To draw the equivalent circuit of a 3 phase induction motor and to
obtain its performance characteristics by conducting No-load and
blocked rotor tests.
NAME PLATE DETAILS:
NAME PLATE DETAILS OF 3ɸ SQUIRREL CAGE INDUCTION MOTOR
KW Rating
5 HP/ 3.5 KW, 0.86pf
Voltage
415 V AC, 50 Hz
Current
7 A
Speed
1440 RPM
Winding type
Squirrel Cage
starter Resistance
5.8 Ω (R-N)
Brake Drum Radius: 0.1 M
APPARATUS REQUIRED:
S.No.
Name of the Equipment
Range
Quantity
Type
1.
Voltmeter
500 VAC
1 No
Digital
2.
Ammeter
20 A
1 No
Digital
3.
Wattmeter
150/300 V 20 A
1 No
Digital
4.
Three phase variac
0 - 415V
20 A
1 No
Core
5.
Tachometer
2000 Rpm
1 No
Digital
INTRODUCTION:
An induction machine can be viewed as a generalized transformer
where the rotor (secondary) voltage and frequency both vary, both of
them being directly proportional to the rotor slip where,
The mechanical power developed is the difference between the air gap
power and the rotor copper loss.
The load resistance represents the mechanical power developed. The
equivalent circuit parameters can be determined by No load test and
Blocked – Rotor test.
NO LOAD TEST::
At no load the machine runs freely with a very small slip which is
sufficient to provide power for windage and friction loss, core loss and also
a certain amount of stator copper loss which has to be accounted for
because of relatively larger no load current (as compared to a transformer).
With low slip, (ℝ_{2}’/
s) > > 𝕏_{2}’ and hence the rotor circuit is literally open
making the rotor current zero.
With these approximations the necessary formula for finding out \(X_m\) will
be given later under procedure for conducting the no – load test
BLOCKED ROTOR TEST:
Blocking the rotor from rotating is equivalent to making s = 1 so that,
R2^{′}(1/s−1)=0
Mechanical power developed is zero at this point.
This test is conducted by applying a low voltage to the stator so as to limit
the current drawn to its full load value.
At this reduced voltage, core loss can be neglected (because flux is low as
[V/f] is low); but the effect of \(X_m\) has to be taken into account as its value
is small as compared to that of a transformer of comparable rating.
CIRCUIT DIAGRAM FOR NO LOAD TEST:
EM LAB SET-UP:
CONNECTION FOR NO-LOAD TEST:
3ɸ AC Supply R to DOL Starter Input R
3ɸ AC Supply Y to DOL Starter Input Y
3ɸ AC Supply B to DOL Starter Input B
DOL Starter Output R to Multi-Function Meter Input R
DOL Starter Output Y to Multi-Function Meter Input Y
DOL Starter Output B to Multi-Function Meter Input B
Multi-Function Output R to Induction Motor U1
Multi-Function Output Y to Induction Motor V1
Multi-Function Output B to Induction Motor W1
Connect U1 to W2, V1 to U2 and W1 to V2
PROCEDURE FOR NO-LOAD TEST:
Connect the circuit diagram.
Ensure that the motor is unloaded and switch on MCB, start the
motor with the help of Direct On Line (D.O.L) starter.
Motor starts to run at rated speed note down the No-load Voltage
(Vo), Current (Io), Power (Wo) from the multi-function meter.
Push OFF button (Red button) in the D.O.L starter, switch off the
MCB.
Measure R1 the stator resistance per phase using multimeter.
OBSERVATION TABLE:
S.No.
V_{0}(V)
I_{0}(A)
W_{0}(three phase)
MODEL CALCULATION:
NO LOAD TEST:-
To obtain no load current and its p.f angle \(\phi_0\), no load test is performed at rated voltage and frequency. Let the readings of ammeter, voltmeter and wattmeter connected in circuit be \(\mathrm{I}_0, \mathrm{~V}_0 \) and \(\mathrm{W}_0\) respectively during no load test.
\( \mathrm{W}_0\) is the 3-phase power consumed by the motor which includes stator copper losses, iron loss and windage and friction loss. However, rotor copper and core losses are negligible.
3ɸ Auto VARIAC Output Terminal R to Multi-Function Meter Input R
3ɸ Auto VARIAC Output Terminal Y to Multi-Function Meter Input Y
3ɸ Auto VARIAC Output Terminal B to Multi-Function Meter Input B
Multi-Function Output R to Induction Motor U1
Multi-Function Output Y to Induction Motor V1
Multi-Function Output B to Induction Motor W1
Connect U1 to W2, V1 to U2 and W1 to V2
PROCEDURE FOR BLOCKED ROTOR TEST:
Connect the circuit diagram.
Block the rotor by tightening the belt.
Switch ON the MCB and apply the voltage gradually by increasing the output of the 3ɸ VARIAC so that the current drawn by the motor under blocked rotor condition is equal to the full load current of the motor.
Record the reading of Blocked rotor Voltage (\(V_{BRL}\)), Current (\(I_{BRL}\)), Power (\(W_{BRL}\)) from the multi-function meter.
Unload the motor by loosening the belts, reduce the applied voltage to zero by varying the three-phase VARIAC and switch OFF the MCB.
OBSERVATION TABLE:
S.No.
V_{BRL}(V)
I_{BRL}(A)
W_{BR} (three phase)
Stator resistance can be measured using a multimeter as \(R_m\)
If the stator is a star connected one, then, \(R_1\) per phase = \(\frac{R_m}{2}\)
If the stator is a delta connected one, then \(R_1\) per phase = \(\frac{3}{2} \cdot R_m\)
However, the above value is DC resistance. \(R_{\text{dc}} = \frac{3}{2} \cdot R_m\)
To compute AC resistance \(R_1 = R_{\text{ac}} = 1.3 \cdot R_{\text{dc}}\)
In the above circuit the stator is connected in delta
Motor equivalent impedance referred to stator \(Z_{\mathrm{BR}}\) per phase = \(\frac{V_{\mathrm{BR}}}{I_{\mathrm{BR}}}\)
Motor equivalent resistance referred to stator \(R_{\mathrm{BR}}\) = \(Z_{\mathrm{BR}} \cos(\varphi_{\mathrm{BR}})\)
= \(\frac{W_{\mathrm{BR}}}{3 I_{\mathrm{BR}}^2}\)
\(R_{\mathrm{BR}} = R_1 + R_2'\) = Sum of series resistances in the stator and rotor (referred to stator)
Motor equivalent reactance referred to stator \(X_{\mathrm{BR}}\) = \(\sqrt{Z_{\mathrm{BR}}^2 - R_{\mathrm{BR}}^2}\)
\(X_{\mathrm{BR}} = Z_{\mathrm{BR}} \sin(\varphi_{\mathrm{BR}}) = X_1 + X_2'\) = Sum of leakage reactances of the stator and rotor (referred to stator)
Rotor resistance referred to stator \(R_2' = R_{\mathrm{BR}} - R_1\)
Rotor reactance referred to stator \(X_2' / 2 = X_1\)
Total stator side current \( I_1 \) = Vectorial addition of no-load and rotor current
\( I_1 = I_0 \angle -\phi_0 + I_2' \angle -\phi_2 = I_1 \angle -\phi_1 \)
Input power \( 3V_{\text{ph}} I_1 \cos \phi_1 = P_{\text{in}} \)
Torque \( T = \frac{P_{\text{out}}}{\omega} \) where \( \omega = \text{rotor speed in radians per sec} = \frac{2\pi N}{60} \)
OBSERVATION TABLE:
SpeedN
N_{s}
0.99N_{s}
0.98
N_{s}
0.97N_{s}
0.96N_{s}
0.95N_{s}
0.9 N_{s}
0.7
N_{s}
0.5
N_{s}
0.3
N_{s}
0.1
N_{s}
0
Slip ‘s’
0
0.01
0.02
0.03
0.04
0.05
0.1
0.3
0.5
0.7
0.9
1
Z_{2}’=
(R_{2}’/s)
+ jX_{2}’
I_{2}’
P_{g}
P_{cu}
P_{out}
I_{1}
Pin
Efficiency η
PF Cosϕ_{1}
Torque T
Equivalent circuit of the induction motor under test:
GRAPHS::
Draw the graphs between
Line current (\(I_1\)) vs. Slip
Torque vs. Slip and rotor speed
Power factor vs. Slip
% efficiency (\(\eta\)) vs. Slip
% efficiency (\(\eta\)) vs. \(P_{\text{out}}\)
RESULT::
Complete the performance
characteristics of the given
3 – phase Induction Motor
LOAD TEST OF INDUCTION MOTOR:
OBJECTIVE FOR LOAD TEST
To obtain the performance characteristics of 3 – phase induction motor by
Load test and by loss summation method
CIRCUIT DIAGRAM FOR LOAD TEST
INTRODUCTION:
On no load, the power factor of an induction motor is very low, and it slowly improves with the load and attains a value around 0.85 on full load.
