Air Gap Values while designing Machine

Air Gap in Electrical Motors: A Necessary Evil

In the design of electrical Motos, the air gap is often referred to as a necessary evil. While it cannot be avoided, it must be minimized as much as possible. If there were no air gap, motors would not start. At the same time, too large a gap severely reduces performance.

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Why Air Gap Matters in Motors

In electromagnetic devices, materials are chosen to offer low resistance (reluctance) to magnetic flux, which reduces the electrical energy required to establish the flux.

However, in rotating machines, an air gap is unavoidable between stator and rotor. Air has very high reluctance compared to iron, so:

• A larger air gap requires higher magnetizing current.

• More electrical energy is needed to produce the required flux.

• It causes undesirable electrical losses.

This is why careful optimization of the air gap is crucial in machine design.

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Rule of Thumb

• Higher motor speed → Larger air gap.

• Slower motor → Smaller air gap.

This compromise balances mechanical safety (too small a gap risks rotor-stator contact) and electrical efficiency (too large a gap reduces power factor and efficiency).

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Why Air Gap Should Be Small

Let’s take the case of an induction motor:

• When supply is given to the stator, flux is induced in the rotor.

• The flux crosses the air gap, which has high reluctance.

• For every unit length of flux path, the mmf requirement in the air gap is much higher than in iron.

👉 Therefore, magnetizing current depends heavily on the air gap size.

Impacts of larger air gap in induction motors:

• Lower power factor.

• Lower efficiency.

• High no-load current (30–40% of full load current).

In contrast, transformers (no air gap) have much lower no-load current for the same rating.

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Air Gap in Different Machines

• Induction Motors → Air gap kept as small as possible (to reduce magnetizing current).

• Synchronous & DC Machines → Larger air gaps are required.

  • Here, the armature magnetic field distorts the main DC field (armature reaction).

  • Larger gaps reduce this effect but sacrifice some efficiency.

This is why synchronous and DC machines have air gaps several times larger than induction motors.

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The Design Contradiction

• Mechanical Requirement → Larger air gap avoids rotor-stator collision and ensures mechanical safety.

• Electrical Requirement → Smaller air gap reduces reluctance, improves efficiency, and lowers no-load current.

Thus, designers must always find an optimum compromise.

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Formula for Air Gap Calculation

A practical formula used for estimating required air gap:

$$\text{Air gap (inches)} = 0.005 + 0.0003D + 0.001L + 0.003V$$

Where:

• $D$ = Rotor outer diameter (inches)

• $L$ = Core stack length (inches)

• $V$ = Rotor peripheral velocity (thousand ft/min)

  $$V = D \times \frac{RPM}{12,000}$$

⚡ Air gap must always be uniform. A non-uniform gap leads to vibrations, noise, and further losses.

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Typical Values of Air Gap

• For standard motors (0.75 kW – 750 kW): 0.2 mm to 5 mm.

This small but critical range directly impacts motor performance, efficiency, and reliability.

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✅ Key Takeaway:
Air gap cannot be eliminated, but minimizing it within safe limits is the cornerstone of efficient motor design. It’s the balancing act between mechanical safety and electrical efficiency that makes the air gap a true necessary evil.

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