Motor/Alternator

[102 fan]

Could someone tell me the reason behind the use of a Motor/Alternator in MK II Air con coaches please?

 

[DelW]

In general terms, motor-alternators were a method of converting DC (e.g.from batteries) to an AC supply, by using a DC motor to drive an AC generator. Development of electronic inverters largely made them obsolete.

 

[Taunton]

I remember in physics class in school (long ago) being told that AC to DC was done with a rectifier (cue drawing of sine waves on the blackboard) but DC to AC "was impossible". I wonder when teaching suddenly realised that inverters were now around.

Incidentally, I did ask the same (long suffering) physics teacher just why electricity was supplied to homes going backwards-forwards, when water was supplied always going forwards. And anyway, didn't it mean that the net effect was zero. I didn't get much of an answer. Anyone care to have a shot?

 

[hexagon789]

Could someone tell me the reason behind the use of a Motor/Alternator in MK II Air con coaches please?

For the electrical requirements, the train heating supply powers these which in turn provide the power for the air-conditioning. Mk2d and e stock have the same type, some Mk2d ones were modified so they could be hauled and heated by 55s and 47401-420 after early experience. The Mk2f are different, I think the same as Mk3 stock.

 

[AM9]

... Incidentally, I did ask the same (long suffering) physics teacher just why electricity was supplied to homes going backwards-forwards, when water was supplied always going forwards. And anyway, didn't it mean that the net effect was zero. I didn't get much of an answer. Anyone care to have a shot?

'Electricity' as in a source of energy that can be used to do work, is a stream of electrons from the negative terminal of the sourcce to it's positive terminal.*
Passing some of this flow through a resistor, heat and or light is generated, by passing it through a coil, a magnetic field is produced, either of these effects can be used to do 'work'. The 'work' can be used whichever way the electrons flow but only for as long as they flow. Thus if the flow is rapidly reversed backwards and forwards, (bearing in mind that its speed is about 0.7 of the speed of light), the supply is effectively continuous to us slower sensing humans. If a unidirectional supply is needed, alternate cycles can be rectified with a single diode/rectifier, or converted to the same direction (with a pair or set of four diodes/rectifiers).
Why is ac supplied in the first place?, - because it is easy to change its voltage by converting it to a magnetic field and back into a voltage in a coil at a different voltage determined by the ratio of the number of turns on the input coil versus the output coil. This is what a transformer does. If necessary, the use of a transformer can electrically isolate the input and output so the supply's voltage with reference to ground/earth can be changed.
* Although the electrons go from what had been called the negative terminal of batteries to the positive one, current flow is normally expressed as going the opposite way from positive to negative, (i.e. the holes where electrons move pass that way.

Hope that isn't as difficult to understand as it was to type.

 

[Taunton]

Thank you. Our master actually had a demonstration transformer which (coming back to our own subject) was a model railway Hammond & Morgan unit, which combined transformer, rectifier and potentiometer all in one. He had prised the casing off and enclosed it in a perspex box with the control knob coming through. He also had a Hornby Deltic with the body taken off so we could see the armature going round, and a 6 foot straight length of Peco track, laid on the front bench of the lab, to send it forwards and back on, and a voltmeter on the wires connecting to the track. Must have made it himself. It was a great little demo.

 

[matchmaker]

I remember in physics class in school (long ago) being told that AC to DC was done with a rectifier (cue drawing of sine waves on the blackboard) but DC to AC "was impossible". I wonder when teaching suddenly realised that inverters were now around.

Incidentally, I did ask the same (long suffering) physics teacher just why electricity was supplied to homes going backwards-forwards, when water was supplied always going forwards. And anyway, didn't it mean that the net effect was zero. I didn't get much of an answer. Anyone care to have a shot?

Rotary Converters can be used to convert DC to AC and they have existed for well over a century.

 

[dubscottie]

Could someone tell me the reason behind the use of a Motor/Alternator in MK II Air con coaches please?

