XMSN coupled generators

[Phoinix]

I've been going through the JAR TR syllabus and stumbled across "generators and generator drives" in the transmission system chapter. I googled it and found that chinook has a transmission coupled generator.

Any other helicopters out there with that kind of generator drive?

Where is the catch of transmission driven generators? I always imagined a combined starter/generator unit. I've only flown light turbine helicopters up to 412 and this is a big "WTF-mate?" for my knowledge.

[industry insider]

Many aircraft have DC starter generators on the engine or engine driven DC generators if they have a separate starter.

Most AC generators are Transmission driven, S-61, S-92, AS332 series and EC225. Not sure about the Chinook as I have never flown it.

[OvertHawk]

in my experience you tend to find that DC power is derived from the normal Starter / Genny engine driven configuration. However, types that also have AC power may have AC generators driven by the main transmission.

I'm sure that others will provide input.

OH

[Phoinix]

Thanks for the input guys. I was convinced it's more efficient to use inverters to transform DC to AC. So, to sum it up, the big ships than have a seperate AC generator, usually transmission driven.

Is it than more efficient to use a AC generator than a big inverter (that needs even additional power supplied by the engine driven DC generator)?

[industry insider]

It depends, an inverter can't usually supply enough AC to run rotor de ice, windshield heating etc. For lighter AC requirements an inverter works fine.

[500e]

Inverter efficiency varies with load.
Normally it will be highest at about two-thirds of inverter’s capacity. (peak efficiency)
The inverter requires power just to run itself, so efficiency of a large inverter will be low when running smaller loads.

[212man]

The Black Hawk also has 2 x transmission driven generators. Doens't work so well if you droop the rotor.

Doesn't it have under-frequency protection? You must be talking about serious drooping to drop offline while airborne (assuming it has air/ground logic in there)

[skadi]

As far as I remember, the Westland Seaking has DC generators, mounted at the transmission. No generator with the engines, only starters. AC is provided via inverters.

skadi

[OvertHawk]

Also - and i'm prepared to be corrected if i'm wrong here, but, if you have anything that requires three phase AC then i think you need an AC genny as i don't think you can get three phase from an inverter.

[mtoroshanga]

The S 61 has AC and DC genes on the gear box, one of which converts to a motor-gene to run the accessory side of the gear box to supply hydraulic power while the rotors are stationary.

[JohnDixson]

212,

You are correct re the S-70 model line having AC Gen underfrequency protection. If I recall, that will keep the main AC Gens online down to something like 67% Nr ( assuming one is airborne and the WOW switch is functional )*. On the ground, the underfreq will drop the Main AC Gens at 94-96% Nr ).

The discussion needs to recognize that many aircraft have an APU and a third AC Generator these days, in addition to some sort of DC battery. Typically this third generator is not driven off the main gearbox.

* On reconsideration, I believe that what triggers the drop off is the under/over voltage protection circuit within the GCU ( Generator Control Unit ). Sorry.

Thanks,
John Dixson

[mfriskel]

It has been a while, but doesn't the UH-1H have a starter gen on the engine and a 2nd generator (stdby) on the transmission? I imagine AH-1 also.
The Apache has two generators mounted to the transmission.

[Phoinix]

S61 got me wondering too, thank you for clearing this one.


Thank you all, I got a bit further with understanding these systems.

[TerjeM]

The Westland Sea King normally has two AC generators installed on the accessory part of the main gearbox. It is possible that some mark numbers has three generators as it is room for it.

Because of an actuated free wheeling unit on engine no 1 it is possible to drive only the accessory part of the MGB without turning the rotor. This will give you hydraulic power and AC power.

All DC power is provided by two TRU (transformer and rectifier unit) or the battery.

There is also a ground inverter, converting 28 VDC to 28 VAC for some instruments.

You will have under frequency protection on the ground only (around 94%).

This how it is on our Sea Kings, but i dont think there is going to be a big difference to other mark numbers.

[FH1100 Pilot]

I'll do you one better: I know they're antiques but there are still a few of them running around, including the ones I'm flying now - the S-55 *only* has a transmission-mounted generator, nothing on the engine at all! (And it would have to be a separate unit, as the radial engine was not able to have a starter/generator unit.) When I questioned it, I was told that not having a generator on the engine facilitated faster engine changes in the field when operated by the military back in the day. Which it does, I guess.

[skadi]

The Westland Sea King normally has two AC generators installed on the accessory part of the main gearbox. It is possible that some mark numbers has three generators as it is room for it.

Because of an actuated free wheeling unit on engine no 1 it is possible to drive only the accessory part of the MGB without turning the rotor. This will give you hydraulic power and AC power.

