About the contactor

 For short, a contactor is an encapsulated relay on steroids.

Just as the encapsulated relay, a contactor has:

• a coil,
• a set of contacts and
• a test button.

However, a contactor is built for heavy duty applications.

You can read about the encapsulated relay here: About the encapsulated relay.

Knowing the contactor

A contactor should be easy to identify. You should see coil terminals, auxiliary contacts (NO or NC or a combination of these), power contacts and a small level working as a test button. Depending on the manufacturer and model, these features can be arranged in different ways.

Most models are design for rail mounting inside a cabinet. However, more expensive models may have other features such as small pilot lights or self-coupling to other components such as overload relays, to say an example.

Images of sample contactors

The coil

The contactor coil can be easily identified by the A1 and A2 terminals. You should  remember that depending on the model, the coil may be for DC or AC and design for a given voltage too.

In the contactor, the coil does the same work as the encapsulated relay: when it is energized a set of contacts is open or closed.

The contacts

In the contactor, two types of contacts appear,

• auxiliary contacts and
• power contacts.

Auxiliary contacts are meant for turning on/of devices of low voltage such as: pilot lights or alarms, for example. On the other hand, power contacts are used for powering equipment requiring high voltaje and representing a potential danger for an operator, such as: motors or electrical resistances, to name some.

In a contactor, the number of contacts are,obviously, specified according to the application of the component. It is common practice that auxiliary contacts are NO but one NO and one NC or only NC contacts can be looked for. You can identify the auxiliary contacts in the contactor by NO or NC legend.

Contacts in a contactor. Manufacturer: WEG, Model: CWB9

On the other hand, power contacts are always NO since these are meant for energizing an equipment. You can identify the power contacts by the letters L1, L2, etc. and T1, T2, etc.. The letters L and T stand for: line and terminal; while the numbers indicate correspondence between each terminal, this is: L1 and T1 are a NO contact, and so on. Also, all NO power contacts are independent so that L1 and T2 do not form a contact.

Specification of a contactor

Specifying a contactor is usually reduced to determine the number of NO/NC auxiliary and NO power contacts. Therefore, you should, first, determine what is to be turn on/off by the auxiliary contacts so that a ladder diagram needs to be well established before anything else.

The power contacts are related to the power diagram or circuit. You need to know if the equipment is single or triple phase or something else so that the number of poles (NO contacts) can be determined.

Rating

Contactor rating usually refers to the NO power contacts since these are the ones to be subjected to high voltage and current. These contacts need to be such that can withstand a given maximum voltage and current intensity.

Again, you need to know the electrical features of the equipment to plug into the contactor so that you can safely use the proper option.

Sometimes, rating is also related to the selection of the coil. However, the coil is selected according to ladder diagram and features of other components in the same circuit.

Any question? Write in the comments and I shall try to help.

Other stuff of interest

• Understanding the law of Boyle
• Measuring atmospheric pressure
• Some examples of temperature instruments
• Minor losses  - Formulas
• What is a process variable?
• What are the most important process variables?
• Time dependence of process variables
• A list of process variables
• Composition variables for mixtures

==========
Ildebrando.

What is plastic injection?

 Plastic injection is a manufacturing technique based on pouring liquid plastic into a mold with a desired form. This process is not knew and it is at the heart of almost all products you see in your daily life: from cars to spaceships passing through dishes and medical devices, for example.

For instance, the plastic bumper of a car is made by plastic injection. The same is true for the caps of some bottles. 

This industrial process requires different machines and different resines or polymers and although it seems a simple process a number of problems arise during operation. Some examples of plastics are,

• polypropilene (PP),
• high density polyethylene (HDPE),
• acrylonitrile butadiene styrene (ABS),
• polycarbonate (PC),
• among others.

• 

  Fig. 01 Here is a photo of the pellets in the form of small cylinders, ready to be used in a plastic injection machine.

Fig. 02 Here is a photo of the bag in which polymer pellets are sold. This is for high density polyethylene (HDPE).

Three important actors

The mold, the machine and the polymer are the most recognizeble actors in this process. Molds are specially designed and manufactured to hold high pressures of the order of thousands of psi. These are very expensive parts of the process. On the other hand, the machine comes in a variety of pressure capacities and automation features. Also, no any mold can work along any plastic injection machine. Finally, the polymer is key because the mold is designed with a given polymer in mind and this has to meet certain physical properties if good quality products are to be manufactured.

Any question? Write in the comments and I shall try to help.

Other stuff of interest

• Understanding the law of Boyle
• Measuring atmospheric pressure
• Some examples of temperature instruments
• Minor losses  - Formulas
• What is a process variable?
• What are the most important process variables?
• Time dependence of process variables
• A list of process variables
• Composition variables for mixtures

==========
Ildebrando.

a and b constants data for the van der Waals equation

 Here are some data for constants a and b appearing in the van der Waals equation:

Gas	Formula	a (atm L2/mol2)	b (L/mol)
Acetylene	C2H2	4.390	0.05136
Ammonia	NH3	4.170	0.03707
Carbon dioxide	CO2	3.592	0.04267
Ethane	C2H6	5.489	0.0638
Ethylene	C2H4	4.471	0.05714
Helium	He	0.03508	0.0237
Hydrogen	H2	0.244	0.0266
Hydrogen chloride	HCl	3.667	0.04081
Krypton	Kr	2.318	0.03978
Mercury	Hg	8.093	0.01696
Methane	CH4	2.253	0.0428
Neon	Ne	0.2107	0.01709
Nitric oxide	NO	1.340	0.02789
Nitrogen	N2	1.390	0.03913
Nitrogen dioxide	NO2	5.284	0.04424
Oxygen	O2	1.360	0.03183
Propane	C3H8	8.664	0.08445
Sulfur dioxide	SO2	6.714	0.05636
Xenon	Xe	4.194	0.05105
Water	H2O	5.464	0.03049

Please, be aware of the units.

