genset

whether i spend 300k or one million Baht on my generator. but having grown up as a poor boy in a poor environment and working hard for whatever i own now i hate wasting a single penny.

Hi Naam,

I appreciate your comments, have attempted to answer your questions to date as honestly as I can and will continue to do so.

so let's look for a solution apart from oversizing

Ok, the first thing we need to correctly size a generator for your needs, is an accurate load calculation based on the starting load requirement of each motor you intend to run and all your other appliances.

Genset

one has to add "the manufacturer wants me to spend a multiple of my dough because he is not willing to produce with an additional minimum of my aforesaid dough the sturdiness of a product i have experienced three decades ago".

At the end of the day Naam, if you size your generator correctly, use it appropriately and service it regularly, it should give you years of trouble free service... I wouldnt like to see you slide into a fit of depression over this now...

Genset

The neutral conductor is not switched.

You can make a working drawing from this information

Hi David96,

You went on to add, (a note at the end of the drawing you subsequently posted), that;

'main neutral and generator neutral are connected to the neutral bar of the switchboard directly, they are not switched'

With respect, I disagree with you on the issue of a permanent neutral connection to a secondary power source (generator) for domestic use.

It is my preference to always treat a standby generator for domestic use (and not designed to sync and run parallel with the utility supply), as a separately derived source of power. As such the star point of the alternator is connected to earth via a grounding electrode at the site of installation and the alternator neutral is bonded to the body of the alternator.

When configured in this manner, the neutrals (utility and generator) should be switched and not permanently connected, if ground fault protection devices are in use.

If the utility supply neutral and the generator supply neutral are both, separately grounded and the neutrals of both sources are permanently connected at the transfer switch; should a ground fault occur, any ground fault current would be split along the neutral into two parallel paths to earth. If the amount of sensed ground fault current does not exceed the settings of any ground fault protection devices fitted (now or in the future) to the faulty circuit, they will be rendered ineffective, quite an important point to bear in mind.

The two main ways to solve this problem are to remove the neutral/earth bond from the alternator, or to switch the neutrals at the transfer switch (both of which would return the full ground fault current through the protection device). In my opinion, switching the neutral is a must.

I understand that MEN (PME) is a consideration, but as it is still not implemented country wide throughout Thailand, I prefer to focus on the viability of the local grounding system and proceed as if MEN is not in place (even if it is).

Genset

...Not sure what happened to the formating of this post, I wrote it in Msoft Word and pasted it into the window... the spacing is a little tight but I cant amend it... sorry.

The attached document explains this problem in more detail and quite clearly...

Gen_earth_grounding_and_4_pole_neutral_switching_Cummins_part_2.pdf

Observer: "Most synchronous generators are designed to support 300% of their rated kVA for 10 seconds. From the OP, [(600 amps starting current)(230 volts single phase)]/1000 = 138 starting kVA"

Genset:"a generator with the correct insulation class (H), may also handle high overcurrent for short periods (such as that mentioned in Naams question), but the potential for alternator insulation failure is high', I followed this up more recently with the comment that things just arent made the way they used to be..."

Gentlemen, assuming both claims are valid it is beyond my comprehension that the supplier of generators (main parts engine and alternator) would try to save cost by being stingy as far as insulation class of the alternator wiring is concerned. in Nigeria i bought the generator together with his twin second hand from a german construction company which withdrew from the country. that was end of 1981, both units were most probably 2-3 years old. and now i am told that after THIRTY years of technical and manufacturing development the products are inferior?

Naam,

No, contemporary alternators are not inferior, far from it, but 300% of rated Kva for 10 seconds equates to an enormous amount of heat being generated in the alternator windings. These recommendations exist to extend the life of an alternator at the end of the day Naam... the manufacturer doesnt want you to burn your alternator out through misuse.

Having said that, competition is rife and the bottom line does lead to cost cutting in some areas which irritate me to be honest. Mecc Alte exciter coils is one such gripe... they need to spend a little more money on improving the insulation of their exciter coils in my opinion, far too many of them short out in only moderate damp conditons and can be a real pain to change sometimes. I have taken to disassembling every Mecc Alte alternator that I fit and spraying additional insualtion on the exciter coil to reduce the frequency of these specific faults.

