Thursday, November 7, 2013

Fun With Generators

So the nights drew in, and so did the clouds in the days.

We had a massive storm the other week in the UK.  A depression barrelled across the Atlantic, whipped up by the jet stream, and smashed into the southern counties of the UK and Wales in the night.

Around 600,000 houses lost power due to trees falling on power lines.

I was prepared.  Batteries fully charged; petrol generator tested and loaded with fuel.  Thankfully, we didn't lose power here but it got me to thinking that I'd never tried to charge the battery from the generator.  The generator would have been just used to provide AC power directly.
With the storm passed and the new Maas PSU capable of delivering up to 765W to the house battery, I decided to test out how well they'd work together.

Now this generator isn't brilliant.  It's a 2-stroke engine and that means you need to add oil to the petrol and all of it is burnt in the engine.  You have to balance having enough oil to lubricate the engine against the volume of smoke the engine produces from burning the oil.  Let's just say I need to work on my fuel mixing skills, as the garden was enveloped in a thin blue mist while this thing was running at 'full power'.

And that brings me to the power output... It's rated at 650W continuous and 720W peak.  But generator manufacturers are notoriously, errr... 'generous' when it comes to their product labelling.

I got a plug-in power meter and used that to measure the generator output and load performance.

The first thing I discovered was that generators can't start large step loads.  I rigged up the Maas charger, dialed in 17A on the limiter and started up the generator with no load.  After letting it warm up a bit, I clamped the positive battery cable on and the load stepped from near nothing (10W) to 450W; or it would have if the generator engine hadn't simultaneously stalled.

So then I tried again, with the current limit dialled down to 5A and then ramped it up.  The generator Voltage started to drop off.  From an essentially no load Voltage of 240V, it dropped off to 195V at 520W.  I didn't ramp up the power any more as the PSU states that the line Voltage should be a minimum of 220V -10% = 198V.  Switch mode power supplies can often fail from under-Voltage supply.  I wasn't going to push it.

At the same time as the Voltage sag, the engine laboured and slowed, reducing the output frequency to 46Hz from 50Hz.  It also didn't like holding the load, with the engine cycling (revving) a bit on a one second-ish cycle.

So I dialled in 450W on the Maas PSU and the generator could handle that (just); cranking out 200-205V and 46-47Hz with the engine droning steadily and less blue smoke from the exhaust.

I let the thing run for about an hour and a half and then tried cranking up the power again.  The line Voltage still sagged to 195V, but the engine seemed happier with no instability (revving).  Even the smoke seemed to have abated somewhat.  I let it run for another 30 minutes or so and no unexpected smoke or outright fire issued :D.

Test over, and the missus complaining about the smell of 2-stroke engine in the garden, I shut it down and dug out my Brother labelling machine to amend the power output labelling on the side of the generator.
Given that I picked this thing up for £30 at a car boot sale (plus a bit more for repairing the broken fuel tap), I'm not unhappy.

But even with a power factor corrected load (it measured 0.98 under load), the generator wasn't able to deliver the charger's full power to the battery.  The generator couldn't manage more than about 70% of it's rated power without unacceptable de-rating of the line Voltage and instability.

So I guess I'm on the look-out for a more chunky machine with at least a 1kW output (claimed) and a 4 stroke engine (to spare the roses another fumigation).

No comments:

Post a Comment