IMPORTANT 'HVAC' FORMULAS

IMPORTANT 'HVAC' FORMULAS 

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TEMPERATURE CHARACTERISTICS - FAHRENHEIT&CELSIUS

TEMPERATURE CHARACTERISTICS

FAHRENHEIT&CELSIUS

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Electric motor Basics - Operating principles

In real life AC induction motors do not consist of magnets but of a physical rotor and stator.

The currents in the stator windings are generated by the phase voltages, which drive the induc-tion motor. These currents generate a rotating magnetic field, also referred to as stator field. The stator rotating magnetic field is determined by the winding currents and the number of phase windings.

The rotating magnetic field form the potential of the magnetic flux. The rotating magnetic field corresponds to electric voltage and the magnetic flux corresponds to electric current.

The stator rotating magnetic field rotates faster than the rotor to enable the induction of currents in the rotor conductor bars, thus creating a rotor magnetic field. The stator and rotor magnetic field generate their fluxes and these two fluxes will attract each other and create a torque, which makes the rotor rotate.

The operating principles of the induction motor are shown in the series of illustrations to your right.

Rotor and stator are thus, vital components in an AC induction motor. Stator and rotor are designed by sophisticated computer design programs. On the next pages, we will have a closer look at how stator and rotor are constructed.

                          Stator flux vs. rotor speed

Stator flux rotates
(i.e. 3000 min-1)

Rotor rotates slower
than the statorflux

i.e. 2900 min-1





The rotating stator flux is caused by the rotating sta-tor magnetic field which is formed by the currents in the different phase windings


                                      Generation of rotor flux

                     The rotor experiences that the stator flux rotates                                                                       at a speed of 3000 - 2900 = 100 min-1                                             

  The difference in speed causes currents to be induced into the rotor.
      These rotor currents generates a rotor flux.
The rotor flux is rotating at a speed of 3000 min^-1 (like the stator flux)


                        Generation of torque

The direction of the rotor flux generates two magnetic poles

The direction of the stator flux generates two magnetic poles.

The attraction of the rotor magnetic north pole towards the stator south pole and vice versa generates a force between stator and rotor. This force constitutes the motor torque that makes the rotor rotate.

Electric motor Basics - Reversing polarity with alternating current

Reversing polarity with alternating current

Magnetic polarity is continuously reversed by means of alternating current, (AC). Later, we will see how the rotating magnet is replaced by the rotor by means of induction. Alternating current is important in this regard, so a brief presentation should be useful:

Alternating current

By alternating current, we mean an electrical cur-rent that reverses in intervals and has alternating positive and negative values.

A rotating magnetic field can be created by using three-phase power. This means that the stator is connected to an AC source which supplies three separate current flows (also known as phases), all of them applied to the same circuit. A complete cycle is defined as having 360 degrees, which means that each phase is different from the others by 120 electrical degrees. They are illustrated in the form of sinus curves such as those presented to the right.

The poles change

On the following pages we will explain how the rotor and the stator interact and thus make the motor turn. In order to illustrate this clearly, we have replaced the rotor by a magnet that turns and the stator by a stationary part with coils. The illustration on your right-hand side, should be considered as a two-pole three-phase motor. The phases are connected in pairs like in a real motor; phase 1 consisting of A1 and A2, phase 2 consisting of B1 and B2 and phase 3 consisting of C1 and C2. When current is applied to the stator coils, one coil becomes a north pole and the other becomes a south pole. So, if A1 is a north pole, A2 is a south pole. The principle we can derive from this is that when the current is reversed, the polarity of the poles is also reversed.

                       The stages of movement.

Three-phase AC

Three-phase power is a continuous series of over - lapping alternating current (AC) voltages.

The rotating magnetic field on slow-motion

Applied alternating current

The phase windings A, B and C are placed 120 degrees apart. The number of poles is determined by the number of times a phase winding appears. Here, each winding appears twice, which means that this is a two-pole stator. It follows, then, that if each phase winding appeared four times, it would be a four-pole stator and so on.

When power is applied to the phase windings, the motor starts running with different speeds depending on the number of poles.

The rotor rotates

The following pages deal with how the rotor rotates inside the stator. Again, we have replaced the rotor with a magnet. Of course, all of these changes in the magnetic field occur really fast, so we need a step-by-step breakdown of the course of events.

To the right, we see how the current in winding A1 creates a north pole at this particular point in time. The magnet moves to make its south pole line up with the stator's north pole.

Having begun its rotation the magnet will try to follow the rotating magnetic field of the stator. .

As the purpose of this process is to keep the mag-net moving, the stator field will now change so that the process is continued. This maintains rotation in the same direction.

We have now begun to touch upon the matter of induction. The next section provides much more detail about this concept.

The phase winding's and number of poles

The phase windings A,

B and C are placed 120

degrees apart.

