2024 Author: Erin Ralphs | [email protected]. Last modified: 2024-02-19 11:41
An induction motor is an electrical machine designed to convert electrical energy into mechanical energy. The design consists of several parts, but today we will consider only the moving part of the electric motor - the rotor. We will also pay attention to how the rotor of an induction motor with a phase rotor is arranged.
Rotor design
Most often, the device of the rotor of an induction motor looks like this: the rotor is a steel shaft, on which plates of cold-rolled anisotropic electrical steel are pressed. The rotor is made of plates, which are isolated from each other by a layer of oxide film. This is necessary to reduce eddy currents that affect motor efficiency.
Types of rotor windings of an induction motor
Next, we will analyze one more point. We have to find out what the rotor windings of an induction motor are, what they are for, varieties, design features, as well as laying methods. There are 2 types of rotor winding: squirrel-cage and phase rotor. A squirrel-cage rotor is more common, it is cheaper to perform than a phase rotor.
Motors with such a rotor require less maintenance than with a phase rotor. The phase rotor is used less frequently, it is a little more expensive in execution, and also requires more frequent maintenance due to the presence of slip rings. Further it will become clear why the engineers introduced this design. Now let's talk more specifically about each rotor.
Squirrel-cage rotor
On the rotor of an asynchronous electric motor there are windings that are filled or soldered into grooves. For machines of low and medium power, the winding material is usually aluminum, and for more powerful ones, copper. This is necessary to create an electromagnet that will, as it were, follow the rotating magnetic flux. The rotor is magnetized under the influence of a magnetic field rotating in space.
This is how it turns out that the rotor has its own magnetic field, which, as it were, follows the rotating magnetic field located in the stator. This design of the rotor windings is called the "squirrel cage". The squirrel cage is in direct contact with the rotor, and, like a transformer, a magnetic field is induced on it, and, accordingly, a certain electromotive force. Despite this, the voltage is zero. The rotor current of an induction motor varies depending on the mechanical load on the shaft. The higher the load, the higher the current flowing in the rotor windings.
Phase rotor
The main part of the structure is arranged like a squirrel-cage rotor. All the same steel shaft, on which plates of electrical steel with grooves are pressed. A feature of the rotor of an asynchronous motor with a phase rotor is the presence in the grooves of not a flooded or soldered winding, but a conventional copper winding laid, as in a stator. These windings are star connected.
That is, all ends are in one twist, and the remaining 3 ends are brought out to slip rings. The phase rotor is made to limit the starting current. Copper-graphite brushes are attached to the slip rings and slide over them. Then, contacts are usually removed from the brushes into a branded box, where the starting current is regulated either by a rheostat or a liquid rheostat by changing the depth of immersion of the electrodes in the electrolyte.
As already mentioned, this measure allows you to limit the starting current. To reduce brush wear, modern electric motors are equipped with a design that, after starting, reclines the brushes and short-circuits all windings to each other. When the engine stops, the brushes return to their place.
Features of maintenance of the drive with a phase rotor
Maintenance of the rotor of an induction motor with a phase rotor is a regular inspection of brushes, slip rings, checking the condition or fluid level in the rheostat. It is also worth inspecting the immersed electrodes. According to the results of the inspection of the rotor of the asynchronousmotor with a phase rotor, if necessary, the brushes must be replaced, but the craftsmen still immediately advise wiping the slip rings and the cavity where the rings are located with a rag. Since the abrasive is electrically conductive, there is a risk of malfunction or even a short circuit.
If slip rings are worn, replace them. If the rings wear too quickly, it means that the brushes are used from the wrong material. They may also have shells, but they are dismantled and then grinded in several passes so that the surface adjacent to the brushes is smooth. This work is done on a lathe to maintain alignment.
Rotation speed
The number of pole pairs sets the rotor speed of the induction motor, it is not more than 3000 feet when connected directly to our network. This is due to the network frequency of 50 Hz. It is with this speed that the magnetic flux rotates in the stator of the electric motor. The rotor behind it is a little late, which is why the motor is asynchronous. The delay is structurally determined and is set separately for each engine.
With 1 pole pair, the rotation speed of the magnetic field will be 3000 rpm, with 2 pole pairs - 1500 rpm, with 4 - 750 rpm. If it is necessary to increase or adjust the number of revolutions per minute without making significant changes, a frequency converter is installed in the design. The frequency converter can output both 100 and 200 Hz. To find the speed, useformula (6050)/1=3000, where:
• 1 – number of pole pairs;
• 60 – constant;
• 50 – frequency;
• 3000 - rotations per minute of the magnetic field at a given frequency.
Suppose we can adjust the frequency of some motor, and raise it to 75Hz. Let's use the formula to find the rotation speed: 1/(6075)=4500 rpm. Now we have analyzed the fact that the rotor speed of an induction motor does not depend on the rotor itself, but depends on the number of pole pairs.
In conclusion, we want to say that electrical machines with a phase rotor are practically never found in domestic use. These machines are intended for industrial use in places where voltage drop is undesirable. This also applies to huge machines, the starting current of which can be up to 20 times the rated current. The installation of such machines implies saving resources and money during installation. The rotation speed is not affected by which rotor in the asynchronous motor: with a phase or squirrel-cage rotor.
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