Electric motors are machines designed to transform electrical energy into mechanical energy. Asynchronous induction electric motors are the most used among all types of motors, as they combine several advantages in their use such as low cost, ease of transport, cleaning, simplicity of control and practical and cheaper maintenance compared to other motors. In this article we will discuss what are the types of induction motors, the principle of operation of asynchronous machines, star closure and a triangle of motors with six terminals.
What is a three-phase motor?
When we talk about applications that require more electrical power to perform work, we cannot use a single-phase or two-phase system as we find in the electrical network in our homes. For these situations, we use three-phase motors that offer three times more power and efficiency, as it is as if we have three single-phase circuits.
In the world most of the energy is distributed in a three-phase manner, this is due to the advantages of the three-phase circuit in relation to the single-phase, see the advantages presented by a three-phase circuit:
1- Three-phase generators use their windings more efficiently – this makes them lighter and more compact.
2- Three-phase motors are also reduced in size compared to an equivalent single-phase motor.
3- The section, that is, the thickness of the cable required in a three-phase system is half the section required in an equivalent single-phase system.
4- A three-phase line can offer different voltages to the user.
The basic operation of a three-phase electric motor
Three-phase motors such as the three-phase induction motor (squirrel cage) have their principle of operation in the angular lag between the three power phases. Observe the angular lag represented by a three-phase network at a frequency of 60 Hz.
This angular lag of the three-phase network generates a magnetic field, rotating in the motor stator coils. This magnetic field is created according to the frequency of the electrical network. The rotating magnetic field generates an electromagnetic induction in the motor rotor, which in turn generates a counter-electromotive force inverse to the original field, thus, the rotor starts to rotate trying to reach the speed of the rotating magnetic field.
The rotor will never reach the speed of the rotating field in this type of motor, if this occurs it loses the rotation torque and stops rotating. We name this speed lag between the rotating field and the motor slip rotor, which is the difference between the synchronous speed of the network and the rotation of the axis of the asynchronous machine.
Observe the slip calculation:
How to calculate the synchronous speed of an engine, just use the formula to calculate the speed:
Types of three-phase motors
There are two main groups of three-phase motors, which work with alternating current, they are the asynchronous motors and the synchronous motors.
Synchronous motors, different from the principle of operation discussed, work without interference from slipping, that is, it is free of the rotor speed lag with the rotating magnetic field, thus it works in synchronous speed, therefore, its speed is in synchronization with the supply network, that is, it is the same speed as the angular speed of the rotating magnetic field in the stator.
These engines do not vary their speed in relation to the axle load. Asynchronous motors are generally used at higher power levels due to their high cost of production in smaller sizes.
In the case of asynchronous motors (Induction motor), the rotor works with a speed lag (slip), in relation to the rotating magnetic field, a factor caused by the 120º angular lag of the three RST phases. These engines vary their speed according to the load applied to the shaft end, due to its practicality and robustness it is the most applied engine in the world.
Six-terminal three-phase motors:
In three-phase squirrel cage motors (asynchronous), we find six output terminals, that is, six cable ends in its connection box. This is due to the configuration of your coils, note the coil layout:
Observe the three main groups of coils, the beginning and end of which are respectively 1-4, 2-5 and 3-6.
Remember that before starting any motor, always perform the coil continuity tests, using a multimeter, this test prevents you from starting a motor with a group of open coils. To perform the test, simply measure the beginning and end of each coil, for example, with the multimeter tips perform the test from terminal 1 to 4 if there is continuity the coil is conducting electrical current, that is, it is in good conditions.
After carrying out the continuous tests, we can now perform the motor closure. We can perform two types of closure: star closure or triangle closure.
The triangle closure
It provides the closing in the Line voltage (to understand more about line voltage see the link), that is, the lowest voltage on the motor plate.
For example: a motor that supports 220V and 380V, when we perform the delta closing, it will support the voltage of 220V. To perform the delta closing, we need to close the terminals 1-6, 2-4 and 3-5, energizing them in the R – S – T phases observe:
The star closure
Star closure allows the motor to withstand higher line voltage, ( to understand more about line voltage ).
When interconnecting terminals 4-5-6, we close a star connection on the motor, so in terminals 1-2-3 we can apply the higher plate voltage. In this way the motor can be connected to a voltage line in the amount of:
220V x √3 = 380V
(the root of three and a constant that comes from the deduction of the lag between the phases of the 120º three-phase system, and we apply the parallelogram rule and using the cosine law to arrive at this deduction), so we do not change the voltage in the winding, even applying the 380V line voltage. See the diagram below:
To fix more about the concepts covered, line voltage and phase voltage be sure to watch the video to understand the differences between the coil closures of the six-terminal motor.
We can understand the importance of three-phase asynchronous and synchronous induction motors, as well as the principle of operation of asynchronous motors. With what we have learned, we hope that you can close the six-point three-phase induction motors without major difficulties, in addition to knowing when we can perform the star closure and when we perform the triangle closure, we expect the concepts of line voltage and phase are well fixed.