If I ask you now what comes to your mind when I speak electric motor, you will probably think of applications like air conditioning, fans, and household appliances.
The electric motor is the main electrical equipment used in industries, currently, it is estimated that almost half of the consumption of electric energy in the world is related to the use of electric motors. The electric motor transforms all electrical energy that enters the cables into mechanical energy by rotating the motor shaft.
But to understand the operation of the motor we need to know first what are the parts of the motor, I will explain the operation using a three-phase induction motor.
Rotor and stator
The three-phase electric motor has two main parts, what we call a stator and what we call a rotor, this construction applies to all three-phase induction motors.
The rotor is fixed on an axis. It corresponds to the rotating part of the machine, that is, it is the part of the engine that will rotate.
The stator is the fixed part of the motor. It is in the stator that the coils responsible for creating the magnetic fields that will be responsible for the rotation of the motor are wound.
The stator and rotor are the two main parts to understand how an electric motor works, but there are still the other parts, which are basically the mechanical construction parts of the motor.
The bearing, in fact, the bearing system are parts that hold the shaft in the motor housing using bearings, The bearing itself and the gaskets.
The bearing reduces friction considerably so that the shaft can rotate.
The gaskets guarantee those who do not infiltrate water and dust, guaranteeing the IP level specified for the engine model.
The housing is the structure that holds the stator and the bearings. Something very interesting, and that few people know is that the vast majority of three-phase motors have side fins that look like those in transformers, these fins are used to dissipate the heat that is generated inside the motor.
The frame also contains the feet for fixing the motor and the point for placing the eye, which is the part used to lift the motor for transportation.
The engine housing varies widely from model to model and has a different construction for certain types of engine use. For example, an engine used for a water pump that has a very different housing than an engine used for a conveyor belt.
A propeller is installed on the rotor, it has a different shape from the traditional one used in fans.
This format was designed to work together with the protective cover to help heat dissipation of the housing, unlike a traditional fan propeller that propels air forward or backward. The propeller used in engines propels air to the sides.
This cover is for protection against touching the propeller. The space between the cover and the fins is part of the project so that the air moved by the propeller passes through the fins, to help with cooling.
As the air is moved by the propeller, the shape of the protective cover and the space between the cover and the fins direct the air to pass through the middle of the fins.
Finally we have the motor connection box, in this box the motor power cables are connected.
The three-phase electric induction motor has a very simple construction and operation, which is why this type of high efficiency motor is so used in the industry.
To understand how it works, it is necessary to know a rule of electromagnetism, every time an electric current passes through an electrical conductor an electromagnetic field is formed in this conductor.
If we wind a conductor in several turns and pass an electric current through this coil, we will have an even greater magnetic field.
If we think of a simple motor with only 3 coils, we can combine these coils and connect them in a three-phase network and we will have three electromagnets producing a magnetic field.
In our drawing we have a stator with 3 coils, coil 1 has tips 1 and 4, coil 2 has tips 2 and 5 and coil 3 has tips 3 and 6.
If we connect ends 4, 5 and 6 to each other we have what would be a star connection and we have ends 1, 2 and 3, that we can connect the phases of a three-phase system.
The three-phase current is formed from three equal alternating currents, except that these currents are displaced from each other in time, this displacement has a difference of one third of the cycle, that is, when one current is at maximum, the other two will not be maximum.
In the figure that appears on the screen we have the three currents I1, I2 and I3 of a three-phase system at the top and at the bottom the magnetic fields that are generated by the stator coils will appear at each moment.
At the first moment T1, the current I1 is zero, I2 is negative and I3 has a value equal to I2 but positive. Thus, the stator coils are traversed by the three currents, establishing a magnetic field directed downwards, with north pole above and south pole below. In our image the pink parts represent the magnetic field.
In the second moment T2, the current I1 has a maximum positive value, while I2 and I3 are negative with half the maximum value. The magnetic field is now directed to the left, the north pole is on the right side and the south pole on the left side. The field had a 90º turn clockwise, in relation to T1.
The third instant T3, the situation is very similar to the instant T1. I1 is again zero, but the polarities of current I2 and I3 have been inverted, now I2 positive and I3 negative. The field turned another 90º, completing 180º in the same clockwise direction.
At the fourth instant T4, the three currents met in opposite positions to those at instant T2, with I1 negative, and I2 and I3 positive. The field has now turned 270º
In the fifth and last instant T5, the situation of currents and the magnetic field is the same as that of T1, the magnetic field goes down again with north pole above and south below completing 360º and the cycle starts again.
In our electrical network, this process is repeated 60 times per second considering a frequency of 60 Hz.
