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Author Topic: Voltage drop under load  (Read 1978 times)

Johndoh

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Voltage drop under load
« on: September 20, 2018, 09:30:10 PM »
Hi guys I have a mongrel diesel generator. Thanks to you guys its working well and making power around 233 volts. Yesterday there was a lot of power outages and she got her first real work rant for 8 hours or so. I noticed the voltage dropped to about 210 volts when the water pump kicked in. It's only a half horse pump. Is this normal? It's 5kw AVR alternator. I should mention that there was a 50hz sticker on the frame when I got it so I checked the speed and it was running at 3600 rpm its no at 3000 rpm. Should I speed it up to 3600 again? Turn up the avr to maybe 245v?  It's extremely noisy at both speeds. I put new brushes and a new avr in it this spring. Any and all suggestions welcome!
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ajaffa1

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Re: Voltage drop under load
« Reply #1 on: September 20, 2018, 11:08:19 PM »
Hi Johndoh, If your generator was set to produce 50 Hz at 3600 RPM, running it at only 3000 RPM will only give you about 42 Hz.

A half horse power pump will pull a lot more than that when it is starting up, it might only pull 1 amp when running but it could, briefly, pull 10 amps while starting. The lower the voltage the higher the current draw will be. Most electric motors have the speed at which they rotate determined by the frequency of the AC supply they are connected to. A 50 Hz motor coupled to a 42 Hz supply will rotate more slowly and pump less water. The head to which you are trying to pump water will also have an impact on how much current it draws.

A lot of smaller generators loose voltage during prelonged running. This is because the wire coils in the alternator warm up under load.The heat increases the resistance in the wire causing the voltage to drop.

My Lister ST2 7.5 KVA generator produces 245 volt when cold, this drops to around 235 volts after about ten minutes. When the domestic water pump kicks in, the voltage will drop to around 220 volt before the governor kicks in to pump more fuel, the voltage then quickly climes back to 235 volts.

hope this helps,

Bob

Johndoh

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Re: Voltage drop under load
« Reply #2 on: September 20, 2018, 11:35:03 PM »
hi ajaffa I was under the impression that 3000 rpm = 50 hz and 3600 = 60 hz? I would be pretty easy to put it back but I was kinda afraid Id blow up the TV but I suppose the AVR is supposed to prevent that? There was a little flicker off the light but I assumed this was normal.
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glort

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Re: Voltage drop under load
« Reply #3 on: September 20, 2018, 11:40:30 PM »
+1 to everything Bob said.

With generators I maintain that frequency is more important than voltage and this is a perfect reason why.  Voltage will always wander a lot more than frequency and if the frequency is right the Volts will be close enough.

I would set the genny speed not by the RPM but the frequency of the output with a Load on it, 2Kw would be about right for this machine.
Run the genny with a 2KW or close to it load and measure the frequency with a Multimeter and set your engine speed from that.   Almost certain that the engine speed will not end up at 3600 exactly but something else.  once you set the speed, unload the engine and see where the frequency goes. Ideally you want it at 51-52 Hz but it will depend on the Governor on the engine.  Just make sure it's not too high.

Would also pay to give the engine a once over. Make sure the air filter isn't clogged which will reduce the engines ability to carry a load.  Same for fuel filter. make sure exhaust isn't restricted with wasps  nests carbon or anything else although after the run you gave it, that would not likely be a problem now. Some diesels have upper limits on the fuel rack as well as idle and run settings so make sure you are getting enough opening there to carry the full load. As things wear they may need more fuel to do the same work when the thing had less hours and compression, vale seats etc were better.
Change the oil for good measure.

If you haven't already, run the thing up to temp, remove the air cleaner and give the thing a good spray down the intake with a litre or more of water to give the thing a good clean out internally.  This can help remove buildup in the cylinder, Rings, ports, Vale seat area and the exhaust.
After all that the thing is probably going to be close as it's going to get to being right apart from the injection timing which is usually set on most small diesels by shimming the pump. Not normally needed to be adjusted ( except for one engine I got that was so advanced it sounded like the piston was hammering the head)  but if the thing is a bit old you could try pulling a shim or 3 and see if the thing has more diesel clack which is a good thing and generally indicates better performance.

