The synchronous reactance of a synchronous motor is an important parameter that represents the combined effect of armature leakage reactance and the effect of armature reaction. Synchronous reactance (Xₛ) is the total reactance offered by the motor to the flow of alternating current.
\[
X_s = X_l + X_{ar}
\]
Table of Contents
Physical Meaning
In a synchronous motor:
- When current flows in the stator (armature), it produces its own magnetic field
- This field interacts with the main rotor field
- The combined effect behaves like an additional reactance
EMF Equation of a Synchronous Motor
The voltage equation per phase is:
V = E + j I_a X_s
Where:
- (V) = Terminal voltage
- (E) = Back EMF
- (I_a) = Armature current
- (X_s) = Synchronous reactance
Components of Synchronous Reactance
(i) Leakage Reactance (Xₗ)
- Due to leakage flux not linking both stator and rotor
- Depends on winding design
(ii) Armature Reaction Reactance (Xₐᵣ)
- Due to interaction of stator flux with rotor field
- Varies with load and power factor
Determination of Synchronous Reactance
It is determined using:
- Open Circuit Test (OCC)
- Short Circuit Test (SCC)