Auxiliary Motor Starting of High-Inertia Machines on Weak Grids
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Solution Overview
Problem
Starting large electric motors in cement plants with unstable power grids is technically challenging due to voltage drops and grid instability, necessitating the use of slip-ring induction motors with sliding contacts and salt baths, which are problematic.
Innovation Solution
A drive system for working machines that includes a main motor and an auxiliary motor, controlled by a frequency converter, allowing for unloading and starting at synchronous speed, reducing torque and current peaks, and using a compact design to avoid grid instability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If direct connection and starting of electric motors on the grid is performed, then the motor can start immediately, but it causes too great a voltage drop and grid instability
Solution Approach 1:
The motor starting process is segmented into two distinct phases: first the auxiliary motor accelerates the high-inertia load to near-synchronous speed, then the main motor is connected to the grid. This segmentation prevents the voltage drop that would occur with direct starting by breaking the starting process into manageable stages.
Solution Approach 2:
The auxiliary motor performs preliminary acceleration of the working machine before the main motor is connected to the grid. This preliminary action brings the rotor to a speed close to synchronous speed, reducing the inrush current and voltage drop when the main motor connects to the grid.
2Reliability
If slip-ring induction motors with liquid starters are used, then grid stability is maintained during starting, but the system becomes complex with sliding contacts and salt bath solutions requiring maintenance
Solution Approach 1:
The complex slip-ring and liquid starter components are completely removed from the system. Instead, a simple auxiliary motor with standard starting equipment is used to accelerate the load, eliminating the need for rotor resistance control and sliding contacts while maintaining grid stability.
Solution Approach 2:
The mechanical slip-ring and liquid starter system is replaced with an electrical solution using an auxiliary motor. The auxiliary motor provides the necessary starting torque through electrical means rather than mechanical resistance control, simplifying the overall system architecture.
3Ease of operation
If slip-ring induction motors with liquid starters are used, then starting control is achieved, but maintenance intervals are frequent due to sliding contacts and cathode wear
Solution Approach 1:
The auxiliary motor is designed as a simple, inexpensive component that can be easily replaced if needed. By using standard motor components instead of complex slip-ring assemblies, the system trades a cheap, simple component for the avoidance of expensive, maintenance-intensive equipment.
Solution Approach 2:
The auxiliary motor automatically performs the starting function without requiring manual intervention or maintenance of sliding contacts. The system eliminates components that require regular maintenance while maintaining full starting control capability through automated motor control.
4Device complexity
If high-inertia working machines are started directly on the grid, then the system is simple, but current peaks and torque peaks cause grid collapse in unstable grids
Solution Approach 1:
The starting system is segmented into two motor stages: an auxiliary motor for initial acceleration and a main motor for continued operation. This segmentation allows the system to overcome high inertia without creating damaging current and torque peaks that would destabilize weak grids.
Solution Approach 2:
The auxiliary motor acts as an intermediary between the grid and the high-inertia load. It provides a buffer that smooths out the current and torque demands, preventing direct peaks from reaching the grid while still enabling the main motor to eventually drive the full load.
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
The system enables efficient starting of high-inertia machines with reduced complexity, cost, and maintenance, while minimizing grid impact and torque surges, and allowing for higher motor speeds and extended starting times.
Implementation Method 1
an auxiliary motor (3) configured as an electric machine... the auxiliary motor is arranged to start and/or accelerate the working machine, in particular up to a synchronous speed of the main motor
Implementation Method 2
at least one main motor (2) which is designed as an electric machine... the main motor is an asynchronous motor, in particular one switched by a medium-voltage circuit breaker, or a synchronous motor
Data Source
AI summary
A drive system for a working machine with a working device which can be unloaded for starting, wherein the drive system comprises at least one main motor, which is designed as an electric machine, in particular as an asynchronous motor, and a motor shaft; an auxiliary motor, which is designed as an electric machine, in particular as an asynchronous motor; and a starting device for starting the auxiliary motor. The auxiliary motor is configured to start and/or accelerate the working machine, in particular up to a synchronous speed of the main motor.