Can 5KVA generator drive this load? - Page 1
Forum section: electronics / beginners

author: Michael George
author_rank: Contributor
timestamp: April 22, 2016, 01:36:19 pm
content: If I buy a 5 KVA generator and a power factor of 0.8 mentioned on its nameplate.From the name plate, I think the rated watts of thegenerator are4KW(5KVA * 0.8 )but I'm not sure.Can I drive a load of 4600 watt that has a power factor of 0.95?The load will consume about 4842 VAI will not exceed the KVA rating but I will exceed the watt rating. Is it ok?Thank you very much,

author: Rob835
author_rank: Newbie
timestamp: April 22, 2016, 02:09:22 pm
content: What kind of load is it? (I'm wondering if it is really 4600 watt constant or if that is a worst possible peak number)My initial reaction is that is too close for comfort.

author: mij59
author_rank: Frequent Contributor
timestamp: April 22, 2016, 02:17:15 pm
content: The apparent power of the generator is 5kVA, the apparent power of the load is 4600/0.95 = 4,84 kVA.BTW, 4370 kVA = 4.370MVA so a tad more.

author: Delta
author_rank: Super Contributor
timestamp: April 22, 2016, 03:20:58 pm
content: It will depend on the power of the prime mover. Is this a Diesel driven genset?From the limited info you have given, I would say "probably yes", it can power that load. Just keep an eye on engine temperature.EDIT: Although if that load is a motor, the genny might struggle to start it...

author: dacman
author_rank: Frequent Contributor
timestamp: April 22, 2016, 03:58:07 pm
content: The generator is rated 5 kVA from a power factor of 1 to 0.8. That would be 5 kW to 4 kW, depending on the PF of the load.

author: Delta
author_rank: Super Contributor
timestamp: April 22, 2016, 05:25:35 pm
content: Is this to power the P.A. system for a Wham! comeback concert?

author: Jeroen3
author_rank: Super Contributor
timestamp: April 22, 2016, 05:28:09 pm
content: You need to tell more about the load to say anything useful about the stability of the system.

author: Michael George
author_rank: Contributor
timestamp: April 22, 2016, 07:03:29 pm
content: Quote from: Rob835 on April 22, 2016, 02:09:22 pmActually, I was asking if I exceed the watt rating but I did not exceed .. Is that correct?To be more specific: Most of the load is light bulbs. Also, There are some computers, router and other electronic devices. The load of 4600 watt is constant, Not the peak.Quote from: Delta on April 22, 2016, 03:20:58 pmYes, It is a Diesel engine that derive a generator (genset).

author: SeanB
author_rank: Super Contributor
timestamp: April 22, 2016, 07:23:23 pm
content: What kind of light bulbs? Most are not very low power when turning on the first time, incandescent lamps draw a lot more current till they heat up, which on large lamps ( or a lot of small ones turned on at the same time) can be pretty big. Discharge lamps are similar, and depending on the ballast and starting method can be from merely 2 times the rated power for a second or two ( choke ballasted fluorescent lamp) with big dips and peaks, or 5times the rated power for a minute or more for things like metal halide lamps. They can produce a very large amount of supply noise, as the current flow can be very fast changing and non linear from cycle to cycle, and even change dramatically during a single cycle.You would have to schedule turn on of the lights in a sequence, starting with the highest power lamps initially and leaving them to warm up, then after warm up and the genset stabilising start switching in smaller loads till max. You computers and such after you have around 20% of the load on, so the regulation of the generator does not go crazy with non linear loading.If your lighting is not a constant load ( flashing lights) then either you only flash under 10% at a time, or you need a bigger genset and a nice set of heater banks to keep the regulation figure in the happy state. This will mean a 10 or 15kVA unit, and a bank of 5kW of heating elements to damp the current spikes. Yes you are turning fuel into heat inefficiently, but there are very few gensets with good load regulation, especially for fast changing loads.

