In this article, we will study more about single-phase motors, with an emphasis on motors with three-wire connections. We will study what single-phase motors are, how they work, what types of single-phase motors, their applications, the advantages and disadvantages of single-phase motors, what is the difference between single-phase and three-phase motors and conclude with the example of how to install a single-phase motor with three wires.
Tips on how to connect a single-phase motor to three wires
Electric motors are responsible for the highest consumption of electric energy in an energy scenario, with that, it is noted the great importance of studies focused on this specific area, aiming to optimize the use of this machine and be more efficient. its operating principles to the engine installation.
Three-phase motors are the most used in the industrial sector, however, in homes, it is common to find single-phase motors, applied in various electronic/electrical equipment, such as refrigerators, freezers, washing machines, electronic gate sensing, among other applications. Therefore, the study of such machines is very important. The installation of single-phase motors depends on several variables, for example, how many terminals it has if there are three / four points, internal connection, where we will discuss throughout the text.
What is a single-phase motor?
The single-phase induction motors operate with a voltage between 220V and 127V, powered by a single-phase electrical energy network. Its field windings are directly connected to a single-phase network, this type of motor transforms the electrical energy consumed into mechanical energy.
Single-phase motors have only one set of coils and are powered by a single-phase alternating current. Electric motors are generally used in places where a three-phase network is not available, or in loads where they do not require much power, for example, fans, drills, printers, refrigerator motors and among other applications.
How do single-phase electric motors work?
Single-phase motors are more difficult to start than three-phase motors, since single-phase motors require only one phase, although there is a variation in the magnetic flux between the field windings and the stator and rotor armatures of the machine, the flow it does not have a lag, since it is aligned with the magnetic field of the stator and does not generate the rotating magnetic field, therefore it does not have a starting conjugate. For the single-phase motor to generate mechanical energy, there must be an angular lag, as it generates the rotating magnetic field and this process is carried out by some methods, such as split-phase windings, motors with a starting and permanent capacitor.
Types of single-phase motors
Split Phase Motor
This type of motor has a main winding and an auxiliary winding, both with a 90-degree lag, the main one is for starting and permanent working regime, the auxiliary winding is useful only when starting, with that, when the motor reaches at a certain speed the auxiliary winding is disconnected from the grid. As the auxiliary is only used to start the machine, if it does not turn off it may cause the engine to burn.
The auxiliary winding creates a lag producing the essential torque to start the rotation, breaking the starting torque and its acceleration until approaching the synchronous speed. This type of motor has a starting torque equal to or slightly higher than the nominal, which limits its application in fractional powers and in loads that require low starting torque. It is an engine suitable for specific applications, such as fans, exhaust fans, office machines, among others.
Starting Capacitor Motor
This type of motor is the most widely used and known, it looks like the split phase, but what differentiates this model is the introduction of a capacitor and
electrolytic, connected in series, as an auxiliary winding for starting single-phase motors. With the inclusion of the capacitor, the lag angle between the winding currents increases, thereby providing high starting torque.
When the motor reaches about 75% to 80% of the synchronous speed, the auxiliary circuit is disconnected, at that moment its operation is the same as a split phase motor.
This single-phase motor has a high starting torque (between 200% and 350% of the nominal torque), the starting capacitor motor can be used in a variety of applications that require a high starting torque. This type of engine is manufactured in powers ranging from ¼ hp to 15 hp.
Permanent Capacitor Motor
In this type of motor, the auxiliary winding and the capacitor are energized during the entire operating time and not exclusively at the start, the capacitor is of the electrostatic type. The effect of the capacitor is to provide conditions in which the flow is similar to that of three-phase motors, thereby increasing the maximum torque, the power factor and the efficiency of the motor.
This motor has a very small starting torque, about 50% of the torque of a split-phase motor, so it is crucial that there is knowledge of where it will be applied, it is recommended that this type of motor be used in light starting applications. . Single-phase motors with permanent capacitor are normally manufactured for powers from 1/50 to 1.5 hp.
Two Capacitor Motor (Start + Permanent)
The motor with two capacitors is the most advantageous and complete model among those that were previously approached, it has a starting as the starting capacitor motor, with high starting and operating speeds, equal to the permanent capacitor motor, therefore, exhibiting better performance and efficiency than other single-phase motors.
However, due to their high production cost, motors with two capacitors are found in powers above 1 hp. It is very common to apply this type of motor in rural areas, where greater power is needed in single-phase installations.
Difference between single-phase and three-phase motor
The main difference that can be observed between a single-phase motor and a three-phase motor, is that in the three-phase motor it can induce a rotating magnetic field in the rotor without the aid of auxiliary coils, because it is fed by a network in three phases out of 120 °.
Single-phase motors are physically larger when compared to three-phase motors of the same power, this is due to the fact that three-phase motors are powered by three phases, thus, the power is three times higher than in a single-phase circuit.
Applications for a single-phase motor
Single-phase motors are generally used in places where there is no three-phase power supply, such as in homes, rural areas, businesses and small industries. As it is an engine that operates at low power, it is recommended for small equipment such as fans, electric drills, printers and many others.
Connection of single-phase 3-wire induction motors
When the motor has four terminals that exit the windings of the machine or three terminals, one of the windings is common, being possible to identify it with the aid of a multimeter, measuring the resistance of two winding points of the single-phase motor. These motors have a terminal with the ohmic resistance approximately twice the others, the same is called auxiliary winding, used to start the motor, when the motor reaches its rated speed of rotation the centrifugal switch is turned off, thus using only the work terminal
The two windings with practically the same resistance show that the motor acts with the permanent capacitor, with the same circuit, but without a centrifugal switch. The following is a video showing the steps on how to identify the coil cables of the single-ended three-phase motor, where the outputs of the single-phase induction motor are identified: