About Starter Generators
Q: What does a starter generator do and how many different types are there?
Briefly, a starter generator rotates the turbine engine fast enough so the fuel can be ignited during start up, then it takes over as a generator providing DC power to the electrical system after start.
There are two types; shunt start and series start. The shunt type can usually be identified by having two large terminals and two small terminals located on the terminal block marked A, B, D and E. For starting, terminals B and E supply current to the armature and stator windings. At the same time, voltage is applied to the shunt field terminals A and D from the Generator Control Unit (GCU). As rotational speed of the armature increases, the magnetic flux generated by the shunt field windings creates a back electromotive force (EMF), which opposes the voltage supplied by the power source. This causes the armature current and output torques to decrease. A feature known as field-weakening reduces the back EMF and provides greater torque at higher speeds.
Three large terminals on the terminal block marked as C+, B and E- can identify series start type units. Shunt field terminals A and D are located on either side of the larger terminals. Terminals C+ and E- supply current to the armature and stator windings during the start cycle and the starter generator works the same as a DC electric motor. The shunt windings are not used in this mode. Following the starter operation, the GCU regulates shunt field current with voltage supplied from generator residual voltage and maintains a nominal system voltage of 28 volts DC.
Q: What are some of the major causes of failure and how can they be avoided?
The primary cause that we see in the shop is premature brush failure, usually caused by brush hang-up. If the brush leads are not properly positioned after a routine brush inspection, they can hang up on the brush holders and prevent the brushes from contacting the commutator as normal wear occurs. Once an air gap appears between the brush and commutator, electrical arcing occurs and causes accelerated brush wear and commutator pitting and burning. Even if the technician positions the leads correctly, they can be disturbed when installing the fan cover or brush band. The Component Maintenance Manual (CMM) should be consulted for each different part number starter-generator for proper inspection procedures and brush lead positioning.
Premature bearing failures are not as common as brush problems but some of these can be eliminated with extra care while handling. Never drop the unit on the drive shaft or put any excessive pressure on the bearings or armature. Also, bearings have a shelf life and a good rule of thumb is to change them at a maximum of three years after overhaul. Improper starting procedures also takes its toll on starter-generators. These units require an inrush starting current between 500 and 1700 amps causing them to get very hot during the start sequence. Consult the aircraft operating manual for maximum starting lengths and minimum cool down periods.
Q: Can starter-generators cause other problems in the aircraft while still delivering the proper electrical output?
Yes. One of the major causes of radio static comes from shorted or open capacitors in the terminal block. This can be caused by an over-voltage condition from a malfunctioning GCU or voltage regulator, improper testing procedures or even a lightning strike. There's also a possibility that shorted capacitors can cause faulty readings from the tach-generator. Radio static can also be a sign of the brushes sparking caused by excessive wear or possibly a shorted armature. Also, vibrations caused by rough or worn out bearings, oversized bearing liners or out of balance armatures can have a serious effect on FADEC (Full Authority Digital Engine Control) systems.
Q: What kind of maintenance or preventive maintenance can be done in the field by the average technician using basic tools?
The most common is brush inspection and the most important is the first one after the unit has been overhauled. For example, if the shortest brush is twenty five percent worn according to the brush wear indicator at 300 hours, then it is safe to assume that the brushes will last 1200 hours. For a safety margin, subtract twenty percent from that and adjust your next scheduled inspection at midlife or according to the aircraft maintenance manual at a minimum. Visually inspect the commutator for burn marks or pitting and while the unit is removed, blow out excessive carbon build-up with compressed air in a well ventilated area. Also, check the drive shaft splines for wear and the shear section for cracks and at the same time, inspect the dampener clutch for wear. This is also a good time to check the terminal block capacitors with a capacitance meter if any of the above symptoms are present. All of the specifications can be found in the CMM.
Brush changes are commonly done in the field but are usually limited to large operators having the proper equipment consisting of a vari-drive and starter test stand. Some technicians change brushes and do the brush run-in and seating with a standard battery charger and testing is accomplished on the aircraft. A good reference for this process is TRW / Lucas's Standard Practice Document # 1006
Q: Have there been any improvements made by the OEMs or any after-market suppliers since starter-generators were first introduced in the early 1960's?
Yes, most of the improvements are in the form of superior brush grades brought on by new technology. A few improvements have been in cooling airflow but these usually require modification to the unit itself. New brush grades can be added during routine overhaul. Goodrich Control Systems of Lima, Ohio (formerly Lear Siegler and Lucas/TRW Aerospace) replaced some of their old technology brushes with long life brushes and incorporated a few design changes for improved cooling on a select number of starter-generators that they manufacture. Aircraft Parts Corporation (APC), based in Farmingdale, New York, introduced a new and improved unit called the "XL Series". Miraj Corporation from Hasbrouck Heights New Jersey, a supplier of after market brushes, has had great results with their "Millennium Series" brush. And NAASCO Northeast Corporation, located in Shirley New York offers their ETR-20 and ETR-25 Mercury Mod starter generator improvements with a 1000-hour brush life guarantee. NAASCO's ETR-25 Mercury Mod is also a cooling modification. Most of these improvements have more than doubled the serviceable brush life of starter-generators.
I don't think we've seen the end of improvements offered for starter-generators. With increasingly more non-OEM companies developing their own improvement programs, OEMs are being forced to improve on their products to remain competitive. This is a winning situation for the operators and wouldn't be possible without competition.