With this construction the motor guarantees a magnetic field that rotates continuously clockwise.
This magnetic field is sufficient to drag the motor rotor in the direction of rotation of the stator’s magnetic field
In a motor with a cage-style rotor, the rotating speed of the rotor is always less than the rotating speed of the magnetic field of the stator.
For this reason, this type of three-phase motor is called asynchronous, that is, the rotor speed and the stator field are not synchronous. This difference in speed between stator and magnetic field is what is called the study of slip motors.
Losses in three-phase induction motors
Electric motors are much more efficient machines than when compared to a combustion engine, but there are still some losses.
Not all electrical energy that is injected into the motor turns into movement, some is lost in heat. We even talked about how the fins on the housing help to dissipate that heat.
The type of metal used in all parts of the engine and the quality of the copper are also essential to reduce losses.
Winding copper losses
We have two main points of losses, the first is in the copper of the windings. And considering the losses in copper we can talk about the following losses:
Losses in the ohmic resistance of the windings:
This type of loss deals with the dissipation of power in the form of heat, due to the Joule effect. This loss in electric induction motors with squirrel cage rotors occurs in the stator, in the copper coils and also in the rotor cage, which are made of aluminum bars. Basically it is the principle of a resistor, the measure that current passes in a conductor the resistance causes heating
Parasitic losses in the winding conductor:
These are losses produced by eddy currents induced in the coil conductors by the dispersion flow. In general, these losses are proportional to the dispersion flow, to the copper mass and to the square of the dimension of each conductor through which the dispersion flow passes. In the principle of magnetism, a conductor through which an electric current passes generates a magnetic field, the opposite is also valid, as we induce a magnetic field of the armature to transform it into an electromagnet, we have a current generated, an eddy current.
Loss in stator metal
The second main point of loss is in the metal of the stator, and considering the stator we have the following losses:
These are losses caused by the property of ferromagnetic materials to have a delay between magnetic induction and the magnetic field.
Losses by eddy or eddy currents:
When an alternating current is flowing through the winding, a variable magnetic field appears in the core. The variation of this field, increasing and decreasing, induces a tension in the nucleus and this electromotive force causes the circulation of eddy currents.
The currents circulate in the ferromagnetic material, causing heating, that is, the energy is generating heat instead of being transferred to the motor load.
The smaller the thickness of the plate and the greater the resistivity of the material, the lower the losses due to eddy currents.
Therefore, the stator is constructed with laminated material, with small thickness and with chemical composition that results in a material of high resistivity.
Induction motor with permanent magnet
The WEG W22 IR5 Ultra Premium Motor is a permanent magnet synchronous motor also called W22 Magnet.
The W22 Magnet motor has a three-phase stator winding similar to the induction motor. If we compare it with the stator of the traditional model, you can see the similarity.
However, the rotor is mounted with permanent magnets instead of the cage. They are made with the combination of Neodymium, Iron and Boron, being called Rare Earth sisters. Here in the cut we can what appears to be a plate is actually the permanent magnet.
Permanent magnets eliminate the need for current induction in the rotor, which is the magnetizing current, after all the magnet is already magnetized.
In this case, without a load, the motor has a very low current value, just to supply the losses.
In addition to the magnetizing current, the W22 Magnet motor also does not require slip compensation, as the axis speed does not vary with the load.
The permanent magnets inserted in the rotor of the motors guarantee a great reduction in the electrical losses, and consequently in the temperature of the motors, allowing reduction of the size of the frame.
Anyone who studies electrical and knows a little about motors and the industry, knows that saving space and reducing temperatures in industrial environments are very important.
W22 Magnet motors have superior performance regardless of speed or load, and can reach up to 30% savings when compared to induction motors driven by frequency inverters.
Efficiency in electric motors
There is no way to talk about engines and not to talk about performance index. We talked about two models of three-phase induction motors that look alike, but have very different yields.
In Brazil, the engine performance index is known as IR and until 2009 there was no regulation for minimum efficiency levels. As a result, the engines were mostly IR1 or Standard performance.
Since 2009, Ordinance No. 553 has established minimum income levels for three-phase induction electric motors. In this way, machine manufacturers and end consumers started to use products that meet at least the level of income IR2.
Now in 2019 a new law will come into force that determines the minimum level of income in IR3 for a power range of 0.16 to 500 hp, from 2 to 8 poles. This law is valid for all engines sold, whether new or used.
And why does it matter? The Brazilian industrial sector consumes about 40% of the country’s electricity, and 70% of the energy used in industry is consumed by electric motors. In this way, more efficient engines guarantee energy savings.
The engines along their trajectory have undergone modifications to always improve performance, precisely so that energy is used in the best possible way.