Why Armature is place on Stator in Synchronous machines

Why Armature Winding is Placed on Stator and Field Winding on Rotor in Synchronous Machines?

In synchronous machines—both synchronous generators (alternators) and synchronous motors—the armature winding is always placed on the stator while the field winding is mounted on the rotor. This construction is not accidental; it offers clear technical and economic advantages.

Let us analyze these reasons step by step with practical examples.

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1. Better Economy of Construction

If the armature winding were placed on the rotor, slip rings would need to carry very high currents at high voltages, making the system bulky, costly, and inefficient.

Example:
Consider a 3-phase, star-connected, 500 MVA, 11 kV synchronous generator:

• Line current,

  $$I = \frac{500 \times 10^6}{\sqrt{3} \times 11 \times 10^3} = 26,244 \text{ A}$$

If this huge current were carried through slip rings, we would require 3 slip rings rated for 26.2 kA each, insulated for a line voltage of 6.35 kV. Additionally, a fourth slip ring would be needed to connect the star point to ground through a neutral resistance.

On the other hand, if the field winding is placed on the rotor, it only handles the DC excitation current at relatively low voltage (usually 100–500 V). Even for large machines, this current rarely exceeds a few thousand amperes. For instance:

• For a 2 MW field winding at 500 V,

  $$I_f = \frac{2000}{0.5} = 4000 \text{ A}$$

Here, only two slip rings are needed, insulated for 500 V only, which is far more economical and practical.

👉 Conclusion: Placing the field winding on the rotor reduces cost, complexity, and insulation requirements.

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2. Lower Insulation Requirements

When the armature winding is on the stator, its terminals are directly connected to the external power system without passing through slip rings.

• Slip rings now carry only low-voltage DC excitation instead of high-voltage AC armature current.

• This makes insulation simpler and allows construction of large synchronous machines up to 33 kV and beyond.

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3. Reduced Brush and Slip Ring Losses

If the armature were on the rotor, four large brushes would be needed to carry massive AC currents. This would increase:

• Contact losses

• Maintenance costs

• Heat generation

By placing the field winding on the rotor, only two brushes are required for the excitation system, handling far smaller currents. This results in lower losses and longer service life.

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4. Higher Output Power Capability

Since the rotor carries only the field winding, it is lighter and experiences less centrifugal stress. This enables:

• Higher operating speeds (higher RPM)

• More compact design

• Higher output power for the same machine size

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5. Mechanical and Thermal Advantages

• Armature winding on stator allows use of larger conductors and heavier insulation, since the stator is stationary.

• Cooling systems such as water jackets or hydrogen cooling can be installed more effectively on the stator than on a rotating rotor.

• This ensures better heat dissipation and higher reliability.

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6. Stronger Armature Tooth Strength

In high-current machines, slots must accommodate large amounts of copper. Cutting deep slots in a rotor weakens rotor teeth, making them prone to mechanical stress and vibration.

By placing the armature on the stator:

• Stator teeth can be made deeper and stronger,

• Resulting in reduced vibration, lower noise, and better mechanical stability.

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7. Lower Rotor Weight and Lighter Bearings

Since the rotor carries only the field winding (low copper content and simple insulation):

• The rotor becomes lighter,

• Bearings carry less load,

• Leading to cheaper construction and longer bearing life due to reduced wear and tear.

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Final Summary

Aspect	Armature on Rotor	Armature on Stator
Slip Ring Current	Very High (AC)	Low (DC excitation only)
Slip Ring Insulation	High (kV range)	Low (few hundred volts)
Losses	High (brush + slip ring losses)	Low
Rotor Weight	Very Heavy	Lighter
Cooling	Difficult	Easier
Mechanical Stress	High	Low
Output Capability	Limited	Higher

👉 Therefore, for technical, economic, and operational reasons, the armature winding is always placed on the stator, and the field winding is placed on the rotor in synchronous machines.

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Disclaimer

This article is intended for educational purposes and provides a general technical explanation of synchronous machine design. Practical design considerations may vary depending on machine size, application, and manufacturer-specific standards. Readers should consult standard references (e.g., IEEE, IEC) and manufacturer guidelines for detailed engineering applications.

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