Correct Answer - Option 2 : inversely proportional to air gap length
Air gap length in a synchronous machine:
- The length of the air gap is a very important parameter because it greatly affects the performance of the machine.
- The air gap in a synchronous machine affects the value of short circuit ratio (SCR) which can affect many other parameters.
The magnitude of magnetizing reactance is influenced by air-gap length as under:
Magnetizing reactance (Xar) = ω Lar
ω = angular frequency
Lar = magnetizing inductance
And, Lar ∝ (Nph2) / (air-gap reluctance)
- So, a synchronous machine with a large air-gap has a low value of Lar and Xar.
- In other words, the magnitude of magnetizing reactance (Xar) is inversely proportional to air-gap length.
- The armature leakage reactance Xal is usually treated as constant.
- As Xar plays a dominant role on synchronous reactance (Xs). So it can be stated that Xs is inversely proportional to air-gap length.
Advantages of larger air-gap:
- As reactance is decreasing with the larger air-gap the stability limit will be increased.
- The smaller value of inherent regulation.
- Better cooling and noise reduction.
- The value of synchronizing power will be higher.
- Less value of unbalanced magnetic pull.
Disadvantages of larger air-gap:
- Larger diameter and hence larger size.
- More amount of field mmf is required.
- Increase in overall cost.