Any question? Write in the comments and I shall try to help.

Other stuff of interest

• Understanding the law of Boyle
• Measuring atmospheric pressure
• Some examples of temperature instruments
• Minor losses  - Formulas
• What is a process variable?
• What are the most important process variables?
• Time dependence of process variables
• A list of process variables
• Composition variables for mixtures

==========
Ildebrando.

Does the state equation matter for Boyle's law?

 First, you need to know that the works on which Boyle's law are based are very old.

Boyle's law is due to the investigations of Robert Boyle by the years 1660! Follow this link if you want read the originial text.

On the other hand, the first equation of state, which is that for the ideal gas law,

$PV=n\text{R}T$

was assembled in 1834 by Emil Clapeyron. Notice, that I said assembled because, as in many fields of science, the ideal gas law is based on the findings of researchers before  1834, Robert Boyle included!

On  the other hand, the second most known equation of state is that of van der Waals,

$\left(P+\dfrac{an^2}{V^2}\right)\left(V-nb\right)=n\text{R}T$

in 1873! Then, what is clear at this point, in answer to the question, is that Boyle's law does not care about the state equation you are using.

Why is it that Boyle's law does not care about the state equation?

The answer is simple. Boyle's law describe the behavior of a system and although it was empirically determined it has been demonstrated its validity in many different systems. However, the state equations are based on Boyle's law (or the findings of Robert Boyle) and worst, the state equations (as the two mentioned early), not always work for every condition.

Any question? Write in the comments and I shall try to help.

Other stuff of interest

• Understanding the law of Boyle
• Measuring atmospheric pressure
• Some examples of temperature instruments
• Minor losses  - Formulas
• What is a process variable?
• What are the most important process variables?
• Time dependence of process variables
• A list of process variables

==========
Ildebrando.

About the encapsulated relay

 An encapsulated relay is a device used for closing NO contacts or openning NC contacts. These NO or NC contacts can be used for auxiliar or power purposes.

On application of the encapsulated relays is for electric isolation between two circuits. One of these can be potentially dangerous (high voltage) while the other one could be safer for operators (low voltage).

Other applications for encapsulated relays are known, such as: interfacing between actutators and controlling devices, etc.

How does an encapsulated relay looks like?

Well it depends on the model you are referring to. For example you may find some like those sell by ABB,

Fig. 01 ABB encapsulated relays.

Among the features that make these kind of relays we may list,

• different number of contacts NO/NC are available. You may have one contact only or four or more,
• there are options for soldering on pcb,
• other models are suitable for reduce space installation in control panels,
• other models are design to resist vibrations.

However, one feature all relays in Fig. 01 share is that these can be mounted on rail DIN.

The encapsulated relays are usually made of two parts. The relay module and the socket. The socket is the accesory that allows the relay module to be firmly mounted to the rail DIN.

Fig. 02 Here is an image of an encapsulted relay showing the pins (bottom metal sticks)

Fig. 03 Socket of an encapsulated relay. Notice that the terminals are numbered and are related to the pin and diagram of the encapsulated relay. These may change from one manufacturer to other.

How does an encapsulated relay work?

In order to give a more precise explanation we should refer to the diagram of an encapsulated relay. Take for instance the one shown below,

Fig. 04 Diagram of and 8-pin encapsulated relay. Numbers in parenthesis are related to the pin of the relay while numbers without parenthesis are related to contact numbers in the socket (see Fig. 03).

Contacts are formed with the terminals available. For example, in Fig. 03 terminals 24 and 21 are a normally open contact NO while 21 and 22 make a normally closed contact NC. As you can see, the encapsulated relay in Fig. 04 has two NO contacts and two NC contacts.

Terminals A1 and A2 are devoted to the coil of the relay. As electricity passes through it the coil is energized and an electromagnet is formed. As the electromagnet is formed the contacts change their position. This is, the NC contacts open and the NO contacts close.

Then, you may use low voltage to operate the relay coil and the contacts for higher voltage so that you are using the encapsulated relay to isolate two different circuits. This is why this device can be considered for safety when designing a control loop.

Fig. 05 Energizing an encapsulated relay

Any question? Write in the comments and I shall try to help.

Other stuff of interest

• LE01 - AC and DC voltage measurement and continuity test
• LE 02 - Start and stop push button installation 24V DC
• Some examples of temperature instruments
• Minor losses  - Formulas
• What is a process variable?
• What are the most important process variables?
• Time dependence of process variables
• A list of process variables

==========
Ildebrando.