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Tony S

Rotary converter.

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Tony S

Rotary converter.


This has got to be one of Nikola Tesla’s most fiendishly complicated machines ever, one I’ve had the dubious pleasure of working on.









A description of one from an American engineer I particularly like “It just sits there, apparently doing nothing other than make heat, dust and noise”. Omitting the fact it converts AC to DC or DC to AC all in the one rotating machine.


They were developed quickly along with the electrification of urban railways. DC being the power of choice as it’s relatively easy to control motor speeds. But as Tesla and Westinghouse had proved AC was the best way to generate and distribute power over any great distance. Early railways would use motor generators, which are inefficient. The rotary converter became king of the hill until the advent of mercury arc rectifiers.


The easiest way to describe a rotary converter is a synchronous rotary switch.





It represents a 2 pole single phase machine that would need to rotate at 3000RPM.


To get a useable machine AC is fed in to a drum winding on the rotor from this DC can be picked off at certain equal spacing using a commutator.





To get an efficient machine more than a single phase is needed. Of course this is where it gets fun.





To get the AC on to the windings slip-rings are used.




The field windings are only to hold the rotor at synchronous speed and adjust power factor. The converter running most efficiently at unity power factor.


Running the converter:

There are two ways to start the machine rotating. Either an AC pony motor or if DC is available from a common bus as a DC shunt motor. If all else failed some converter stations used batteries to get the ball rolling.


Both have their problems, the pony motor can only get the machine to near synchronous speed, DC to the field is increased to lock the rotor in to synchrony.  Using the converter as a shunt motor to start the AC sliprings can be out of phase to the main AC bus and there is a possibility of over speeding the machine causing mechanical damage. Paralleling to the AC bus is by a synchroscope or by using the dim bulb method.


This picture is of a Westinghouse converter you can see the pony motor to the right.




So how do we get it started?


I’ll start with the simple method (says he), the pony motor.





The converter starts as a normal DOL motor in reverse, the supply is connected to the rotor. In series with the rotor windings is another set of sliprings to feed the squirrel cage star wound pony motor.

Once up to near synchronous speed the pony motor is disconnected and the secondary rings shorted out forming a star point.

As the DC stator field builds up to it locks the rotor to synchronous speed.

Adjust the transformer tap changer (if fitted) to give the correct DC O/P voltage.

Close the DC breaker.

Adjust the brushes to the commutation point. You’ll soon know if the commutation is out it looks like bonfire night around the brush boxes.


The not so simple method.


Adjust field to max.

Close the DC breaker. The converter then starts as shunt motor at low speed.

Adjust the field current down to speed the rotor up.

Switch the synchroscope on between the converter and the AC bus.

Adjust the field/speed to give equal phasing.

Close the AC breaker.

Adjust the brushes to the commutation point.


You can see the commutation adjusting wheel in this picture. The commutation point is the point on the rotor winding where the current changes direction between the magnetic poles. This is the same for DC motors and generators as well as the converter. Just to chuck a spanner in the works this commutation point varies slightly with load. There is a small neutral point mid way between the points of maximum magnetic flux, IE the pole pieces. This neutral point may only be a few commutator segments wide. In this narrow gap the bushes sit across several segments giving a little leeway either side that is safe for the brushes to sit. Get the alignment wrong and there will be a firework display as armature segments are shorted out. The brushes and armature bars will literally be eaten away and there is also the possibility of flashover between the DC brush arms.







This gives an idea of the size of a medium sized unit. The picture also shows the insulating shrouds to prevent flashover between brush arms.




One major problem is the frequency the converter works at. It’s happiest running at low speed / frequency for several reasons. The one in my drawing would be a two pole machine so for 50Hz it would have to run at 3000RPM, 60Hz makes it worse, 3600RPM. I don’t know about you but I wouldn’t fancy being around a machine at that speed knowing the rotor is bolted together. The other problem is the peripheral speed of the armature. The faster a machine runs the harder it is to get good commutation. Brush wear is accelerated. So to slow things down more poles are added just as you would do with squirrel cage motor.

Another approach used by the railways with their own power stations, generate at a lower frequency. London underground and much of the southern region used 25Hz.



A bit of history:


Tesla’s Westinghouse rotary converter technology was first deployed in 1894 following the Niagara Falls AC power station being connected to the city of Buffalo. Edison had previously ruled the roost with his DC distribution system and DC was still required by some buildings/factories. Rotary converters became the interface between the systems transferring power both ways.


In New York the Westinghouse Company having convinced city developers to adopt the AC system before it was ready found themselves having to use Edison stations to feed DC power to rotary converters to feed the expanding AC system.


No doubt Edison wouldn’t have be so pleased when this same connection was back feeding in to the DC systems forcing the local stations to close. At this point he didn’t have a lot of say in the matter as NY State had taken control of the distribution. This followed the total chaos and upheaval as the two companies ripped up the same streets.



Three wire DC.


+220V/0V/-220V was a common distribution system. 440V for motors, 220V for lighting, etc.


Three wire is only possible from a rotary converter with its own transformer.




When running up to speed using the pony motor the secondary of the transformer is earthed via a NER (neutral earth resistor). Once running and paralleled to the DC busbars the transformer is solidly earthed.


A rotary converter starting. Note the switchgear, the HSE would have a field day :swear:



© Tony S



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