In simple terms, the air conditioning requires a 3 phase AC supply which the loco cannot provide.
The MG set converts the DC or single phase AC supply into the 3 phase AC plus lower voltage AC for lighting etc.
The use of a MG set also supplies a constant power output (in most cases).
As mentioned certain locos caused issues.

 

[Nym]

A Motor-Generator doesn't generate AC. You need a Motor-Alternator.

 

[apk55]

Until the 1980s the only really practical and reliable way to convert a high voltage DC supply (500V upwards) to a lower and isolated voltage for control, lighting ventilation and possibly air-con was to use a motor generator or alternator set. A dc motor would be mechanically coupled to a generator or alternator which would provide the isolated output. Practically all DC units would employ them and they were also used to convert ETH supplies on a coach to supply lighting etc.
However they were heavy, bulky and high maintenance items. Remember they would be rotating all the time a unit was in service so the commutator wear would be high often even more than the traction motors.

After the 1980s power electronics had developed to the point where high voltage semiconductor switches became available (such as IGBTs) so voltage converters similar to those used in domestic appliances became practical.

 

[AM9]

Slightly off-topic but valid as an illustration of the dependence on mechanical generators for ac supplies, the dial tone that was heard when picking-up the handset of a telephone connected to an automatic exchange was as recently as the 1970s produced by a rotary 25Hz generator driven by a dc motor fed by the 48V DC batteries in the exchange. It was even possible in some smaller exchanges to lift the receiver in the quietest hours (say before 06:00 on a Sunday) to hear the generator spinning up to speed, indicating that there was nobody else dialling at the time. Once reliable solid state generators were available in the '70s, they rapidly replaced those rotary generators in existing electromechanical exchanges allowing the 25Hz to be raised to 33Hz (which better suited the earpieces) and the parallel rapid introduction of electronic exchanges meant that the tone was generated by the individual subscriber's line cards.
It was incredible that the rotary converter survived so long as it must have been an ongoing maintenence issue as well as quite energy inefficient to be powered by batteries.

 

[Taunton]

If you ever went into a telephone exchange before 1980 the noise was extraordinary, with all the electro-mechanical units, thousands of them, in there. It all sounded purely mechanical.

 

[DelW]

A Motor-Generator doesn't generate AC. You need a Motor-Alternator.

Getting into semantics here, but I'd suggest:

• A dynamo produces DC
• An alternator produces AC
• A generator produces electricity (of either flavour)

Certainly on construction sites we referred to diesel "generators" which were definitely producing AC.

It was incredible that the rotary converter survived so long as it must have been an ongoing maintenence issue as well as quite energy inefficient to be powered by batteries.

There was no alternative until electronics were developed that could handle the necessary power.

Is it still the case that inverters produce roughly a square wave, or can they now simulate sine wave? IIRC motor-alternators continued to be used after inverters were developed, in situations where pure sine-wave AC was required.

 

[apk55]

Modern inverters can produce quite a good simulated sine wave. Inverter generators such as you can buy from Machine Mart use them to convert a variable speed alternator output to a fixed frequency AC.
A lot of modern air-con units actually use a variable speed drive for the compressor where the incoming supply is rectified and then converted to variable frequency and voltage output, so in this instance a DC input could be used.
Same applies to lighting a lot of modern LED fittings could equally run on DC.

 

[AM9]

Getting into semantics here, but I'd suggest:

• A dynamo produces DC
• An alternator produces AC
• A generator produces electricity (of either flavour) ...

I think that the name 'dynamo' is an antiquated name for a DC generator, largely perpetuated by the device's use in motor vehicles (until about the '70s). The ac equivalent is a 'Magneto' which has a fixed coil and a moving magnet, (yet most people called that a "dynamo"), - older members here will remember their use as a cycle lamp generator. When the wheel was turning very slow, the lamp(s) would pulsate in sympathy with the ac waveform.

