assuming "power tool" is a brushed motor (most hand-held tools) and "electric motor" is an induction motor (bigger stationary machines).

Unless you're a woodwork shop, assume "power tool" for all.

If you _really_ care, the electrical FAQs at

formatting link
are good on power tool motors.

Reply to
Andy Dingley

A simpler way to think about it is to think of your genny as having a current rating rather than a power rating. Since the power rating at unity power factor is given as 650 watts, we can deduce that the current rating is 650 watts divided by 230 volts, which is about 2.8 amps.

Most (or at least "many") appliance rating plates give a full-load current rating. If this is 2.8 A or less, the genny should be able to run the appliance OK (although it might struggle to start some things with 'larger' motors).

Reply to
Andy Wade

The only bit of te telly at risk from voltage surges is teh swtched mode PSU. Most are ramarkably tolerant.

SAafer, but arguable as to how necessary.

Worth putting a meter on the gennie and seeing how high the voltage goes. Most mains side TV stuff is rated at 400v = peak voltage of 280vAC.

Reply to
The Natural Philosopher

'Leakeage' inductance.

Reply to
The Natural Philosopher

There have been enough discussions about power factor but I feel none are likely to be helpful to the lay man.

Generators are rugged beast and can cope with a great deal of abuse. You can typically draw more than the rated power though the engine will start to struggle. It is the power which determines whether the engine can cope or not where power factor will have little effect.

When a load with a poor power factor is used the "useful" current is higher than indicated by power alone. These give rise to copper losses in the generator part itself and creates heat. If you can get a true RMS meter reading Amps then it becomes easy to determine whether if the VA loading is exceeded. If the load is intermittent then internal heating effects are unlikely to be a problem.

For the purists, current phase lag or lead is not the only cause of reduced power factor. Many power supplies draw a non sinusoidal current and reduce power factor accordingly. Old PCs would have this characteristic!

My experience is that I have never damaged electrical stuff using a generator. To be honest I'm not convinced a surge protector is going to do much and have never used one!

Reply to
Fred

HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.