Let me qualify the remark I made, if I may... I regularly see alternators (belt driven PTO Tractor sets mostly), which are thirty + years old. These belong to old farmers who work the things hard, constantly (and dont like to spend money...!). I see them when the belts need changing, these things are overwound and heavily insulated, I have no doubt that they will be providing power to the milking parlours of future farming generations for years to come... 'They just dont build them like they used to...' (is the term I should have used), is a reference to the fact that these generators take a lot of stick and last for years because they were overbuilt and not because they were technically superior.

Genset.

Or another way: Most synchronous generators are designed to support 300% of their rated kVA for 10 seconds. From the OP, [(600 amps starting current)(230 volts single phase)]/1000 = 138 starting kVA. The generator was rated at 70kVA, so 300% x 70kVA = 210kVA available to support starting current for 10 seconds. We can see that the generator had more than enough capacity to start the specified loads.

Thank you Interested Observer...

I have been so busy over the past few days that some of these questions, deserving of specific answers have slipped through the net so to speak, thank you for picking this up in more detail.

I did respond to this question directly to Naam by email on 17/11 and gave the following (and, as I dont know if I'm coming or going at the moment, woefully brief answer); 'a generator with the correct insulation class (H), may also handle high overcurrent for short periods (such as that mentioned in Naams question), but the potential for alternator insulation failure is high', I followed this up more recently with the comment that 'things just arent made the way they used to be...'

Genset

The best way to solve this conundrum, is to purchase that tong tester (clamp on ammeter with peak hold function) we discussed earlier and take a simple reading of each of your AC 'comfort' circuits on startup, that should put this baby to bed once and for all....

i have been phoning around already. result: "clam testa no habb"

For anybody in need of electrical components or test equipment... attached is a list of electrical distributors in Bangkok.

Genset.

Merlin_Gerin_Square_D_Telemecanique_Distributors_Bangkok.xls

i have been phoning around already. result: "clam testa no habb"

... try an electrical supply store instead of a fishmongers... lol...

If you can make it to Bkk, I can give you the address of a place you can buy one...

Genset

correct me if i am wrong in my thinking that only a huge overdimensioned generator could handle the starting amps of the consumers on all three phases EXCEPT if the big consumers (aircons) are handled with timers and other gadgets which switch -before generator start- all of them first OFF and then ON one by one.

Hi Naam,

The short answer to your question is yes... Although the generator would seem to be oversized, calculating your load requirement based on the starting current of all the motors served by your existing 3 phase distribution board (if a whole house solution is required) is the normal method for sizing the generator.

Although in reality, the peak load requirement (of every motor starting simultaneously) may never actually be realized; sods law unfortunately states, that if you select an undersized generator, it would...

If a system designed to inhibit the simultaneous starting of all air conditioner motors, or a generator sized to accommodate the combined peak load requirement of all your appliances, are discounted; linking your critical (comfort) circuits to a separate distribution board (single phase would work as long as these circuits do not include any 3 phase motors), supplied via an automatic transfer switch by a single phase generator and sized to meet the peak load requirement of the aforementioned critical (comfort) circuits, would represent a simple and effective solution. If configured in this way, the transfer switch design would differ slightly from normal, in that the utility supply would feed your existing 3 phase distribution board and the generator supply would feed the secondary (comfort circuits) distribution board via interlocked contactors. This configuration would ensure that both supplies could not be connected simultaneously.

Genset sent me a pdf-file from a renowned manufacturer of alternators. it says:

"When a motor is started, a large surge of current is drawn by the motor. This starting current is equivalent to the motors

locked rotor or stall current and is 5 to 10 times normal full load current."

Naam.... dont panic... the document refers to an alternator sized in the 300 to 400Kva range (not the 30 to 40Kva alternators we are discussing for your application) and as such would be used to power large, 3 phase motors for commercial applications and not small motors such as the type found in your home. The paragraph quoted should not be taken literally, I sent it to you to reinforce the idea that a calculation based on a multiple of 3 (for startup current) was realistic based on your earlier comments, doubting the veracity of 'expert' opinions regarding the rough calculation of startup current for motors posted in the thread.

The best way to solve this conundrum, is to purchase that tong tester (clamp on ammeter with peak hold function) we discussed earlier and take a simple reading of each of your AC 'comfort' circuits on startup, that should put this baby to bed once and for all....

