Why Air Gaps Between Rotor and Stator Matter in Electric Motors

Why is there an air gap between the rotor and stator of an electric motor?

In an electric motor, it is essential to maintain a small air gap between the rotor and the stator to ensure smooth operation. This air gap allows the rotor to rotate freely within the stator chamber. While there is no direct electromagnetic connection between the rotor and stator, once the stator winding is energized, the rotor and stator are magnetically linked, similar to the principle of a transformer. This linkage facilitates the conversion of electrical energy into mechanical energy.

The size of the air gap has a significant impact on the performance and reliability of the motor. A larger air gap increases the magnetic reluctance, resulting in higher excitation losses, increased excitation current, and a decrease in the motor's power factor. This degrades the motor's performance. To minimize excitation current and improve the power factor, the air gap should be as small as possible.

However, if the air gap is too small, it can cause an increase in the harmonic magnetic field, which leads to higher stray losses and noise. This also reduces both the maximum torque and starting torque of the motor. Additionally, a small air gap may cause the rotor and stator to collide during operation, resulting in a phenomenon known as "scuffing," which complicates starting and reduces operational reliability. It also poses challenges during assembly.

For small asynchronous motors, the typical air gap ranges from 0.25 to 1.5 mm, while for medium-sized motors, it ranges from 0.75 to 2 mm. The gap between the rotor and stator is usually set during the manufacturing process and is not likely to change unless the rotor experiences issues such as scuffing, which could increase the gap. In general, a motor's large current draw can result from several factors, which can be summarized as follows:

1.Misalignment of the positioning magnet: Solution: Re-bond the magnet.

2.Uneven gap between the carbon brushes: Solution: Adjust the brush gap.

3.Short circuit between the positive and negative terminals: Solution: Eliminate the short circuit.

4.Inter-turn short circuit: Solution: Rewind the wire.

5.Short circuit in the commutator segments: Solution: Clean and polish the commutator.

6.Tight fit between the shaft and bearing: Solution: Grind the shaft.

7.Electric vehicle not starting: Solution: Avoid driving in such conditions.

8.Heavy load or high driving resistance: Solution: Assist with manual power.