Motor Starter Voltage Control for Semiconductor Switch Protection
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Solution Overview
Problem
Motor starters face challenges in managing the blocking voltage of semiconductor switches when switching off and on three-phase synchronous motors, leading to potential damage due to excessive voltage stress, requiring semiconductor switches with higher than necessary maximum allowable blocking voltage.
Innovation Solution
A motor starter with a control unit that ensures the voltage applied to the semiconductor switch is below its maximum allowable blocking voltage before switching off and on, using a two-step process to manage the energy supply through a current path with a semiconductor switch and an electromechanical switch element, allowing for safe operation with lower blocking voltage semiconductor switches.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the semiconductor switch is used to switch off the electric motor, then the energy supply can be interrupted, but the voltage stress on the semiconductor switch increases to the total of two peak voltages which exceeds its maximum allowable blocking voltage
Solution Approach 1:
The control unit delays the closing of the switch element until the voltage generated by the electric motor has decreased below the maximum allowable blocking voltage of the semiconductor switch. This preliminary timing action prevents excessive voltage stress before the switching operation occurs, resolving the contradiction between safe switching and voltage stress protection.
Solution Approach 2:
The switch element acts as an intermediary between the supply network and the semiconductor switch. It controls the timing of voltage application to the semiconductor switch, ensuring that the switch element is closed only when the motor-generated voltage is below the semiconductor switch's blocking voltage threshold, thus protecting the semiconductor switch from excessive voltage stress.
2Object-affected harmful factors
If a semiconductor switch with higher maximum allowable blocking voltage is used, then the voltage stress problem is resolved, but the cost of the motor starter increases
Solution Approach 1:
The control unit changes the timing parameter of the switching operation by delaying the closing of the switch element until the motor voltage has naturally decreased. This parameter change allows the use of lower-cost semiconductor switches with lower blocking voltage ratings, as the voltage stress is controlled through timing rather than requiring high-voltage-rated components.
Solution Approach 2:
The control unit replaces the need for high-voltage-rated semiconductor switches with a timing-based control system. Instead of relying on the semiconductor switch's inherent high blocking voltage capability, the system uses the control unit to manage the timing of the switching operation, substituting component specification requirements with control logic.
3Productivity
If the switch element is closed immediately after opening, then the switching speed is high, but the voltage applied via the switch element exceeds the maximum allowable blocking voltage of the semiconductor switch
Solution Approach 1:
The control unit performs a preliminary check of the voltage condition before closing the switch element. It waits for the motor-generated voltage to decrease below the semiconductor switch's blocking voltage threshold before allowing the switch element to close, ensuring both reliability and reasonable switching speed.
Solution Approach 2:
The control unit continuously monitors the voltage generated by the electric motor and uses this feedback information to determine the appropriate timing for closing the switch element. This feedback mechanism ensures that the switch element is closed only when voltage conditions are safe, maintaining both reliability and optimal switching speed.
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 approach reduces the maximum blocking voltage required for semiconductor switches by half, preventing damage and lowering costs while ensuring safe and efficient switching operations.
Implementation Method 1
the first current path comprises a semiconductor switch and an electromechanical switch element, wherein the semiconductor switch and the switch element are connected in series
Implementation Method 2
the first current path comprises a semiconductor switch and an electromechanical switch element
Implementation Method 3
The other voltage source is the voltage induced by the electric motor, which is present for as long as there is still a magnetic field in the electric motor
Data Source
AI summary
A motor starter includes a control unit and a first current path, via which energy can be supplied to a downstream electrical motor. The first current path includes a semiconductor switch and an electromechanical switch element, the semiconductor switch and the switch element being connected in series. In order to provide a cost-effective, safe motor starter, according to an embodiment of the invention the control unit is designed such that, in order to produce an energy supply via the first current path in a first step, it ensures that the voltage currently connected via the switch element, in respect of a previous opening of the switch element, lies below the allowable maximum blocking voltage of the semiconductor switch. Subsequently in a second step, the switch element first closes and then switches the semiconductor switch to be conductive.