Genset

Genset,

can it route the generator to the phase which is gone? if yes, why using electronics which nobody can service in Thailand within a short time if something goes wrong instead of using good old manual switches?

Hi Naam,

I have never before been asked to configure a generator in the manner you are suggesting but I see no reason why it could not be done.

As it is your wish (in the event of a dropped phase) to substitute one of your utility phases (either/or L1, L2, L3) with a single phase standby generator supply, the main issue (as Crossy mentioned with his comment on interlock and safety devices), is ensuring that the single phase generator supply and the phase/s in question are never connected simultaneously.

In my opinion, the safest form of manual transfer switch, is a 3 position 'break before make' rotary switch. The utility supply is fed into one side of the switch, the generator supply is fed into the other, the output from the switch then carries the supply from whichever feed is selected, to the distribution board. In position one, the utility supply is fed to the distribution board. The second position 'breaks' the connection between both the uility supply and the generator supply (this ensures that both the utility supply and the generator supply are isolated from each other and from the distribution board during transfer), and in the third position the generator supply is fed to the distribution board.

To achieve the effect you are suggesting, you would need 3 separate, 3 position 'break before make', single phase rotary transfer switches (imagine them side by side in a line if you will...). The first of your three utility supply phases (L1) would be fed into the utility supply side of the first transfer switch, the second utility phase (L2) into the utility supply side of the second switch and the third utility phase (L3) into the the utility supply side of the third switch. The single phase standby generator supply, would be fed into the secondary supply side of the first rotary switch, it would then be looped to the secondary supply side of the second and third switches. The output from each of the three transfer switches are fed into your 3 phase distribution board MCB to be distributed between your circuits, as per your normal three phase utility supply.

In the event that you drop a utility phase (L3 for example), after starting the generator and checking that you have a stable suppply, you would then manually switch the third rotary transfer switch from position one (utility) to position two, this breaks the utility connection and no power (utility or generator) is available to the circuits on your L3 phase. Selecting position 3 (generator) restores power to your L3 circuits from the generator. When the L3 utility supply phase is restored and stable, you may then move the selector from position 3 (generator) to position 2 breaking the supply to your L3 circuits once more, then select position 1 (utility) restoring the utility supply phase to your L3 circuits, you may then shut down the generator.

Because the generator supply is looped between each rotary transfer switch, the generator supply is able to replace whichever phase has dropped out... you may even (as long as the generator is sized appropriately), use the generator to supply two, or even all three phases should a complete power outage occur (this is only appropriate if you are not running any three phase appliances in your home, under no circumstances should you attempt to operate any three phase appliances using a single phase generator supply).

Just a quick word on your comment regarding the servicability of electronics in Thailand (specifically in regard to the ComAp Automatic Transfer Switch Controllers I use when manufacturing auto transfer switch panels). The authorised distributor of ComAp products in Thailand is a small family company (40 years in business) in Bangkok called Daven Company. I spent some considerable time with the owner (a very engaging 70 year old man) and his technicians this year, they are a very friendy and knowledgable bunch. They speak good English and offer great support for these products (replacement and/or repair if necessary).

I should mention the advantages of an automatic transfer switch, just for the record. I know you mentioned the economic viability of using a three phase generator as a secondary supply when only one phase has dropped out. The advantages are that the entire procedure is carried out automatically (obviously), not only in the event of a dropped phase (voltage asymmetry) but also utility supply under/over voltage, under/over frequency, and over current. It also negates the need for you to get up and start the generator and carry out a manual transfer should this occur in the middle of the night (or other inconvenient times), or if you are not available (away from home) and the task falls to your wife or staff. Also, regular start up and running of the generator on load (in the event of a dropped phase) is a good thing for the generator; if not used in this manner, you should still exercise the generator regularly (once a week, on load if possible) to keep the generator in peak condition.

Hope this helps, if anybody has an alternative solution to Naams requirement, I'd be interested to hear it.

Genset.

Crossy,

What is the huge duty on imported panels, for those of us who are considering going this route. Any idea?

Hi gregb,

I have no idea how much these guys charge for their technology and/or installation service, but this is the website for the Thai-German Solar Company, they are located in Khon Kaen http://www.tgs.co.th they may be able to help you out with the information you need.

