Modern equipment uses slightly different types of electric motors. Different in design, characteristics and principle of operation, all these engines are selected for each specific case according to their parameters. At the same time, quite often in devices and equipment, electric motors are needed with the ability to connect to a single-phase network. One of the suitable options is a capacitor motor, the device and principle of operation of which we will consider within the framework of this article.
Content:
- Device and principle of operation
- Types of capacitor motors
- How to choose a capacitor for a starting capacitor
- Scope of practical application
Device and principle of operation
Speaking of capacitor asynchronous motors, we will primarily talk about electric motors, originally designed for connection to a single-phase network. This has something in common with two-phase or three-phase motors, converted to connect to a conventional single-phase 220 volt network. But the essential difference between these electric motors is that here
capacitor acts as a prerequisite for the electrical circuit and the inclusion of such an asynchronous motor in a three-phase 380 volt network is simply impossible.The design and operating principle of a capacitor motor are based on physical properties asynchronous motor, but to create a driving force and rotation of the magnetic field, a starting capacitor is included in the winding circuit.
In terms of its structure, it does not differ from a conventional asynchronous device and includes:
- Fixed stator in a massive casing with working and starting windings.
- Shaft-mounted rotor driven by an electromagnetic field generated by the stator windings.
Both parts of the electric motor are connected to each other by rolling or sliding bearings (bushings) fixed in the stator housing covers.
According to the principle of operation, a capacitor motor, as noted above, refers to asynchronous - movement is carried out due to the creation of an electromagnetic field by the stator windings, shifted relative to each other by 90 degrees. The only difference from three-phase asynchronous electric motors is the capacitor included in the circuit, through which the second winding of the electric motor is turned on.
A conventional asynchronous motor, when connected to the network, begins to work with a starting winding. After the rotor has gained speed, the starting winding is turned off and only the working winding continues to work. The disadvantage of such an electric motor with a starting winding is the starting moment, when the rotor starts to speed up. It is important for the electric motor that at this moment there is no load, or the load is small. The starting torque is lower than that of three-phase motors of similar power.
In the connection diagram of a capacitor induction motor, there is a phase-shifting capacitor. When connected to the network through a capacitor, a phase shift of 90 degrees occurs in the second winding (in practice, a little less). This contributes to the fact that the rotor is turned on with the maximum possible torque.
This starting ensures that the engine is turned on both at idle speed and under load. This is very important for connecting the motor under load. In practice, according to this scheme, a motor is connected from a washing machine of old models. At the moment of starting, the engine should begin to rotate the water in the tank, and this is a significant load on the electric motor. In the absence of a starting capacitor, the engine will not start, it will hum, warm up, but will not work.
Types of capacitor motors
The connection diagram, in which a capacitor induction motor is started only from a starting capacitor, has one significant drawback. During operation, the magnetic field does not remain circular or elliptical, performance drops, and the motor heats up. In this case, for an optimal mode, a working capacitor is included in the circuit, which provides a constant phase shift, and not only at the moment of start-up.
Note that two groups of capacitor motors can be distinguished:
- A capacitor is needed only for starting, then it is called a starting capacitor. These are usually low-power devices.
- A capacitor is needed for continuous operation, in this case it is called a working capacitor. In machines of high power (several kW), there may not be enough torque for starting under load, and then an additional starting capacitor is connected. Most often this is done using the PNVS button.
You can learn more about the connection diagram and how to distinguish these types of single-phase motors in the following video clip:
In the international classification, designations are used for the types of capacitor induction motors:
- Motor with capacitor start / winding (inductance) (CSIR);
- Motor with capacitor start / capacitor operation (CSCR);
- Permanently split capacitance (PSC) motor.
How such a scheme works is easy to imagine: a large-capacity starting capacitor provides engine start, and after gaining power, a worker with a smaller capacity ensures the most suitable operating mode and rotation speed rotor.
For special cases, when it is necessary to maintain the required rotor speed at different loads for working capacitors, different capacitors are selected with the possibility of switching them.
To change the direction of rotation, in other words, turn on reverse, you need to swap the ends of one of the windings. It is convenient to use a 6-pin toggle switch for this.
How to choose a capacitor for a starting capacitor
It should be said right away that the capacity of the starting and working capacitor (or only the working capacitor if the starting capacitor is not needed) is usually indicated on the motor nameplate. At the same time, the exact data characteristic of this particular electric motor with its design and operation features are indicated.
If the nameplate is overwritten or missing, then it is possible to calculate the capacity of the working and starting capacitor for a single-phase capacitor rather not by the formula, but by the mnemonic rule:
The sum of the working and starting capacitor should be 100 μF per 1 kW of power (70% starting and 30% operating). If the motor is 1 kW, then the working capacitor is needed at 30 μF, and the starting capacitor - at 70. And the capacitors themselves must be designed for a voltage greater than in the supply network. Usually choose about 400 volts.
But in the literature, you can also find recommendations that the capacity of the starting capacitor should be 2 times greater than the capacity of the working capacitor.
How to check the performance of a capacitor will be suggested by the article posted on our website earlier - https://samelectrik.ru/kak-pravilno-proverit-rabotaet-li-kondensator.html
Scope of practical application
Asynchronous capacitor motors are used in household electric fans, refrigerators, some modern washing machines, almost all washing machines made in the USSR. But in hoods, motors with shaded poles without a capacitor are more often used, however, models can also be found with the type of electric motor under consideration.
In addition to household appliances, their scope of application also extends to pumps with a capacity of up to 2-3 kW, compressors and various machines with a single-phase power supply, in general, for everything that needs to rotate and work from 220 Volts.
So we examined what a capacitor motor is, how it works and what it is for. We hope the information provided has helped you understand the issue!
Related materials:
- How to connect a three-phase motor for 220 and 380 volts
- Online calculation of energy in a capacitor
- What is the difference between direct current and alternating current