... There was no alternative until electronics were developed that could handle the necessary power.
Is it still the case that inverters produce roughly a square wave, or can they now simulate sine wave? IIRC motor-alternators continued to be used after inverters were developed, in situations where pure sine-wave AC was required.

The simple two-step inverter produces very a large third harmonic content making it unsuitable for anything other than a very low power device in a non-EMC sensitive area. Theoretically, it is possible to feed the square wave through a filter but such a filter would be very heavy and expensive, especially for any appreciable power. Most lower cost inverters have a modified square wave which produces a three-step waveform and it is feasible to filter the harmonic contend to a level that allows EMC legal use on low-powered devices.
Conceptually, rotary generators produce better shaped waveforms with lower harmonics, but a sychronous load can trash the slip rings in the generator in quite a short time.

 

[john prytherch]

May I thank the contributors to this thread, and in particular AM9, for the clearest and best explanation of basic electrical things that I have ever seen. It is very helpful to me in trying to comprehend what I have always only very partially understood.

May I ask another question please, which I think is what this thread is about. I confess that although I'm mad about railborne transport and normally use it most days, I haven't been on a train or tram since 6 March because of some nuisance caused by a pesky virus. So I'm a bit rusty. But, I think I have observed that some, or even many, DC powered trains drawing the juice from third or third and fourth rail systems, and. I think also, some or nearly all DC powered trams, have internal fittings, mainly fluorescent tube lights, that run on AC at 50 Hz. I am thinking initially of Merseyrail 507s and 508s but I think there are many more applicable types. I'm sure I've sometimes heard a 50Hz ac hum coming from some lights and other things on such vehicles. So do some DC powered trains and trams have AC fittings inside?

If the answer to that question is Yes, then presumably the AC is produced from the DC by the motor alternators referred to in this thread. These are presumably rotating all the time and are among the things I can hear whirring under the train.

I get the impression that an electronic inverter is a more modern device that somehow achieves the same result. Do some modern trains and trams have these? Do they make a sound?

John Prytherch

 

[AM9]

May I thank the contributors to this thread, and in particular AM9, for the clearest and best explanation of basic electrical things that I have ever seen. It is very helpful to me in trying to comprehend what I have always only very partially understood.

May I ask another question please, which I think is what this thread is about. I confess that although I'm mad about railborne transport and normally use it most days, I haven't been on a train or tram since 6 March because of some nuisance caused by a pesky virus. So I'm a bit rusty. But, I think I have observed that some, or even many, DC powered trains drawing the juice from third or third and fourth rail systems, and. I think also, some or nearly all DC powered trams, have internal fittings, mainly fluorescent tube lights, that run on AC at 50 Hz. I am thinking initially of Merseyrail 507s and 508s but I think there are many more applicable types. I'm sure I've sometimes heard a 50Hz ac hum coming from some lights and other things on such vehicles. So do some DC powered trains and trams have AC fittings inside?

If the answer to that question is Yes, then presumably the AC is produced from the DC by the motor alternators referred to in this thread. These are presumably rotating all the time and are among the things I can hear whirring under the train.

I get the impression that an electronic inverter is a more modern device that somehow achieves the same result. Do some modern trains and trams have these? Do they make a sound?

John Prytherch

I'm not sure exactly when rotating motor alternators for 'hotel' needs moved over to be replaced in trains, but up until the mid-seventies, high power semiconductor circuits were expensive to deploy where reliability was needed and we're largely only used where their economy or space/weight/cooling requirements were critical e.g. on aircraft.
It seems that all Mk1 stock had MAs and also the odds and ends between them and the MkIIIs. Early MkIIIs like the 455s had rotary alternators , (some of them do still) but I think that by the time the 319s arrived, power electronics was becoming commodity kit and static inverters became the norm.
In recent years, as more and more electronics has been introduce into the railway (safety systems, signalling, comms., etc.) the importance of electromagnetic compatibility has been a driver in the design. The requirements for that are now defined in much more legal terms as poor designs, in extremis could even affect passenger personal electronics like heart monitors.

