Solid State Reversing AC Motor Starter Powered by DC Source
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Solution Overview
Problem
Nuclear power plants face challenges in using DC motors due to maintenance difficulties and the need for qualified motors in harsh environments, while AC motors are already qualified but require a DC power source for operation in safety systems.
Innovation Solution
A solid state reversing AC motor starter powered from a DC source, utilizing a power bridge with solid state gates, an AC power logic circuit for pulse width modulation, and a motor starter logic circuit to control motor direction and duration, ensuring safe and reliable operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If DC motors are used to operate valves in nuclear power plants, then the motors can be directly powered from the 125VDC battery source, but the motors require periodic maintenance of brushes and are difficult to qualify for harsh nuclear environments
Solution Approach 1:
The patent introduces an AC motor starter as an intermediary device that converts DC power from the nuclear plant's battery source into three-phase AC power. This allows qualified AC motors to be operated from DC sources without direct connection, thereby avoiding the maintenance and qualification issues associated with DC motors while maintaining compatibility with the DC power source. The motor starter includes a DC input stage, inverter circuitry, and control logic that together enable AC motor operation from DC batteries.
Solution Approach 2:
The invention changes the electrical parameters (voltage type, frequency, phase) between the power source and the motor. The system transforms 125V DC into variable frequency three-phase AC output suitable for AC motor operation. This parameter transformation allows the use of AC motors (which are already qualified for nuclear environments) instead of DC motors, resolving the contradiction between power source compatibility and motor reliability.
2Reliability
If AC motors are used in nuclear applications, then the motors are already qualified for harsh environments and have lower maintenance requirements, but they cannot be directly operated from the nuclear station's DC battery source
Solution Approach 1:
The AC motor starter serves as a mediator between the DC battery source and AC motors. It includes a DC-AC converter that takes DC input from the 125VDC batteries and produces three-phase AC output to drive AC motors. This intermediary device enables the use of reliable, pre-qualified AC motors in nuclear applications without requiring them to be directly connected to DC sources.
Solution Approach 2:
The patent replaces the need for mechanical DC motor brush assemblies with an electronic conversion system. Instead of using DC motors with brushes that require maintenance, the system uses solid-state electronics to convert DC to AC, eliminating mechanical wear components while maintaining the reliability benefits of AC motors in harsh nuclear environments.
3Adaptability or versatility
If a solid state power bridge is used to convert DC to three-phase AC, then the system can operate AC motors from DC sources, but the switching frequency must be higher than the desired output frequency requiring pulse width modulation
Solution Approach 1:
The motor starter employs periodic switching action through pulse width modulation (PWM). The solid state gates switch on and off at high frequency in periodic cycles, with the duty cycle varied to control the output voltage and frequency. This periodic switching, controlled by timing circuits, enables the conversion of DC to variable frequency three-phase AC while managing the complexity through systematic control patterns.
Solution Approach 2:
The system uses dynamic pulse width modulation to control the power bridge output. The duty cycle of the switching gates is dynamically adjusted to produce the desired AC waveform characteristics. This dynamic control allows flexible adjustment of output frequency and voltage to match motor requirements while maintaining efficient DC-to-AC conversion.
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
Enables the use of qualified AC motors in nuclear safety systems by providing a reliable and maintainable AC motor starter that can operate from a DC power source, meeting stringent safety requirements and reducing maintenance burdens.
Implementation Method 1
at least six solid state gates respectfully having a gate output individually switchable between an on and off state
Implementation Method 2
A capacitor input filter is provided across the DC input
Implementation Method 3
a capacitor output filter is connected across each phase of the three phase output
Implementation Method 4
The AC power logic circuit varies the duration of the on state of the respective gates so that an output voltage of the three phases can be made to approximate a sine wave using pulse width modulation
Data Source
AI summary
A solid state reversing AC motor starter that employs a DC power source and a power bridge employing insulated gate bipolar transistors that provide an ungrounded three phase AC output. The AC output is connected to the power input of the motor starter, which requires simultaneous RUN and Directional Commands to initiate valve actuation.