Additional information

Additional information

Using polyester resin

Polyester is the plastic substance used in fibreglass work for building boats, car body parts, etc. Various things are added to it to make it work better for various jobs. Talk to your supplier and explain what the resin is to be used for. Your supplier should be able to help you.

Hardeners

There are two systems used to harden polyester resin, and each system uses two chemicals. For resin casting and most fibreglass work we use peroxide and cobalt. ('Body filler pastes' use the other system.)

Cobalt is a purple fluid. Ask the supplier to mix the right amount of cobalt into the resin. After it is mixed, the resin must be stored in the dark, or it will harden.

Peroxide is a hazardous chemical. Avoid contact with skin. Store in a PVC container, in the dark, below 25 degrees С Never mix it with cobalt (except for the cobalt already in the resin), or it will explode. Mix very small quantities (about 1-2%) of peroxide with resin or it will overheat.

Wax-free 'Air inhibited' resin 'B'

This type of resin is used for 'gel-coats' on boat moulds, where the resin is going to be built up in stages. We do not recommend using this resin for the PMG. Any exposed surface will remain tacky indefinitely. Ask for resin 'A', or better still 'casting resin'.

Thixotropic additive

A special powder of very light silica is often added to resin to make it thicker, so that it is easier to spread it with a paint brush. This powder is not needed for casting resin . If it is already added, it does no harm.

Styrene monomer

Approximately 35% of the resin as supplied is styrene monomer. This is used for thinning the resin. It causes the smell. It is possible to add a little more styrene monomer (1 0%) to make it more liquid.

Pigment

Pigment is used to colour the casting, if a coloured finish is desired. Add pigment to the first mix, which will be on the outside of the casting. Add no more than 1 0% pigment to the mix. It is not necessary to add pigment to the resin. Without pigment, the casting is transparent and the coils are visible.

 

Fibreglass

The resin has almost no strength without fibreglass. It is available in sheets of 'chopped strand mat' (CSM). It is also possible to buy just chopped strands, and to mix them with the resin. This is useful for the magnet rotor castings. Add a little resin to the fibreglass, and press out all the air bubbles, before adding more resin.

Talcum powder

Talcum powder is a cheap filler which can be mixed with the resin after the peroxide has been added. It makes the resin mixture much cheaper, and a little thicker. Resin can be mixed with up to twice its own weight of talcum powder. The powder also helps to reduce the heat build-up in large resin castings.

Using painted moulds in Peru

Mould preparation

Polyurethane varnish

Ordinary paint should not be used on moulds. Better to use nothing. If possible, use polyurethane varnish. This will prevent moisture coming out of a mould made from wood, plaster or clay. Smooth the varnish off with sandpaper before polishing it.

Polish

Polish the mould several times before using it first time. Rub all the polish off with a rag and then leave it some hours and do it again. Silicone polish is not compatible with PVA release agent. Use wax polish.

PVA Release agent

Paint this over the mould and let it dry. It forms a sheet of PVA, which greatly helps to separate the casting from the mould.

 

Using the PMGfor hydro power

The PMG can also be used for charging batteries from small hydro turbines. It will be ideal for low head, low power sites, because it is efficient even when producing only a few watts. It can also be used for higher head higher power sites, because it is capable of high power outputs at high rpm.

There is a danger of rust damage to the magnet rotors in a very humid or wet environment such as in a hydro application. It is advisable to galvanise or plate the steel components with zinc.

Low head sites

Here are some examples of conditions where the PMG could work without

modification (connected delta). It would need a simple 'impulse' runner mounted on

the front magnet rotor.

10	5
5	5
200	100
27	23
440	360

Head (metres) 1 0

Flow (litres/second) 1

Net Power(watts) 40

pcd runner (cm) 37

speed (rpm) 325

High head, high power

At higher rpm, higher power is available from the PMG. Doubling the speed can also double the output voltage and the current, offering four times as much power with the same efficiency as before. The PMG may overheat under these conditions, so it may be better to keep the current the same, and have better efficiency. Much will depend on whether the water is used for cooling.

In any case, increasing the speed improves the PMG's power handling abilities considerably. It would be risky to run a wind turbine at high speeds, because of the problem of gyroscopic forces on the rotors, but this problem does not arise with hydro power, because the shaft axis is fixed.

If higher voltage is not required, then the stator winding can be changed to give 12 volts (as before) at the higher speed, but deliver higher current without overheating. This is done by connecting the coils of each phase in parallel instead of in series. Or the coils can be wound with fewer turns of thicker wire. This is better still, because parallel and delta connections can suffer from parasitic internal currents.

Do not use the star/delta connection (diagram 49) for hydro power where the speed is constant. There is no advantage.