No-load Test on 3-ph Squirrel Cage Induction Motor and Determine the Equivalent Circuit Parameters

Experiment No.: 3

Experiment Name:

No-load Test on 3-ph Squirrel Cage Induction Motor and Determine the Equivalent Circuit Parameters.

Objectives:

To conduct no-load test on 3-ph Squirrel Cage Induction Motor
To determine the equivalent circuit parameters

Theory:

In the no load test, the machine is run at no load, the slip is then quite small and hence the total rotor resistance (R₂’ / s) becomes quite large Due to this: we can ignore the rotor current I₂’ and say that the rotor (i.e. the secondary side of the machine) is open circuited. The motor is operated at the rated voltage and the stator current, voltage, and power input are noted. Let the per phase values of these quantities be I₀, V₁, and P₀, respectively. Out of P₀, let the core losses be P_i, Thus, the approximate equivalent circuit then becomes as shown in the figure below, for the no load condition.

In this, it is true that the current Io flows through the stator impedance (R₁+jX₁), but since the no load current is quite small, the equivalent circuit in the figure above is acceptable. Thus from this equivalent circuit.

Thus, with the three observations during the no load test, the two parameters R₀ and X_m are calculated.

The two short circuiting keys K, and K, are to be closed while starting the motor. After the motor runs at normal speed, the keys are opened. This is necessary to avoid any damage to the low current ammeters and current-coils due to the heavy currents during starting.

Circuit Diagram:

Equivalent Circuit of Induction Motor under No Load:

Observation Table:

Sl. No.	Voltage (V₁)	Line Current (I₀)	Phase Current (I₀)	W₁	W₂	W₁+W₂ x M.F	cos φ = (W₁+W₂)/√3.V₁.I₀	sin φ = sin (cos^-1 φ)	I_w = I₀ cos φ	I_μ = I₀ sin φ	R₀ per phase = V₁/I_w	X_m per phase= V₁/I_μ
Unit	Volt	Amp	Amp	Watt	Watt	Watt			Amp	Amp	Ω	Ω
1.	401.2	1.07	0.617	-50	70	70-50= 20×4 = 80	0.107	0.994	0.066	0.613	3509.6	377.87
2.	397.5	1.08	0.623	-50	70	80	0.107	0.994	0.066	0.619	3477.22	370.75

Calculation: as per data in serial no. 1 in observation table:

I_w = I₀ cos φ = 0.617×0.107 = 0.066 A

sin φ = sin (cos^-1φ) = sin 83.86 = 0.994

I_μ = I₀ sin φ = 0.617×0.994 = 0.613 A

R₀ per phase = V₁/I_w = (401.2/√3)/0.066 = 3509.6 Ω

X_m per phase= V₁/I_μ = (401.2/√3)/0.613 = 377.87 Ω

Apparatus Used:

Sl No.	Name of Apparatus	Specification	Quantity	Makers name
1.	3 ph Squirrel Cage Induction Motor	2 HP, 415V, 50 Hz, 3 A, 1440 rpm with brake drum.	1	M.E.W.
2.	Clampmeter	0-200A AC	1	Metravi
3.	Digital Multimeter as Voltmeter	0-750 V AC, 0-1000 V DC, 0-10 A	1	Akademika
4.	Digital Tachometer	Non contact type	1	Metrix+
5.	Single phase Wattmeter	0-750-1500-3000 W, 0-150-300-600 V, 0-5-10 A,	2	MECO-G