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

Electrical Contactors AC1, AC2, AC3, AC4

Electrical Contactors Utilization Categories:-

Electrical Contactors are categorized according to the current making and breaking values for contactors.
This will depend on the following:

1. The type of load to be controlled (Inductive load, Lighting load, Motor load, Resistive load, Capacitive load)
2. The operating cycle conditions.

For any applications there are different contactors available depending upon load application which every electrical engineer you must know before designing any systems. 

You may have often seen the ratings described on contactor name plate depicting various parameters. One of most important to be considered are as described in this article. 

• AC1 - These type of contactors are used for Non-inductive or slightly inductive loads they are used for Heaters which are resistive loads.
• AC2 - These type of contactors are used in Starting of slip-ring motors
• AC3 – They are particularly used for Starting of squirrel-cage motors and switching off only after the motor is up to speed.
• AC4 - Starting of squirrel-cage motors with inching and plugging duty. Rapid Start/Stop.
• AC11 - Auxiliary (control) circuits i.e. they don’t have power contacts.
• AC-6a- Switching of Transformers.
• AC-6b- Switching of Capacitor banks

Figure below shows the AC-3 contactor:-

You can see how name plate shows the AC-3 contactor applications at different Voltages.

Difference between AC1 and AC3 Contactors:-

There is most frequently asked question that what is difference between AC1 and AC3 contactor??
As clear from above it is very much clear that AC1 contactors are used for Non Inductive loads and are used for heaters , resistive loads.

But AC3 contactors are used for starting of Squirrel Cage Induction motors, They are also used for squirrel cage motors where breaking occurs while motor was running. There are so many examples of using of these contactors which are as below:-

Example of use: all squirrel cage motors, lifts, escalators, conveyors, bucket elevators, compressors, pumps, mixers, air conditioning units, etc.

Making and Breaking capacity of Circuit breaker

There are two types of currents which every electrical engineer must know before selecting a Circuit breaker.
These type of current are known as :-

1. Breaking current or Breaking capacity.
2. Making Current or Making Capacity.


You may visit also at link below for MCB Characteristics:-
http://electrialstandards.blogspot.in/2014/04/mcb-characteristics-and-applications-of.html

Breaking capacity :-

Breaking capacity of a circuit breaker or fuse is defined as the interrupting capacity of Circuit breaker or fuse that they can interrupt without destroying that equipment. The short circuit current which can occur under short circuit conditions should not exceed the rated breaking capacity of the apparatus. If breaking. Otherwise breaking of the current cannot be guaranteed.

Thus Breaking capacity of the circuit breaker refers to the maximum current in rms value the circuit breaker can interrupt.

Breaking capacity is also in the order of kA.

On MCCB’S Breaking current is specified on their name plate. Breaking current is what is taken in due consideration before designing any Electrical Systems.

Making Capacity:-

Making capacity of a circuit breaker is the maximum current which the breaker can conduct at the instant of closing. 

This making capacity is considered to the peak value of the first cycle when there is an imaginary short circuit between the phases.

When there is a short circuit in the line and the breaker is closed, the peak value of the first cycle is the most severe from an electrical perspective. 

Making current value is in kA. 

The making capacity is expressed as a peak value as the dc offset during fault conditions is taken into 

account.

The making capacity of the circuit breaker is usually greater than the breaking capacity of a circuit breaker as breaking an electric circuit is difficult due to arcing which occurs and which has to be quenched.

Making Current is usually 1.5 to 2 times greater than breaking current.

Though Making Current isn't specified on MCCB name plate but Making current is more important for design aspect and design companies must take due care of Making current before designing the MCCB.

To know about direction of fault current visit at:-
http://electrialstandards.blogspot.in/2014/04/fault-current-direction.html