Compressor Unit with Shared VFD for Startup Inrush Current Control
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Solution Overview
Problem
Conventional refrigeration systems face high inrush currents during compressor startup, necessitating costly transformers to mitigate this issue, and existing solutions with variable frequency drives (VFDs) are expensive and inefficient at rated speed operation.
Innovation Solution
A compressor unit with multiple compressors and a common variable frequency drive, where each compressor is independently connected to the VFD during startup and acceleration, then switched to a three-phase grid voltage supply at rated speed, utilizing power modulation means like slide valves or guide vanes to manage power according to load needs, thereby avoiding high inrush currents and reducing VFD size and cost.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a variable frequency drive is permanently connected to supply power to the compressor at rated speed, then the inrush current is controlled, but the system efficiency decreases and costs increase
Solution Approach 1:
The variable frequency drive performs preliminary action by controlling the compressor during startup and acceleration phases only. Once the compressor reaches rated speed, the VFD disconnects and the compressor operates directly on grid power, eliminating continuous energy losses while maintaining reliable inrush current control during the critical transition phases.
2Reliability
If a variable frequency drive is permanently connected to supply power to the compressor, then the inrush current is controlled, but the system cost increases
Solution Approach 1:
The VFD is used only temporarily during startup and acceleration phases, then disconnected. This preliminary action approach allows the system to pay for VFD operation only when necessary for inrush current control, rather than maintaining continuous VFD connection and its associated costs throughout rated speed operation.
3Loss of energy
If the compressor is directly connected to grid voltage supply at rated speed, then the system efficiency is maximized, but the inrush current during startup becomes excessively high
Solution Approach 1:
The variable frequency drive performs preliminary action during startup and acceleration to control inrush current. Once the compressor reaches rated speed, the system transitions to direct grid connection for efficient operation, combining the benefits of both approaches at different operational stages.
Solution Approach 2:
The system dynamically switches between two operational modes: VFD-controlled mode during startup/acceleration to manage inrush current, and direct grid connection mode during rated speed operation to maximize efficiency. This dynamic transition optimizes performance across different operational phases.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration effectively reduces inrush currents, lowers costs by eliminating the need for transformers, and enhances efficiency by up to 3-4% compared to systems with continuous VFD operation at rated speed, while maintaining reliability and flexibility in load management.
Implementation Method 1
a common variable frequency drive, configured to be connected to a three phase grid voltage supply
Implementation Method 2
a compressor motor, particularly an alternating current induction motor, with a three phase supply line
Data Source
AI summary
A compressor unit (2) for use in a refrigeration circuit, comprises at least two compressors (8, 10); and a common variable frequency drive (6), configured to be connected to a three phase grid voltage supply (4); each compressor (8, 10) having an inlet port (12) configured to be in refrigerant communication, via a suction line, with an outlet of an evaporator; an outlet port (14) configured to be in refrigerant communication, via a pressure line, to an inlet of a condenser; a compressor motor (18), particularly an alternating current induction motor, with a three phase supply line (32-1, 32-2); a power modulation means (24) for controlling the power of the compressor (8, 10) according to its load needs; wherein each compressor (8, 10) is switchable, independently from the respective other compressor(s) (8, 10), to be connected, during start-up and acceleration of the respective compressor (8, 10), to the common variable frequency drive (6), and to be connected, during rated-speed operation of the respective compressor (8, 10), to a common three phase grid voltage supply (4).