# In a synchronous machine, the magnitude of magnetising reactance is:

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In a synchronous machine, the magnitude of magnetising reactance is:
1. directly proportional to air gap length
2. inversely proportional to air gap length
3. directly proportional to air gap length squared
4. constant

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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.

Magnetizing reactance:

The magnitude of magnetizing reactance is influenced by air-gap length as under:

Magnetizing reactance (Xar) = ω Lar

Where,

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.

• 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.

• Larger diameter and hence larger size.
• More amount of field mmf is required.
• Increase in overall cost.