Slip test on Synchronous Machine

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 ( X q ) is different from reactance along direct a xis ( X d ). Normally X q < X d 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. Salient pole rotor driven at a speed N r > N s Field Poles and rotating mmf of armature windings should be rotating in same direction t o avoid measurment of negative reactance. Both ends are connected to a CRO to get output Oscillograms .

4. At one instant , whe the peak of armature m.m.f is in line with field poles or direct axis, t he 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 ( X d ) 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 a rmature current per phase gives q-axis synchronous reactance ( X q )

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(N s – N r ) 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