Genset

In my photo studio we used the big Bowens capacitor power packs which claimed to have a peak draw rated at 15amps. We would run two of these plugged into a 13amp extension lead protected by a 13 amp fuse, and a total of six or seven on a 30amp ring main. Never a problem. I know it's not motors which is what we're talking about in this thread, but this Quoted Max/Peak drawn current business baffles me too.

Hi Marvo,

I'm not sure your explanation has really clarified what I think you were trying to say... I had to read it myself a couple of times to get my head around it... how about this..

"I understand that my example refers to the load drawn by a capacitor power pack and not the peak load drawn by motors, which is the current focus of this thread..."

Hopefully this will avoid a string of posts from readers tearing their hair out in response to your last missive...

Genset...

David,

thanks to your explanations i am well aware what the function of a soft starter is. but the question i asked has nothing to do with any soft starter. anyway, the question is academic and not really relevant for the problems which have to be solved and "Genset" was kind enough to answer it.

I decided to answer Naam, directly by email, in an effort to pour cold water on the discussion of soft starters within the thread.

i also want to keep my installation as simple as possible that in case of a problem either myself or a qualified electrician can lay a hand on. anything that contains chips or is computerised should in my view be avoided when living in the technical wilderness of Thailand. member "Genset" has sent to me by mail the description of a most beautiful gadget which would handle automatic/remote start-up in case of brownouts or phase failures.

I have attached a copy of the data sheet for the 'gadget' to which Naam refers. Manufactured by ComAp, this is an Automatic Transfer Switch controller (True RMS sensing), I use a variety of ComAP controllers including IntelliAMF (Automatic Mains Failure with Generator controls) and IntelliGen (Single Genset Parallel to Mains operation (Synchronized operation)). The IntelliATS is the most basic controller with a simple remote start command to start the generator (monitors under/over voltage and frequency for single and three phase applications), but still very, very reliable. If you have an existing, manual start (electric start) generator, this unit can be retrofitted (to any generator with remote start ability) to enable automatic transfer switching.

additional second thoughts have also come up as far as a 3-phase generator is concerned. i normally face a brownout or complete loss of a single phase only and have still two phases with normal voltage. therefore i think it's an economical nonsense to switch off the "good" phases too and serve all three phases with a 3-phase generator.

It should be possible to have the (single phase) generator switch to any of the phases in my house as needed. i had a similar but very simple setup in a house (in another "high-tech" country) decades ago. the house was wired single phase but we had a 3-phase incoming supply and used whatever phase was "on" or delivering the correct voltage.

opinions please and thanks in advance.

Reasonable comment...

and from your next post...

"legally in Oz or UK". i wish i had photos of the setup i described (using whatever phase was convenient). people like you and Crossy would have slight heart attacks

If you decide to use a single phase genset, configured to address the problem you have cited, I would ask you to consider asking either Crossy or Elkangorito (if they dont mind...) to assist you in designing a method of achieving this safely prior to genset installation.

Genset

InteliATS_NT_Datasheet_2009_02_CPLEIANT.pdf

To show the effect of having a reduced voltage starter (soft starter) on the size of a genset.

David96...

At last count, I think this is four attempts you have now made to force an explanation of this issue... I have stopped banging my head against the wall for 5 minutes to ask you AGAIN... to please drop this now

As Naam has stated, on more than one occasion, "the question i asked has nothing to do with any soft starter. anyway, the question is academic and not really relevant for the problems which have to be solved"

Further, you proceeded offer the following advice;

To show the effect of having a reduced voltage starter (soft starter) on the size of a genset.

and...

Advantages of soft starter.

Low starting currents , smooth starting, full motor protection, reduction in genset capacity.

Costs. Cost of a soft starter against the cost of a larger genset. And if you have a larger genset you have spare capacity in kVA for other purposes.

The point you have missed by dragging out this subject and thrashing it to within an inch of its life... is the reason why the question of whether Naams AC units contained soft starter motors, was raised to begin with.... namely the possible effect of non-linear loads on the operation of a generator/avr and why they may present a problem for anybody considering the installation of a generator... You've gone round full circle and ignored the original reason for this subject being raised.

For any reader who may be confused by this string of posts regarding soft starters (I'm sure they're lined up around the block..) please begin by reading Crossys post #85, Naams response #86 and my post #87, that should (I hope), clear up any confusion.

Although I do appreciate your efforts to pitch in with detailed information, I'm afriad your obsession with this subject will have readers chewing through their own arms to get away from the thread, so please, please, please.... enough...!

