Generators/Alternators in commercial and GA
A guess, but if you fed 3 phase into an alternator (one without the diode pack) and energised the field winding, it would rotate wouldn't it?
That could be your starter motor for something like a national grid generator where there is always plenty of 3 phase already present on the network.
That could be your starter motor for something like a national grid generator where there is always plenty of 3 phase already present on the network.
Join Date: Jan 2014
Location: N5109.2W10.5
Posts: 720
Likes: 0
Received 0 Likes
on
0 Posts
A guess, but if you fed 3 phase into an alternator (one without the diode pack) and energised the field winding, it would rotate wouldn't it?
"The 787's engine-start and APU-start functions are performed by extensions of the method that has been successfully used for the APU in the Next-Generation 737 airplane family. In this method, the generators are run as synchronous starting motors with the starting process being controlled by start converters."
AERO - 787 No-Bleed Systems
Forgive me - your punctuation is slightly confusing, but the principle of using an alternator as a motor is very simple, once you have a source of 3 phase AC*.
The field coil will follow the rotating field provided by the stator coils, once it is suitably energised with DC, so the alternator works in reverse, becoming a 3 phase motor.
You would probably need to limit the current flowing in the stator coils because their impedance would be very low, but this could all be done in the GCU.
*On most airliners the APU supplies this.
The field coil will follow the rotating field provided by the stator coils, once it is suitably energised with DC, so the alternator works in reverse, becoming a 3 phase motor.
You would probably need to limit the current flowing in the stator coils because their impedance would be very low, but this could all be done in the GCU.
*On most airliners the APU supplies this.
There are two basic types of generators:
Dynamo: The output comes from rotating coils connected to a commutator, (which is a rotating switch fixed on the end of the shaft), and excited by fixed field coils (or sometimes permanent magnets). It produces DC.
Alternator: The output is taken from fixed stator coils, excited by a rotating field coil. It produces AC, (usually 3 phase).
Two other variants of alternators are:
DC Alternator: Has a built in diode pack. It produces DC.
Magneto: The rotating field is provided by a permanent magnet.
Dynamo: The output comes from rotating coils connected to a commutator, (which is a rotating switch fixed on the end of the shaft), and excited by fixed field coils (or sometimes permanent magnets). It produces DC.
Alternator: The output is taken from fixed stator coils, excited by a rotating field coil. It produces AC, (usually 3 phase).
Two other variants of alternators are:
DC Alternator: Has a built in diode pack. It produces DC.
Magneto: The rotating field is provided by a permanent magnet.
Join Date: May 2001
Location: A few degrees South
Posts: 809
Likes: 0
Received 0 Likes
on
0 Posts
To uplinker: your alternator will indeed become a relatively low power synchroon motor, but until you have the rotor up to speed, you need precise voltage and frequency regulation.
For that reason I say it is not that simple, to just 'reverse' a standard alternator into a motor if you have AC power at hand.
For that reason I say it is not that simple, to just 'reverse' a standard alternator into a motor if you have AC power at hand.
From wiki .....
When attached to......... other alternators, an alternator will dynamically interact with the frequency already present on the grid, and operate at a speed that matches the grid frequency. If no driving power is applied, the alternator will continue to spin at a constant speed anyway, driven as a synchronous motor by the grid frequency. It is usually necessary for an alternator to be accelerated up to the correct speed and phase alignment before connecting to the grid, as any mismatch in frequency will cause the alternator to act as a synchronous motor, and suddenly leap to the correct phase alignment as it absorbs a large inrush current from the grid, which may damage the rotor and other equipment.
........................as any mismatch in frequency will cause the alternator to act as a synchronous motor, and suddenly leap to the correct phase alignment as it absorbs a large inrush current from the grid, which may damage the rotor and other equipment.
