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What role does air gap flux play in a motor?

Date:2023-11-09   Author:XINDA MOTOR
The air gap is the non-magnetic part of the magnetic circuit that is usually in series with the rest of the circuit so that a significant portion of the magnetic flux flows through the gap. The air gap can be filled with non-magnetic materials such as gas, water, vacuum, plastic, wood, etc., and not necessarily filled with only air.

Motor air gap

The stator and rotor are the two main components of the motor. They are magnetically coupled . Energy is transferred from both sides through the machine through the magnetic field. A small air gap can bring low noise and smaller unbalanced magnetic pull. The diffusion of magnetic flux into the surrounding medium due to increased air gap reluctance can lead to flux fringing effects, an unwanted phenomenon that increases proximity and eddy current losses in conductors located near the air gap. 

The air gap causes a reduction in the flux linkage between the rotor and the stator, which is considered a loss. Since the magnetic flux between the stator and rotor windings is reduced, thereby weakening the transfer of magnetic energy, a large air gap means more magnetizing current and a low power factor . The positive side of the large air gap has a better impact on the overload capacity of the motor.

The air gap may be an integral part of ensuring the correct performance of the device, it should be as small as possible, the form, shape and size of the air gap depends on the type and shape of the magnetic circuit, which is determined by the working principle, performance, size, efficiency and many other technologies factors determine. In rotating machinery , an air gap is usually not required due to the physical movement required between the stator and rotor, but it is unavoidable and the minimum practical air gap for  industrial machines is about 0.2 mm.

In relays, the air gap is usually an integral part that facilitates movement between the stationary parts (such as windings and cores) and the active armature, mechanically driving the connection or disconnection of the main electrical contacts. In traditional transformers, air gaps are usually avoided and the role of the transformer is to instantaneously transfer energy from the primary winding to the secondary winding without the need for energy storage.
Motor air gap

The air gap is the gap between the rotor and stator of a motor. A "free and open space" physically separates the two motor components. Since the rotor and stator are not magnetically coupled, there is an air gap between them. 

When properly supplied (depending on the type of machine), a magnetic field is established and connects the stator and rotor. Due to the air gap, some part of the magnetic field passes through the rotor or the stator, but not both. This part of the magnetic flux is called Leakage or air gap flux (because it passes through the air gap alone). This leakage flux plays no role in power transmission because it is not connected to either the stator or the rotor, and the current generated to establish the leakage flux causes a loss of power in the machine.

Both the stator and the rotor are made of magnetic materials (usually silicon steel). The higher the magnetic permeability of the core medium, the smaller the magnetic resistance, and the lower the magnetic permeability of air. The larger the magnetic resistance, the longer the air gap, and the lower the magnetic flux leakage. Larger results in less power. Therefore, the air band length is kept as short as possible to provide separation between the rotor and stator and provide the required mechanical balance of the machine. A free-spinning rotor reserves approximately 2 mm of clearance as a sacrificial loss of magnetic force, thereby enabling normal mechanical operation of the motor or generator.

In synchronous and DC motors, two independent magnetic fields interact in the air gap, and the AC magnetic field generated by the armature (stationary in the synchronous motor and rotating in the DC motor) distorts the DC magnetic field supply, reducing efficiency, Reducing motor performance, increasing the air gap will reduce the effect of "armature reaction", therefore the air gap of these machines will be several times larger than that of induction motors. 

In an induction motor, the electromotive force induced in the rotor winding is a mutually induced electromotive force. When the electromotive force induced in the rotor is generated by mutual induction, the induction motor can be regarded as a rotary transformer. If the air gap is larger, the leakage magnetic flux will be larger. larger, the smaller the mutual magnetic flux, thereby reducing the electromotive force, current and torque of the rotor.

In a synchronous motor, the magnetic flux is set individually through the field winding. The electromotive force induced in the stator armature winding is not generated by mutual induction, but is a dynamically induced electromotive force due to the relative motion between the magnetic field and the conductor. Therefore, it is not Consider air gaps. For salient pole motors, the air gap will be much larger in the area between the poles.

The air gap required to separate the rotating rotor from the stator in a generator should be as small as possible to reduce magnetizing power requirements, but should be large enough to prevent contact between the two, manufacturing tolerances in their dimensions, or due to mechanical deflection and Movement caused by loose support bearings.

in conclusion

In any case, the gap must be large enough to ensure that the eccentricity of the rotor relative to the stator does not cause the shaft stiffness to be affected by unbalanced magnetic pull forces, which could cause the rotor to strike the stator. Commonly used empirical calculations include rotor circumferential speed, core length and rotor diameter. Increasing the air gap will increase the magnetizing current and also reduce stray load losses.

There are few design guidelines for selecting the air gap size that is most suitable for any rotating machine. For induction motors, practical values of 0.2 to 5 mm are typical in the rated power range of 3/4 to 750 kW, the higher the motor speed. , the larger the gap.