Slip Test on Synchronous Machine

A Salient-pole Synchronous Machine’s Direct-axis and Quadrature-axis Synchronous Reactances are Measured using the slip test, a simple no-load test. The 3-phase stator winding of the Synchronous machine is Subjected to a low voltage at the Machine’s rated Frequency during this test. Unexcited and left open Circuited is the field winding.

Circuit Arrangement for Slip Test

The circuit Arrangement for slip test is shown in Figure-1.

An Auxiliary motor drives the rotor at a speed that is either slightly slower than or faster than the Synchronous speed. The Stator’s rotating magnetic field should rotate in the same direction as the rotation of the shaft. The voltmeter connected across the open circuited field winding terminals reads a very small voltage, which Indicates that the rotor is rotating in the correct direction.

The rotor is running at a speed (NR) close to the synchronous speed (𝑁𝑆) and there will be a small slip between the rotating magnetic field produced by the armature and the actual salient-field poles. Thus, the relative speed between the armature MMF and the salient field poles is equal to the slip speed (NS − NR).

Since the armature MMF moves slowly past the actual field poles, hence there will be an instant when the peak of the armature MMF wave is in line with the axis of the actual salient field poles as shown in Figure-2.

The axis of the field poles is direct axis or d-axis. In this position, the reluctance offered by the small air gap is minimum and this results in a minimum magnetising current 𝐼_𝑚𝑖𝑛 indicated by the line ammeter A.

In this position, the armature flux linkage with the field winding is maximum. And the rate of change of this flux linkage is zero. Hence, the voltage induced in the field winding is zero. Therefore, the d-axis can be located on the oscillogram as shown in Figure-4.

From this figure, the direct axis synchronous reluctance is given by,

𝑋𝑑= 𝑎𝑏/𝑐𝑑

After one-quarter of slip cycle the peak armature wave is in line with the quadrature axis. In this position, the reluctance offered by the larger air gap is maximum as shown in Figure-3.

A large magnetising current is needed to produce the same air gap flux because of the high reluctance. The line ammeter A also records the maximum current (Imax). The armature flux linkage with the field winding is also zero in this position,. And its rate of change is at its highest. The induced voltage across the field winding is therefore at its highest level. As a result, the q-axis can also be found on Figure 4’s oscillogram. The quadrature axis synchronous reactance is given by this figure:

𝑋𝑞= 𝑎′𝑏′/𝑐′𝑑′