Constant Speed Drive for Variable APU Speed and Constant Generator Frequency
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Solution Overview
Problem
Conventional aircraft auxiliary power unit (APU) systems are inefficient in fuel consumption due to fixed operating speed, which cannot be varied without affecting the frequency of power output, leading to suboptimal performance at different altitudes and resulting in fuel inefficiency at sea level.
Innovation Solution
The implementation of a constant speed drive (CSD) unit between the APU drive shaft and the generator allows for varying the APU operating speed within a target range while maintaining a constant rotational speed for the generator, ensuring the target frequency of 400 Hz is maintained, thereby optimizing fuel efficiency across different altitudes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the APU operates at a fixed speed to maintain constant generator output frequency, then the power frequency remains standardized (400 Hz), but fuel consumption efficiency deteriorates at different altitudes
Solution Approach 1:
The system transitions from a fixed-speed APU operation to a variable-speed operation through the introduction of a constant speed drive (CSD). The CSD allows the APU to dynamically adjust its rotational speed based on operating conditions (altitude, power demand) while maintaining a constant rotational speed input to the generator, thereby resolving the contradiction between frequency stability and fuel efficiency.
Solution Approach 2:
The constant speed drive (CSD) acts as an intermediary mechanism between the APU and the generator. It decouples the variable-speed APU output from the constant-speed generator input, allowing independent optimization of each component's operating point for maximum efficiency while maintaining standardized power output frequency.
2Use of energy by moving object
If the APU speed is varied to optimize fuel efficiency at different altitudes, then fuel consumption improves, but the generator output frequency becomes variable and non-standardized
Solution Approach 1:
The system enables dynamic APU speed adjustment for fuel efficiency optimization while using the constant speed drive to maintain a constant rotational input to the generator, ensuring standardized frequency output despite varying operating conditions.
Solution Approach 2:
The CSD serves as a mechanical intermediary that translates variable-speed APU rotation into constant-speed generator drive, allowing the APU to operate at optimal speeds for fuel efficiency while the generator receives a constant rotational input that maintains standardized frequency output.
3Use of energy by moving object
If a constant speed drive is introduced to decouple APU speed from generator speed, then fuel efficiency improves and frequency stability is maintained, but device complexity increases
Solution Approach 1:
The constant speed drive is introduced as a dedicated intermediary component between the APU and generator. This modular approach allows the complex speed-decoupling function to be isolated in a single unit, simplifying the overall system architecture while achieving the desired performance benefits.
Data Source
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AI summary
An aircraft power system includes an auxiliary power unit (APU), a generator, and a constant speed drive (CSD). The APU drives an APU drive shaft (110) at a first rotational speed during a first condition and a second rotational speed during a second condition. The generator is rotatably coupled to a generator shaft and produces an AC voltage having a target frequency in response to rotation of the generator shaft at a target rotational speed. The CSD unit receives the first rotational speed from the APU drive shaft (110) and rotates the generator shaft at the target rotational speed based on the first rotational speed. The CSD further receives the second rotational speed from the APU drive shaft (110) and rotates the generator shaft at the target rotational speed based on the second rotational speed.