 

[apk55]

It was the development of Insulated Gate Bipolar Transistors IGBTs which became available in suitable ratings (eg 2KV voltage and 100A current) in the 1990s. These can be turned off and on by a simple drive circuit.
Before that other semiconductor switches of a similar rating required complex drive circuits (Gate Turn Of Thyristors (GTO)) or complex power circuitry.

 

[AM9]

It was the development of Insulated Gate Bipolar Transistors IGBTs which became available in suitable ratings (eg 2KV voltage and 100A current) in the 1990s. These can be turned off and on by a simple drive circuit.
Before that other semiconductor switches of a similar rating required complex drive circuits (Gate Turn Of Thyristors (GTO)) or complex power circuitry.

Does that mean that the class 319s were the first UK mainstream class to have solid state power conversion from the outset, which I assume used GTOs?

 

[43096]

I'm not sure exactly when rotating motor alternators for 'hotel' needs moved over to be replaced in trains, but up until the mid-seventies, high power semiconductor circuits were expensive to deploy where reliability was needed and we're largely only used where their economy or space/weight/cooling requirements were critical e.g. on aircraft.
It seems that all Mk1 stock had MAs and also the odds and ends between them and the MkIIIs. Early MkIIIs like the 455s had rotary alternators , (some of them do still) but I think that by the time the 319s arrived, power electronics was becoming commodity kit and static inverters became the norm.
In recent years, as more and more electronics has been introduce into the railway (safety systems, signalling, comms., etc.) the importance of electromagnetic compatibility has been a driver in the design. The requirements for that are now defined in much more legal terms as poor designs, in extremis could even affect passenger personal electronics like heart monitors.

I believe the Class 442s had static converters from new for their auxiliary supplies (lighting, heating etc). Their traction equipment was resolutely 1960s, though, being ex-REP equipment.

 

[Taunton]

I'm not sure exactly when rotating motor alternators for 'hotel' needs moved over to be replaced in trains, but up until the mid-seventies, high power semiconductor circuits were expensive to deploy where reliability was needed and we're largely only used where their economy or space/weight/cooling requirements were critical e.g. on aircraft.
It seems that all Mk1 stock had MAs and also the odds and ends between them and the MkIIIs. Early MkIIIs like the 455s had rotary alternators , (some of them do still) but I think that by the time the 319s arrived, power electronics was becoming commodity kit and static inverters became the norm.
In recent years, as more and more electronics has been introduce into the railway (safety systems, signalling, comms., etc.) the importance of electromagnetic compatibility has been a driver in the design. The requirements for that are now defined in much more legal terms as poor designs, in extremis could even affect passenger personal electronics like heart monitors.

I wonder if this extreme validation of compatibility extends beyond the railway world. I was aware of the issues with rail signals, but not that it has moved on to heart monitors.

So what about elsewhere. Lifts in buildings? Escalators? Electric cars?

Which was the first stock to have motor alternators? Someone wrote elsewhere here that the EPB/CEP Southern stock from the early 1950s was the first, but I seem to recall the pre-refurb CEP units had lighting etc at traction DC voltage. I don't recall Mk1 hauled stock having anything at all, they were essentially just on batteries charged by axle-driven belts.

 

[apk55]

Before motor alternators trains used motor generator sets which did the same thing - provided an isolated stable lower voltage supply for lighting and control circuits. They were definitely used in the 1930s for example the 1931 MSJ&A stock used them,

 

[GRALISTAIR]

What a brilliant thread. I might have to pin this somewhere suitable. Thanks for posting.

