This blogs on Automatic Transmissions that we done in practical.
Automatic transmissions purpose depending on how its driven and its load e.g moving from stand still, towing, is to select a suitable gear ratio either a high ratio or low ratio automatically. Gear ratios in automatics can also be driven manually like a manual transmission e.g 1, 2, 3. drive or L, 2, 3, drive.
For an automatic transmissions, there are three major sections,
Torque Converter: A torque converter is a fluid coupling with a stator that work together for one cause, torque multiplication forward or reverse acceleration. When engine is rotating so is the torque converter. Without stator, torque multiplication wouldn't be achieved and would just be a fluid coupling which has a constant ratio of 1:1 which means impeller and turbine rotating at close to same speed . Torque converter has four major parts. Impeller, turbine, stator and housing. The housing (impeller side) is bolted to the fly wheel of the engine which rotates the torque converter. When engine starts rotating, impeller will rotate and sling with a great force hydraulic fluid from centre to the outside vanes and will enter in the turbine outside vanes. This is done by centrifugal force known as rotary oil flow.Without a stator, the fluid will enter the impeller and hit the front (wrong side) of vanes and slow it down. With a stator and its one way clutch allowing it to rotate one way and lock the other, the fluid flow from the turbine will hit the stator and its job is to redirect fluid flow so that when fluid goes back to impeller it hits the back of the vanes and assists it to rotate to produce a high vortex oil flow for torque multiplication. Maximum vortex oil flow is archived when impeller is rotating at full power and turbine is stationary e.g acceleration from standstill. When both impeller and turbine are rotating at same speed this is coupling point which then the one way clutch will then let stator turn with impeller and turbine. This happens because fluid pressure from the impeller to turbine has lessened therefore pressure on the one way clutch is no longer and also no torque multiplication. 
Mechanical gear system: Also known as the gear section is made of one way clutches, shafts, gears, front and rear multi plate clutches, front and rear bands, and compound planetary gears. The components for automatic transmission of a rear wheel drive is as pictured without valve body. The clutches engage to connect to drive parts of gearing e.g sun gears while bands and one way clutch holds other parts of the planetary system stationary e.g planet/pinion carrier. Gear ratios is archived by the use of different combinations of clutches and bands . The multi plate clutch Compound planetary gears are made of four parts, primary sun gear (small), secondary sun gear (large), internal gear and planet carrier or pinion carrier with a long set and short set of pinions. When either planet carrier, sun gears or ring gear is stationary, the other conponets will rotate/walk around them e.g for the primary sun gear to rotate, the front clutch would be engaged which in turn will kick in one way clutch to hold stationary the planet carrier. In this example the primary sun gear rotates with the engine direction, as sun gear is meshed to primary pinions they too turn but the one way clutch is engaged which at this point causes the pinion carrier to be held stationary, primary pinions rotate in opposite direction to engine direction and they too are in mesh with secondary pinions which rotate them in the direction with the engine which then turns the ring gear by being in mesh through the ring gears inside helical teeth. As ring gear rotates with engine direction it supplies power flow through to the output shaft (1st gear). This is shows how a automatic transmission works its mechanical components to select its different gears while in motion.
Hydraulic systems: The main part of the system consist of a pump, governer, pressure regulator values, manual value, throttle value and shift value. PUMP: Is usually located in the cover of the transmission which draws oil from the sump which feeds torque converter, hydraulic system and transmission cooler. The inner ring gear connect to the housing of torque conveter so it turns with engine. The outer gear is turned by the inner gear and as it rotates oil is drawn up from the sump which then feeds hydraulic system.
GOVERNER: This value tells the transmission how fast the car is traveling and is located on the output shaft. Inside the governer is a spring loaded value that opens up depending on how fast governer is spining, more spin more opening of value. The pump feeds oil to governer through output shaft. The faster the car goes, the faster the governer value opens which then gives higher pressure of fluid it lets through.
MANUAL VALUE: This is whats connected to shift lever, depending on what gear is being selected, this value feeds hydraulic circuits that force certain gears into play for instance if shift lever was in thrid value feeds a circuit so overdrive doesn't kick in.
SHIFT VALUE: This supplies hydraulic pressure to the bands and clutches to engage each gear it also ddetermines when to shift from 1st to 2nd. shift value is pressurized with fliud from the governer on one side and the other from the throttle value and rotate fluid one of two cicuits which controls the gear car runs in. Each shift responds to particular pressure range so if car is traveling fast the 2nd and 3rd shift will take over because the pressure from governer is enough to trigger that value.
PRESSURE REGULATOR VALUE: There are two main pressure values found in the value body Primary value takes fluid under pressure from the pump which supplies sufficient fluid to system to maintain system pressure. beyond this point the fliud is reguulated pressure also known as line pressure which goes to manual value for redistribution to other values and components. Secondary value provides pressure for the torque converter which is converter pressure. Some fluid from secondary value is used for lubricating various parts of transmission.
THROTTLE VALUE: This monitors engine load. As driver accelerates on the pedal the engine will take up load the more pedal is depressed the more engine takes on load. When pedal is depressed, the movement is transmitted to throttle value which increases hydraulic pressure to the opposite end of shift value which moves and cause a downshift in gears.
