Generator Constan Speed Drive

Photo showing the components of the Constant Speed Drive unit.

General

The AC electrical power system of modern civil jet airplanes is basically a 3-phase 400-Hz, 115/200- volt, generating and distribution system.

Each engine driven generator is driven through a variable ratio transmission. The transmission supplies the torque to drive the generator at a constant speed from the variable speed accessory drive pad located on the airplane engine.

Accordingly, the purpose of the constant speed drive may be stated as the conversion of the varying speed of the jet engine to a constant speed, so that the generator it drives will produce current at 400Hz within narrow limits. Constant speed drive consists essentially of a hydraulic transmission with mechanical controls governing the output rotation speed. The transmission is capable of either adding to or subtracting from the speed received from the gearbox in order to provide constant output speed to keep the generator on frequency. Mechanical (flyweight) governor action keeps the generator output close to 400Hz.

Constant Speed Drive

The constant speed drive consists essentially of two positive displacement, axial slipper piston type hydraulic units, and a mechanical axial geared differential, a speed governor, an all attitude reservoir, an air separator, a charge filter, a generator drive disconnect, a scavenge filter, an oil cooler, two speed sensors, two temperature bulbs and an integral hydraulic system. All of these components are integral to or mounted on the constant speed drive except the oil cooler. The input speed can vary from 3500 rpm to 9000rpm. The output constant speed can be from 6000rpm to 8000rpm depending of the generator type, in order to produce 400Hz. The output rating of the constant speed drives, is in the area of 100 horsepower.

Functional Description

Each CSD consists essentially of two positive displacement axial slipper piston type hydraulic units, and a mechanical differential, which performs the speed summing function. The hydraulic units are the same in physical size, one unit having a variable hydraulic displacement unit and a variable angle wobbler and the other having a fixed angle wobbler and, therefore a fixed displacement. The hydraulic units rotate independently and are positioned on opposite sides of a common stationary port plate. The variable displacement hydraulic unit runs at a fixed ratio with respect to the transmission input speed. Because the wobbler angle of the variable displacement unit is continuously variable in both directions (from full positive wobbler angle, to zero angle, to full negative wobbler angle), the displacement, of the variable displacement hydraulic unit, is continuously variable, from zero to full rated displacement in both directions. The fixed displacement hydraulic unit is driven by oil delivered by the variable displacement hydraulic unit. The fixed displacement hydraulic unit, will therefore run at any speed, from zero to full rated speed in either direction. The working pressure between the two hydraulic units is proportional to the torque being transmitted to the generator.

At the lower input speeds, the variable displacement unit acts as a hydraulic pump to supply flow to the fixed unit which is added to the input speed through the differential. At the straight through input speed, torque is transmitted directly through the differential unit and the fixed unit is not rotating. The variable displacement unit wobbler will be slightly offset from the zero angle so that some pumping will be accomplished and leakage loses made up. At input speed above straight through, the variable angle wobbler is set to allow negative displacement of the variable displacement hydraulic unit. The working pressure, in this case, is manipulated to allow the fixed displacement hydraulic unit to be motored by the differential and subtract from the input speed. The variable displacement unit is acting as a motor. The multiple piston hydraulic unit in the mechanical differential type CSD unit handles only a portion of the power transmitted, therefore it is reduced in size. Since power loss is less in mechanical differentials than for multiple piston type hydraulic units, heat rejection is low resulting in high efficiency.

CSD Mechanical Differential and Hydraulic Units

The differential is a folded type with planet gears in the center and input and output ring gears on the outside. The planet gears rotate about their own axes and also revolve about the centerline of the planet gear carrier. The planet gear carrier is driven by drive input. The variable displacement hydraulic unit is also driven by the transmission input. The fixed displacement hydraulic unit is hydraulically coupled to the variable displacement unit and is connected to the differential through the input ring gear. The output ring gear of the differential is connected to the transmission output. Constant speed of the output ring gear is maintained by either adding to or subtracting from the speed of the planet gears by controlling the direction of rotation and speed of the input ring gear. The gear and pumps are driven by a constant speed output gear.

Differential

The differential consists of a carrier shaft, two planet gears, and two ring gears - input and output ring gear. At any speed and load, a torque is imposed on the output ring gear by the output gear. Input torque is supplied by the input gear turning the carrier shaft. If there were no torque on the input ring gear, it would run freely at whatever speed would allow the output ring gear to stop. Because the ring gear to carrier shaft ratio is 2:1, the speed of the input ring gear at this condition would be double that of the carrier shaft. Since a given output speed is desired, the input ring gear must be constrained. If the input ring gear is constrained to zero speed, the output ring gear will run at double the carrier shaft speed. If the input ring gear is forced to rotate in a direction opposite to that of the carrier shaft, the output ring gear will run at a speed more than double that of the carrier shaft. If the input ring gear is allowed to rotate in the same direction as the carrier shaft, the output ring gear will run at a speed less than twice that of the carrier shaft. Thus, the differential is a speed summer or adding device which is controlled through the input ring gear to add to or subtract from the speed of the engine gearbox to achieve the desired output.

HYDRAULIC UNITS

Variable Hydraulic Unit

The variable hydraulic unit consists of a cylinder block, reciprocating pistons, a variable angle wobbler, and a control piston. The variable unit is connected to the engine by direct gearing; consequently, the speed of the cylinder block is always proportional to the input speed and rotation is always in the same direction.

Fixed Hydraulic Unit

The fixed hydraulic unit consists of a cylinder block, reciprocating piston and a fixed angle wobbler. The direction of rotation and speed of the fixed hydraulic unit is determined by the volume of oil pumped or received by the variable hydraulic unit. This volume of oil is determined by the angular position of the variable wobbler and the speed of the variable block. Constant output is maintained through operation in the phases described below:

(a) Overdrive

If the input speed supplied to the drive is lower than that needed to produce the required output speed, the transmission hydraulically adds the necessary speed to the speed of the engine gearbox through the differential. When the drive is adding speed hydraulically it is operating in overdrive. When the transmission is operating in overdrive, the variable hydraulic unit will function as a hydraulic pump. To enable the variable unit to pump oil, the governor ports control oil to the control piston, which in turn positions the wobbler so oil will be compressed as the pistons are forced into the rotating cylinder block. This high pressure, (working pressure) oil is ported to the fixed hydraulic unit, which now functions as a hydraulic motor. High pressure oil pumped from the variable unit, forces the fixed unit pistons to slide down the inclined wobbler face thus causing the cylinder block to rotate. The block's rotation forces the input ring gear to turn in a direction opposite to the carrier shaft rotation and adds to the speed of the engine gear box through the differential thus maintaining constant output speed.

(b) Straight through Drive

If the input speed supplied to the drive is sufficient to produce the required output speed, the transmission drives the generator directly through the differential. As the input speed increases and the need to add speed decreases, the governor will port less oil to the control cylinder until the variable wobbler is in a position approximately normal to the pistons. When the face of the variable wobbler is approximately perpendicular to the pistons, no oil, (except that required to provide for power losses due to leakage), is pumped by the variable hydraulic unit. At this time the cylinder block stops rotating and the transmission is operating in straight through drive.

(c) Underdrive

When the transmission is operating in underdrive, the variable hydraulic unit functions as a motor and the fixed hydraulic unit functions as a pump. The variable wobbler in the variable hydraulic unit is positioned so the variable hydraulic unit can receive oil from the fixed hydraulic unit. The fixed hydraulic unit's pistons are forced into the cylinder block as they slide up the inclined wobbler face thus pumping high pressure oil to the variable hydraulic

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