I'm not familiar with the article you make reference too. Was there a circuit diagram, if so could you send a copy here, then we might be able to sort you out.

Car dynamos were shunt wound generators, one side of the field and armature were connect to the frame. No doubt you have lifted those connections and made two new terminals. You will need four terminals in total, two small ones for the field and two larger ones for the armature The field winding should be separate and be connected straight across the battery, full 12 volts. The fire element should be in SERIES with the armature bit by bit via the studs, one end to positive the other to negative. As you notch up the resistance should get less and motor speeds up until eventually, the arm armature its self is straight across the battery full 12 volts, that will be maximum speed. There are better speed control circuits around using electronics, I think Neil the editor might be able to assist in that area, better than me.Let me know how you get on.Ted

It sounds to me as if you have the variable resistance in series with the field winding. As the field current is reduced so the armature has to spin faster to generate the required back emf.

Russell.

If you are controlling a shunt wound generator (like a car dynamo), the output current will increase with the field (wound rotor) current. The battery is pretty much a constant voltage in this case. Similarly, if you are driving a shunt wound / wound field motor (which is essentially the same electrical machine as the dynamo above), the maximum speed will occur at minimum field current and the maximum torque will occur at maximum field current and low speed.

I think I've got that right – and I think it's what Russell is saying. Intuitively, if the field current is cranked right up, the back emf generated in the armature reaches battery voltage at a low speed and you can't then drive any current into the armature (current = torque). Backing the field current off reduces the back emf and allows current to be driven into the armature at a higher speed.

Murray

Edited By Muzzer on 13/01/2015 10:57:01

Back again, initially forget about varying field current (weak fielding) that come after you have completely notched up if at all. Follow my instructions and get the thing running, then play if you feel like doing. In days gone by when DC drives were around Face plate starter were used. On stud one, a current was applied to the field (full 220 volts) then there was about 10 brass studs mounted on some thick black insulating material in a circular fashion. Behind each stud was a resistance wire, such that as the arm was moved across the studs resistance was lowered, until at last stud the armature itself was connected directly across the 220 volts. Then and only then, was weak fielding applied and it was electrically inter locked so that you couldn't attempt to start the motor on a weak field. Weak fielding give speeds above normal and not all motors were balanced well enough to weak field with safety. As some one has already said modern permanent motors are much better, and more efficient than the old dynamo, no field current. Unfortunately the resistors are wasting energy ( heat) and that is not you really want, its cheap, cheerful and energy wasteful. Also, if my memory serves me well, the size of the resistance wire (CSA) varies, thicker at the beginning. I don't think soldered connections will last long . Can you make good mechanical connections as an alternative? Ted

I think we are (almost) all furiously agreeing here – you need to reduce the field (rotor) current to speed the motor up once it is moving, otherwise it will reach a (slow) speed and stay there. Whether you put one or more resistors in series with the field winding to achieve that or control the field current with a PWM controller (or even a linear one), the effect should be the same.

Murray