Function Principle, 3-Phase Synchronous Motor

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The Twin Engine uses 3-phase synchronous motors in the form of CISG and ERAD (Electric Rear Axle Drive).

The main components of a 3-phase synchronous motor consist of a stator, a rotor, and a control unit. The function principle is to create a rotating magnetic field in the stationary stator windings where the permanently magnetized rotor is forced to rotate. The load can then be connected to the rotor, e.g. an electrically-driven rear axle.

The rotor can be used conversely to generate charging current or brake force by means of voltage being induced in the stator windings as an effect of the rotation. Depending on the situation, ERAD is used in the Twin Engine both as an individual brake and as a brake in combination with the friction brakes.

Stator and rotor 

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The stator consists of three phases (windings), which are wound as coils 1, 2 and 3. They are jointly connected at connection 4, a so-called Y-point. The three phases are internally offset 120°. In the example, each of the stator windings (each phase) consists of a pair of poles, i.e. two poles. The rotor is permanently magnetized and has 1 north pole and 1 south pole.

A current flow is created, and hence the magnetism in the coil, when the phases are connected to a voltage source.

Control unit 

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By means of six transistors, and based on a DC voltage from the high-voltage battery, it is possible to build up the desired AC voltages for the different AC loads. In cars with Twin Engine, IGBT transistors (Insulated Gate Bipolar Transistor) are used for ERAD and CISG.

The transistors open and close the phase circuits in different patterns, so that the current through the respective phase changes direction and varies in intensity.

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The polarity of the magnetic fields also shifts by means of the winding for the coils and the alternating current direction. The amperage and the direction of the current are usually illustrated by sinusoidal curves, one for each phase.

By controlling the currents through the three phases in different patterns, a rotating magnetic field is obtained. When the rotor is positioned in the stator's rotating magnetic field, the rotor will start to rotate. Alternatively, the rotor is allowed to induce voltage in the coils, which results in a generator effect.

For the control units to be able to control the transistors correctly, accurate information is required about the rotor's position in relation to the stator windings as well as the rotor's angular velocity. For this reason, the synchronous motor is equipped with a sensor. A resolver is used for ERAD, while a simpler RPS (Rotor Position Sensor) is used for CISG.

Function principle, motor effect 

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The electric motor's rotation speed is controlled by controlling the base curve form in the voltage, and thus the resulting current. The base curve form, also called the fundamental frequency, creates a rotating magnetic field where the fundamental frequency corresponds to the speed of the magnetic field. The fundamental frequency can be illustrated as a sine wave.

The higher the fundamental frequency, the higher the rotor's rotation speed. This takes place by controlling the transistors, which open and close the circuits in different patterns. By controlling the speed of the rotating magnetic field the rotor will also follow to the equivalent extent. The rotor's maximum rotation speed is 12 000 RPM.

Since the stator and rotor have 10 poles each, five sine waves for one phase will correspond to one revolution. If five sine waves for one phase took 1 second then the magnetic field's rotation speed would be 1 revolution per second, i.e. the rotor would have rotated one "mechanical" revolution.

The transistors open/close each respective phase leg with a frequency of 10kHz, a so-called PWM control. By closing the circuit for shorter or longer periods an adapted voltage level is achieved for each situation, and thus also the corresponding current intensity, which in turn generates a desired magnetic field. The magnetic field in turn gives the rotor an equivalent torque.

If the rotor speed is 2 revolutions per second (equivalent to 10 sine waves per second), then each sine wave will be "chopped up" 1000 times (10 000 Hz/10 = 1000 Hz).

If the rotation speed increases to 2000 revolutions per second (equivalent to 1000 sine waves per second), then each sine wave will be "chopped up" 10 times (10 000 Hz/1000 = 10 Hz).

Since the frequency is fixed, the number of pulses per sine wave will vary accordingly, depending on the rotation speed. If the rotation speed increases then there will be fewer pulses per sine wave.

Courtesy of VOLVO CARS CORPORATION

Courtesy of VOLVO CARS CORPORATION

In a basic mode where no driving or braking torque is sought, the currents in the stator windings are regulated in such a way that the stator's magnetic field rotates at a speed equivalent to that of the rotor.

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Courtesy of VOLVO CARS CORPORATION

The most efficient motor effect is obtained by controlling the stator's magnetic field to be aligned 90° before the rotor's magnetic field. In this position it is possible to regulate the motor's drive torque by regulating the current in the stator windings. However, it is not possible to align the stator's magnetic field 90° before the rotor's magnetic field at rotation speeds that are too high.

Courtesy of VOLVO CARS CORPORATION

Courtesy of VOLVO CARS CORPORATION

For a rotation speed above around 4000-5000 RPM, a field weakening current needs to be formed and the stator's magnetic field is aligned here more than 90° before the rotor's magnetic field. This is done to counteract the rotor's magnetic field which otherwise limits an increase in rotation speed. The output, which is a result of torque and angular velocity, is held constant while the torque decreases with increasing rotation speed.

Function principle, generator and brake force 

The electric motor can also have a braking effect on the rotor. This is used both when brake force is sought and to generate current.

Depending on the situation, ERAD is used in the Twin Engine both as an individual brake and as a brake in combination with the friction brakes.

Both ERAD and CISG in the Twin Engine also function as generators by means of the rotor's magnetic field being used to induce voltage in the stator windings, which results in a braking effect.

The principle of operation is the same as for a 3-phase alternator. To create energy costs energy. In a conventional 12 V alternator the charge control normally takes place by means of regulating the rotor's magnetic field and thereby controlling the charging voltage.

Since CISG and ERAD have permanently magnetized rotor parts where the magnetic field cannot be regulated, the same IGBT transistors (Insulated Gate Bipolar Transistor) that are used to give the motors a driving torque are also used for charge control. During charge control the transistors work at a frequency of 10 kHz, i.e. the same as for motor control.

Courtesy of VOLVO CARS CORPORATION

Courtesy of VOLVO CARS CORPORATION

In order to achieve a braking effect, the transistors in the motor's control unit are controlled in such a way that the rotor's magnetic field induces a voltage in the stator windings. The induction, when the rotor's magnetic field is aligned before the stator, brakes the rotor at the same time as induced voltage can be used for generating. By means of regulating the ratio between the stator's magnetic field and the rotor's magnetic field, the size of the induction can be adapted, and thereby also the braking torque.