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Author Topic: Tinkering with an ST Alternator  (Read 4271 times)
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Energy Self Enlightenment

« on: January 21, 2007, 10:56:09 AM »


ST alternators, a generic Chinese design, are becoming available from a variety of suppliers in the UK at low cost. They are cheap and cheerful and open to a whole host of experimental possibilities.  Available from 3kW to about 10KW (and more) they are well matched to the output of diesel based generator systems.

On Friday afternoon I thought I'd try some external field excitation on my 3kW ST alternator.

The Lister was plodding away at 325 rpm (Stealth mode) spinning the alternator at just 750 rpm.   I wanted to see what range of voltages I could get out of the ST,  with the idea of using it as a universal battery charger, or the means to run at variable speed depending on the load.

As the alternator was only running at half speed, the power output would be 25Hz, and the internal excitation winding had not yet reached the point where it self excites from the residual magnetism in the rotor.

I croc-clipped a dc power supply to the slip-rings and varied te amount of excitation from 3V dc to 36Vdc.  The field windings have a resistance of about 20 ohms - so the currents were not going to be much more than 2 amps at most.

Starting at 3V excitation, I measured 12.5Vrms across the alternator stator windings  (connected in series for UK 230V output).   With this I successfully lit a car headlamp bulb at full brightness - though a bit flickery at 25Hz!

I then worked up the voltage range with the following results:

Vdc        Vrms

3.0V       12.5V
3.3V       15.0V
12.0V     62.5V
21.7V     97.5V
30.0V     112V
36.0V     120V

So with a simple dc supply, controlled by the PWM output of a PIC or similar microcontroller using a power mosfet, it would be possible to get the ST to produce any voltage output, which could then be rectified and used for charging the inverter batteries be they 12V, 24V, 48V or 96V system.

It also means that the alternator output could be stabilised for battery charging, regardless of what rpm the engine was running at.  Whilst the ac frequency would vary, this would be of little consequence when charging batteries.   

Using a double pole changeover contactor, the stator coils of the ST could be switched from series to parallel connection - allowing half the voltage but up to twice the current, should your battery system require this.

It also allows the Lister to run at two distinct speeds,  325 rpm and 650 rpm.  325 rpm used when there is little demand for electrical output, and allowing greater fuel savings to be made.   In this mode the Lister uses less than half a litre of fuel per hour, but supplies enough current to keep the inverter batteries topped up with a float charge.

When there is a demand on the inverter - say someone turns on a 3kW kettle, or the washing machine has just turned it's heater on,  then the aternator windings could be switched to parallel, and the fuel rack opened to meet the extra demands.  When the load is satisfied, the rack position and field excitation voltage could be adjusted so that the engine meets the demands of the load, with little net current actually being drawn from the inverter battery.

This scheme would be practical in households where there is a fairly low baseload, for much of the day - of  perhaps 300 to 500 W, and the occasional demand for 3kW from heating devices.  If you are anything like me, caffeine addict - I tend to boil a kettle for about 2 minutes every hour.

BTW.  The variable voltage, variable frequency ac from the alternator, could be used to run the coolant circulation pump at a range of speeds  - just an afterthought.


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