ATR 42-500 Electrical System
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ATR 42-500 Electrical System
Hi,
Can anyone explain why the main electric system on ATR 42-500 aircraft is DC not AC when according to a lot of online resources AC machines are lighter in weight and simpler in design? Is it because we dont want a CSD/IDG incorporated in a turbo prop? Or is it because DC system can be simpler in wiring by utilizing aircraft fuselage as common ground?
Thanks in advance.
Can anyone explain why the main electric system on ATR 42-500 aircraft is DC not AC when according to a lot of online resources AC machines are lighter in weight and simpler in design? Is it because we dont want a CSD/IDG incorporated in a turbo prop? Or is it because DC system can be simpler in wiring by utilizing aircraft fuselage as common ground?
Thanks in advance.
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One of the advantages for medium sized aircraft is that the DC generator is also a starter. No additional starter motor required, and you can start the engine directly from batteries. It is also easier to step down DC voltage if required (e.g. 28VDC to 5VDC).
If the generators were AC type, it would mean that a CSD should be fitted as well in order to provide stable 115VAC at 400Hz, despite power/turbine speed changes.
If the generators were AC type, it would mean that a CSD should be fitted as well in order to provide stable 115VAC at 400Hz, despite power/turbine speed changes.
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I'm not in the aviation business but I will comment anyway with a wind turbine perspective :-)
Many modern machinery need power electronics to control speed anyway, so doing for a DC distribution system will eliminate an additional DC/AC and AC/DC conversion.
Since the driving machine has variable speed, something (CSD/DFIG/Full scale converter) has to be added to convert to a fixed frequency AC.
The most modern and weight and power effective way is to use a PM Generator with a AC to AC converter. (This is in reality a AC to DC to AC converter)
Many systems like (hydraulic) pumps and fans can benefit from having PM motor with a variable frequency drive, since the weight of systems are reduced when the rotational speed is higher.
The efficiency is also improved since the speed can be adjusted, and a PM machine has lower loss than an induction machine.
Another advantage is that DC buses can be connected together without any need for synchronization. (Some fault isolation relays with hall sensors are need)
A disadvantage is that a lot of industrial components like circuit breakers are only rated for 50 to 60 Hz.
Many modern machinery need power electronics to control speed anyway, so doing for a DC distribution system will eliminate an additional DC/AC and AC/DC conversion.
Since the driving machine has variable speed, something (CSD/DFIG/Full scale converter) has to be added to convert to a fixed frequency AC.
The most modern and weight and power effective way is to use a PM Generator with a AC to AC converter. (This is in reality a AC to DC to AC converter)
Many systems like (hydraulic) pumps and fans can benefit from having PM motor with a variable frequency drive, since the weight of systems are reduced when the rotational speed is higher.
The efficiency is also improved since the speed can be adjusted, and a PM machine has lower loss than an induction machine.
Another advantage is that DC buses can be connected together without any need for synchronization. (Some fault isolation relays with hall sensors are need)
A disadvantage is that a lot of industrial components like circuit breakers are only rated for 50 to 60 Hz.
Last edited by HighWind; 17th Nov 2013 at 11:50.
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Yes, the ATR is also fitted with AC generators, dubbed "AC wild" for wild frequency, i.e. unregulated.
In cruise, the AC frequency is at or close to 400Hz anyway, but the AC is only used to supply power hungry equipment that does not need stable 400Hz, like ice protection, windshield, pitot, hydraulic pumps, etc. Therefore it does not require the additional (and heavy) IDG/CSD whatever, as no specific frequency is vital to make the said equipment work.
In cruise, the AC frequency is at or close to 400Hz anyway, but the AC is only used to supply power hungry equipment that does not need stable 400Hz, like ice protection, windshield, pitot, hydraulic pumps, etc. Therefore it does not require the additional (and heavy) IDG/CSD whatever, as no specific frequency is vital to make the said equipment work.
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ATR and General turboprop Electrical power design
From your question I take it you are not trained on ATR as Pilot or engineer?