author: Red Squirrel
author_rank: Super Contributor
timestamp: April 22, 2016, 07:47:08 pm
content: Seems too close to comfort. Personally I would size a generator 2x what you plan to use. Gives some room for expansion as well, and high startup current loads like motors.

author: Zero999
author_rank: Super Contributor
timestamp: April 22, 2016, 08:15:37 pm
content: Quote from: Michael George on April 22, 2016, 07:03:29 pmQuote from: SeanB on April 22, 2016, 07:23:23 pmI hope they're not incandescent lights. You should avoid them. Fluorescent or LEDs with power factor correction would be ideal but even if they aren't power factor corrected, it'll still work out more efficient than crappy incandescent.

author: Jeroen3
author_rank: Super Contributor
timestamp: April 22, 2016, 08:30:30 pm
content: Is the diesel also capable of providing the 5 kW?Problems with the mechanical power are often dismissed as electrical issues with the regulator. Because that is where you observe the problems.Note that if the generator it cheap (made of chinesium), you cannot run it on full load for long periods. It will overheat and short circuit.QuoteNot with stock regulator no, you just need a proper regulator. And you are not supposed to switch hundreds of amps very often. They do this on ships often, switching large ballast pumps. It makes the lights flash for a bit. You'd need your tongue at the right angle to adjust the regulator correctly.

author: dacman
author_rank: Frequent Contributor
timestamp: April 22, 2016, 11:43:05 pm
content: If you are going to use electronic devices (such as a router or even controllers for things like a furnace), then you need to look into THD. Since it's a diesel generator, you need to look into "wet stacking".

author: rx8pilot
author_rank: Super Contributor
timestamp: April 22, 2016, 11:55:08 pm
content: I would expect the frequency to be very unstable and the noise to be very high. If you can get past the inrush current of the lights, they wont care - but your electronics would likely freak out if they work at all. The little SMPS's are generally not capable of dealing with the wild and crazy power that comes from a small 'putt-putt' generator. The Honda's (and a few others) have sine wave inverters for the output meaning they are clean as a whistle.As for the wattage - I would not trust the plate on the side. The 'rating' is subject to the discretion of the test team who are often pushed by the marketing people to give them an unrealistic number. Assuming you are running at close to the max load, the system would most likely be stressed in some way - heat, stability, noise, regulation, etc. The only real way to answer is to test it, but by then you already own it.

author: dacman
author_rank: Frequent Contributor
timestamp: April 23, 2016, 02:53:07 am
content: Most cheap generators have way too much THD for electronics. The generators aren't perfectly sinusoidal under perfect conditions. If the generator is not THD rated, then I would not trust it. I'd guess that a common THD value for an unrated generator is 15% to 20%. They're ok for driving nails and cutting boards, but they are not ok for things like powering households under emergency situations, or powering a computer center with a bunch of lights. I've seen generators rated at 6% THD that were supposedly ok for electronics.From: IEEE Std 519-1992, IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems"Computers and allied equipment, such as programmable controllers, frequently require ac sources that have no more than 5% harmonic voltage distortion factor , with the largest single harmonic being no more than 3% of the fundamental voltage. Higher levels of harmonics result in erratic, sometimes subtle, malfunctions of the equipment that can, in some cases, have serious consequences."

author: Brumby
author_rank: Unknown
timestamp: April 23, 2016, 04:46:02 am
content: In my book, there are two basic considerations ...1. Headroom. Make sure you have a supply that is capable of supplying more than you need.2. Power quality. It has to be fit for the purpose.From the numbers you have given, I would not be at all comfortable running this setup for any extended period. Also, you certainlywouldhave to have a planned and staged power-up sequence to have any hope of it getting up. Running too close to the limits is bound to cause problems. Then there's a fundamental question: Just how accurate are the ratings? Marketing departments tend to be optimistic, if not downright deceitful.If Ihadto run this setup, I'd be watching it very closely ... the whole time it was running.The power quality is another factor for which isn't mentioned - and that's something that can be affected by running at the limits.As for exceeding ANY parameters, I believe you are courting pain.Let's just say that if I was signing off on this, I would not consider it acceptable.

