Step 6: Speed Control (part 5 - Improving speed regulation and low speed torque)
An optional further increase can be had in speed regulation and low speed torque by effectively converting the machine to a direct current shunt wound motor. To do this we rectify the speed controller output and supply the motor with DC (direct current). We also use a very low voltage (and high current) to energize the field coil.
As shown in the diagram, we still keep the 240V DC running through the field coil as well, this gives the benefit of reducing starting current surges. It is important to wire the two rectifiers such that they are pushing current in the same direction through the field coil (see diagram).
The transformer, being the most substantial electronic component is definitely worth scavenging (A new transformer is likely to set you back a good 15pounds). To our shame, in Britain at least, you will not have any trouble finding one. Likely places for the transformer are junked stereos, amplifiers, power supply boxes (though most wall warts are not up to it), and other electronic devices that routinely get thrown away.
To do this we will need the optional components mentioned earlier - a transformer (100VA should suffice - but see what you can find) and bridge rectifiers (these get quite hot, high current ones are a good idea). After flirting with the idea of using a rewound microwave transformer (and discarding it as overkill), I came across an old-school Spectrum power supply at the local tip (which is essentially a transformer with a rectifier and smoothing capacitor) - just the ticket! It is only rated (on the case) to 2.1A at 9V DC and I was needing more like 5A, but after testing it seemed the rectifying diodes were the weak link, and the transformer, when not completely sealed in plastic, as it was originally, actually runs nice and cool when delivering a steady 4.2A.