We are going to study about single-phase motors, giving emphasis and focus to single-phase motors with 04 terminals (four wires). Let’s find out what single-phase motors are, what are the characteristics of single-phase motors, the basic difference between single-phase motors and three-phase motors, the advantages and disadvantages of single-phase motors and finally how to supply and connect the single-phase motors.
What is a single-phase motor?
Single-phase motors receive this name, because they work at a voltage between 220V and 127V they receive electrical energy from a single-phase network and their field windings are directly connected to a single-phase network, these motors transform electrical current into mechanical energy to perform different types of job.
Single-phase induction motors are the solution for places where there is no availability of the three-phase electrical network, they have only one set of coils and their supply is usually made by one or two phases of alternating current, they are applied in loads that require low power such as fans, refrigerator motors, drills, and many others.
The operation of a single-phase motor?
Single-phase motors do not have the same facility to generate a rotating magnetic field as three-phase motors, so they need an auxiliary winding to generate an angular lag, these auxiliary coils are positioned in order to create a “fictitious second phase” allowing the formation of a rotating field necessary for the engine to start. Capacitors are present in single-phase motors to assist in the creation of this rotating magnetic field, as they are able to advance the current by 90 °, there are generally two types of operation for capacitors: those that work in steady-state and those that start. after the match it is disconnected).
Observe the diagram of a single-phase electric induction motor:
Types of single-phase induction motors
Split Phase Motor
These single-phase induction motors have a main and an auxiliary winding as previously informed. The main winding is used for starting and permanent work regime, whereas the auxiliary winding is used only for the moment of the motor starting. The two windings are mounted on the motor 90 ° out of phase, in the space of the motor stator.
The auxiliary winding creates a phase shift, this lag produces the torque required for the initial rotation, breaking the motor starting torque and accelerating it until it approaches the synchronous speed. When the motor reaches its synchronous speed of rotation, the auxiliary winding is disconnected from the mains using a switch type switch, this switch is usually actuated by a centrifugal force (centrifugal switch). These motors have a starting torque equal to or slightly higher than the nominal, which limits their application in fractional powers and in loads that require low starting torque, such as fans and exhaust fans, small polishers, emery wheels, hermetic compressors, small pumps centrifuges, dishwashers, etc. It is an engine suitable for specific applications.
Motor with Starting Capacitor
This single-phase induction motor is the most widely used and known type of motor, it is similar to the split-phase motor, however to improve the starting torque, an electrolytic capacitor is installed, connected in series with the auxiliary starting winding. When the motor reaches its synchronous speed, the auxiliary circuit opens, at this point its operation is the same as that of the 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: compressors, pumps for swimming pools, rural equipment, industrial air conditioners, tools in general. it is used in equipment that requires high starting torque.
Permanent Capacitor Motor
In this type of single-phase induction motor, the auxiliary winding and capacitor are fed into the circuit at all times and not only at startup. The effect of this capacitor is to create flow conditions very similar to those found in three-phase motors, thereby increasing the maximum torque, efficiency and power factor.
This type of motor has a very low starting torque of about 50% of that of a single-phase split-phase motor, so it is necessary that its application is well studied, it is recommended for applications that require light starts, such as: fans, hoods, blowers and office machines.
Because they do not contain moving parts and have no moving contacts, it is an engine that does not require much maintenance. Due to its robustness, simplicity, versatility of use (it allows instant reversion), in addition to its performance being more efficient than the others, it is a type of engine that has an increasing demand.
Two Capacitor Motor (starter + permanent)
The most complete single-phase induction motor, which uses the advantages of the previous models, it starts like the starting capacitor motor (high torque) and can break high load torque and operate in the same regime as the permanent capacitor motor, presenting better performance and income. Due to the high cost of production, they are usually found in power above a horse, where an engine with good performance under load and at start-up is required. This type of motor is common in rural applications, where considerably higher power is required in single-phase installations.
Difference between single-phase and three-phase motor
The main difference between single-phase motors for three-phase motors is that the three-phase motors, being powered by a network in three phases out of phase by 120 °, can induce a rotating magnetic field in the rotor, without the aid of auxiliary coils.
Single-phase motors are larger compared to a three-phase motor of the same power, this is due to the three-phase motor being powered by three phases, so it is as if the motor had three single-phase powers of the same intensity in its circuit, that is, its power is three times more than in a single-phase circuit.
Connection of single-phase induction motors.
When there are four terminals or four ends of the windings or 3 (in this case one of them is the common one), we can measure the ohmic resistance of the two windings using a multimeter, the auxiliary winding (starting winding) will almost always have about twice the working winding resistance, the centrifugal switch is turned off after starting, when the engine reaches approximately its rotation speed.
When we find two windings with approximately the same resistance, we know then that it is a motor that works with a permanent capacitor, the closing is likewise but there is no centrifugal starting switch in the circuit.
We have a step-by-step video on how to identify the cables of a single-phase three-point motor, see how we perform the identification of the single-phase induction motor outputs:
We can close the four-terminal motor in two ways, in parallel when we close terminals 1 and 3 and feed with a phase and close terminals 2 and 4 powered with neutral. In this closing, the motor can be powered by a lower voltage in the case of 127V:
In the second mode, we close terminals 3-2, isolating them (in this mode we close the two coils in series), and supply terminal 1 with one phase and terminal 4 with another phase. In this way we have the motor closing at the highest voltage it will support the 220V voltage.