By conducting the load test on three phase induction motor, the performance of the motor viz. slip, power factor, input, efficiency etc. at various loads can be studied.
The induction motor is located by any of the following methods :
1. Brake test
2. By connecting a d.c. generator
In case of loading by connecting a d.c. generator, the induction motor is connected to a d.c. generator. The generator is loaded by a lamp bank. Thus inturn an induction motor is loaded. The Fig. 1 shows the experimental set up for conducting load test on three phase induction motor using a d.c. generator.
 |
| Fig. 1 Load test on three phase induction motor |
On induction motor side, ammeter reads line current and voltmeter reads line voltage VL. The two wattmeters are connected as per the two wattmeter method hence,
Pin = Power input = W1 + W2
On generator side, the ammeter reads load current and voltmeter reads terminal voltage Vt.
By varying the lamp bank, load on generator i.e. load on induction motor can be varied. The induction motor can be star or delta connected and can be squirrel cage or slip ring type. The speed readings are taken using tachometer. The load is increased till induction motor carries rated line current. The following observation table is prepared,
Calculations : The output of induction motor is input to a d.c. generator.
Output of d.c. generator = Vt x IL W
Assume ηgen = 80 %
... Pout of induction motor = Pin of d.c. generator
= Pout of d.c. generator / ηgen = (Vt IL )/ηgen W
Pin of induction motor = W1 + W2 W
cos Φ = Pin/(√3VL IL )= (W1 + W2)/(√3VL IL) = power factor
where Ns = 120f / P for a given motor
For various loads above parameters are obtained.
As the load on the induction motor increases,
1. The output of motor increases.
2. The power factor increases.
3. The efficiency increase upto certain load and then decreases.
4. The speed decreases marginally.
5. The slip increases.
6. The input current increases.
The various performance characteristics can be obtained as shown in the Fig. 2.
Output of d.c. generator = Vt x IL W
Assume ηgen = 80 %
... Pout of induction motor = Pin of d.c. generator
= Pout of d.c. generator / ηgen = (Vt IL )/ηgen W
Pin of induction motor = W1 + W2 W
cos Φ = Pin/(√3VL IL )= (W1 + W2)/(√3VL IL) = power factor
where Ns = 120f / P for a given motor
For various loads above parameters are obtained.
As the load on the induction motor increases,
1. The output of motor increases.
2. The power factor increases.
3. The efficiency increase upto certain load and then decreases.
4. The speed decreases marginally.
5. The slip increases.
6. The input current increases.
The various performance characteristics can be obtained as shown in the Fig. 2.
 |
| Fig. 2 |
The graphs indicate the behaviour of various performance parameters against output of the induction motor and not shown to the scale.
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- Circle Diagram of a 3 Phase Induction Motor
- No Load Test
- Blocked Rotor Test
- Construction of Circle Diagram
Downloads
Examples on Circle Diagram
According to the experiment, if load increases, efficiency also increase upto an extent and then decreases why!?
ReplyDeleteAccording to the experiment, if load increases, efficiency also increase upto an extent and then decreases why!?
ReplyDeleteBecause at the hihgest point it is 70% Percent loading ,which is the optimal loading for 3 phase induction motor
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