All DC power is provided by two TRU (transformer and rectifier unit) or the battery.

There is also a ground inverter, converting 28 VDC to 28 VAC for some instruments.

You will have under frequency protection on the ground only (around 94%).

This how it is on our Sea Kings, but i dont think there is going to be a big difference to other mark numbers.

Yes, thats right. Seems it was too long ago for me to remember well. Just checked my old books about that.

skadi :O

[riff_raff]

Phoinix,

If the generator/alternator is driven from the MRGB, as opposed to an EMAD, the generator may likely be an Integrated Drive Generator (IDG). IDG's have an integrated variable speed transmission so that they can produce constant frequency AC power from a varying input shaft speed. While IDG's are efficient and reliable, they are also large and heavy. So mounting them on the MRGB, with a lower speed PTO, makes things easier.

Turboshaft engines also have high speed Permanent Magnet Alternators (PMA's) sometimes mounted on the engine accessory gearbox (EMAD). The PMA produces variable frequency AC power, which must be converted to a constant frequency. The advantage of the PMA is that it is very lightweight and can operate at high speeds.

With multi-engine rotorcraft, the main advantage to driving the generators from the MRGB is due to the fact that the generators will continue to be driven as long as the rotor is turning, even after the loss of engine(s) operation. However, the drawback to this particular MRGB accessory drive arrangement is that the generators also cannot be utilized as starter motors, due to the unavoidable presence of an overrun clutch between the MRGB input and engine.

Finally, large rotorcraft may also employ a separate Auxiliary Power Unit (APU). The APU is typically a small turboshaft engine that provides bleed air pressure, hydraulic pressure, electrical power, etc. while the aircraft is on the ground. The APU bleed air and/or electrical power can be used for engine starting, among other things.

Regards,
riff_raff

[RotaryWingB2]

As well as everything mentioned above, some aircraft have alternators, belt driven from the transmission. :eek:

Basically any medium/large aircraft that has the potential to require large AC loads (winch, aircon, lots of IFE toys, etc) has the ability to be fitted with an AC generator, the Dauphin has it as an optional extra for example.

The British AH-1 has air driven starter motors, and as such all AC/DC power is developed by the transmission mounted generators. (then rectified, distributed etc)

[Shawn Coyle]

British AH-1??? Pictures please!!!

The Polish W-3A has two transmission-driven generators - one AC and one DC. The AC is mainly for the blade de-ice. The engines have an electrical starter, not a starter generator.

[HarvestReaper]

Most multi engined heavy helicopters i.e. Puma : Super puma L / L1 / L2 / 225 primary power supply is 3 phase 115v 400 hertz from two alternators driven from the main gearbox. Constant frequency is assured by the fact that main rotor rpm is constant. Both machines supply seperate bus bars that are not coupled due to the fact that synchronisation of the machines is not possible. DC power is obtained from two transformer rectifier units (TRU) and a 28v battery. No inverter is installed due to the fact that if one alternator fails the remaining machine is capable of supplying all ac loads including the TRU,s
The S61N has a similar system but is older and not so refined, DC being provided by a TRU and a motor generator also driven by the MGB.

[Aser]

Ad the Sea Knight to the list.
I remember reading about a Ch-46 accident, where the generators went off-line due the rpm droop...

[MightyGem]

British AH-1??? Pictures please!!!

Shawn, the British Army's Apache has the designation AH1.
AgustaWestland Apache - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/AgustaWestland_Apache)

[JohnDixson]

InTheWeeds,

The 67% value that I quoted arose from a discussion many moons ago re a non-SA accident wherein the test instrumentation dropped off due to this issue during the event.

At the time our electrical power guys gave us an analytical number which I recalled as 67%.

Turns out a test was done later on, with the following result:

Several years ago we performed a UH-60L experiment on the ground with full electrical load (including blade de-ice dummy load) with the WOW switches wedged to the flight mode. Bringing PCL’s back slowly, we demonstrated that generators remained on line below 70% Nr. At about 68%Nr the #1 and #2 converter circuit breakers popped causing loss of VIDS displays.
Looks like the Army folks down at Ft Rucker may be putting out some bad info. ( And I was wrong in thinking it was the GCU undervoltage rather than the converters which provided the drop off ).

In any case, one would not want the electrical system protection knocking off the AC power source in flight at Nr numbers in the 80's.

Thanks,
John Dixson

[MLHeliwrench]

214ST has two C-box mounted DC Generators. The C-box is a separate Bell component from the GE CT7 Engines. (Same engine design as Black Hawk)

The engines have separate electric starters.

[brett s]

CH-47's also have transmission driven generators (plus one on the APU on D & newer models), the engines have hydraulic starters.