Join Date: Jul 2013
Location: Maryland USA
Posts: 133
Likes: 0
Received 0 Likes
on
0 Posts
Someone who used to sell alternators and engineer DC systems here:
"Generator" is both a generic term for anything that gets spun to generate electricity AND a specific term for a unit that uses the brushes to provide a DC output. This works by having the current generated in the rotating part of the unit (the rotor) and having contacts on the rotor spaced correctly to always have + and - on the same brushes. Advantage is this is a rugged setup that can stand a lot of abuse. Running the current through the brushes is a disadvantage because they spark, generating radio noise, and wear out. Also for mechanical reasons they have a lower maximum RPM that alternators. Generators also make fairly good DC motors if you feed power into them and vice-versa. This can be seen on any aircraft with a starter-generator. 24 volts into it spins the engine and the running engine causes it to produce 28 volts (24 volts batteries charge with 28 volts
) I used to fly a Cessna that had a starter-generator to fire up the PT-6 engine. This is also a danger for a piston engine airplane with the traditional car-type electric starter. If the starter does not disengage you now have a very powerful totally unregulated generator running 
Alternator is in theory a word for generators that produce AC. In practice it is hardly ever used that way. What it usually means is a unit designed to produce DC using built in diodes. Alternators are superior in many ways to generators for engine-driven DC power. The spinning part of the alternator is the field that carries a small amount of DC to provide the magnetic field. Thus there is no large amount of current through the brushes and the contacts are not segmented, thus eliminating the sparking. The stator - fixed windings - is where the power comes from. Alternators can spin faster too, so they can be geared to spin faster than the engine and charge better at idle speed. It is very confusing because alternator=AC, but DC comes out of it. In almost all cases rectifiers internal to the case change the output to DC. An alternator cannot become a motor powered by a battery. The only way to make one become an electic motor would be to bypass the dioes and feed it with 3 phase AC.
It was LONG ago when I worked at Piedmont, but if memory serves the 727s and 737s did not have alternators as the word is commonly used. They had AC producing generators, battery chargers run from the AC bus, and the APU provided air to start the engines. The APU might have had a starter-generator, I remember starting them off the ship's battery when the generator cart had gone missing, but can't recall how I set up the switches after that.
"Generator" is both a generic term for anything that gets spun to generate electricity AND a specific term for a unit that uses the brushes to provide a DC output. This works by having the current generated in the rotating part of the unit (the rotor) and having contacts on the rotor spaced correctly to always have + and - on the same brushes. Advantage is this is a rugged setup that can stand a lot of abuse. Running the current through the brushes is a disadvantage because they spark, generating radio noise, and wear out. Also for mechanical reasons they have a lower maximum RPM that alternators. Generators also make fairly good DC motors if you feed power into them and vice-versa. This can be seen on any aircraft with a starter-generator. 24 volts into it spins the engine and the running engine causes it to produce 28 volts (24 volts batteries charge with 28 volts
) I used to fly a Cessna that had a starter-generator to fire up the PT-6 engine. This is also a danger for a piston engine airplane with the traditional car-type electric starter. If the starter does not disengage you now have a very powerful totally unregulated generator running Alternator is in theory a word for generators that produce AC. In practice it is hardly ever used that way. What it usually means is a unit designed to produce DC using built in diodes. Alternators are superior in many ways to generators for engine-driven DC power. The spinning part of the alternator is the field that carries a small amount of DC to provide the magnetic field. Thus there is no large amount of current through the brushes and the contacts are not segmented, thus eliminating the sparking. The stator - fixed windings - is where the power comes from. Alternators can spin faster too, so they can be geared to spin faster than the engine and charge better at idle speed. It is very confusing because alternator=AC, but DC comes out of it. In almost all cases rectifiers internal to the case change the output to DC. An alternator cannot become a motor powered by a battery. The only way to make one become an electic motor would be to bypass the dioes and feed it with 3 phase AC.
It was LONG ago when I worked at Piedmont, but if memory serves the 727s and 737s did not have alternators as the word is commonly used. They had AC producing generators, battery chargers run from the AC bus, and the APU provided air to start the engines. The APU might have had a starter-generator, I remember starting them off the ship's battery when the generator cart had gone missing, but can't recall how I set up the switches after that.
Join Date: Sep 1998
Location: wherever
Age: 55
Posts: 1,616
Likes: 0
Received 0 Likes
on
0 Posts
A typical brushless ac generator as used on commercial aircraft work like this:
Mounted on a single rotating shaft are a permanent magnet (PMG), an exciter coil and a main field electromagnet around which are a PMG stator, and exciter field winding and a main field stator. Rotation of the PMG induces an alternating current in the 3-phase windings of the PMG stator. This alternating current is supplied to the generator control unit (GCU) where it is rectified into direct current. The GCU voltage regulator uses this rectified DC to control the current in the windings of the generator’s exciter field. The magnetic field produced in the exciter field induces a current in the rotating windings of the exciter armature (rotor). This current is converted to DC by the rotating rectifier assembly on the rotor. The resultant current is supplied directly to the windings of the main field to produce a rotating magnetic field. This rotating magnetic field induces an alternating current (AC) in the windings of the main generator stator. This is fed via appropriate contactors the aircraft busbars. In most cases the rotation speed of the armature is controlled by a constant speed drive (CSD) to give a constant frequency (400Hz) but latest design has binned the CSD to save weight so output is "frequency wild" as we used to call it or "variable frequency as they say these days.
Mounted on a single rotating shaft are a permanent magnet (PMG), an exciter coil and a main field electromagnet around which are a PMG stator, and exciter field winding and a main field stator. Rotation of the PMG induces an alternating current in the 3-phase windings of the PMG stator. This alternating current is supplied to the generator control unit (GCU) where it is rectified into direct current. The GCU voltage regulator uses this rectified DC to control the current in the windings of the generator’s exciter field. The magnetic field produced in the exciter field induces a current in the rotating windings of the exciter armature (rotor). This current is converted to DC by the rotating rectifier assembly on the rotor. The resultant current is supplied directly to the windings of the main field to produce a rotating magnetic field. This rotating magnetic field induces an alternating current (AC) in the windings of the main generator stator. This is fed via appropriate contactors the aircraft busbars. In most cases the rotation speed of the armature is controlled by a constant speed drive (CSD) to give a constant frequency (400Hz) but latest design has binned the CSD to save weight so output is "frequency wild" as we used to call it or "variable frequency as they say these days.
Join Date: Jul 2013
Location: Maryland USA
Posts: 133
Likes: 0
Received 0 Likes
on
0 Posts
Honda and others use "wild frequency AC" to make those nice quiet camping generators. Instead of holding a constant 3600 RPM (for 60 Hz). they vary engine speed with load and use a built in AC>AC inverter to hold the output frequency to 60 Hz. Same deal as you just described.
Join Date: Feb 2005
Location: flyover country USA
Age: 82
Posts: 4,579
Likes: 0
Received 0 Likes
on
0 Posts
Not that the subject hasn't been thrashed to within 25.4mm of its life, but to summarize:
1) Simplest is an alternator providing AC output - probably 3 phase, frequency varying with the engine rpm (wild frequency). (May be called a "generator" in defiance of most norms)
2) Said alternator may be packaged with a set of solid-state diodes, which convert AC to a kind of sloppy DC. Typically employed in automobiles for about the last 50 years, and more recently in light aircraft.
3) If a commutator and brushes is packaged in the case to convert the AC to DC, it's a traditional generator from nearly a century ago.
4) If fixed-frequency AC is required, as in transports starting from the 50s, a variable-ratio hydromechanical drive (CSD = constant-speed drive) may be employed driving the 1) alternator. If the CSD and alternator are packaged as a single unit, it's called an IDG (integrated drive generator).
1) Simplest is an alternator providing AC output - probably 3 phase, frequency varying with the engine rpm (wild frequency). (May be called a "generator" in defiance of most norms)
2) Said alternator may be packaged with a set of solid-state diodes, which convert AC to a kind of sloppy DC. Typically employed in automobiles for about the last 50 years, and more recently in light aircraft.
3) If a commutator and brushes is packaged in the case to convert the AC to DC, it's a traditional generator from nearly a century ago.
4) If fixed-frequency AC is required, as in transports starting from the 50s, a variable-ratio hydromechanical drive (CSD = constant-speed drive) may be employed driving the 1) alternator. If the CSD and alternator are packaged as a single unit, it's called an IDG (integrated drive generator).
Join Date: Apr 2011
Location: USA, Vermont
Age: 79
Posts: 15
Likes: 0
Received 0 Likes
on
0 Posts
Or do they use the generator as a starter motor??
barit1,
In regard to your 4th item, I think Boeing has broken that mold. The B787 uses simpler direct geared variable frequency AC alternators in a variable frequency AC to DC, then inverted back to whatever fixed AC frequency is desired for the AC buses.
barit1,
In regard to your 4th item, I think Boeing has broken that mold. The B787 uses simpler direct geared variable frequency AC alternators in a variable frequency AC to DC, then inverted back to whatever fixed AC frequency is desired for the AC buses.
Join Date: Feb 2005
Location: flyover country USA
Age: 82
Posts: 4,579
Likes: 0
Received 0 Likes
on
0 Posts
In regard to your 4th item, I think Boeing has broken that mold. The B787 uses simpler direct geared variable frequency AC alternators in a variable frequency AC to DC, then inverted back to whatever fixed AC frequency is desired for the AC buses.
I'm not aware which (or both?) the 787 employs.
