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Author Topic: Voltage drop under load  (Read 18192 times)

guest23837

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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!

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

guest23837

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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.

guest23837

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Re: Voltage drop under load
« Reply #3 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?

ajaffa1

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Re: Voltage drop under load
« Reply #4 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 #5 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

guest23837

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Re: Voltage drop under load
« Reply #6 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

guest23837

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Re: Voltage drop under load
« Reply #7 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

Hugh Conway

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Re: Voltage drop under load
« Reply #8 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
1978 Royal Enfield (glutton for punishment by Indian iron)
1963 BMW R-27 project

dieselspanner

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Re: Voltage drop under load
« Reply #9 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.

guest23837

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Re: Voltage drop under load
« Reply #10 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

guest23837

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Re: Voltage drop under load
« Reply #11 on: September 21, 2018, 08:57:36 AM »

mikenash

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Re: Voltage drop under load
« Reply #12 on: September 21, 2018, 09:31:51 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

I have a 5kVa gen head here that's 20-years old.  3000 RPM for 50 Hz (actually recommends 3150 RPM unloaded).  And 3600 RPM (actually 3750 unloaded) for 60 Hz

guest23837

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Re: Voltage drop under load
« Reply #13 on: September 25, 2018, 03:15:15 PM »
So I did a load of checks and tests and reread all the posts here. Seems ajaffa et al was on the money. The vessel for the pump is 10 feet for the generator the extension lead is 50 feet and about 42 feet was coiled. Rookie mistake on my part, thanks to all

guest23837

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Re: Voltage drop under load
« Reply #14 on: September 25, 2018, 04:26:43 PM »
Glort the things you can learn on forums like this are staggering. Like I posted earlier my house was wired for a heavy duty water pump but the connection was never used. My "electrician" has wired a connection to it that simply backfeeds to the house. I have to turn off the main switch before connecting the generator bit of research says he lied and I DO need a changeover switch. Grrr