Efficiency starts from zero on no load; increases with load, reaches a maximum at about 80% of rated load and then starts decreasing.
THEORY:
The a.c. motors are classified as single and three phase induction motors, synchronous motors, and some special purpose motors.
Out of all these types, three phase induction motors are widely used for various industrial applications.
The important advantages of three phase induction motors over other types are self-starting property, fairly good power factor, good speed regulation, and robust construction.
The working principle of three phase induction motors is based on the production of rotating magnetic field.
CONNECTION:
3ɸ AC Supply R to DOL Starter Input R
3ɸ AC Supply Y to DOL Starter Input Y
3ɸ AC Supply B to DOL Starter Input B
DOL Starter Output R to Multi-Function Meter Input R
DOL Starter Output Y to Multi-Function Meter Input Y
DOL Starter Output B to Multi-Function Meter Input B
Multi-Function Output R to Induction Motor \( U_1 \)
Multi-Function Output Y to Induction Motor \( V_1 \)
Multi-Function Output B to Induction Motor \( W_1 \)
Connect \( U_1 \) to \( W_2 \), \( V_1 \) to \( U_2 \), and \( W_1 \) to \( V_2 \)
PROCEDURE:
Connect the circuit as per the circuit diagram.
Switch ON the MCB and start the induction motor with the help of the D.O.L starter.
Induction motor starts to rotate at rated speed.
Note down the readings of wattmeter, ammeter, and voltmeter on no-load.
The induction motor current is increased by tightening the brake drum till the rated current is attained. At each current, Voltage (V), Current (IL), Power input (Pin), Speed (N), S1 & S2 (spring balance readings) are noted.
Reduce the load on the motor and finally unload it completely, then press the starter ‘stop’ button and turn off the MCB.
Note down the effective diameter of the brake drum in meters (R).
OBSERVATION TABLE:
Constant losses
\( P_{\text{const}} = \text{No load input power} – \text{corresponding input power during no-load test} \)
M. F. = Multiplication factor of Wattmeter = \( \left(\frac{VI \cos \varphi}{\text{FSD}}\right) \)
FSD = Full scale divisions
S.NO.
V
IL
PIN
N
Spring Balance
T
PO
Slip
PF
% η
S1
S2
S1-S2
MODEL CALCULATIONS:
$$
\begin{aligned}
\text{Input power drawn by the motor} (\text{Pin})&=\text{W} \\
\text{Torque} (\text{T})&=9.81(\text{S}_1-\text{S}_2) \text{R} \text{N-m} \\
\text{Output power in watts} (\text{P}_0)&=2 \pi \text{N} \text{T}_{\text{sh}} / 60 \text{Watts} \\
\text{Efficiency} \eta&=\text{P}_0 / \text{P}_{\text{in}} \\
\text{Slip}&=\left(\text{N}_{\text{s}}-\text{N}\right) / \text{N}_{\text{s}}\left[\right. \text{where} \left.\text{N}_{\text{s}}=(120 \text{f}) / \text{p}\right]; \text{p being number of poles} \\
\text{Power factor of the induction motor} (\text{PF}) &= \operatorname{Cos} \phi_1=\text{P}_{\text{in}} / \sqrt{3} \text{VI}_{\text{L}}
\end{aligned}
$$
MODEL GRAPH:
Draw the graphs between
% efficiency (\(\eta\)) Vs Output
Torque Vs Output
Power factor Vs Output
% Slip Vs Output
Torque Vs Slip ( on a separate sheet )
Result:
LOAD
EFFICIENCY
PF
SLIP
LINE CURRENT
1/4
1/2
3/4
FULL LOAD
QUESTIONS:
What are the methods of starting induction motor?
What are the types of induction motor?
Why is a cage motor preferred as compared to the slip ring motor?
What are the differences in characteristics obtained from load test and performance prediction using equivalent circuit?