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PLC - Programmable Logic Controller

PLC
Schneider Modicon
Programmable logic controllers are electronic devices used to control a process. It consists of CPU; Input and output devices, power supply, communication and redundancy units. PLCs are preprogrammed based on their application and put into operation. A PLC accepts inputs from the field, process the input based on the programming done and outputs the commands to the field equipment. It can also accept commands from the plant operators through SCADA and give commands to the field equipment. The features of PLCs vary from one manufacturer to the other but the basic architecture remains the same.

AC motor stall
Induction motor Torque-Speed Characteristics
Stalling is a condition at which a motor stops rotating. This condition occurs when the torque required by the load is more than the maximum torque (Breakdown torque) that can be generated by the motor. At this condition, the motor drains the maximum current and the speed comes zero.
DC Motor Torque speed Characteristics
Every motor is designed to spin a particular amount of load. When the load is further increased above its overload region, the speed of the motor gradually reduces and the motor stalls. The Slip during stalling of a motor is 100%. Since the motor is not rotating, there will be no back EMF and so the power consumption will be the maximum. 

Reasons for stalling of motors:

1. Electrical: Missing out of one phase (two phasing) or single phasing of input supply
2. Mechanical: Rotor jam, overload or load unable to move.

Stall Current

Stall current is the current drawn by the motor at locked rotor condition. It is the highest current a motor can draw and is proportional to its rotor resistance. If stall current is drawn by a motor for a longer time, motor gets overheated causing damage of winding.

Stall torque 

Stall torque is the load torque at which the motor shaft stops rotating. It is also known as locked rotor torque.

Related Searches

Induction Motor Stalling
Motor Stall Protection
Motor Stall Fault
Motor Stall Current
Safe Stall Time Of Motor
Motor Stall Torque Calculation
Maximum Continuous Stall Torque
Locked Rotor Protection Relay


As the name indicates, starters are used to start or stop a motor. It consists of an electromechanical switch and protection devices (overload and/or short-circuit protection).

Horsepower and kilowatt are two different units of measuring power. Watts is the SI unit of power. It is a well-defined and normally used unit.

This is a simple tool to instantaneously convert kilowatt into horsepower.

How to convert kilowatts into horsepower?

Mechanic / Hydraulic horsepower to kilowatts

One mechanic or hydraulic horsepower is equal to 0.745699872 kilowatts:
1 hp(I) = 745.699872 W = 0.745699872 kW
Therefore,
P(kW) = 0.745699872 ⋅ P(hp)

Electrical horsepower to kilowatts

One electrical horsepower is equal to 0.746 kilowatts:
1 hp(E) = 746 W = 0.746 kW
Therefore,
P(kW) = 0.746 ⋅ P(hp)

Metric horsepower to kilowatts

One metric horsepower is equal to 0.73549875 kilowatts:
1 hp(M) = 735.49875 W = 0.73549875 kW
Therefore,
P(kW) = 0.73549875 ⋅ P(hp)

Horsepower to kilowatts Convertor


 
hp
   
Result in kilowatts: kW





The following are few standard KW to HP conversions


Kilowatt (KW)
Mechanic horsepower
(hp(I))
Electric horsepower (hp(E))
Metric horsepower (hp(M))
0.1KW
0.080HP
0.080HP
0.082HP
0.1KW
0.121HP
0.121HP
0.122HP
0.1KW
0.161HP
0.161HP
0.163HP
0.2KW
0.241HP
0.241HP
0.245HP
0.3KW
0.335HP
0.335HP
0.340HP
0.4KW
0.496HP
0.496HP
0.503HP
0.6KW
0.738HP
0.737HP
0.748HP
0.8KW
1.006HP
1.005HP
1.020HP
1.0KW
1.341HP
1.340HP
1.360HP
1.1KW
1.475HP
1.475HP
1.496HP
1.5KW
2.012HP
2.011HP
2.039HP
2.0KW
2.682HP
2.681HP
2.719HP
2.2KW
2.950HP
2.949HP
2.991HP
3.0KW
4.023HP
4.021HP
4.079HP
4.0KW
5.364HP
5.362HP
5.438HP
5.5KW
7.376HP
7.373HP
7.478HP
7.5KW
10.058HP
10.054HP
10.197HP
11.0KW
14.751HP
14.745HP
14.956HP
15.0KW
20.115HP
20.107HP
20.394HP
18.5KW
24.809HP
24.799HP
25.153HP
22.0KW
29.502HP
29.491HP
29.912HP
30.0KW
40.231HP
40.214HP
40.789HP
37.0KW
49.618HP
49.598HP
50.306HP
45.0KW
60.346HP
60.322HP
61.183HP
55.0KW
73.756HP
73.727HP
74.779HP
75.0KW
100.577HP
100.536HP
101.972HP
90.0KW
120.692HP
120.643HP
122.366HP
110.0KW
147.512HP
147.453HP
149.558HP
132.0KW
177.015HP
176.944HP
179.470HP
160.0KW
214.564HP
214.477HP
217.540HP
200.0KW
268.204HP
268.097HP
271.924HP
250.0KW
335.256HP
335.121HP
339.906HP
315.0KW
422.422HP
422.252HP
428.281HP
355.0KW
476.063HP
475.871HP
482.666HP
400.0KW
536.409HP
536.193HP
543.849HP
500.0KW
670.511HP
670.242HP
679.811HP
560.0KW
750.972HP
750.670HP
761.388HP
630.0KW
844.844HP
844.504HP
856.562HP
710.0KW
952.126HP
951.743HP
965.332HP
800.0KW
1072.818HP
1072.386HP
1087.698HP
900.0KW
1206.920HP
1206.435HP
1223.660HP
1000.0KW
1341.022HP
1340.483HP
1359.622HP

