This document discusses performing a slip test on a salient pole synchronous machine to determine the direct and quadrature axis reactances. [1] The test involves driving the machine at a speed slightly less than synchronous speed and measuring voltages and currents along the direct and quadrature axes. [2] This allows calculating the direct axis reactance Xd when the magnetic fields are aligned, and the quadrature axis reactance Xq when they are 90 degrees out of phase. [3] Oscilloscope measurements provide more accurate results than voltmeter-ammeter methods.

1. PERSENTATION ON
SLIP TEST ON
SALIENT POLE SYNCHRONOUS MACHINE

2. Slip test is performed on salient pole synchronous machine to determine the direct axis
reactance and quadrature axis reactance.
As in a salient pole synchronous machine , air gap is not constant ; like in cylindrical rotor
type rotor ; reactance around quadrature axis (Xq) is different from reactance along direct
axis (Xd).
Normally Xq < Xd
Reactance depends upon air gap.
Salient Pole type synchronous machine Non Salient Pole (cylindrical) type induction machine

3. The synchronous machine is driven by a separate prime-mover (or motor) at a speed slightly
different from synchronous speed .
The field winding is left open and a voltage of reduced magnitude and of rated frequency is
impressed across armature terminals.
Both ends are connected to a CRO to get output
Oscillograms.
Field Poles and rotating mmf of armature
windings should be rotating in same direction
to avoid measurment of negative reactance.
Salient pole rotor driven at a speed Nr > Ns

4. At one instant , whe the peak of armature m.m.f is in line with field poles or direct axis,
the reluctance offered by small air gap is minimum .
At that instant the impressed terminal voltage per phase divided by corresponding
armature current per phase gives d-axis synchronous reactance (Xd)
After one-quarter of slip cycle , the peak of armature m.m.f wave acts on the interpolar
region or q-axis and the reluctance offered by long air gap is maximum.
At that instant , the ratio of armature terminal voltage per phase to the corresponding
armature current per phase gives q-axis synchronous reactance (Xq)

5. Oscillograms produced are shown below

6. When armature m.m.f. wave is along the direct axis , the armature flux passing through
open field winding is maximum , therefore , induced field emf is zero.
When armature m.m.f is along q-axis , the armature flux linking the field is zero ,
therefore , the induced e.m.f is maximum.
The readings should be taken as per phase values .

7. If oscillograms can’t be taken , then an ammeter and a voltmeter can be used across the
machine to get desired values of voltages and currents
While performing the test , the slip(Ns – Nr) should be made as small as possible to
avoid large errors in measurments.
Oscillogram method is more oftenly used than voltmeter-ammeter method as:-
• Large slip speeds can be achieved
• Elemination of inertia effect