Load test on squirrel cage induction motors.

Experiment No.: – 02

Title of the Experiment: –

Objectives: –

To draw performance curve of induction motor and plot graphs output vs efficiency, torque vs speed, output vs slip, torque vs starter current.

Apparatus Required: –

Sl. No.	Name	Specifications	Quantity
1	Three phase Squirrel cage Induction Motor with Spring balance load	2 HP, 415V, 50Hz, 6A, 1440 rpm, 20kg spring balance load	1 no.
2	Three phase auto-transformer	(0-500) V	1 no.
3	Voltmeter	(0-500) V, MI	1 no.
4	Ammeter	(0-10) A, MI	1 no.
5	Wattmeter	(0-1500) W, EMMC	2 no.
6	Tachometer	Digital Type	1 no.
7	Connecting Wires	PVC Insulated Copper	As per required

Circuit Diagram: –

Theory: –

The load test on induction motor is performed to compute performance i. e torque, slip, efficiency, power factor etc. . During this test the motor is operated at rated voltage and frequency and slowly increase the spring balance load on motor pully.

Precautions: –

Ensure all connections are secure and insulated.
Avoid overheating the motor beyond its rated capacity.
Maintain proper ventilation to avoid overheating.

Procedure: –

Connect all equipment’s as per circuit diagram.
At starting keep three phase auto transformer at zero output voltage.
Now run the motor at rated voltage and speed with no load condition.
By increasing the load slowly record the readings of Ammeter, Voltmeter, Wattmeter, Speed and Load.
Increase the load till rated current of the motor and plot the graphs.

Observation Table: –

Sl. No.	V (in Volts.)	I (in Amps.)	W₁ (in Watts.)	W₂ (in Watts.)	Input power P_i= W₁ +W₂(in Watts.)	Load S₁ (in Kg.	Load S₂ (in Kg.	W = S₁-S₂ (in Kg.)	Speed N (in rpm)	Output power P₀ (in watts.)	Torque (in N-m)	% Efficiency Ƞ= (P₀/P_i)* 100	Slip S= (N_s-N_r)/N_s
1
2
3
4
5
6
7

Calculation: –

\[\color\green{(i)\; Input\; power \;to \;the\; squirrel\; cage\; Induction \;Motor}\] \[\color\red{P_i\;=\;\sqrt{ 3}*V*I*cos\;\phi}\] \[\color\red{= \;W_1\;+\;W_2}\] \[\color\green{(ii) \;Torque \;(T)\; =}\;\color\red{ (S_1-S_2)*9.8*r}\] \[\color\green{Where,}\] \[\color\magenta{S_1 \;and\; S_2 \;are\; spring \;balance \;load \;in \;Kg.}\] \[\color\magenta{r \;is \;the\; effective \;radius \;of \;the\; pully\; in\; m.}\] \[\color\green{(iii)\; Output\; power\; (P_0) \;=\;}\color\red{ \dfrac{2\pi*N*T}{ 60} \;watt.}\] \[\color\green{(iv) \;Slip \;(s)\; = \;}\color\red{\dfrac{N_s-N}{ N}}\] \[\color\green{where, }\] \[\color\red{N_s\;=\; \dfrac{120*f}{p}}\] \[\color\green{p \;is \;the\; no.\; of\; poles.}\] \[\color\green{(v) \;Efficiency\; of\; motor\; =\;}\color\red{\dfrac{output\; power}{ input\; power} * 100}\]

Plot graphs:-

Efficiency vs Output power.
Torque vs Slip.
Output power vs Slip.
Torque vs Speed.

Conclusion: –

To be written by student.

Some Viva questions:

How are the meter ratings selected for this experiment ?

Answer: In a load test on a three-phase induction motor (IM), the selection of meter ratings depends on the expected operating conditions of the motor, such as voltage, current, and power.

Why does one of the wattmeter read -ve at starting ?

Answer: In a load test on a three-phase induction motor (IM) using the two-wattmeter method, one wattmeter may read negative (–ve) at starting due to the power factor being low and close to zero.

A wattmeter shows negative when its current and voltage coils produce a phase difference greater than 90°, indicating reverse power flow.

This happens because of the phase shift in the current at low power factors.

The negative reading doesn’t mean power is reversed; it just indicates vector summation of power in the two-wattmeter method.

What are the two types of 3-φ induction motors and what is the difference between the two ?

Answer: Three-phase induction motors are classified into two types based on the rotor construction:

Wound Rotor Induction Motor (WRIM) / Slip Ring Induction Motor

Squirrel Cage Induction Motor (SCIM)

Feature	Squirrel Cage Induction Motor (SCIM) 🐿️	Wound Rotor Induction Motor (WRIM) 🎯

Rotor Construction

Rotor consists of copper or aluminum bars shorted at both ends by end rings (like a cage).

Rotor has three-phase windings similar to the stator, connected to external resistors via slip rings.

Starting Torque

Moderate (lower than WRIM)

High (can be adjusted using external resistors)

Speed Control

Limited (speed variation is minimal)

Good speed control by adjusting rotor resistance

Slip at Full Load

Low (~2-5%)

Higher (~5-10%)

Efficiency

High, because rotor resistance is low

Lower, due to rotor copper losses

Maintenance

Low (no brushes, rugged construction)

High (brushes and slip rings require maintenance)

Applications

Fans, pumps, compressors, conveyors

Cranes, hoists, elevators, rolling mills

What is the value of slip at starting ?

Answer: The slip (S) at starting of an induction motor is 100% or 1.

What are the advantages and disadvantages of squirrel cage induction motor ?

