# Magnetisation Curve (Synchronous Machines)

Hi All,

Today I conducted a lab in which we plotted the open circuit voltage (V[oc]) and closed circuit current (I[sc]) versus the excitation current in the rotor (I[x]).

From the data (plots are attached to thread), it can be seen that saturation occurs in the V[oc] versus I[x] curve however the I[sc] versus I[x] curve remains linear.

Why is it that saturation occurs only with V[oc] and not with I[oc] in the stator?

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gerbi
Gold Member
Open-circuit curve (stator voltage vs excitation current) test is performed (or calculated) at zero stator current and 0-1,2 of rated stator voltage. In this case saturation of magnetic circuit is clearly visible.

Short-circuit curve (stator current vs excitation current) test is performed at shorted stator terminals 0-1.0 of rated stator current. Stator voltage (internal stator voltage) is quite low during the test, so you wont observe effects of saturation.

So why saturation is dependent on voltage ? Remember that equation: E=4,44*f*ψ*z
where: E-voltage (stator), f-freuqency, ψ-magnetic flux, z-number of turns

Frequency is static, numer of turs also. Only voltage and flux chanches, so: E≈ψ
The higher voltage, the higher flux is required. Higher flux means higer saturation.

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