Design of an asynchronous electric motor: mechanism design from A to Z! Application and characteristics of modern electric motors (160 photos)

Concept of an asynchronous electric motor

As can be seen in the photo of an asynchronous motor, such a unit is an electric machine, the purpose of which is to convert electricity into mechanical energy. In other words, such equipment, consuming electric current, produces torque. It is this that allows you to rotate many units.

The name "asynchronous" means "non-simultaneous". If you study the description of asynchronous motors, you will notice that in such devices the rotor rotates at a lower frequency than the electromagnetic field of the stator.

This lag or, as it is also called, slip can be calculated using the following formula:

S = (n1 - n2)/ n1 - 100%, where

n1 – frequency of the stator electromagnetic field;

n2 – shaft rotation frequency.

Selecting the core, shape and number of rotor slots

Large dk2 values ​​apply to motors with a large number of pole pairs.

When the core is directly seated on the rotor shaft, the internal diameter of the rotor is calculated using the formula

(53)

Short-circuited rotor windings can be cast or welded. In some extremely rare cases, cells are isolated from the rotor core. The short-circuit rings and radiator vent blades are cast as one piece with the rod.

Modern engines with power up to 200 kW have single-cage squirrel-cage rotors with pear-shaped slots, and more powerful engines have deep-slotted rotors.

Aluminum and its alloys are used as a material for filling squirrel-cage rotors (Table 10). Aluminum casting alloys with increased resistivity are used in asynchronous motors with increased slip, increased starting torque, multi-speed and other special designs.

Fig. 17 IM rotor slots

The most critical step in calculating the rotor is choosing the number of rotor slots. This is due to the effect on the rotor of higher harmonics m.m.f., which are formed both due to the gearing of the rotor and stator, and due to their mutual gearing and induce EMF and higher-order currents in the rotor, worsening the operating and starting characteristics of motors and causing additional noise and vibration.

Z2 ≠ 6pq+2q; Z2≠ Z1+2p; Z2 ≠ 2Z1+2p; Z2 ≠ ½ Z1+p; Z2 ≠ Z1+p

The following ratios are especially unfavorable:

They should be avoided in any case.

In order to slightly reduce the harmonics of the m.m.f. tooth order, it is necessary, as noted above, to provide a bevel by an amount approximately equal to one tooth division of the stator or half of it, which makes it possible to significantly reduce the amplitudes of high-order harmonics close to the order of tooth harmonics. With beveled grooves, the possibilities for choosing the number of grooves expand.

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Structural solution of an asynchronous type electric motor

Stator, rotor, bearing shields and bearings, fan, terminal box - all these are structural elements of an asynchronous motor.
The stator is a stationary structural part on which the winding is located. It is she who creates the electromagnetic field.

The rotor is the movable component of the device. It is here that an electromagnetic moment is created, which promotes the movement of both the rotor itself and the actuator.

The cores of the two elements described above are made of electrical steel with a thickness of 1/2 mm. Insulation must be present: on the stator, its role is played by the varnish film, and on the rotor, by scale. The rotor winding is most often made of aluminum.

Today, two types of asynchronous electric machines are produced - single- and three-phase. As for the latter, they are divided into:

Electric motor disassembly procedure

You should start disassembling the unit after it is disconnected from the electrical network, i.e. you need to pull the plug out of the socket and remove the wires. If there are capacitors in the control circuit of the electric motor, their terminals should be discharged.

There should be no shocks or distortions during disassembly; a certain sequence must be followed. It is necessary to disassemble the electric motor after it has been disconnected from the rotating mechanism. Care should be taken when disassembling the unit to avoid damage to the commutator, brushes, windings and other components of the electric motor.

Before disassembling the unit, you need to mark the locations of the covers relative to the housing using marks, and if there is no key, mark the position of the fan relative to the shaft, otherwise the shaft may be unbalanced.

Devices with wound rotor

The phase rotor consists of a shaft with a core equipped with 3 windings. Some of the ends, when connected, form a star, and the rest are attached to slip rings that supply electric current.

The widest area of ​​use is for three-phase electric motors with a squirrel-cage rotor.

Stator winding, slot and yoke

Number of stator slots. Preliminary selection of tooth division t 1
is carried out according to Fig. 1.4. [3]

At h=150 mm, the 2nd zone is selected at m

Possible number of stator slots.

Then the stator tooth pitch.

Number of conductors in the slot.

