Even-Phase Motor Drive Circuit for Lower EMI and Frame Current
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Solution Overview
Problem
Variable speed drives for large motors face challenges due to unwanted electronic interference and high costs associated with matching and sourcing many transistors in parallel, which are necessary to manage high currents and reduce electromagnetic interference (EMI).
Innovation Solution
The solution involves driving phase pairs using individual transistors, arranged in a specific configuration to minimize current flow into the motor's frame, thereby reducing EMI. This configuration includes a four-transistor bridge with capacitors and diodes to manage reactive energy and freewheeling currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If many transistors are connected in parallel to handle high current for large motors, then the power handling capability is improved, but the cost and complexity of matching and sourcing transistors increases
Solution Approach 1:
The patent divides the motor phases into pairs and assigns each phase pair to a separate H-bridge module. Each module handles two phases independently, allowing the system to scale by adding modules rather than increasing the number of transistors within a single module. This segmentation reduces the complexity of matching and sourcing transistors while maintaining high power handling capability.
2Ease of operation
If transistors are switched on and off to control motor speed, then the ability to adjust motor operating point is improved, but electromagnetic interference and frame current increase
Solution Approach 1:
The patent introduces a neutral point as an intermediary connection between phase pairs in the H-bridge configuration. This neutral point provides a return path for capacitive frame currents, preventing them from flowing through the motor chassis and causing EMI. The intermediary structure allows transistor switching for speed control while managing the harmful frame current effects.
Solution Approach 2:
The patent converts the harmful frame current into a beneficial feature by using the capacitive coupling between phases to create a balanced push-pull voltage application. The frame current that would normally cause EMI is instead used to establish a neutral reference point that actually reduces net frame current flow, as equal and opposite voltages are applied to phase pairs.
3Use of energy by moving object
If rectangular pulse voltage is applied to motor windings for speed control, then the efficiency of voltage utilization is improved, but the fast transitions cause significant current through winding capacitance
Solution Approach 1:
The patent employs periodic switching of transistor pairs in the H-bridge configuration to apply rectangular pulse voltages to phase pairs. By systematically alternating between different transistor pairs and phase pairs, the system maintains efficient voltage utilization while distributing the capacitive current effects across multiple switching cycles and phase pairs, reducing peak harmful currents.
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 effectively minimizes electromagnetic interference (EMI) and reduces the cost and complexity of sourcing transistors by using individual transistors rather than expensive modules, while also allowing for more efficient cooling and higher power handling capabilities.
Implementation Method 1
Capacitor are connected to the positive and negative supply busses. The capacitors are required to absorb the reactive energy of the windings.
Implementation Method 2
Diodes are connected across each transistor to provide a freewheeling path for winding current when the opposite transistor on the same leg is turned off.
Implementation Method 3
The transistors in the variable speed drive or either on or off. As a consequence, the output voltage of the drive is a rectangular pulse of voltage
Data Source
AI summary
Electric motors are controlled by electronic circuits called variable speed drives which supply the motor with adjustable voltage and adjustable frequency to power the motor at its best operating point. Variable speed drives output rectangular pulses of voltage to the motor. The fast edge of these pulses produce unwanted electrical noise called electromagnetic interference (EMI). The invention consists of a motor with an even number of phases and a balanced drive circuit to reduce electromagnetic interference (electrical noise). By using an even number of phases, one phase will be going negative at the exact instant another phase is going positive, and the noise current can flow across windings but not back to the supply. This will greatly reduce unwanted current and EMI in the frame of the system.