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Performance Characteristics of AC 3 Phase Motors

AC 3 phase motors are a type of electric motor powered by 3 phase alternating current. Alternating current is where the electric current in the power supply periodically switches its direction. In a 3 phase system, there are three current supply lines. The current in each line alternates at the same frequency but at a different phase to the current in the other two lines.

Torque-Displacement Characteristic
- Torque is the force generated by the rotating electromagnetic field in the motor. Displacement is the actual amount of rotation of the motor axle. The torque-displacement characteristic of a motor can be drawn as a two-dimensional graph. Some applications require high initial torques, while others require a constant displacement over a wide span of different torques. It is important to consider the torque-displacement characteristic to choose a suitable 3 phase motor in both types of application.
Hysteresis
- Hysteresis is the tendency of a system to exhibit path-dependency. This means the state of the system is affected by it history. Hysteresis occurs in AC electric motors because there is a tendency for the magnetic fields generated in the motor to remain the same. This is a problem because AC electric motors need a changing magnetic field to work. Therefore hysteresis represents a limit on the efficiency of the motor. The level of hysteresis is an important characteristic to consider when selecting a motor because it affects the efficiency of the motor.
Armature Loss
- Within AC electric motors there are wires wrapped around the rotor and the stator. The stator is the part of the motor that remain stationary, and the rotor is the part that rotates. These wires carry an electric current and exhibit resistance to this current. Because the wires exhibit resistance, they also dissipate power as heat. The formula for the relationship between power, current and resistance is P = R.I^2. This represents a loss of efficiency in the motor because power dissipated as heat in the wires within the motor is not converted into rotation. This loss of power is called armature loss.
Bearing Friction
- The bearings are those parts of the motor that connect the rotor to the stator. When the motor is operational and rotating friction is generated in the bearings. As the motor increases its rotational speed the amount of friction generated in the bearings increases proportionally. Bearings have to be carefully designed to minimize the amount of energy wasted in the bearings. Bearing friction is another important characteristic to consider when selecting an AC motor.

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