Three-Phase Motors

Three-phase Motors

Three-phase currents and even three-phase alternating currents are produced by generators with a stator containing three coils offset at 120°. Three alternating currents offset at 120° with the same frequency are available at the terminal clamps on the stator coils.







In motor vehicle/hybrid technology, these three phases are referred to as u, v and w. The connections for these three coils are often designated as u1, u2, v1, v2 and w1, w2.

The voltage/time diagram below clearly shows the phase offset. The time t is specified in degrees here. The full circle of 360° represents a complete phase of the sinusoidal signal waveform.







Due to the phase offset, the three-phase motor does not require the auxiliary phase that the alternating-current motor requires. The rotary field acting around the stator carries the rotor along, which means that the speed depends on the number of poles and the frequency (see alternating-current motor).

In hybrid technology, the speed of the three-phase motor is controlled by variations in the frequency. The frequency range that the power electronics require therefore depends on the number of pole pairs on the stator as a fixed variable and the maximum speed of the combustion engine, possibly in conjunction with an increase or reduction in the gear ratio. The phase sequence u, v, w indicates the rotational direction of a three-phase motor. If phases 2 and 3 are exchanged, the direction of rotation is reversed (see illustrations, star -> clockwise, delta -> counter-clockwise).

There are always two variations for switching all six coil connections on a three-phase motor (star or delta connection). The availability of two versions means that only three cables need to be routed to the motor or generator:

Star connection:







The addition of the three voltages u, v, w in the voltage/time diagram shows that the sum of the three voltages equals zero at all times.







One side of all coils can therefore be short-circuited as a star point. The individual coils are connected to a voltage that corresponds to the respective distance in relation to the zero line.

Delta connection:







The coils on a delta circuit are connected in series. Each coil is therefore connected to a voltage that corresponds to the respective distance of two phases.

The voltage/time diagram shows that the voltages are much higher than those of a star connection.







As a result, the currents and voltages from motors/generators with a delta connection increase by a factor of 1.73 (square root of 3) compared to the star connection. Power levels increase threefold with a delta connection. Coils used in Porsche hybrid modules are always connected in a delta configuration.