BMW 3 Owners & Service Manuals

BMW 3: Power electronics for activation of the electrical machine

BMW 3 Series G20 (2018-2024) Training manual / Electrical Machine Electronics / Power electronics for activation of the electrical machine

The power electronics for the activation of the electrical machine are mainly made up of the bidirectional DC/AC converter. It is a pulse converter with a 2-pin DC voltage connection and a 3-phase AC voltage connection. This DC/AC converter can work as an inverter and conduct electrical energy from the high-voltage battery unit to the electrical machine when it is to work as a motor.

However, the DC/AC converter can also work as a rectifier and transfers electrical energy from the electrical machine to the high-voltage battery unit. This operating mode occurs during brake energy regeneration (energy recovery) in which the electrical machine works as an alternator and "generates" electrical energy.

Power electronics for activation of the electrical machine
Operating modes of the bidirectional DC/AC converter

  1. Schematic diagram
  2. Schematic diagram with components
  1. High-voltage battery unit
  2. Operating mode as inverter, electrical machine works as an engine
  3. Operating mode as rectifier, electrical machine works as an alternator
  4. DC/AC converter
  5. Current sensors
  6. Electric motor

The operating mode of the DC/AC converter is defined by the EME control unit. The EME control unit also receives the setpoint values (essential input variables) from the DME control unit for which torque (amount and sign) the electrical machine should supply. From this setpoint value and the current operating condition of the electrical machine (engine speed and torque) the EME control unit determines the operating mode of the DC/AC converter, as well as the amplitude and frequency of the phase voltages for the electrical machine. According to these specifications, the power semiconductors of the DC/AC converter are activated in sync.

In addition to the DC/AC converter, the power electronics also contains current sensors in all 3 phases on the AC voltage side of the DC/AC converter. Using the signals from the current sensors, the EME control unit monitors the electrical power which is used in the power electronics and electrical machine and what torque the electrical machine generates. The control loop of the Electrical Machine Electronics is closed by the signals of the current sensors and the rotor position sensor in the electrical machine.

The performance data of the Electrical Machine Electronics and the electrical machine were coordinated in development. The Electrical Machine Electronics can deliver a continuous electrical power of 55 kW and a temporary maximum power of 83 kW. In order to avoid overloading the power electronics, there is also another temperature sensor at the DC/AC converter. If an excess temperature of the power semiconductor is identified using this signal, the EME control unit reduces the power delivered to the electrical machine in order to protect the power electronics.

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