Induction motors not kept running in Star Connections

Induction motors are not kept on running in Star connections due to following reasons:-

In Star connections, Phase current= Line current and

Phase voltage= Line voltage/ √3.

As Power = √3X Line Voltage XLine Current X Power Factor

This means that during starting voltage get reduced by factor 1/√3.

So current is also get reduced by factor 1/√3.

So Torque also gets reduced by factor 1/3 as Torque is directly proportional to square of voltage.

Due to reduced voltage Motor unable to take-up higher loads.

So motor power get reduced by 1/3 times then in Delta connections that is why motor is not kept running in Star connections. As motor is get under utilized.
So Motor will kept on running at reduced power and means motor will run at low efficiency

Star Delta Starter and applications

Motors up-to 7.5 KW are started using direct online starters but above 7.5 KW induction motors are started using Star-Delta Starters as if motors above 7.5 KW are started directly then there will be voltage disturbances in line due to large starting current surges. So let’s discuss how this Star-Delta Starter works.

Working Principal of Star-Delta Starter:

First of all you must know what Star and Delta connections are:-

Star Connection or Wye Connections:-

In Star connections, 

Phase current= Line current and

Phase voltage= Line voltage/ √3.

This means that during starting voltage get reduced by factor 1/√3.

So Torque also gets reduced by factor 1/3 as Torque is directly proportional to square of voltage.

Current also get reduced by 1/√3 in induction motor if motor is Started in Star connections.

If You want to know about Direct online starter first then click link here

Delta Connections:-

In Delta connections

Phase Voltage = Line Voltage and 

Phase current= Line Current/ √3.

After Starting motor in star connections then transition is done from Star connections to Delta connections after certain delay.

Star-delta Starter Consists following components:

1)     Main, star and delta contactorsà 3 No’s

2)     Timer Star-Deltaà 1 No.

3)     Overload relayà 1No.

4)     Main MCCB or Fuse Unit

Power Circuit of Star Delta Starter:

Main Power circuit of Star-Delta Starter is as Shown below:-

Add caption

In this Circuit there are three power contactors  C1 for Main C2 for Delta and C3 for Star.

Control Circuit of Star-Delta Starter (Open Transition):

Control circuit for Star-Delta Starter is as shown below:-

In that circuit when Start PB is pressed Star and Main contactor gets holds and motor get started in Start connections and after certain delay which was set in timer star contactor get disengage and delta contactor comes in line.

Star and Delta contactor NC’S are take in line both in Delta and contactor line so that only 1 contactor either star or Delta may comes in line.

In Star Delta Starter since Six leads goes at Motor end so in each lead phase current will flow so Overload relay used for Star-Delta circuit is usually for phase current not line current.

There will be always question arises that why Induction motor kept running in Star then visit link below:-

http://electrialstandards.blogspot.com/2014/04/induction-motors-not-kept-running-in.html

 Application of Star-Delta Starter:

In Most of industries for motors above 7.5 KW Star-Delta Starters are used. Star Delta Starters are used as during starting start current get reduced by 1/3 times and also torque get reduced by 1/3 times.

They have low maintenance cost compared to VFD or Soft-Starters which will be discussed later.

Direct online Applications Reverse Forward, Limit & Level Switch, Remote Starters

Earlier we discussed about Direct Online Starter Basic Circuit.

For Basic Circuit diagram of Direct online visit at link below:-

http://electrialstandards.blogspot.in/2014/04/direct-online-dol-starter.html

Now let’s discuss about Direct online Starters following circuits:-

1.       Reverse forward direct online Starter.

2.       Limit Switch & level Switch circuit in DOL.

3.       Remote Start/Stop Direct online Starter Circuit.

Let’s discuss their circuitry one by one:-

1.     Reverse Forward Direct Online Starter:-

Power and control circuit for Reverse forward Direct online starter is as below:-

In control circuit C1 contactor is used as forward contactor and C2 contactor is used as Reverse contactor

And also Reverse contactor NC is used in series with Forward contactor starting and also forward contactor NC is used in circuit of Reverse contactor so that while motor is operating in one direction it’s other direction contactor could not hold, Which otherwise leads to Short circuit.
Reverse forward applications for DOL starter are used boilers and other applications in an Industry.

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2.       Limit Switch & level Switch circuit in DOL:-
        This type of application is used for houses for water level controllers and Limit switches are used in industries for switching off the motor when limit is reached. This type is used in Air compressors, gas compressors etc. where at a certain pressure limit motor get switched off. 

Circuit for the same is as given below:-

3. Remote Start/Stop Direct online Starter Circuit:-

Remote Start/Stop Direct online circuit is as below:-

For taking 3 wires are taken from starter to remote.

1 wire for stop, 1 for start and 1 as common.
These applications are used where there is need for starting and stopping the motor near operator.

Direct Online (DOL) Starter

Different Starting methods are used for starting motors depending upon the rating of motor.

Direct online starter is simplest starter used for starting of Induction motors and in short it is also known as DOL starter.

Direct online starters are used only for motors upto 7.5 KW as they draw heavy inrush current during starting of motor. In these starters full voltage is comes across motor and draws 6-8 times rated current at starting of motor. Due to heavy current drawn during starting of motor these starters are used only for motors upto 7.5 KW rating only.

Main Parts of Direct online starter:-

Direct online Starter consists of following parts:-

1.       Electromagnetic Contactor

2.       Overload relay

3.       Main MCB

4.       ON/OFF switches

All Starters has two circuit diagrams :-

1.       Power circuit Diagram

2.       Control circuit diagram

Let’s discuss one be one:-

1.      Power circuit diagram:-

Power circuit is usually also called three phase diagram. In That diagram Overload relay comes at end of circuit and is below:-

DOL Power Circuit

Above diagram is known as power circuit diagram as ON/OFF switches are not shown in this diagram, instead only 3-Phase wiring is shown. In this diagram you can easily see that Overload relay is connected nearest to motor end so that if any abnormality occurs then Overload get tripped immediately and protects both motor and circuit.

2.              Control Circuit diagram

In control circuit O/L Relay comes at starting of circuit and is shown as below:-

Direct Online Starter Control Circuit

In this circuit as we see O/L Relay NC is used and Stop Push Button NC is used.

Let’s discuss how this DOL Control circuit works as power circuit will not work until Control circuit gets right.

Whenever start PB is pressed Phase 1^st goes through Overload relay NC and then goes through Stop PB and  Start PB and thereafter holds power contactor now as soon as Power contactor get holds  it’s NO becomes NC and thereafter power supply  continue to flow to power contactor through 13-14 point and Start PB goes to initial position.

In above control circuit 220V circuit is shown but  it  may be 440V circuit depends upon Power contactor coil voltage supply.

This is how a Simple DOL Starter works.

For other Direct online Starter applications  visit at link below:-

http://electrialstandards.blogspot.in/2014/04/direct-online-applications-reverse.html

Advantages of DOL Starter:-

1.                These are most simple type of starters.

2.                Most economical and cheapest starter.

3.                Maintenance cost is very low.

Disadvantages of DOL Starter:-

1.        Draws very high inrush current at starting i.e. 6-8 times rated current of motor which restricts its use to motors upto 7.5 KW only.

2.         It will put high Thermal Stress on motors that will reduce motor life.

3.       High Starting current will leads to dips in voltage.

To know Circuit of Star-Delta Starter click below:-
http://electrialstandards.blogspot.in/2014/04/star-delta-starter-and-applications.html

You may add anything into this DOL Starter it such as Indicators, Remote Start/Stop, Level Switch, Limit switch, Reverse forward etc. can be easily drawn from above circuit which will be discussed later on in next post.

Battery Rating Calculation

Batteries are usually connected in Series as in Series Voltage get added up and current remains the same.

BATTERY RATING CALCULATION

The formula employed rating calculation is as stipulated below:-

FORMULA EMPLOYED:

                                               

Capacity of Battery (AH) = (DC Current X Duration in Hrs)/( %age capacity utilization)

Where, DC Current = (UPS (KVA) x 1000 x Load Power Factor (=1))/( Inverter efficiency x End Voltage)

 Hence, VAH = AH x Nominal Voltage

 For e.g.: for 3 KVA UPS, 1 hour Backup having 18 no. batteries:

DC Current = (5 x 1000 x 1)/ (0.95 x 10.5 x 18) =27.85A

Battery “AH” =   (27.85 x 1.0)/ (0.62) = 44.91

Hence , 18 no.s of 12V/42AH batteries will suffice .

TOTAL VAH = 18 x 12 x 42 = 9072 VAH

The following assumptions have been made in the above calculations:-

1. DC Voltage – 216 for 5 KVA

2. End cell voltage / battery of 10.5 V

3. Load Power Factor = 1.0

4. Inverter efficiency = 95%

% Capacity utilization is: 

Duration % Capacity utilization

½ hr (30 min’s) à52%

1hrs à 62%

2hrs à74%

3hrs à83%

4hrs  à85%

• The amp-hour is a unit of battery energy capacity, equal to the amount of continuous current multiplied by the discharge time, that a battery can supply before exhausting its internal store of chemical energy.
• An amp-hour battery rating is only an approximation of the battery's charge capacity, and should be trusted only at the current level or time specified by the manufacturer. Such a rating cannot be extrapolated for very high currents or very long times with any accuracy.
• Discharged batteries lose voltage and increase in resistance. The best check for a dead battery is a voltage test under load.

Three phase Motor Insulation classes

Maximum allowable temperatures of various types of insulation

Insulation Class	Maximum Permissible temperature in Degree Celsius
Y	90
A	120
B	130
F	155
H	180
C	ABOVE 180

Insulation classes types are explained in details as below:-

1.        Class-Y insulation: These type on insulations can Withstands a temperature of up to 90°C these type of insulations are made of Cotton, silk, or paper.

