Starters of induction motor and protection equipment

Starting methods and protection of equipment Electrical machines

K.Sateesh kumarSnist

Electrical Work shop labObjectives:

Starting methods of induction machines

Operations on induction machines

Protection of induction machines

Oil testing

Construction of Induction and Dc machines

List of Events (Experiments)1 DIRECT ON-LINE STARTER

2 FORWARD and REVERSE STARTER WIRING AND TESTING

3 STAR-DELTA STARTER WIRING AND TESTING SUITABLE FOR 5 HP MOTOR

4 INCHING (JOGGING )CIRCUIT FOR AC MOTOR

5 INTERLOCKING OF GROUP OF DRIVES

6 STUDY OF PHASE FAILURE RELAY (SINGLE PHASE PREVENTOR)

STARTER FOR TWO SPEED, TWO WINDING (SEPARATE WINDING) MOTOR

7 3-PHASE SQUIRREL CAGE INDUCTION MOTOR DISMANTLING, ASSEMBLING AND TESTING

8 1-PHASE CAPACITOR START CAPACITOR RUN INDUCTION MOTOR DIS-MANTLING, ASSEMBLING AND

TESTING

DC MACHINE DIS-MANTLING AND ASSEMBLING AND TESTING

9 WIRING UNDERVOLTAGE RELAY TO A DOL STARTER

WIRINING OF SLIPRING MOTOR STARTER AND TESTING

10 TESTING OF DIELECTRIC STRENGTH OF TRANSFORMOR OIL

Why ?

Cause PercentageOverloads 30 %Contaminants 19%Single – Phasing 14%Bearing Failure 13%Old age 10%Rotor failure 5%Miscellaneous 9%

44% of motor failure problems are related to HEAT

Electric Shock and Human Perception

Current Reaction

1 Milliampere Perception level

5 Milliamperes Slight shock felt; not painful but disturbing

6-30 Milliamperes Painful shock; “Let go” range

50-150 Milliamperes Extreme pain, respiratory arrest, severe muscular contraction

1000-4300 Milliamperes Ventricular fibrillation-a cause of cardiac arrest and sudden cardiac death

10,000 + Milliamperes Cardiac arrest, severe burns and death

Paying attention !!?

PRE-CAUTIONSWearing of shoes and apron is ESSENTIAL

Never work alone in the lab/workshop

Keep away from all moving parts

Stay away from the open terminals & do not allow any loose connections

Never touch any live terminals, while experiment is being conducted.

Switch on the supply, only after getting the circuit checked by the guide.

Maintain discipline ALWAYS.

Importance of Protection from Single PhasingTo maintain the same power input to the motor during single phasing the current in the remaining two phases will increase by1.73 times.

Single phasing of the motor results in superposition of negative sequence of current flow over the positive sequence through the motor windings.

These negative sequence currents are of double frequency as compared to positive sequence currents.

As the iron losses are proportional to frequency, the negative sequence currents cause higher iron losses and therefore greater heating of the rotor. Rotor heating is further increased due to skin effect (due to higher frequency).

This undetectedheated. This excessive heat can finally cause burnout of the motor

To avoid failure of the motor due to single phasing, phase failure relay or commercially known as single phase preventer is used. heating can damage the rotor and due to convection, the stator winding also gets

Importance of Protection from Under-Voltage and Over-Voltage

Power systems loads are MVA loads (motors, uninterrupted power supplies, etc.). As the voltage decreases, the load current increases while the power system transfer capability decreases.

An overvoltage is a sustained system voltage in excess of transformer capacity, motor, generator, or reactor voltage rating. Dielectric breaks down.

Dielectric Testing of Oil Transformer oil or insulating oil is usually a highly-refined mineral oil that is stable at high temperatures and has excellent electrical insulating properties.

It is used in oil-filled transformers, some types of high voltage capacitors, fluorescent lamp ballasts, and some types of high voltage switches and circuit breakers. Its functions are to insulate, suppress corona and arcing, and to serve as a coolant

Transformer oils are subject to electrical and mechanical stresses while a transformer is in operation. In addition there is contamination caused by chemical interactions with windings and other solid insulation, catalyzed by high operating temperature.

The original chemical properties of transformer oil change gradually, rendering it ineffective for its intended purpose after many years. Oil in large transformers and electrical apparatus is periodically tested for its electrical and chemical properties, to make sure it is suitable for further use

Starting of Induction Motor

For induction motors, the starting torque is approximately proportional to

the square of the starting current drawn from the line. I∝ 2

This starting current is proportional to the applied voltage (V)

Torque can also be considered to be approximately proportional to the

applied voltage. V∝ 2

An induction motor will develop far too much torque when connected

directly to the supply.