Genset

what size gen should i consider buying for a small job such as this? what specs should i consider most? what price should i expect to pay in bkk?

Hi Maccaroni Man,

If you have some spare time, try to read through the thread from start to finish, there is a lot of information within, regarding machine specs and prices for the Kipor range I am promoting.

It sounds like you havent purchased the pump yet; decide which pump best suits your needs and base your generator size on the power requirements of the pump you purchase (again, there are references to calculating generator size according to your load requirement within the thread).

Genset.

Here is some information on AC soft starters suitable for single phase.

http://www.pne.com.au/products/istart.shtml

Soft starting motor controls will in the future replace many DOL and existing reduced

voltage starters.

Hi David96,

The link and additional information is interesting and the subject of soft starters for AC units may warrant a thread all of its own, I am sure there is interest out there amongst potential users of this technology; but as we have now established that Naams AC units do not feature non-linear/variable speed motors and as this thread exists to provide information and answers related specifically to the installation and operation of generators; I would appreciate very much, if we could please stay on topic and resist the temptation to parse these issues further (beyond the specific concerns of those considering the installation of a generator) within this thread. I'd hate to lose the attention of those with an interest in generators who have read this far...

Genset

If individual & independent timers are used in each a/c unit, it will not prevent the chance that all or some of the a/c units could start at the same time.

Agreed...

I made reference to this in post #24 in response to Naams initial suggestion of manually turning each AC circuit on, one by one, after generator start up to avoid the AC units starting up 'en masse' (this was prior to the suggestion of using timers, but still applies).

In reality (given that Naam is contemplating a generator sized in an appropriate fashion), this will most probably not be an issue (never say never though...); although the timers would initially provide a delay between the start up of each AC unit after generator startup and transfer to generator supply. As each individual room warms and cools at a variable rate (especially during extended generator run time in the event of a sustained power outage), it becomes increasingly likely that the warming/cooling cycles of the AC units will randomly sync and two or three units (more if you're unlucky) may indeed start up simultaneously.

Genset

# Derating (oversizing) the generator where the load is predominantly non-linear.

# The possible addition of a small linear load (a load bank sized for approx 10% of the alternators rating for example) where a load is predominantly non-linear so that some resistive load is on the generator as the non-linear loads begins the ramp on process. This should aid in stabilizing the system somewhat.

# The use of a single phase or three phase, true RMS sensing AVR. This should provide reliable voltage regulation for generator sets serving non-linear loads. They will sense voltage level more accurately, regardless of the distortion of the voltage waveform.(the SR7 AVR from Mecc Alte is one such AVR, circa 500 sterling).

# The use of an alternator featuring Class H insulation. This provides additional thermal protection to offset alternator over-heating caused by non-linear loads.

AAARRRGGGHHHH...

...apologies Naam...

These recommendations were posted as an example of steps which may be taken, if/when it becomes apparent that a predominantly non-linear load is affecting generator/avr operation.

As you have now clearly stated that your AC units do not feature non-linear/variable speed motors, this should not apply to your prospective project and should be read in the context of the posts which preceeded it.

Genset

Genset, just out of curiosity, what is the "standard" insulation class of generator windings?

An "inverter class" motor has a minimum Class F insulation. There are other Class "combinations" that can be effective, which depends upon the design of the machine. Please be aware that I'm talking about induction motors.

Hi Elkangorito,

Synchronous alternator windings are manufactured using all four classes of insulation. The most common for standby and prime power applications being Class F and Class H. It really is incumbent on the generator distributor to give the best advice (regarding the most appropriate insulation class) to a client, based on the proposed application of the generator. The Kipor SS (ultra silent) range feature Class H insulation.

Genset

nope Gentlemen! none of my units have variable speed compressors. when we built the house 4 years ago i opted for straight ac/ac compressors as we had ongoing problems with inverter units installed in an apartment the Mrs. owns in another country. they were installed in 2002 (?) when the technology was rather new and the technicians of the supplier were rather helpless

by the way, i called a friend in Germany, he very much doubts that the amp readings i posted here are correct and suggested, no matter what the new readings are, that "Genset" installs for each unit individual "delay relays", each with a time difference of one minute.

Hi Naam,

You beat me to my reply to Crossy, but my previous post is still relevant with regard to homes where predominantly non-linear loads are present. The news that your AC units do not feature inverter technology (given the percentage of your load taken up by AC units), is great from an operational viewpoint. You would, of course, still need to nail down your load requirement. If you dont already own a tong tester (clamp on ammeter), you may wish to pick one up, if you do, try and get a true RMS version with peak hold function as suggested by InterestedObserver, they're relatively cheap and worth having.

Genset

.

I'd like to begin this post by thanking all those who have contributed to this thread.

In addition to offering fundamental advice and promoting a specific line of generators, my goal here, has been to prove that a market exists within the expatriate community for the services I intend to offer. I have received a flood of emails that I am slowly working my way through and significant interest does indeed appear to exist; interest which in large part seems to have been ignited by this thread.

Another reason for my posting in a sub-forum occupied by a knowledgable collection of individuals, was the hope that I could draw on your collective experience and highlight any potential issues specific to the use of generators in Thailand, in anticipation of rolling out a comprehensive service in 2011.

As discussed earlier in the thread, one such issue that has been raised, may be the propensity for expatriate homes to house numerous UPS systems, variable speed motors (in the case of the Daikin AC units cited by Naam) and other rectifier based, non-linear loads.

I'm wondering what the interesting VI waveforms produced by these VVVF drives will do to the AVR on the Genset. Ref our conversation on UPSs and PC PSUs with active PF correction.

Any A/C load will likely be a significant percentage of the overall consumption along with CFL lighting and switching PSUs.

After consulting with my tech mates at Mecc Alte, their opinion on the most likely effect of the 'interesting VI waveforms' on the AVR, is one of voltage oscillation and frequency instability; a condition which will further serve to exacerbate the problem.

Their recommendations included:

• Derating (oversizing) the generator where the load is predominantly non-linear.
  
• The possible addition of a small linear load (a load bank sized for approx 10% of the alternators rating for example) where a load is predominantly non-linear so that some resistive load is on the generator as the non-linear loads begins the ramp on process. This should aid in stabilizing the system somewhat.
  
• The use of a single phase or three phase, true RMS sensing AVR. This should provide reliable voltage regulation for generator sets serving non-linear loads. They will sense voltage level more accurately, regardless of the distortion of the voltage waveform.(the SR7 AVR from Mecc Alte is one such AVR, circa 500 sterling).
  
• The use of an alternator featuring Class H insulation. This provides additional thermal protection to offset alternator over-heating caused by non-linear loads.
  

I dont want to overstate the problem, there are thousands of generators operating satisfactorily, in prime and standby applications throughout Thailand; but in the event that a load is comprised totally, or in large part of non-linear loads, the only way to know for sure how a generator will perform (regardless of whether the machine is worth 200k or 2m baht...) is to get on site and 'test' the generators ability to operate satisfactorily with the proposed load prior to purchase and installation.

I will contact the distributor next week and discuss the possibility of providing a generator specifically for the purposes of testing the ability of the generator to operate with predominantly non-linear loads, where a potential expatriate customer has shown interest in a purchase.

I welcome and look forward to your opinions and comments on this issue; particularly those of InterestedObserver, who has a lengthy and comprehensive experience in generator operation and control.

Genset

Hi Marvo,

Wish I'd seen this thread earlier, I would like to have given you a copy of this document.

For anybody out there considering a bore hole, the attached document is an interesting read and will improve your knowledge of the process and the importance of drilling depth, proper screening, casing to prevent collapse and sealing the borehole to prevent contamination of the aquifer.

Genset

Lifewater_Drilling_Manual_2004.pdf

Honourable Experts Elkangorito et Genset, Esqs.,

may i humbly add actual (measured in my presence) starting amps?

Hi Naam,

If you measured the current drawn by these circuits directly through the use of a tong tester or the like, I see no reason to question them.

My area of expertise... is the manufacture and installation of generators and not, unfortunately, the specific characteristics of AC motors (why some are more efficient than others).

When calculating the load requirement for a home or business, my preference (if on site), would be to use a tong tester to accurately measure the current drawn by motors such as these on startup. If I am offering advice by email, I am looking for the house/business owner to provide me with as much information as possible, the locked rotor amps (startup) or full load amps (running) if available; and if not, the watt rating stamped on the data plate. If the latter is the only information available, I would normally mulitiply this figure by 3 to give me an approximate startup requirement. For the type of small motors typically found in the home, this general rule of thumb allows me to ensure that the generator selected, is of a sufficient size to handle their power requirements on startup (even if oversized a tad) and is not undersized in relation to your overall load requirement (the less desirable of the two options).

