SIDDHARTHA INSTITUTE OF ENGINEERING & TECHNOLOGYEEE Department – EM-II LAB

Experiment No: 5 III/IV EEE (I Semester) Page

CIRCLE DIAGRAM OF 3-Φ INDUCTION MOTOR FROM NO LOAD & BLOCKED ROTOR TESTS.

Aim : - To draw the Circle diagram of a 3-Phase Induction motor.

Apparatus: -S.No. Meter Range Type Quanti

ty1. Voltmeter 0-600V MI 12. Ammeter 0-10A MI 1

3. Wattmeter

150/300/600V-5/10A-UPF

Dynamometer 2

Name Plate Details: -Three-Phase Induction Motor

Volts : 415 volts. Amps : 7.8 Amps.P.H./ Cycle : 3/50. Rating : Cont.KW/HP : 3.75/5 HP Connection : Delta.RPM : 1440Circuit Diagram: -

SIDDHARTHA INSTITUTE OF ENGINEERING & TECHNOLOGYEEE Department – EM-II LAB

Experiment No: 5 III/IV EEE (I Semester) Page

Theory: -The locus of the stator current of an induction motor is a circle under

certain reasonably valid assumptions. This locus may be dawn using the test data obtained from the no-load and blocked-rotor test and the value of stator resistance.(a) No-Load test: - If the motor is run at rated voltage and frequency without any

mechanical load, it will draw power corresponding to its no-load losses. The current drawn (No-Load current) will have two components, the active component and the magnetizing components, the former being very small as the no-load losses are small. The power factor at no load is, therefore, very low. No-load test gives one point on the current locus.

(b) Blocked-rotor test: - This test affords a second point on the current locus, and is analogous to the short-circuit test on a transformer. The stator is supplied with a low voltage of rotated frequency, and, with the rotor blocked (i.e., physically prevent from moving) the power input, current input and voltage applied are recorded. The data when converted to rated voltage gives the short-circuit current and the power factor.

The power input during blocked-rotor test is almost wholly consumed in the stator and rotor copper losses. From the short-circuit current and the power input, therefore, the total equivalent resistance of the stator and rotor can be obtained.

From this if the stator resistance (which may actually measured) is subtracted, the remainder is the rotor resistance referred to stator.

If R is the D.C. resistance of the stator, its A.C. resistance or effective resistance will usually be higher on account of the skin effect. Usual range of effective resistance is 1.05 to 1.2 times D.C. resistance, the exact value depending upon the frequency and conductor size.

From the values of the no-load and blocked-rotor currents at rated voltage, the corresponding power factors and the equivalent stator and rotor resistances, the complete circle diagram may be drawn.

With V1 as reference, OA is plotted equal to no-load current I0 at an angle Ø0 lagging, where Cos Ø0 = W0 / (√3V0I0), and W0 is the no-load power input at rated voltage and frequency.

The blocked –rotor test-data is converted to values that would have been obtained if this test were performed at rated voltage. If VS is the voltage at which blocked-rotor rest is performed, IS the current input, and PS the power input, the values converted to rated voltage V would be

Ise = IS (V /VS) Pse= PS (V/VS)2

Cos Øse = PS / (√3VS IS) = Pse / (√3VIse)

SIDDHARTHA INSTITUTE OF ENGINEERING & TECHNOLOGYEEE Department – EM-II LAB

Experiment No: 5 III/IV EEE (I Semester) Page

OC equal to Ise at an angle Øse is now drawn. A and C are then two points on the current locus, of which AC is a chord. The line AL is drawn at right angle to OV through A. The point X, where the right-bisection of chord AC meets AL, is the center of locus-circle and XA is the radius.

CF, which is equal to Isc Cos Øse, represents to a certain scale, the power input to the motor √3VIse Cos Øse (If on the current scale 1cm = x amperes, and V is the rated line voltage, the power scale is 1 cm = √3V x watts, for a 3-phase motor). The power intake under blocked-rotor –conditions is consumed as no.load losses (assumed constant), stator copper loss and rotor copper loss. Ef, which is equal to I0 Cos Ø0, represents on the power scale, no-load loss √3 I0 Cos Ø0. The balance of the power equal to CE is, therefore, the copper loss. If D divides CE such that CD/DE= R2

1/ R1 where R21 is the rotor resistance referred to phase,

perphase, and R1 is the effective stator resistance per phase, than CD= rotor copper loss at standstill, and the output line, and line AD the torque line. For any operating point P on the circle diagram, the entire performance may be obtained by drawing a perpendicular PT. For this operating condition,

Input current = OPPower output = PQRotor copper loss = QRStator copper loss = RSFriction, windage and iron losses = STPower input = PTTorque in synchronous watts = PRSlip = rotor copper loss/ rotor input =QR/PRPower factor = PT/ OP

SIDDHARTHA INSTITUTE OF ENGINEERING & TECHNOLOGYEEE Department – EM-II LAB

Experiment No: 5 III/IV EEE (I Semester) Page

Procedure: -1. Connect the circuit as per the circuit diagram.2. Apply full voltage to the motor terminals and the rotor is

made to run at no load speed.3. Note down the readings of voltmeter, Wattmeter and

ammeter.4. Now block the rotor to conduct blocked rotor test.5. Block rotor by a belt and apply 20 % of full load voltage and

is so adjusted such that full load current flows in the stator.6. Note down the meter readings.

Observation: -

No Load Test Blocked Rotor TestV0

(Volts)I0

(Amp)W0

(Watt)VS

(Volts)IS

(Amp)WS

(Watt)

Model Graph:-

Result: - Circle diagram is plotted from the readings.