3-phase, 2-pole motor      3-phase, 8-pole motor

When an AC

supply is applied

Current flow in the

positive direction

Current flow

at zero

Current flow in the

negative direction

    Time 1          Time 2          Time 3

Electric motor Basics - Induction

Induction

The previous sections have established how an ordinary magnet, would rotate inside a stator. Alternating current AC motors have rotors inside them, not ordinary magnets. Our analogy is not far off, however, the rotor is polarised. This is caused by induction, where current is induced in the rotor conductor bars. The rotor is then polarised due to electromagnetism.

Induced voltage

The rotor basically acts just like a magnet. When the motor is switched on, a current flows through the stator winding and creates an electromagnetic field that rotates and cuts across the rotor bars. This induce current in the rotor bars which then create a electromagnetic field around the rotor and a polarisation of the rotor.

In the previous section, we substituted a magnet for the rotor for the sake of simplicity. We can do the same with the stator. The rotor field does not appear out of thin air; it is also the result of induc-tion. Induction is a natural phenomenon which happens when a conductor is moved through a magnetic field. The relative motion of the conduc-tor and the magnetic field causes an electric cur-rent in the conductor; a so-called induced current flow. This induced current in the rotor creates a magnetic field around each rotor conductor bar. As the three-phase AC power supply makes the magnetic field of the stator rotate, the induced magnetic field of the rotor will follow this rota-tion. The rotor is connected to the motor shaft, so naturally the motor shaft will rotate with it. If, for example, the motor is connected to a pump, it will begin pumping.

This is why AC motors are often called AC induction motors or IM (induction motors).

When power is applied to the stator, it generates an expanding magnetic field that cuts across the rotor conductor bars and induce a rotor current.

The magnetic fiels of the rotor is created by the current flow induced in the rotor conductors

                                                   Rotating magnetic field of stator

Electric motor Basics -Magnetism,Electromagnetism,Rotation from magnetism

Basic motor concepts

This section will look at how motors work. The objective is to provide basic information to serve as a background for more detailed studies. We will take a look at the concepts of magnetism, AC (alternating current), electromagnetism, motor construction, and torque.

Magnetism

All magnets share two characteristics: they attract metals such as iron and steel, and they will move to point north-south if nothing obstructs them. Another very important feature of magnets is that they all have a north pole and a south pole: unlike poles attract each other, whereas like poles repel each other.

Magnetic lines of flux

We can visualise the magnetic field – the invisible force that makes magnets behave the way they do – as lines of flux moving from the north pole to the south pole. In some cases, the north and south poles are not as easily identifiable as in the classic bar or horseshoe magnets. This is certainly the case with electromagnetism.

Electromagnetism

A magnetic field is created around an electri- cal conductor when an electric current is passed through it. This is known as electromagnetism, and the physical rules for ordinary magnetism also apply here. The magnetic field moves around the conductor.

Magnetic field around a conductor     The more current, the stronger the magnetic field 

The magnetic field around electrical conductors can be strengthened by winding them into a coil around an iron core. When the wire is wound into a coil, all the flux lines produced by each turn of wire join up to form a single magnetic field around the coil. The greater the number of turns of the coil, the greater the strength of the magnetic field. This field has the same characteristics as a natural magnetic field, and so also has a north and a south pole. But before we dig any further into the world of magnetism, let us have a closer look at the main components of an electric motor: the stator and the rotor.

It possible to reverse the poles by reversing the direction of the current

Rotor:

The rotating part of the motor, rotates with the motor shaft by moving with the magnetic field of the stator.

Stator:

The stator is the stationary electrical part of the motor. It contains a number of wind-ings whose polarity is changed all the time when an alternating current (AC) is applied. This makes the combined magnetic field of the stator.

Rotation from magnetism

Quite apart from their strength, the advantage of having a magnetic field which is created by a current-carrying coil is that it makes it possible to reverse the poles of the magnet by reversing the direction of the current. This ability to reverse the poles is precisely what we use to create mechani-cal energy. What follows is a brief look at how this works.

It possible to reverse the poles by reversing the direction of the current

Opposites attract

Like poles repel each other while unlike poles attract. Simply put, this fact is used to generate constant movement of the rotor by continuously changing the polarity in the stator. You could think of the rotor as a magnet which is capable of rotating. This will keep the rotor moving in one direction, and the movement is transferred to the motor shaft. In this way, magnetism is used to convert electrical energy into mechanical energy.

Engineers Guide to Pharmaceuticals Production

Engineers Guide to Pharmaceuticals Production

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What Is A Rotary Air Compressor?

What Is A Rotary Air Compressor? 

A rotary-screw compressor is a type of gas compressor that uses a rotary-type positive-displacement mechanism. They are commonly used to replace piston compressors where large volumes of high-pressure air are needed, either for large industrial applications or to operate high-power air tools such as jackhammers.

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Screw type compressor