Having said that, I am happy to see that your AC motors are so efficient, aside from saving you money when running on the utility supply; should you choose to purchase a generator, they will place less stress on the engine and alternator when running on the generator supply.

Genset

thanks, amazing! based on that i am considering a "preventive" installation within the coming months.

The amazing thing is that when I eventually begin selling these generators directly, I will be able to knock another 15% off the price...

Genset

Apologies for the delay in responding to posts, but I'm working flat out, 7 days a week at the moment.

If I may, I'd like to present an example of calculating generator size. If I have left anything out, I hope that Genset will correct me.

Again, this is merely an example.

Thank you Elkangorito, your explanation of how to calculate the surge load for motors typically found in the home is a very useful addition to the thread. Historically, when L.R.A and F.R.A data-plate info is not available, I have multiplied the running watts by 3 where small motors of the type in your example are present, but I am happy to defer to your experience and electrical knowledge in using a multiple of 4 as you have suggested. As a rule of thumb, because most homes contain appliances with motors such as those listed in Elkangorito's example (and because L.R.A and F.R.A. may not be available on the motor data-plate), I will always post the Kw (alongside the Kva) rating of a machine, based on a power factor of 0.8, to enable accurate sizing and generator selection based on your load requirement.

"Home" generators typically have 2 ratings...a "continuous" rating & a "surge" rating. You may also notice that generators are sized in VA (Volt Amps) & usually not in kilowatts...there is a good reason for this. If you want to know the reason, feel free to ask.

This is a very timely post, as you've opened the door on generator ratings for 'home' use; a subject that I touched on in my original post, but would like to discuss in more detail now if I may, as it can be a source of confusion not only for those seeking information about generators, but also… unfortunately… some who sell them.

When you're considering the purchase of a generator, the very least you should expect is good, accurate information to help you select a generator best suited for your needs, and not just some guy trying to move a boatload of cheap generators, who has little if any more knowledge about the product than you. The misapplication of an under sized generator is the most common problem I see out there and is normally the result of poor advice from the seller/distributor to a customer, an incorrect load calculation, or no load calculation at all.

Let's look at the ISO 8528-1:2005 ratings for generators and then discuss how each rating translates into home use.

There are four ratings; we are only interested in the first three with respect to 'home' use, they are;

ESP – Emergency Standby Power Rating

LRP – Limited Time Running Power (generally referred to as a Standby Rating)

PRP – Prime Power Rating

ESP – Emergency Standby Power Rating

Light duty, portable, single cylinder sets running at 3000rpm; rated to run for a maximum 200hrs a year (that's 3.8hrs a week) with a variable load at the alternators maximum load rating, no overload permitted.

LRP – Limited Time Running Power Rating ('Standby Rating')

Heavier duty, normally a permanent installation, multiple cylinder engines, running at 3000rpm or 1500rpm; rated to run for up to 500hrs a year (that's 9.6hrs a week maximum) with a constant load at the alternators maximum load rating, no overload permitted.

PRP – Prime Power Rating

Heavy Duty, permanent installation, multiple cylinder engines, running at 1500rpm; rated to run for an unlimited period of time with a variable load at the prime rating of the alternator; typically an overload of 10% in any 12 hour period is permitted.

To better understand why a particular rating is attributed to a generator for home use and more specifically, why it is important that LRP Standby Ratings should be limited to a maximum 500hrs operation a year and PRP Prime ratings are not, we need to look at aspects of the engine and alternator.

Voltage and Kva ratings of alternators are dictated by a number of factors; the length and size of the copper windings, the length of the lamination stack and the amount of cooling air passing through the alternator etc.

The class of insulation material used to protect the copper windings is a key factor. Resistance of the copper wire making up the alternator windings causes its temperature to rise as a current is passed through it… the more current, the more heat; the more heat, the shorter the life expectancy of the insulation material, with sufficient heat the insulation material will fail and the alternator will burn out.

NEMA (The National Electrical Manufacturers Association) recognizes four classes of insulation material for use in alternators (A, B, F and H). The difference between each class being the maximum temperature at which the copper windings can operate for each of two duty cycles; continuous use and standby use.

