PERSENTATION ON SLIP TEST ONSALIENT POLE SYNCHRONOUS MACHINE
Slip test is performed on salient pole synchronous machine to determine the direct axisreactance and quadrature axis reactance.As in a salient pole synchronous machine , air gap is not constant ; like in cylindrical rotortype rotor ; reactance around quadrature axis (Xq) is different from reactance along directaxis (Xd). Normally Xq < XdReactance depends upon air gap. Salient Pole type synchronous machine Non Salient Pole (cylindrical) type induction machine
The synchronous machine is driven by a separate prime-mover (or motor) at a speed slightlydifferent 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
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 correspondingarmature 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 interpolarregion 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 correspondingarmature current per phase gives q-axis synchronous reactance (Xq)
When armature m.m.f. wave is along the direct axis , the armature flux passing throughopen 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 .
If oscillograms can’t be taken , then an ammeter and a voltmeter can be used across themachine to get desired values of voltages and currentsWhile performing the test , the slip(Ns – Nr) should be made as small as possible toavoid 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