Number of effective conductors:

where a 1 =

1 the number of parallel branches in the winding is equal to one, and the rated current of the stator winding.

The number of turns in a winding phase.

Final linear load value.

The average diameter of the insulated wire d = 1.785 mm.

We specify the current density, A/mm 2

Dimensions of groove, tooth and groove insulation . The total number of conductors in the slot.

Free groove area

In modern machines, as a rule, trapezoidal grooves are used for random windings, since in this case the active zone of the machine is used in the best possible way. The dimensions of the grooves must be such that the teeth have parallel walls.

Outer diameter: D

a=255 mm

Stator yoke height, m

Rice. 1. Sketch of the stator trapezoidal groove.

Principle of operation

The operating principle of an asynchronous electric motor with a squirrel-cage rotor is as follows: when current is supplied to the stator windings, a magnetic flux arises, which, when rotating, contributes to the generation of current and a magnetic field in the rotor. The rotor and stator fields, interacting with each other, set the engine rotor in motion.

Equipment with a wound rotor has a similar operating principle. Therefore, we will not re-describe the entire operating process of the device.

How to disassemble the electric motor of a rotary hammer or drill

Failure of electric motors in power tools is a very common occurrence. To carry out repair work independently, the average person should replace the graphite brushes and clean the motor commutator. For these purposes, it is impossible to do without disassembling a non-working or poorly functioning tool.

Disassembly steps:

• Disconnect the unit from the power supply;
• Unscrew the existing bolts or screws and remove one half of the plastic housing, which does not contain the unit itself;
• To replace the brushes, you need to unscrew the bolts that secure the brush holder and remove the pressure spring;
• To remove the commutator, first remove the brushes, and then unscrew the bolts that secure the bearing;
• Next, the unit must be lifted and the armature removed from the stator winding.

Photo of an asynchronous electric motor

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Engine parameters

The parameters of an asynchronous motor are the active and inductive resistance of the stator windings R

1,
X
1, rotor
R
1,
X
1, mutual inductance resistance
X
12 and calculated resistance
R
12 (
R
?), the introduction of which takes into account power losses in the stator steel.

To calculate the active resistance, it is necessary to determine the average length of the winding turn, m

, consisting of the sum of straight grooves and curved frontal parts of the coil, is determined by the formula:

Accurate calculation of the length of the frontal part of the winding is labor-intensive, so it is necessary to use empirical formulas.

The formula for calculating the frontal part of random windings is given:

Total length of winding phase conductors, m

,

Let's calculate for the stator:

Let's calculate for the rotor:

Winding phase active resistance:

Let us calculate the reduced rotor resistance determined by the formula:

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TECHNICAL REQUIREMENTS FOR THE ELECTRIC MOTOR SHAFT.

Classification and technical requirements for shafts. The shaft in an electric machine is the most loaded part that transmits torque to the actuator. The reliability and quality of operation of an electric machine depend on the strength and rigidity of the shaft. The shafts have a stepped shape with steps decreasing in diameter towards both ends.

Fig 1. Electric motor shaft. a) electric motor shaft of series 4A; b) traction motor shaft.

The design of the shafts (Fig. 1) depends on the nature of the engine operation. The traction motor shaft is more loaded, so the transition from one stage to another is smooth, in the shape of a radius called a fillet. This achieves a reduction in stress concentration at the transition points. The shaft of the electric motor of a single series at the transition points of the steps has a slight reduction in diameter, designed to allow the wheel to come out during grinding. A keyway is provided to secure the core package to the shaft. For small-diameter shafts, knurling is used instead of a keyway. The shafts of electrical machines are made of carbon steel grade 45 (GOST 1050 - 60). For the most loaded shafts, alloy steel grade 20KhNZA or 30KhGSA is used. To obtain a fine-grained structure, the shaft blanks are subjected to heat treatment (normalization). The shaft is the most precise part of an electric machine. Most of its surfaces are manufactured according to the 2nd class of accuracy of the hole system and the 7th class of cleanliness (GOST 2789 - 59). The shaft steps for the bearing must be made especially accurately. When manufacturing steps according to the 2nd accuracy class, the sum of ovality and taper should be no more than half the manufacturing tolerance. The shaft drawings also indicate permissible deviations on the location of individual surfaces. For example, for a shaft (Fig. 1a), the deviation from the alignment of the bearing journals should not be more than 0.015 mm.

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Electric motor AIR characteristics