As mentioned in the preceding threads,Turboprops (ATR included) primarily employ 28VDC electrical power.Most turboprops use DC generators, supplemented by one or more batteries to power equipment and services such as avionics, exterior lights, and motors, that draw a heavy electrical load. Some of the DC power is changed inverted to 28V , 400 Hz AC power for certain Avionic requirments, indicator synchronization functions,lighting, as well as general "hotel" services modes such as galley, toilet and cabin cleaning etc.
The heart of most DC systems is the battery.It provides electrical when Generators are not working..asuch as at pre flight(before GPU connection of engine 2 start in Hotel mode in the case of ATR) and during emergency operations.
The distribution of DC and AC power is divided into functional groups (gangs) referred to as "buses" which give priority to certain equipment during normal, abnormal, and emergency configurations.Distribution buses branch out from central locations, each protected by a circuit breaker. Main buses serve most of the aircraft's electrical demands. Essential busses feed power to equipment having top priority.
As mentioned in the preceding threads,Turboprops (ATR included) primarily employ 28VDC electrical power.Most turboprops use DC generators, supplemented by one or more batteries to power equipment and services such as avionics, exterior lights, and motors, that draw a heavy electrical load. Some of the DC power is changed inverted to 28V , 400 Hz AC power for certain Avionic requirments, indicator synchronization functions,lighting, as well as general "hotel" services modes such as galley, toilet and cabin cleaning etc.
The heart of most DC systems is the battery.It provides electrical when Generators are not working..asuch as at pre flight(before GPU connection of engine 2 start in Hotel mode in the case of ATR) and during emergency operations.
The distribution of DC and AC power is divided into functional groups (gangs) referred to as "buses" which give priority to certain equipment during normal, abnormal, and emergency configurations.Distribution buses branch out from central locations, each protected by a circuit breaker. Main buses serve most of the aircraft's electrical demands. Essential busses feed power to equipment having top priority.
Last edited by Trackdiamond; 22nd Nov 2013 at 16:57.
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General Electrical System Architecture on ATR aircraft.
The electrical power generation on ATR aircraft(upto 500 series at least) is provided by the following sources:
- Main and Emergency Batteries
- 2 engine-driven DC starter/generators
- 2 AC wild (ACW) frequency generaors
- 2 external power units (AC and DC)
In addition, 2 static inverters (supplied by DC system) provide CONSTANT frequency AC power.
The ACW electrical system can also supply DC electrical system through a Transformer Rectifier Unit (TRU).This available on 500 series onward but feature as an opton in some 400 and 300 series. ATRs.
The electrical distribution is ensured by electrical buses which feed the electrical equipment demanding. The Dc system has 11 busses!
Two separate networks (left and right side) run individually and can be connected in case of generation failure thanks to bus tie connectors (BTC)
- Main and Emergency Batteries
- 2 engine-driven DC starter/generators
- 2 AC wild (ACW) frequency generaors
- 2 external power units (AC and DC)
In addition, 2 static inverters (supplied by DC system) provide CONSTANT frequency AC power.
The ACW electrical system can also supply DC electrical system through a Transformer Rectifier Unit (TRU).This available on 500 series onward but feature as an opton in some 400 and 300 series. ATRs.
The electrical distribution is ensured by electrical buses which feed the electrical equipment demanding. The Dc system has 11 busses!
Two separate networks (left and right side) run individually and can be connected in case of generation failure thanks to bus tie connectors (BTC)
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ATR Electric Query
Thanks all for your replies.
Trackdiamond I am a trained engineer on ATR 42-500. I have no problem with understanding the architecture or working of the electrical system. Rather I was wondering why turbo props including ATR use DC as primary system instead of AC as on larger commercial jets. This unfortunately is something that the instructor failed to address during the type course. In light of the above answers I can only deduce that the biggest advantage lies in using the DC electrical machine as a starter as well as a generator.
Trackdiamond I am a trained engineer on ATR 42-500. I have no problem with understanding the architecture or working of the electrical system. Rather I was wondering why turbo props including ATR use DC as primary system instead of AC as on larger commercial jets. This unfortunately is something that the instructor failed to address during the type course. In light of the above answers I can only deduce that the biggest advantage lies in using the DC electrical machine as a starter as well as a generator.
