Analysis of the structure and principle of BMW F18PHEV motor

The electric motor in the F18 PHEV is a permanently excited synchronous motor. It converts the electrical energy of the high-voltage battery into kinetic energy, thereby driving the vehicle. The vehicle can be driven in electric mode at speeds up to 120km/h and can also support the engine, for example, during overtaking (acceleration function), or actively support the engine’s torque when shifting gears.

The electric motor in the F18 PHEV is a permanently excited synchronous motor. It converts the electrical energy of the high-voltage battery into kinetic energy, thereby driving the vehicle. The vehicle can be driven in electric mode at speeds up to 120km/h and can also support the engine, for example, during overtaking (acceleration function), or actively support the engine’s torque when shifting gears.

Conversely, during braking and in coasting mode, the electric motor converts kinetic energy into electrical energy and supplies it to the high-voltage battery (energy recovery).

As shown in the figure below, the hybrid assembly is integrated as a separate component in the transmission bell housing, occupying the space for the hydraulic torque converter in the transmission housing. The main components of the motor are rotor and stator, interface, rotor position sensor, and cooling device.

Analysis of the structure and principle of BMW F18PHEV motor

F18 PHEV Motor Installation Location and Auxiliary Components

1—high voltage battery unit; 2—motor—Electronic servo control system; 3—anti-loosening ring; 4—motor cover plate; 5—auxiliary torsional shock absorber; 6—release clutch; 7—motor; 8—hollow shaft

The hybrid system in the F18 PHEV is a so-called “parallel hybrid system”. Both the engine and the electric motor are mechanically connected to the drive wheels. When the vehicle is driven, the two drive systems can be used individually or simultaneously. The internal structure of the motor is shown in the figure below.

Analysis of the structure and principle of BMW F18PHEV motor

Internal structure of the F18 PHEV electric motor

1—stator; 2—permanent magnet; 3—rotor; 4—hollow shaft with release clutch housing

The electric motor (traction motor) structure in the F18 PHEV takes the form of an internal rotor. “Inner rotor” means a rotor with permanent magnets arranged in a ring shape inside. The windings that generate the rotating field are located outside and form the stator. The motor of the F18 PHEV has 8 pole pairs. The stator is fixed above a flange on the hollow shaft of the rotor, which is embedded in the input shaft of the gearbox (below).

Analysis of the structure and principle of BMW F18PHEV motor

F18 PHEV Motor Interface

1-Gearbox bell; 2-Temperature sensor; 3-Coolant passage; 4-Coolant interface; 5-Rotor position sensor electrical interface; 6-High pressure interface

There are four motor ports on the automatic transmission housing, which are respectively used for temperature sensor, two coolant pipes, rotor position sensor, and high-voltage wire. The sensor installation location is shown in the figure below.

Analysis of the structure and principle of BMW F18PHEV motor

F18 PHEV Motor Sensor Mounting Location

1—temperature sensor; 2—rotor position sensor rotor; 3—rotor position sensor stator

In order for the motor-electronic servo system to correctly calculate the amplitude and phase of the stator winding voltage and generate the voltage correctly, the exact position of the rotor must be known. The rotor position sensor undertakes this task. Its structure is similar to that of a synchronous motor, and it has a specially shaped rotor and a stator, the rotor is connected to the rotor of the motor, and the stator is connected to the stator of the motor. Motor – The electronic servo system evaluates the phase voltages generated by the rotor screwing into the stator windings and calculates the rotor position angle.

The components of the electric motor are not allowed to exceed a certain temperature during operation. Monitor motor temperature with a temperature model and a temperature sensor. The sensor is designed as a variable resistor with a negative temperature coefficient (NTC) and measures the coolant outlet temperature on the automatic transmission housing. The higher the NTC, the lower the resistance value.

The motor-electronic servo system analyzes the temperature sensor signals, compares these signals with a calculated temperature model, and reduces the motor power if the motor temperature approaches the maximum allowed. A separate temperature sensor is no longer installed on one stator winding.

To ensure the temperature reliability of the electric motor in all conditions, the electric motor is cooled with coolant in the F18 PHEV. For this purpose, the electric motor is connected in the coolant circuit of the engine (below).

Analysis of the structure and principle of BMW F18PHEV motor

Coolant circulation for the F18 PHEV engine and electric motor

A—coolant-air heat exchanger (electric motor—coolant circulation of electronic servo control system); B—electrical coolant pump (motor—coolant circulation of electronic servo control system, 80W); C—coolant thermal expansion balance tank (Electric motor – coolant circulation of electronic servo control system); D – electric motor – electronic servo control system EME; 1 – coolant – air heat exchanger (coolant circulation of engine and electric motor); 2 – electric fan; 3 – cooling Liquid thermal expansion balance tank (coolant circulation of engine and electric motor); 4-characteristic curve thermostat; 5-electrical coolant pump (coolant circulation of engine and electric motor, 400W); 6-engine oil cooler; 7-exhaust gas Turbocharger; 8-engine; 9-electric motor; 10-heater for air heating; 11-double water valve; 12-electric heating device; 13-electrical coolant pump for heating circulation loop; 14-electrical switching valve; 15—Motor thermostat

To cool the stator windings, there is a cooling channel between the stator carrier and the housing of the automatic transmission, through which the coolant flows from the engine cooling circuit. The cooling channels are sealed forward and backward by two sealing rings, respectively. The transmission oil cools the rotors, and the oil mist of the transmission oil absorbs the heat and dissipates it to the atmosphere on the transmission oil cooler (below).

Analysis of the structure and principle of BMW F18PHEV motor

Cooling unit for F18 PHEV electric motor

1—coolant-air heat exchanger; 2—motor thermostat; 3—motor; 4—automatic transmission housing; 5—motor coolant pipeline; 6—stator bracket

The motor comes with a thermostat to adjust the coolant inlet temperature to the optimal range of about 80°C. This adjustment is necessary because the motor operating temperature is lower than the engine operating temperature. The thermostat is regulated by a paraffin thermostatic element that expands according to the coolant temperature. At this time, there is no electric control, and the operating state of the thermostat is shown in the figure below.

Analysis of the structure and principle of BMW F18PHEV motor

F18 PHEV Motor Thermostat Operating Status

A—thermostat closed; B—thermostat partially open; C—thermostat open; 1—coolant from the coolant-air heat exchanger; 2—coolant to the motor; 3—coolant from the engine come here; 4 – thermostat

When the coolant temperature is low, the thermostat is closed. This is the case, for example, during the warm-up phase. At this point, the thermostat blocks the coolant in the coolant-to-air heat exchanger, sending engine coolant to the electric motor. In this way, the optimum operating temperature can be reached quickly.

Due to the high engine coolant temperature, the thermostat is partially opened. This causes the high temperature coolant from the engine to mix with the low temperature coolant from the coolant-to-air heat exchanger. In this “mixed mode” the coolant temperature is self-regulated in the coolant supply line to the electric motor, keeping it in the optimum temperature range of about 80°C.

If the coolant temperature of the coolant-to-air heat exchanger rises additionally, the thermostat opens completely. This occurs, for example, when the engine thermostat opens a large coolant circuit. The thermostat shuts off the coolant line from the engine due to the extra warming. Now all the coolant from the coolant-to-air heat exchanger goes to the electric motor.

The Links:   LB084S02-TD01 LMS700KF01-001