PERSENTATION ONSLIP TEST ON

SALIENT POLE SYNCHRONOUS MACHINE

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 directaxis (Xd). Normally Xq < Xd

Reactance 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 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 Nr > Ns

Field Poles and rotating mmf of armature windings should be rotating in same directionto avoid measurment of negative reactance.

Both ends are connected to a CRO to get output Oscillograms.

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)

Oscillograms produced are shown below

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 .

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