 

[Taunton]

Account I read of the 4-SUB was that the lights were traction voltage (and thus needed a CM&EE fitter to change them), and there wasn't really anything else. No starting bells, it was still green flag into the 1980s. The control voltage (70v?) was achieved by a potentiometer, like the volume control on a radio, sure that wasted some in the resistance but it was the best they had.

 

[AM9]

I wonder if this extreme validation of compatibility extends beyond the railway world. I was aware of the issues with rail signals, but not that it has moved on to heart monitors.

So what about elsewhere. Lifts in buildings? Escalators? Electric cars?

Which was the first stock to have motor alternators? Someone wrote elsewhere here that the EPB/CEP Southern stock from the early 1950s was the first, but I seem to recall the pre-refurb CEP units had lighting etc at traction DC voltage. I don't recall Mk1 hauled stock having anything at all, they were essentially just on batteries charged by axle-driven belts.

I wasn't inferring that operators of rail installations were specifically required to demonstrate that heart monitors were unaffected by electromagnetic interference (EMI)*, but the technical and legal scope of EMC requirements are far reaching, although the EU has been instrumental in much of the gradual tightening of requirements resulting in improvements. Governments of industrialised countries across the world are increasingly aware that those that ignore the issue will find that it will cost them dearly in trade.
* EMC design and qualification is a complex and expensive process but the principles of demonstration are fundamentally simple. Mention has been made upthread of the cheap inverters that can be bought to drive ac devices from dc souces. It's very easy to use modern inverters to generate a square wave that's good enough to drive a power tool or some types of fan, but in doing so, this square wave may create havoc in a domestic environment with FM radio, maybe TV, some hearing aids etc., whilst the inverter user would be oblivious to the consequences. The EMC requirements are written for certain types of electric equipment with due consideration given to their location and intended use.
So for a device that would normally be sensitive to interference, it would have a maximum specified level that it should work irrespective. Correspondingly, a potential radiator of EMI will have a maximum level of allowable interference, which will be a few dB below that for which the susceptible equipment, thereby ensuring that no borderline passes on either type of device will create problems.
A heart monitor would be required to operate correctly in the presence of considerable interference for obvious reasons, but that would be safe in the 'assumption' that radiators stay within their limits.

Apologies for the wordy response but it is difficult to explain from first principles.

 

[Taunton]

I'm reminded of an account from about 1970, from Stanford University in San Francisco. The biology department were doing a study into the very small electrical impulses in plants. They devised and started the experiment, but found the results were being distorted by some unexpected interference at a certain frequency. Stanford had one of the largest nuclear linear accelerators in the world, but the interference did not coincide with its operation. The one thing they seemed sure of was it was man-made, as it only happened 07.00 to 17.00 on Mondays to Fridays. It was traced - to the initial trial runs of the BART system. 1000v DC third rail. This was, however, 5 miles away at nearest point from the university building.

 

[Journeyman]

Account I read of the 4-SUB was that the lights were traction voltage (and thus needed a CM&EE fitter to change them), and there wasn't really anything else. No starting bells, it was still green flag into the 1980s. The control voltage (70v?) was achieved by a potentiometer, like the volume control on a radio, sure that wasted some in the resistance but it was the best they had.

4-SUB control circuits all operated at line voltage. 70v circuits were an EPB thing.

 

[AM9]

I'm reminded of an account from about 1970, from Stanford University in San Francisco. The biology department were doing a study into the very small electrical impulses in plants. They devised and started the experiment, but found the results were being distorted by some unexpected interference at a certain frequency. Stanford had one of the largest nuclear linear accelerators in the world, but the interference did not coincide with its operation. The one thing they seemed sure of was it was man-made, as it only happened 07.00 to 17.00 on Mondays to Fridays. It was traced - to the initial trial runs of the BART system. 1000v DC third rail. This was, however, 5 miles away at nearest point from the university building.

Heavy ground currents can travel a long way in certain substrates. I remember in hobbyist magazines projects to try and communicate voice signals via ground currents.