At the instant of start-up, there are some un-necessary effect on

electrical and the mechanical components.

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Un-Necessary Electrical Effects

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A heavy current surge on the electrical supply which can

be severe enough to cause voltage dips and flickering

lights.

Burning of contacts due to high currents which are many

times the motor full-load current.

Problems Arises by these Effects

Greater chance of unscheduled shutdowns

Oversized mechanical and electrical components to

cater for the power surge on start up.

Short component life.

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Solution of the problems….. Starter

By Adjusting voltage during starting, the current drawn by the motor and thetorque produced by the motor can be reduced and controlled.

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Function of starter

Start and stop the motor. Limit inrush current where necessary. Permit automatic control when required Protect motor and other connected equipments from over

voltage, no voltage, under voltage, single phasing etc. Motor overload protection

Types of Starter for 3-Ph Induction motors

For slip-ring induction motors: Rotor rheostat starter

For squirrel cage induction motors: D.O.L starter Primary resistance starter Auto transformer starter Star delta starter

Other starters: Soft starters

Starting of slip ring induction motor

In case of slip ring induction motors it is possible to add

external resistance in rotor phases as wound rotor has 3-

-phase star connected winding to limit the starting high

current.

D.O.L.(direct on line) starter

Components of D.O.L. starter :Components of a DOL Starter

Overload Unit (Thermal type)

Main pole terminals 1, 3 & 5

When motor overheats due to overload conditions, main poles latch open

Auxiliary contacts also latch open and when interlocked within control circuit prevents motor restarting by itself when cool.

Main pole terminals 2, 4 & 6

N/OAuxiliary contacts97 & 98

N/C Auxiliary contacts95 & 96

Reset button

Red pushbutton can be used to reset

Components of a DOL Starter:Start and Stop pushbuttons

Start button is green and flush mounted

Stop button is red and protruding

N/O contact N/C contact

Emergency Stop button has a red mushroom head which latches in and must be turned to release

Contacts at the back of switches can be either N/O or N/C

04/17/2023 cole.celestino/motor control fundamentals

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THERMAL OVERLOAD RELAY

Thermal overload relay is an electrical component used in motor control applications which provide protection for motor against excessive current.

Line terminals

resetSet value dial

Auxiliary contact(s)


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THERMAL OVERLOAD RELAYPrinciples of operation:

This component has a bimetal strip that is made of two dissimilar metals bonded together. The two metals have different thermal expansion characteristics, so the bimetal strip bends at a given rate when heated.

As current rises, heat also rises. The hotter the bimetal strip becomes, the more it bends. In an overload condition, the heat generated from the heater will cause the bimetal strip to bend until the mechanism is tripped, stopping the motor.


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OVER/ UNDER VOLTAGE RELAY

This component protects the motor against over voltage and under voltage situation.

Typical installation diagram for over/under voltage protection


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OVER/ UNDER VOLTAGE RELAY OPERATION

11 12 21 22

21 24

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Motor contactor

Over-voltage alarm

Under-voltage alarm

PHASE LOSS/ PHASE SEQUENCE RELAY

Phase control relays monitor:■ Phase sequence■ Loss of one or more phases■ Unbalance level

In 3-phase system, phase lossIs the event when one or two phases is/are disconnected in the main supply. Phase sequence on the other hand is when two of the phases are interchanged resulting to a reverse rotation of a 3-phase motor.

Phase loss is mandatory protection to every motor operation. Phase sequence is for selective purposes only such as pumps that reversing the motor direction may cause serious damage to the equipment.


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PHASE LOSS/ PHASE SEQUENCE WIRING DIAGRAM

Motor contactor

PL/PS NC contacts

PHASE PROTECTION

RELAY

Star-delta starter

Star-delta starter

Advantages of Star-Delta starter:

The operation of the star-delta method is simple and rugged It is relatively cheap compared to other reduced voltage

methods. Good Torque/Current Performance. It draws 2 times starting current of the full load ampere of the

motor connected

Disadvantages of Star-Delta starter:

Low Starting Torque, only 33% starting torque Six Terminal Motor Required (Delta Connected). It requires 2 set of cables from starter to motor.

Difference between DOL/Star delta /Autotransformer

Sr. DOL Starter Star delta starter Auto transformer starter

1 Used up to 5 HP Used 5 HP to 20HP Used above 20 HP

2 Does not decrease the starting current

Decreases the starting current by 1/3 times

Decreases the starting current as required

3 It is cheap It is costly It is more costly

4 It connects directly the motor with supply for starting as well as for running

It connects the motor first in star at the time of starting in delta for running

It connects the motor according to the taping taken out from the auto transformer

THANK YOU

Starters of induction motor and protection equipment

Engineering