From Alan Belson:

I mentioned the Rotary Converter several times in the Transverter thread. The concept of this machine may be a lttle difficult to grasp at first. Well, it was for me! Here's some further clarifying thinks on the subject.

[Linked Image]

Click here for larger image

My sketch shows a schematic of a simple shunt-wound dc dynamo, 1 pole-pair, 2 circuit winding, where the field windings are permanently coupled across a dc supply while the machine is running. Such a machine will also run as a motor if connected across a suitable dc supply.

Permanent magnets could replace the field windings in a small unit, of course, but the latter's coils draw a relatively low current. Easy and economic speed adjustments using a rheostat are thus possible by varying the field current. A dc dynamo is fundamentally an ac machine with a commutator and brushes to rectify the alternating current it produces, so it follows that if we were to imagine adding a set of slip rings and brushes to such a machine, [ say on the opposite end of the armature from the dc brushgear, for convenience as shown] and then tapped into the armature windings, we could generate an alternating current as well as a dc one. Imagine such a machine uncoupled from its engine. If we connected it to a dc supply as a dc shunt-wound motor, it would run up to a certain speed, commensurate with the field-winding volts and back-emf in the windings equalling the applied dc volts, less the losses in the armature, brush gear etc. But equally, an ac voltage would appear across our new slip rings, with a hz and number of phases determined by the rpm attained, the number of slip rings and the numbers of pole pairs and taps.

Let’s also imagine that, by strokes of luck, midnight, lightening and genius on our part, the armature rpm, the phases, the voltage and the hz of our generated ac are of the exact same values and sine wave quality as those supplied to us by those nice folks at the POCO. We could then synchronise and connect up their ac output with ours.

Safety Note. Please, don’t try this at home, remember we’re only imagining this! There are many dangerous issues to be aware of here, apart from the obvious risk of an electric shock . The motoring speed of a Rotary Converter is restrained by the field windings' "back-emf" in certain circumstances. Loss of the ac voltage when running with a weak field-current, whilst connected to an external dc supply, would result in the machine possibly spinning up to a centrifugal burst. Real Rotary Converters incorporated centrifugal switching and overspeed cut-out devices on both sides to prevent this. The dc side must attain correct polarity at start, and again in real machines steps were taken to make sure + really was + . I will not go into the detail of how we would sychronise our ac outputs, but it is easily achieved, and particularly with poly-phase machines, just as easy to get spectacularly and disasterously wrong! Finally, Rotary Converters seriously overheat if run at anything other than near-unity power factor. At 0.9 P.F., for example, winding conductor heating increases by 23%.

You will see that our imaginary machine has one pole-pair as drawn. To achieve 60hz would require its armature to turn 3600 prm. If the pole-pairs increase in number, the number of phases increase and the rpm for a particular hz falls, so a 12-pole machine spins 600 rpm at 60hz and is normally wound 6-phase, requiring star/double star transformer connections to obtain a 6-phase supply from a POCO 3-phase. To connect our baby into a 120v single-phase supply would require us to operate at acv x 1.414 = about 170vdc. Should we require to operate at a different dc voltage from 170v, a transformer would be necessary on the ac side, tapped for small adjustments of say +/-2.5% steps, as the relative voltage ratio in the Rotary Converter is near enough fixed, less losses.

Now that we are all paralleled and motoring sweetly, [ 3600rpm on the tacho. I just dreamt I added! ], let’s imagine switching off the dc supply to the commutator and switching in a resistive load, such as a suitable-voltage incandescent bulb on the dc side, as sketched above. Note that our field current remains intact as an assured supply [ from a battery perhaps ]. Our machine will now start generating on the dc side, lighting the bulb. This event should slow down our armature rpm of course, but we are paralleled into an ac system now and our machine tries to remain in sync with it. The angular lag caused by the armature slowing now causes current to flow from the ac system through our machine’s armature to try to maintain our speed. Hence our machine motors on the ac side and generates on the dc side. Since we are both motoring and generating in the same conductors, the arrangement is of direct conversion, that is 100% from ac to dc. The thought to cling on to, is that both dc and ac currents are flowing in the same conductors simultaneously. It is evident that the dc voltage at the commutator bars must exist in the armature windings as well as in the external dc circuit, and so too will the ac current incoming from our slip rings.

We might now mentaly add, [ since it costs nothing! ] , a dash more sophistication with a synchronous ac pony-motor nailed to our shaft, to bring our rig smartly up to the correct rpm when starting. Then we could run it up from the ac side without worrying about excessive amps draw on our armature, nor need a dc supply on hand when starting, although a dc start would be possible. This is exactly the arrangements that the electric subways used to switch in extra units during rush-hours.

You will have noted that our machine is reversible; it can in theory take a suitable dc supply and feed converted ac into the grid.

We just thunk a Rotary Converter!