Developing a Rotating Magnetic Field

The principles of electromagnetism explain the shaft rotation of an AC motor. Recall that the stator of an AC motor is a hollow cylinder in which coils of insulated wire are inserted (Picture 1).

Picture 1: Stator of AC motor

Stator Coil Arrangement

The following diagram on Picture 2 shows the electrical configuration of stator windings. In this example, six windings are used, two for each of the three phases. The coils are wound around the soft iron core material of the stator. When current is applied, each winding becomes an electromagnet, with the two windings for each phase operating as the opposite ends of one magnet. In other words, the coils for each phase are wound in such a way that, when current is flowing, one winding is a north pole and the other is a south pole. For example, when A1 is a north pole, A2 is a south pole and, when current reverses direction, the polarities of the windings also reverse.

Picture 2: Stator coil configuration

Stator Power Source

The stator is connected to a three-phase AC power source. The following illustration on Picture 3 shows windings A1 and A2 connected to phase A of the power supply. When the connections are completed, B1 and B2 will be connected to phase B, and C1 and C2 will be connected to phase C.

Picture 3: Stator connected to phase A of a three-phase source


As the following illustration shows, coils A1, B1, and C1 are 120° apart. Note that windings A2, B2, and C2 also are 120° apart. This corresponds to the 120° separation between each electrical phase. Because each phase winding has two poles, this is called a two-pole stator (Picture 4).

Picture 4: 2-pole stator windings


When AC voltage is applied to the stator, the magnetic field developed in a set of phase coils depends on the direction of current flow. Refer to the following chart shown on Picture 5 as you read the explanation of how a rotating magnetic field is developed. This chart assumes that a positive current flow in the A1, B1 or C1 windings results in a north pole.

Picture 5: Current flow direction chart

Start

In the following illustration shown on Picture 6, a start time has been selected during which phase A has no current flow and its associated coils have no magnetic field. Phase B has current flow in the negative direction and phase C has current flow in the positive direction. Based on the previous chart, B1 and C2 are south poles and B2 and C1 are north poles. Magnetic lines of flux leave the B2 north pole and enter the nearest south pole, C2. Magnetic lines of flux also leave the C1 north pole and enter the nearest south pole, B1. The vector sum of the magnetic fields is indicated by the arrow.

Picture 6: Start of a motor - diagram

Time 1

The following chart on Picture 7 shows the progress of the magnetic field vector as each phase has advanced 60°. Note that at time 1 phase C has no current flow and no magnetic field is developed in C1 and C2. Phase A has current flow in the positive direction and phase B has current flow in the negative direction. As the previous chart shows, windings A1 and B2 are north poles and windings A2 and B1 are south poles. The resultant magnetic field vector has rotated 60° in the clockwise direction.

Picture 7: Time 1 diagram

Time 2

At time 2 (Picture 8), phase B has no current flow and windings B1 and B2 have no magnetic field. Current in phase A is flowing in the positive direction, but phase C current is now flowing in the negative direction. The resultant magnetic field vector has rotated another 60°.

Picture 8: Time 2 diagram

360° Rotation

At the end of six such time intervals, the magnetic field will have rotated one full revolution or 360° (Picture 9). This process repeats 60 times a second for a 60 Hz power source.

Picture 9: 360° Rotation diagram

Synchronous Speed

The speed of the rotating magnetic field is referred to as the synchronous speed (NS) of the motor. Synchronous speed is equal to 120 times the frequency (f), divided by the number of motor poles (P):

NS = 120 * f / P

The synchronous speed for a two-pole motor operated at 60 Hz, for example, is 3600 RPM.

Synchronous speed decreases as the number of poles increases. The following table on Picture 10 shows the synchronous speed at 60 Hz for several different pole numbers.

Picture 10: Synchronous speed at 60 Hz for different pole numbers