Air gap in rotating electrical machine is a coupling medium to transfer electrical energy from stator to the rotor or vice versa. It is also required for free rotation and proper cooling of the machine. Generally air gap length is kept as low as possible due to several reasons. But if we compare the air gap length of Synchronous Machine to that of Induction Machine, we notice that it is more in case of Synchronous Machine as compared to Induction machine.

**Why Air gap length is more in Synchronous Machine?**

Air gap length in synchronous machine is kept more as compared to induction machine due to the following reasons:

**To Increase the Stability Limit of the Synchronous Machine**

Let us understand how stability limit is increased by longer air gap length. Longer air gap length means more reluctance offered to the magnetic flux. More reluctance means lesser value of armature reaction flux and hence lesser value of armature reaction reactance (X_{ar}). Apart from this, armature leakage flux will also reduce due to longer air gap length. This in turn reduces the value of armature leakage reactance (x_{al}).

Thus the Synchronous Reactance (X_{s}) which is defined as the sum of armature reaction reactance and armature leakage reactance i.e. X_{s} = (X_{ar} + x_{al}) also reduces. The power flow in synchronous machine is given by power angle curve,

P = (E_{f}V_{t}Sinδ) / X_{s}

From the above power angle curve, the maximum power transfer capability of machine increases due to reduced value of synchronous reactance X_{s}. Since stability limit is the maximum possible power flow when the synchronous machine is operating with stability, therefore stability limit is increased by having longer air gap length.

**Smaller Value of Voltage Regulation**

The voltage regulation of synchronous machine becomes better with reduced value of synchronous reactance X_{s}. This can be easily understood by the following expressions:

**E _{f} = V_{t} + I_{a}(r_{a} + X_{s})**

where E_{f}, V_{t}, r_{a} and I_{a} are excitation emf, terminal voltage, armature resistance and armature current respectively.

Voltage Regulation = (E_{f} – V_{t}) / V_{t}

Therefore longer air gap length improves the voltage regulation of synchronous machine.

**Higher Value of Synchronizing Power**

Synchronizing power is inversely proportional to the synchronous reactance X_{s}. Therefore with increase in air gap length, the synchronous reactance decreases and hence synchronizing power increases.

In addition to the above mentioned reasons, longer air gap length also facilitates better cooling and reduction in noise.

It seems from the above mentioned advantages of longer air gap length that we can have any length of air gap in synchronous machine. But it is not true. Larger field mmf is required to set up working flux in the machine. This required more copper and hence size as well as cost increases. Also, we reduced value of synchronous reactance increases the level of fault current. Therefore, an optimum value must be chosen for efficient operation of machine.

**Why Air Gap Length is kept Minimum for Induction Machine?**

Well, till now we have discussed the requirement of longer air gap length in synchronous machine. The concept is though not applicable for induction machine. In Induction Machine (IM) effort is made the minimize the air gap length.

An Induction Motor works on the principle of Electromagnetic Induction. The induced EMF in the rotor winding or rotor bars is due to mutual induction caused from the rotating magnetic field produced by the stator winding. Thus, an IM can be treated as a Rotating Transformer as the EMF induced in the rotor is by Mutual Induction.

Longer air gap means increased reluctance and leakage flux. Therefore the value of air gap flux reduces. This in turn reduces EMF induced in the rotor bars, rotor current and torque. Apart from this, longer air gap length will also cause to have poor power factor and hence associated losses will also increase. In fact, power factor is inversely proportional to the air gap length for Induction Machine.

It must be clearly understood that IM is a self-excited machine and excitation power is drawn from the supply main only. But in Synchronous Machine, separately field excitation is provided. Therefore, increasing the air gap length do not affects the power factor rather degree of excitation affects the power factor of Synchronous Machine.

Could you please describe that why in Synchronous machines leakage is less when air gap is more? Ain’t even Synchronous machines rotate because of interaction between rotor and stator fluxes? If they have to interact then they need to do that in air gap(at a common location) and if air gap will be more, leakge reactance will be more. Then why do we say that in Synchronous machines leakage reactance decreases with longer air gaps?