Continuous Use:

Running 24 hours a day, 7 days a week, the maximum operating temperature of the copper windings permitted by the NEMA insulation classes in degrees C are; Class A-100, Class B-120, Class F-145 and Class H-165.

Standby Use:

As standby use involves less running hours (that's the maximum 500hrs a year for standby use remember), the NEMA insulation classes allow the alternator windings to operate at up to 25 degrees C hotter; Class A-125, Class B-145, Class F-170 and Class H-190. The ability of the copper windings to operate up to 25 degrees C hotter for a limited period, allows the alternator to generate more power, hence the higher standby rating.

The maximum operating temperature of the copper windings is made up of two parts; the ambient environmental temperature and the temperature rise resulting from the operation of the alternator. If the ambient temperature is 40 degrees C, then deducting that figure from the maximum operating temperature for each class, gives us the temperature rise permitted for each class due to use.

A 1500rpm engine can be rated for use as a standby or prime mechanical power source, but a 3000rpm engine can only ever be used as a standby mechanical power source…these high revving engines are simply not designed to operate as a prime mechanical power source; this is concrete, regardless of what any dealer or distributor tries to tell you…

So…to simplify and summarize I will use the example of two Kipor generators, the KDE12STA and KDE16SS.

The KDE12STA is a 3000rpm, single phase generator. The alternator features class F insulation and the complete generator (engine and alternator combination) is rated as an LRP set for Standby Power use only. It should be operated at its maximum load rating for up to a maximum of 500hrs a year (9.6hrs a week maximum) and should give you good reliable service over the long term. However, as we have discussed, if you choose to operate this generator at or above its maximum load rating for a period exceeding the maximum runtime, you should fully expect to experience problems with the under rated mechanical power source (engine), or for the alternator insulation to fail and burn out at any time. It is a false economy to 'save' money by purchasing a standby generator only to then commit it to a prime power role. This is misapplication of the generator and you should fully expect to experience problems.

Note; if sufficiently rated, this same alternator driven by a 1500rpm machine could be used as a prime power source, as long as the heat in the copper windings does not exceed the class F temperature rise for continuous use. This is achieved by reducing the maximum output of the alternator from the maximum standby rating to the prime rating stamped on the alternator.

The KDE16SS is a 1500rpm, single phase generator. The alternator features class H insulation (the highest of the NEMA insulation classes) and the complete generator (engine and alternator combination) is rated for use as both a PRP set for Prime Power use and an LRP set for Standby Power use (13Kva/10.4Kw and 15.5Kva/12.4Kw respectively) according to application. The KDE16SS may be operated as a Prime Power source, at its Prime Power rating, continuously, for an unlimited period. However, as with the KDE12STA, if used as a secondary Standby Power source, it should be operated for up to a maximum 500hrs a year at its maximum Standby rating, operating this generator at or above its maximum load rating for a period exceeding the maximum runtime will eventually result in failure of the class H insulation and subsequent failure of the alternator.

Regardless of whether you are considering a generator for a Prime Power or Standby application, I would strongly recommend you follow my earlier suggestion that your generator be sized so that your maximum load requirement falls between 70 and 80% of the alternators rated output (70 to 80% of the Prime rating for Prime Power applications and/or 70 to 80% of the Standby rating for Standby Power applications). Among the benefits of sizing a generator in this way, are a reduction in the amount of fuel used, reduced strain on the engine, reduced thermal stress on the alternator insulation and a reduction in noise pollution; it also allows capacity for growth in your electrical needs.

My motivation for discussing this now, other than to offer advice to readers, is the result of a recent blizzard of emails between myself, the manufacturer and the distributor in Thailand, regarding the rating for the SS (ultra silent) range of Kipor machines. The manufacturer had initially represented their posted rating as being for Standby use (language and terminology issues), but I now have the accurate ratings sorted out and have amended my earlier posts to reflect the change.

I hope I haven't muddied the waters with this explanation; these are important points regarding generator sizing and selection, if you found it a little confusing please try giving it another read through.

Genset

to make a long story short i sent you an e-mail with my mobile number. please call me tomorrow as i am presently in Macau and will return to Thailand this evening.

Hi Naam,

E-mail received, I will send you a response later today.

Genset