Voltage division rule

Voltage division rule is one of the basic rules of circuit analysis. It is applicable to all series circuits and combination circuits. Series circuit always acts as a Voltage divider. In a series circuit, the same current flows through each resistance. Hence the voltage drops across each resistor are proportional to their ohmic value. 


Image result for series circuit examples

Current flow in a circuit is determined by the impedance offered by the circuit components, which is in turn determined by the way circuit components are connected. Series connection and parallel connection are the two simplest ways of connecting a circuit.
Ohm’s law is one of the basic principles of electricity. It relates the basic parameters of electricity, Current and voltage, to each other. Georg Ohm, after whom the law was named, conduct a few experiments on circuits containing different lengths of wires and found that the voltage applied and current are directly proportional. He derived a complex equation and published it along with his results in the book Die galvanische Kette, mathematisch bearbeitet in 1827.
Miniature Circuit breaker

Miniature Circuit Breaker (MCB) - Principle of operation

Miniature circuit breakers normally known as MCBs, are mechanically operated switches cum electro-mechanically operated automatic circuit protection devices. They are used to interrupt a circuit during overload and short circuits. They are used as an alternative of fuses. The advantage of MCBs over fuses is that MCBs are reusable even after interruption of the circuit after an overload or short circuit. Moreover, MCBs are more sensitive to faults than fuses.

Contactor- Construction, Working Principle, Application

Contactor- Construction, Working Priciple, Application
Low Voltage Contactor
An electrical contactor is a switching device, widely used for the switching of motors, capacitors (for power factor correction) and lights. As the name indicates contactor are used to make or break contacts as like an ordinary on-off switch. The only difference is that the contactors have an electromagnet that holds the contacts when energized whereas switches do not have it.

The basic operation of a contactor is similar to that of a relay but contactor contacts can carry much more current than relays. Relays cannot be directly used in circuits where current exceeds 20 amperes. In such conditions contactors can be used.  Contactors are available in a wide range of ratings and forms. Contactors are available up to the ampere rating of 12500A. Contactors cannot provide short circuit protection but can only make or break contacts when excited.


Table of contents

1.       Construction
1.1.    Electromagnet
1.2.    Contacts
1.3.    Enclosure
2.       Operating principle of Contactors
3.       Arc Suppression
4.       Categories
5.       Application
5.1.    Motor Starters
5.2.    Capacitor Switching
5.3.    Lighting Control
6.       Selection of contactors
7.       Checking a Contactor

 Constructional features of Contactor

A contactor consists of an electromagnet, contacts and spring enclosed inside an enclosure. In some contactors, economizers are provided to reduce power consumption. Certain arrangements for arc extinction is also made inside for making and breaking operation of contactors.

Electromagnet / Contactor Coil

Conventional Low voltage contactor coil
Conventional Low voltage contactor coil
Hollow Cylindrical Type Coil
Hollow Cylindrical Type Coil


Electromagnet is the key component in contactors without which it cannot function. It requires an additional supply for excitation. It drains negligible current from the supply during excitation. These electromagnets will be hollow cylindrical in shape. A rod (armature) with spring return arrangement will be placed in the hollow cylindrical electromagnet. In some contactors this electromagnet is split into two halves. One of the halves is fixed and the other is movable. Movable power contacts are fixed to the movable electromagnet. Under normal condition, these two halves of electromagnets are held apart using a spring in between.
Conventional laminated soft iron Magnetic Core
Conventional laminated soft iron Magnetic Core
Solid Steel Core
Solid Steel Core
In contactor with AC coil, the electromagnetic core is made up of laminated soft iron to reduce eddy current losses and in contactor with DC coil, the electromagnetic core is made up of solid steel/ soft ironic core since there is no risk of eddy current loss in DC.

Contacts

In a contactor there are two sets of contacts , of which one is stationary and the other is moveable. Silver tin oxide (AgSnO2), silver nickel (AgNi) and silver cadmium oxide (AgCdO)are the normally used contact materials. There materials have high welding resistance and stable arc resistance. Silver cadmium oxide and silver nickel are used in contactors of less ampere rating whereas Silver tin oxide is used in contactors of high ampere rating and in DC contactors.

 The movable set of contacts is attached to the armature or movable electromagnet. Contact material must withstand mechanical stresses, arcs, erosion and must have very low resistance.

Enclosure

Electromagnet and contacts are packed inside an enclosure made of plastic, ceramic or Bakelite, which protects it from dust and external environment and ensures safe opening and closing of contacts.

Arc extinction is a major part of contactor operation. AC arcs can be easily extinguished since it passes through zero twice for every cycle.  DC contactors used magnetic blowouts or specially designed arc chutes for arc extinction.

Operating principle of Contactors

Symbol of Contactor
Symbol of Contactor
The operating principle of a contactor is very simple. Whenever the electromagnetic coil is energized, an electromagnetic field is produced.  This electromagnetic field attracts the metallic rod (armature) towards the gap in the hollow cylindrical magnet. In contactors with split electromagnets, the movable half of the electromagnet is attracted towards the fixed electromagnet. This action closes the contacts. The contacts remains closed as long as the electromagnet remains excited. When the coil is de energized, moving contact is pushed back to its normal position by the spring. Contactors are designed to open and close contacts rapidly. Moving contacts may bounce as it rapidly makes contacts with the fixed contacts. Bifurcated contacts are used in some contactors to avoid bouncing.
The input to the contactor coil may be AC or DC (available in various voltage ranges starting from 12Vac/ 12Vdc to 690Vac). A small amount of power is drained by the contactor coil during its operation. Economiser circuits are used to reduce the power consumed by the contactor during its operation.
Contactors with AC coils have shading coils. Otherwise, the contactor may chatter every time the alternating current crosses zero. Shading coils delay demagnetization of the magnetic core and avoids chattering. Shading is not required in DC coils as the flux produced is constant.

Arc Suppression in contactors

DC contactors
DC contactors
Arc occurs between the contacts every time when contacts are closed or opened under load. Arc formed during the breaking of a load is more destructive and may damage the contacts, hence reducing the life of the contactor. In addition to that high temperature of arc degrades the gases surrounding the contacts and forms harmful gases such as carbon mono-oxide, ozone etc.  This may affect the mechanical durability of the contactors. Several methods are adopted for control and extinction of arcs.

Vacuum Contactor
Vacuum Contactor
As mentioned earlier, DC arcs are more severe compared to AC arcs. In DC contactors magnetic blowouts are used to propagate the arcs towards specially designed arc chutes and extinguishing it by splitting it. Contactors used in low voltage AC applications (690Volt or less), atmospheric air surrounding the contacts extinguishes the arc. For medium voltage and high voltage applications vacuum contactors are used to avoid the risk of arc.

Contactor Categorization

 Contactors are categorised based on the type of load (IEC utilisation categories - 60947) and current and power rating (NEMA size).
Few important IEC utilisation categories are below:
AC-1: Non-inductive or slightly inductive and resistive heating type of loads
AC-2: Starting of slip ring induction motor
AC-3: Starting and switching off Squirrel-cage motors during running time
AC-15: Control of AC electromagnets.
AC-56b:- Switching of capacitor banks
DC–1: Non-inductive or slightly inductive and resistive heating type of loads
DC-2: Starting, inching and dynamic breaking of DC shunt motors
DC-3: Starting, inching and dynamic breaking of DC series motors
DC-13: Control of DC electromagnets

NEMA size:

NEMA size is based on the maximum continuous current and horse power rating of the induction motor controlled by the contactor. In NEMA standard contactors are designated  as size 00,0,1,2,3,4,5,6,7,8,9.

Application of contactors

Motor Starters:

Contactors are used in motor starter either Direct-on-line or Star Delta along with thermal overload relays or motor protection circuit breakers. Even in our homes, one can find it inside the pump starters.
DOL Starter
DOL Starter

Switching of capacitor Banks     


In capacitor banks, capacitor contactors are used for switching of capacitors based on the correction requirements. Capacitor switching contactors are specially designed to control high transient currents during switching of contactors.

Capacitor Switching Contactor
Capacitor Switching Contactor

Lighting control

Modular contactor used for switching of small Lighting loads
Modular contactor used for switching of small Lighting loads
Contactors are used in the switching of street, commercial and residential lights. Especially timer controlled lighting systems uses contactors for switching. Latch type contactors are also available. In these type of contactors, two coils are available, one for opening and the other for closing. Closing coil closes the contacts, when excited and cuts off the supply to the coil. Contact is then held closed mechanically. Second coil is used for opening the contacts.


Selection of contactors

Contactors are selected based on the following:
  1. Application – based on IEC utilisation category.
  2. Load current and voltage.
  3. Control voltage available – For selecting coil voltage of contactors.

Checking of contactor

Contactor can be checked whether it is “open” or “closed” using an ohmmeter. If the resistance between the input and output terminals is infinite then the contactor is opened and if the ohmmeter reading is zero then it denoted that the contacts are closed.

Also Read:

The difference between Relays and contactors.


Related Searches:

1. How does a contactor work?
2. How do Contactors work?
3. Operation of contactors
4. What is a contactor?