2.       Class-A insulation: These type of insulations can Withstands a temperature of up to 105°C, these type of insulations are made from reinforced Class-Y materials with impregnated varnish or insulation oil. Insulation consists of materials such as cotton, silk, and paper when suitably impregnation or coated or when immersed in dielectric oil.

3.       Class-E insulation: These types of insulation materials can Withstands a temperature of up to 120°C .

4.       Class-B insulation: These type of insulations can Withstands a temperature of up to 130°C. This has a form that inorganic material is hardened with adhesives. These types of Insulations consist of materials or combinations of materials such as mica, fiber, asbestos, etc with suitable bonding, impregnation or coating substance.

5.       Class-F insulation: These type of insulations can Withstands a temperature of up to 155°C these type of insulations consists of materials or combination of materials such as Mica, fiber, Asbestos etc. Actually these are made of Class-B materials that are upgraded with adhesives, silicone, and varnish of higher thermal endurance.

6.       Class H insulation: These types of insulations can Withstands a temperature of up to 180°C. These types of insulations consist of inorganic material glued with silicone resin or adhesives of equivalent performance. These are also made Class-B Materials with suitable Impregnation or coating of Silicon resins or Adhesives.

7.       Class-C insulation: These types of insulations can Withstands a temperature of up to 180°C or higher. This type of Insulation materials are made of 100% inorganic material. These type of Insulation consists of materials or combinations of materials such as mica, porcelain, glass quartz and asbestos without or with an inorganic binder

Thus we have studied details about insulation materials that are used in Motors. By adopting an insulation technique of higher thermal endurance, electric instruments can be downsized.

There is one small example which will tell you how important are motor insulations are in designing motors.

A motor operating at 180 Degrees Celsius will have an estimated life of

1.     250-300 hours with a Class A insulation

2. 1500-1800 hours with Class B insulation
3. 7000-8500 hours with Class F insulation
4. More than 10000 of hours with Class H insulation

Class F Insulation is most commonly used for 50HZ Motors.

INSULATION RESISTANCE:-

The insulation resistance plays an important role in designing any motor  and usually insulation resistance is affected by moisture and dirt that will play an important indicator of deterioration from such cause.

The insulation resistance may be measured by non-destructive test.

The resistance value may be read directly from mega ohmmeter or indirectly by calculation using voltmeter-ammeter method, or from voltmeter reading alone.

The insulation resistance values of DC machine are generally more sensitive to change in humidity than are those of AC winding, this might be due to greater number of leakage paths in the armature and fields of DC machines.

METHOD OF MEASUREMENT OF TEMPERATURE RISE BY RESISTANCE:-

The hot temperature is determined from the following formula:

T2 = R2 / R1 (T1 + 234.5) – 234.5

R1 = resistance of winding cold

R2 = resistance of winding hot

T1 = temperature of winding cold OC

T2 = temperature of winding hot OC

That temperature rise can be measured by installing temperature detectors during manufacturing of machines .These will be the form of thermo-couples or resistance thermometer.

This provide important details about any machine as during operation of motor if temperature rises after a prescribed limit then it can be very easy to identify any problem in motors.

Mechanical and electrical Life of Circuit Breakers

Circuit breakers whether it's ACB, VCB, MCCB etc. you must know about no. of Mechanically  and electrical operations/Life of that circuit breaker.

These are described by Manufacturer in its detailed technical specification.

For An electrical engineer these are very important while designing their electrical systems:-

Mechanically Operations or Mechanical Life:-

As clearly from the name mechanical operations of a Circuit breaker is no. of times a Circuit breaker is made ON/Off.

Electrical Operations or Electrical Life:-

These are those operations in which Circuit breaker is Made ON/OFF or Trip at available power supply.

For a MCCB life of MCCB is getting lower lower as rating of MCCB is getting increased:-

For example for MCCB's data is below:-

at Rated Current upto 100A-- Mechanical life is 8500 Operations and Electrical Life is 1500 Operations

at Rated Current from 100A to Max 315A --Mechanical life is 7000 Operations and Electrical life is 1000 Operations

at Rated Current from 315A to Max 630A --Mechanical life is 4000 Operations and Electrical life is 1000 Operations

at Rated Current from 630A to Max 800A --Mechanical life is 2500 Operations and Electrical life is 500 Operations

For ACB'S No. of operations details is below:-

at Rated Current upto 1600A-- Mechanical life is 20000 Operations and Electrical Life is 5000 Operations

at Rated Current from 1600A to Max 3200A --Mechanical life is 15000 Operations and Electrical life is 5000 Operations

at Rated Current from 3200A to Max 5000A --Mechanical life is 10000 Operations and Electrical life is 2000 Operations

at Rated Current from 5000A to Max 5300A --Mechanical life is 10000 Operations and Electrical life is 2000 Operations