What sort of Genny actually is it? One of those red or black China vertical Cylinder yanmar Clones?

Johndoh

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Re: Voltage drop under load
« Reply #4 on: September 20, 2018, 11:55:46 PM »
Hi Glort its a real yanmar! I serviced it new air and fuel filter and its runs well with no smoke. I don't have a meter that can measure frequency so I can only rely on the RPM. I maybe shouldn't have lowered the speed? It ran the other day for about 8 hours and apart from the drop in voltage all seemed well. The voltmeter on the generator stayed at 230 ish but the separate voltmeter I have on the little homemade board showed a drop. Maybe its faulty?
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ajaffa1

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Re: Voltage drop under load
« Reply #5 on: September 21, 2018, 12:02:26 AM »
Hey Johndoh, the alternator should have a metal plate on it stating the voltage and frequency at a given RPM.

A lot of generators run at 1500 rpm for 50 Hz and 1800 RPM for 60 HZ (USA), This would equate perfectly to 3000rpm for 50HZ and 3600 for 60 HZ.

Bob


ajaffa1

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Re: Voltage drop under load
« Reply #6 on: September 21, 2018, 12:32:52 AM »
Just a thought on the voltage drop. A 5 KVA generator kicks out a maximum of about 20 Amps. A standard UK extension lead has only 2.4 mm wire in it. This would only be rated at about 15 Amps. Pulling 20 Amps through it would cause it to heat up increasing the resistance and lowering the voltage at your distribution board. The longer the cable the more voltage drop. Leaving any part of an extension lead rolled up causes induction making things worse.

Bob

glort

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Re: Voltage drop under load
« Reply #7 on: September 21, 2018, 12:43:00 AM »
Have a look on fleabay for multimeters.  They are so cheap now it's crazy.
I received 3 new ones yesterday.  I have a perversion for the things.  They even come in funky colours now.  Might put them all in that spare display cabinet I have and make them a decorative feature in the lounge room.  Mrs has all her ornaments and vases and other cra... trinkets in there, only fair I get  to put my touches on our home decor.

You should get a meter  that does frequency for about $10 whatever your equivalent of that is. Should equate to less than a feed at maccas or a coffee and cake at a Cafe.  Plus the multi meters are non fattening so you'll still look good for the Mrs, won't upset your Diabetes, cholesterol, heart, kidneys or anything else us old farts have to concern ourselves with!   :embarassed:

They are a handy thing to have for this very purpose and many others. Spending 20 bucks/euro will get you one with a heap of other useful features as well. 

Johndoh

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Re: Voltage drop under load
« Reply #8 on: September 21, 2018, 12:50:58 AM »
Hey Johndoh, the alternator should have a metal plate on it stating the voltage and frequency at a given RPM.

A lot of generators run at 1500 rpm for 50 Hz and 1800 RPM for 60 HZ (USA), This would equate perfectly to 3000rpm for 50HZ and 3600 for 60 HZ.

Bob

Hi Bob
It used to be a silent type generator in an enclosed frame I believe there was a plate beside the 50hz sticker but that stuff was taken off and dumped. I'm thinking I should get a kill a watt, are they any good?
PS I'm not using a standard extension lead there has been a 32 amp plug installed because theres a big output socket on the generator I am using a heavy caravan/camper van hook-up lead, the power is disconnected in the house before the generator can supply power.
Paul
« Last Edit: September 21, 2018, 12:56:06 AM by Johndoh »
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glort

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Re: Voltage drop under load
« Reply #9 on: September 21, 2018, 12:58:00 AM »
Just a thought on the voltage drop. A 5 KVA generator kicks out a maximum of about 20 Amps. A standard UK extension lead has only 2.4 mm wire in it. This would only be rated at about 15 Amps. Pulling 20 Amps through it would cause it to heat up increasing the resistance and lowering the voltage at your distribution board. The longer the cable the more voltage drop. Leaving any part of an extension lead rolled up causes induction making things worse.

Bob

This is a good point and one I come up against with my solar setup.

I'm amazed UK leads are 2.4mm wire. Most leads here are only 1MM conductors but rate the cables to 2400W which may be OK for a 1M lead but for a 20 or 25, it's BS. The biggest lead I can find in standard, non 3 phase configuration is 1.5MM which is still marginal but rated " heavy duty".  I have looked far and wide but 1.5 is the largest conductor area I can find.

The resistance works both ways, as well as voltage drop when pulling through a lead, you get voltage rise when trying to push down it. Nothing I have tried in a commercial lead is really up to snuff in the longer 20-25M lengths for their rating.
I just bought some 2.5MM builders cable, cut a long length of that and put some 15A plugs on it. Used 15a not because of the rating so much Pins are the same size bar the earth which I file down)  but because the terminals are bigger to get the larger diameter wiring into and make good connection. Still a trick though.

If you look on google for resistance and power ratings of leads, You'll likely be surprised at the power fall off and the diameter you need to properly feed higher loads.  As said, If the voltage is low on the genny end, on a longer lead it's going to be dismal time it gets to the load and the amps will go up making the problem cascade for the worst. 
In this respect, getting your voltage up to 250 if you are not going way over the frequency will be a good thing. The more volts the less amps.
The upper limit for voltage is 253 on a 230V circuit so You'll still be in spec.

Johndoh

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Re: Voltage drop under load
« Reply #10 on: September 21, 2018, 01:18:36 AM »
Hi Glort,
I can easily raise the voltage on the AVR but would it help? The house was wired for a 2 HP water pump to draw water for cattle but the pump was never upgraded and the landlord sank another well for the animals. The wires are about 6mm similar to an electric shower cable and the hook-up cable is about 5 mm so fairly heavy.(including insulation so maybe 4 and 3 mm actual cable) This is the connection the guy wired up. It's not really too big of a problem I suppose, until recently i was using capacitor generators and extension leads and all was ok but then I knew no better until I started reading stuff on this forum, which got me worried/thinking/panicking etc

P
« Last Edit: September 21, 2018, 01:24:34 AM by Johndoh »
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Hugh Conway

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Re: Voltage drop under load
« Reply #11 on: September 21, 2018, 01:29:27 AM »
Hello Paul
One of those Kill-a-Watt plug in meters are very handy to check volts and freq from any receptacle, as well as load from anything plugged into it. They are just handy to have around. Also as Glort says, meters are cheap.
I have wired in V and Hz meters before my main  generator switch so I can see if all is well before going on-line. Then an ammeter is good to have in the circuit too. Usually 1500 or 3000 rpm will give 50Hz and 1800 or 3600 rpm for 60Hz/
I am clueless re an AVR as my gen set-up has a PMG, so both Hz and voltage are rpm dependent. Better to run over a bit on Hz than under. Low Hz (like low voltage causes more heat build-up) I do run some 50Hz rated appliances at 60Hz with no problem, but have been told that going the other way can cause overheating.
Cheers,
Hugh
JKson 6/1  (Utterpower PMG ) Off-grid
Lister 6/1 Start-O-Matic engine......running with PMG
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dieselspanner

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Re: Voltage drop under load
« Reply #12 on: September 21, 2018, 08:40:10 AM »
Put one of these on it, a 20 min job to knock up a bracket and two bits of wire out of the 'come in handy box'

Cheers

Stef
Tighten 'til it strips, weld nut to chassis, peen stud, adjust with angle grinder.

Johndoh

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Re: Voltage drop under load
« Reply #13 on: September 21, 2018, 08:41:31 AM »
Hi Hugh
I'm going to order a proper multi-meter with hz function today. I will also recheck the rpm and increase the voltage to about 240v. The vessel for the water holds about 50l it has a diaphragm so the pump didn't have much to do. I have been looking at small generators on the interweb and it seems you can buy 120/240 v 3600 rpm generators which is really confusing for me. This was an old generator when I got it for cheap but it was running at 3600 and making power with no ill effect before I checked the revs so maybe thats as it should be? A little knowledge is a dangerous thing in my case!

P
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Johndoh

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Re: Voltage drop under load
« Reply #14 on: September 21, 2018, 08:57:36 AM »
It is better to light a candle than to curse the darkness