In most cases, the power source for residential buildings is a single-phase electrical network with a voltage of 220 volts. Therefore, in everyday life, electrical appliances are used that have an asynchronous single-phase motor in their design - a mechanism with a power of no more than 1500 watts.
Content
- Design and principle of operation
- Capacitor device
- Bifilar engine
- Direction of rotation
- Collector motors
Design and principle of operation
In terms of its structure, a single-phase electric motor is similar to a three-phase one. They both have a starter and a rotor. The main difference lies in the number of windings (there are two of them for a single-phase).
The rotor starts moving when the winding creates a magnetic field. Its peculiarity lies in the fact that it does not rotate as in a three-phase motor, but pulsates, therefore can be decomposed into two:
-
Direct. It moves with synchronous speed in the same direction as the rotor, forming the main electromagnetic element. - The opposite. It rotates in the opposite direction to the movement of the rotor, therefore the rotor speed is negative relative to the electromagnetic field.
Due to the emergence of an electromotive force, currents begin to flow through the rotor with frequencies proportional to slip. In this case, the value of the frequency of the reverse field current significantly exceeds the value of the current frequency in the forward field.
An increase in inductive resistance leads to the fact that the current in the reverse field demagnetizes the magnetic flux. Because of this, the torque directed against the rotation of the rotor is small.
A non-rotating rotor keeps the axis stationary between the two magnetic fields, so the motor does not run. To start it, you need to spin the rotor, forcing the axis to move. The rotor must rotate in a circular magnetic field created by two types of winding (starting and working). In order to achieve the maximum resulting torque, magnetomotive forces must be:
- are equal;
- perpendicular;
- offset by 90 degrees.
If these conditions are not met, the magnetic field will take the shape of an ellipse. The increased braking torque of the reverse field will cause a decrease in the value of the resulting torque.
The required phase shift of the magnetomotive forces is created using phase-shifting elements. The role of such an element can be a coil, a capacitor or an active resistance. A single-phase asynchronous electric motor with active resistance is widespread. The winding in it has a reduced cross-sectional area, which makes it possible to increase the resistance. Due to the fact that the starting winding works for a short time, it does not have time to fail.
Capacitor device
In motors, where a capacitor serves as a phase-shifting element, the winding functions constantly. The connection of such a device can be carried out according to different schemes. The first, implying the presence of a capacitor in the starting winding circuit, guarantees a good engine start, but does not provide sufficient power (it turns out to be much less than the nominal).
A capacitor located in the working winding circuit gives the opposite result. With good performance, the engine starts poorly. Connecting two capacitors at the same time is the most efficient because it combines the advantages of the first two circuits. Wherein a push trigger is used, which turns on the capacitor only during the start-up period. To start motors with one capacitor, a regular button, automatic machine or toggle switch is suitable.
In order for a capacitor single-phase electric motor of an asynchronous type to show high efficiency, it is necessary to correctly calculate the capacity of the capacitors. When calculating, one should be guided by the rule that 1 kW of motor power corresponds to 0.7-0.8 μF of the capacity of the working capacitor. For a starter, the indicators will be 2-3 times higher. The operating voltage of the capacitors must be 1.5 times the mains voltage.
Bifilar engine
An asynchronous device that works without a capacitor (only due to the starting and working winding) is also called bifilar. To connect it, you must use a push starter, which has a in the middle, the contact closes for a hold period, and the rest are in a closed state. When marking such switches, a number is used that indicates the current strength for which they are designed.
Before you start connecting the device, you should determine the type of winding by measuring the resistances on each of them. The coil, the indicator on which will be higher, is the starting coil, the other is working.
After the measurements have been carried out, it is necessary to connect the ends of the windings and connect them to the extreme terminal of the switch. Connect the free end of the working winding to the second terminal, connect the middle terminal with the remaining wire of the starting coil. Connection should only be made with terminals located on one side of the push starter. The ones on the other side are for the power cord and the jumper coming from the terminal with the work wire.
When all connections are made, you need to carry out a test run of the engine. Having connected the plug to the outlet, press the start button and hold it until the motor picks up speed. After a few seconds, the button must be turned off.
Direction of rotation
In some cases, when the engine is turned on, the shaft starts to rotate in the wrong direction. To solve this problem, you will have to change the position of the terminals in the place where one was connected to the switch, and the other to the end of the working winding.
Rotation of the shaft in both directions can be ensured by installing the reverse toggle switch. It should have two or three working positions and six pins. In the process of installing such a toggle switch on a capacitor motor, it is necessary to provide for the possibility of switching it during the operation of the motor. The central terminals need to be connected to the wires of one of the windings, the extreme ones must be connected diagonally and two wires removed. Connect the retracted wires to the places where the ends of the winding were. After that, the motor will spin in both directions.
Collector motors
In addition to asynchronous, some electrical appliances can be equipped with collector motors that have structural differences. Mandatory elements of their design are special brushes and a copper drum divided into sections. The advantage of motors of this type is the high number of revolutions at the time of start and after acceleration. The direction of movement of the shaft in them is regulated by changing the polarity.
Also in such electric motors the rotation speed can be changed. This property allows the use of collector motors in various household and construction equipment. Their disadvantage is the strong noise emitted during operation. Asynchronous ones work with less noise.
