To determine the equivalent circuit of a 3- ɸ induction motor and calculate various parameters of induction motor with the help of circle diagram.

Aim

To determine the equivalent circuit of a 3- ɸ induction motor and calculate various parameters of induction motor with the help of circle diagram.

Apparatus Required:

Name plate details:-

Power rating : 5Hp

Voltage : 400V

Current : 6.8A

Speed (RPM) : 150

Frequency : 50Hz

PF : Lagging

3- ɸ Auto transformer Details:

Input Voltage: _____415_____________ (Volt)

Output Voltage: ___0-470_____________ (Volt)

Current:__________________ (Amp)

Circuit Diagram:

Procedure:-

No Load Test:

1. Connections are made as per the circuit diagram
2. Ensure that the 3- ɸ variac is kept at minimum output voltage position and belt is freely suspended.
3. Switch ON the supply. Increase the variac output voltage gradually until rated voltage is observed in voltmeter. Note that the induction motor takes initially, so, keep an eye on the ammeter such that the starting current current should not exceed 7 Amp
4. By the time speed gains rated value, note down the readings of voltmeter, ammeter, and wattmeter
5. Bring back the variac to zero output voltage position and switch OFF the supply.

Blocked Rotor Test:

1. Connections are as per the circuit diagram.
2. The rotor is blocked by tightening the belt.
3. A small voltage is applied using 3- ɸ variac to the stator so that a rated current flows in the induction motor.
4. Note down the readings of Voltmeter, Ammeter and Wattmeter in a tabular column.
5. Bring back the Variac to zero output voltage position and switch OFF the supply.

Observation:

No Load Test

Blocked Rotor Test

Measurement of stator winding resistance (r1):

Circuit Diagram:

Blocked Rotor Test

Procedure to find r 1:

1. Connections are made as per the circuit diagram
2. Switch ON the supply. By varying the rheostat, take different readings of ammeter and voltmeter in a tabular column.
3. From the above readings, average resistance r1 of a stator is found

Measurement of Stator resistance:

1. Connect the circuit as per the circuit diagram shown in fig (2).
2. Keeping rheostat in maximum resistance position switch on the 220 V Dc supply.
3. Using volt-ammeter method measure the resistance of the stator winding
4. After finding the stator resistance, Rdc must be multiplied with 1.6 so as to account for skin effect i.e. Rac = 1.6 Rdc.

Model Calculation:

G=W₀/3V ₂, Y₀=I₀/V, B₀=Y₀²-G₀²

Z₀₁=V_SC/I_SC, R=W_SC/3_XI_SC², X ₀₁=√Z₀₁²-R₀₁²

For circle diagrm:

cosΦ ₀= W₀/√3 V₀I₀, Φ₀ = cos-1(W₀/√3 V₀I₀)

cosΦ_SC =W_SC/√3 V_SCI_SC, I_SN=I_SC(V/V_SC);

Precautions:

1. Connections must be made tight
2. Before making or breaking the circuit, supply must be switched off
3. Using volt-ammeter method measure the resistance of the stator winding

Result:No load and blocked rotor tests are performed on 3-Φ Induction motor.

Via Questions:

1. Explain why the locus of the induction motor current is a circle.
2. What is the difference between the transformer equivalent circuit and induction motor equivalent circuit
3. What are the reasons in conducting no-load test with rated voltage and blockedrotor test with rated current?
4. Why do you choose LPF wattmeter in load test and hpf wattmeter in blocked rotor test?
5. How do you reverse the direction of rotation of induction motor?
6. What are the various applications of this motor?