DC Electrics - Generators and Alternators — Page 88, Lesson 84

I want to walk you through the characteristics of the series wound DC generator, and then we'll look at commutator ripple. Let's start with the series wound DC generator. In this type of generator, three things are connected in series — that means they're all in one single path, one after the other. Those three things are: the armature, which is the rotating coil; the field coils, which are wire wrapped around the pole pieces to add strength to the magnetic field; and the external circuit, which is the load that the generator is supplying. Because they're all in series, the same current that flows through the armature and the external circuit also flows through the field coils. That's a key point — the field current and the load current are the same thing. Now, since the field current — which is also the load current — is large, the generator can produce the required strength of magnetic flux with a relatively small number of turns in the field windings. So the field coils don't need many turns because the current through them is already high. Here's how it behaves: as the load draws more current from the generator, that additional current also flows through the field coils, which increases the field strength. A stronger magnetic field then generates more voltage in the armature winding. So initially, more load current means more voltage. But there's a limit. A point is soon reached, labelled point A on the characteristic load curve, where further increase in load current does not result in greater voltage. Why? Because the magnetic field has reached saturation point. Saturation point is the point where no more magnetic lines of force can be absorbed by the pole pieces — the iron core of the field magnets simply cannot hold any more magnetic flux. Because a constant voltage is required for aircraft systems, the series generator cannot be used. Its voltage varies with load, and that's not acceptable for powering aircraft electrical systems that need a stable, regulated voltage. Now let's move on to commutator ripple. Commutator ripple is the term given to the fluctuation of the voltage output of a DC generator. As the armature loop rotates, the voltage rises and falls — it's not perfectly smooth DC. This ripple effect is particularly noticeable at low RPM, when the generator is turning slowly. How can we reduce commutator ripple? By increasing the number of coils in the armature, or increasing the number of field coils, or indeed both. More coils mean the voltage peaks and troughs overlap and smooth each other out, so the pulsating or ripple effect of the DC produced by a generator can be reduced. The text mentions that a diagram compares a single coil armature with a multiple coil armature to illustrate this smoothing effect, and that diagram is available to you on screen. So to summarise: the series wound generator has armature, field coils, and load all in series, so load current equals field current. It produces more voltage as load increases, until the field saturates at point A. Because it cannot provide constant voltage, it's not used in aircraft. And commutator ripple is the voltage fluctuation inherent in DC generators, reduced by using more coils in the armature or field.