Electric Machine Gear Control for Field Weakening Thermal Management
Solution Overview
Problem
In motor vehicles with electric drives, permanent-magnet electric machines face voltage overload issues at low nominal voltages, leading to inefficiencies when operating in field weakening conditions, as existing methods limit the machine's operating range by shifting transmission gears, which can cause overheating and reduced efficiency.
Innovation Solution
An electronic control unit adjusts the gear ratio of the transmission based on temperature values and driving strategies to delay or prevent gear changes during field weakening, reducing the rotational speed of the rotor and minimizing thermal load on the stator and power converter, allowing for more current to be used for torque without overloading the power supply.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If field weakening is used to operate the electric machine at low nominal voltages, then the operating range is extended, but the stator and power electronics overheat due to active energization
Solution Approach 1:
The patent changes the control parameter from active field weakening energization to passive rotor speed reduction through transmission gear ratio adjustment. By modifying the gear ratio, the rotor speed is reduced, which passively reduces the induced voltage and current feedback to the power supply, avoiding active stator energization and the associated heating while maintaining operation at low nominal voltages.
2Temperature
If transmission gear shift is used to avoid field weakening conditions, then thermal load is reduced, but the operating range of the electric machine is limited
Solution Approach 1:
The patent implements dynamic adjustment of the transmission gear ratio based on real-time monitoring of electric machine operating conditions. The control unit continuously adjusts the gear ratio to maintain the electric machine operating point below the field weakening threshold, dynamically preventing thermal overload while maximizing the usable operating range through adaptive transmission control.
3Reliability
If rotor speed is reduced to prevent voltage overload, then current feedback to power supply is reduced, but the drive train efficiency decreases
Solution Approach 1:
The patent introduces the transmission gear ratio as an intermediary mechanism between the electric machine and the vehicle drive train. By adjusting the gear ratio, the system indirectly controls the rotor speed and induced voltage, providing a buffered solution that prevents direct voltage overload on the power supply while maintaining better efficiency compared to direct speed reduction of the electric machine.
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 enables reliable operation of permanent-magnet electric machines in field weakening conditions by reducing thermal loads and preventing voltage-induced current overload, thus enhancing the efficiency and operational range of the drive train, especially at low nominal voltages like 48 V.
Implementation Method 1
the voltage induced by the rotor in the stator is directly proportional to the rotational speed of the rotor
Implementation Method 2
by suitable energization of the stator, an air gap field component can be generated, which counteracts the exciter field generated by the permanent magnet of the rotor
Data Source
AI summary
A method for operating a drive train of a motor vehicle, which utilizes at least one permanent-magnet electric machine (EM) as a motor vehicle drive source, is provided. A transmission (G) with different gear ratios is arranged in a power path between the electric machine (EM) and driving wheels (DW) of the motor vehicle. A power converter (LE) is associated with the electric machine (EM) and can operate the electric machine (EM) in a field weakening condition. The method includes prematurely carrying out, delaying, or preventing a changeover of the gear step of the transmission (G) specified by a driving strategy depending on a temperature value of the electric machine (EM) in order to reduce, not increase, or increase with delay a rotational speed of the electric machine (EM). An electronic control unit (ECU) for carrying out the method and a motor vehicle with the control unit (ECU) are also provided.