author: Kappes Buur
author_rank: Regular Contributor
timestamp: April 23, 2016, 07:03:58 am
content: Quote from: Michael George on April 22, 2016, 07:03:29 pmI hope you are also looking at how effective the generator is in protecting against varying power levels and how clean the output waveform is.Most inexpensive generators produce a waveform which is closer to a square wave, which is not suitable for most IT equipment.As already mentioned, size the generator at ca twice the load requirement.

author: Delta
author_rank: Super Contributor
timestamp: April 23, 2016, 11:25:06 am
content: Out of interest, why is power quality so important for SMPS-based equipment (computers for example). As the front end of a SMPS is bridge rectifier and reservoir cap - I'd have thought it wouldn't really matter...

author: SeanB
author_rank: Super Contributor
timestamp: April 23, 2016, 11:45:30 am
content: Important because of crest factor and waveform shape leading to either a very high voltage on the capacitor as it rectifies the peak, and with a bad waveform shape the diodes conduct over a very small angle, leading to very high peak currents in them. As well active PFC expects something approximately sinusoidal, give it something else or DC and it can self destruct.Choke ballast loads and resistive loads are very forgiving of power quality, simply because they are effective low pass filters and harmonics are generally well suppressed by them. A resistive load is a very good true RMS responding item, it does not care if it is 240VAC 50Hz, 240VDC, 400VDC with a 50% duty cycle or 1kV with a 20% duty cycle, all are going to generate roughly the same heat inside. Apply that to a SMPS and only the first 2 will be usable, the third might cause issues and the last will blow it sky high in a few seconds.

author: bktemp
author_rank: Super Contributor
timestamp: April 23, 2016, 12:29:50 pm
content: Quote from: SeanB on April 23, 2016, 11:45:30 amWhen applying PWM to a resistive load the power scales linearly with the duty cycle, but not the voltage, because P = U²/R:400VDC with 50% duty cycle at 400Vdc = 282.8Vrms1kV with 20% duty cycle = 447VrmsSo you need 400VDC @ 36% duty cycle for 240Vrms or 1kV @ 5.76% duty cycle.

author: Jeroen3
author_rank: Super Contributor
timestamp: April 23, 2016, 12:36:51 pm
content: Is it a backup generator? If so, how are you going to ramp up the load. You cannot do a 0-100% load step. Voltage and frequency will plummet, causing a blackout.

author: _Andrew_
author_rank: Regular Contributor
timestamp: April 23, 2016, 02:29:20 pm
content: Is that 5kva the generators prime or standby rating? It is very common for manufacturers to advertise the standby rating as it makes the generator look more powerful than it is.The standby rating is typically the absolute maximum load the generator can handle for 1 hour before it gets grumpy.The prime rating is the maximum loding that the generator can chew away at for hours on end. So do check to see what the actual rating is.Running a generator at its maximum loading is not good. It leaves no headroom for alternating loads, thrashes the engine and burns exsesive fuel. Most generators are designed around being rated at 75% load. Most fuel usage figures are given for this rating.The type of voltage regulation is important. Small generators tend to be the capacitive resinant type rather than AVR controlled although there are more small inverter generators appearing on the market. Idely go for self exciting AVR controlled.A crude rule of thumb is a typical western household can be potentially powered of a 15kva.You should draw up a itemised list of all the items and there total power requirements that you would like to power. Also work out how much of it you are lightly to want to have on at the same time. From this you will have the bases you need to work out the sizing of the generator.If you are looking to spend the money on a new generator then it may well be worth your time to look at ex-standby generators with with a 100 or so hours on them. You may well find a nice John Dear, Deutz, Cattapiller, Scorpion, ASKA, FG Wilson, Cummings, SDMO 10kva acoustically canopied unit with internal bund.