Answer: Advantages and Disadvantages of Squirrel Cage Induction Motor 🐿️

✅ Advantages

Simple and Rugged Construction – No slip rings, brushes, or external resistors, making it highly durable.
Low Cost – Manufacturing is cheaper due to its simple design.
Low Maintenance – No brushes or commutators, reducing wear and tear.
High Efficiency – Minimal losses due to low rotor resistance.
Reliable Operation – Less prone to failure, making it ideal for continuous use.
Self-starting – No need for external starting mechanisms.
Compact Size – Smaller in size compared to other motors of the same power rating.
Operates at Constant Speed – Suitable for applications where speed variation is not required.

❌ Disadvantages

Fixed Rotor Resistance – Unlike wound rotor motors, resistance cannot be adjusted to improve torque performance.

Low Starting Torque – Not suitable for applications requiring high initial torque (e.g., heavy loads like cranes).

Limited Speed Control – Speed cannot be easily varied, as it depends on supply frequency.

High Starting Current – Can cause voltage drops in the power system.

Not Suitable for Variable Load Applications – Works best for constant-speed applications.

What is the condition for maximum torque in an induction motor ?

Answer: The torque (T) in an induction motor is given by:

\[\color\red{T\; \propto\; \dfrac{s R_r}{(R_r^2 + X_r^2)}}\]

where:

s = slip
R_r = rotor resistance
X_r = rotor reactance

For maximum torque, the denominator should be minimized, which happens when: R_r=X_r

i.e., Rotor resistance = Rotor reactance

What are the different losses in an induction motor ?

Answer: The different losses in an induction motor can be categorized into the following types:

Stator Losses (Copper Losses), Rotor Losses (Copper Losses), Core (Iron) Losses, Mechanical Losses, Stray Load Losses.

Total Losses = Stator Losses + Rotor Losses + Core Losses + Mechanical Losses + Stray Load Losses

Gives one applications of 3-φ squirrel cage induction motor ?

Answer: One common application of a three-phase squirrel cage induction motor is in pumps used for industrial and agricultural purposes. These motors are widely used in water pumps, irrigation pumps, and centrifugal pumps due to their high efficiency, reliability, and low maintenance requirements.

Explain a typical Torque-slip characteristic.

Answer: The torque-slip characteristic of an induction motor represents how the developed torque varies with the slip (S). It is crucial for understanding motor performance under different operating conditions.

Slip is defined as the difference between the synchronous speed (N_s) and the rotor speed (N_r), expressed as a percentage of N_s

\[\color\red{\;S \;=\; \dfrac{N_s – N_r}{N_s} \times 100}\]

Slip varies from 0 (at synchronous speed) to 1 (at standstill).

The torque (T) vs. slip (S) curve can be divided into three regions:

(i) Low-Slip Region (0 < S < 0.05) – Normal Operating Region

The motor operates normally in this range (S≈0.01−0.05).
Torque is directly proportional to slip.
The motor is efficient, with low rotor current and minimal losses.

(ii) Medium-Slip Region (0.05 < S < 0.2) – Stable Region

Torque increases with slip but at a decreasing rate.
The motor can handle moderate overloads efficiently.

(iii) High-Slip Region (S > 0.2) – Unstable Region

Torque reaches a maximum (Breakdown Torque, T_max) at around S≈0.2.
Beyond this point, torque decreases with an increase in slip.
If the load is too high, the motor may stall (stop running).

What is the effect of increased rotor resistance on the performance of an induction machine ?

Answer: Increasing the rotor resistance in an induction machine (especially in wound rotor induction motors) has the following effects:

Improved Starting Torque
Higher Slip for Maximum Torque (Shifting of T_max)
Reduced Efficiency at Full Load
Smoother Speed Control
Reduced Starting Current

What is slip of an induction motor ?
Answer: The slip speed expressed as the ratio of synchronous speed is defined as slip.

\[\color\red{\;S \;=\; \dfrac{N_s – N_r}{N_s} \times 100}\]

An induction motor is generally analogous to ?
Answer: It is analogous to a winding rotating transformer with its secondary circuit closed

What are the operating modes of 3-φ induction motor ?
Answer: Motoring mode
Generating mode
Braking mode

State the advantages of skewing ?
Answer: It reduces humming and hence quite running of motor is achieved. It reduces
magnetic locking of the stator and rotor

How can the direction of rotation of the motor be reversed ?
Answer: By transposing or changing over any two line leads.

What happens if the air gap flux density in an induction motor increases ?
Answer: The increase in air gap flux increases iron loss and hence efficiency decreases.

Why the starting torque of squirrel cage induction motor is low ?
Answer: The resistance of a squirrel cage motor is fixed and small as compared to its reactance which is very large especially at the start. At standstill, the frequency of the rotor
currents equals the supply frequency. Hence, the starting current of the rotor though
very large in magnitude, lags by a very large angle behind E2, with the result that the
starting torque per ampere is very poor

What is meant by plugging in induction motor ?
Answer: It is a type of electric braking used in induction motors where the braking torque is produced by interchanging any two supply terminals. Here, the direction of rotation of
the rotating magnetic field is reversed with respect to the rotation of the motor. The
electromagnetic torque developed provides the braking action and brings the rotor to a
quick stop.

✅ Advantages

❌ Disadvantages

(i) Low-Slip Region (0 < S < 0.05) – Normal Operating Region

(ii) Medium-Slip Region (0.05 < S < 0.2) – Stable Region

(iii) High-Slip Region (S > 0.2) – Unstable Region

Share this:

Like this: