The device and principle of operation of the induction motor. Asynchronous motor type: operating principle, description and function

Like most electric motors, an AC induction motor (AC) has a fixed outer part, which is called a stator, and a rotor rotating inside. Between them there is a carefully calculated air gap.

How it works?

The design and operation of asynchronous motors, like all others, are based on the fact that rotation of the magnetic field is used to drive the rotor. Three-phase BP is the only type of motor in which it is created naturally due to the nature of the food. In DC motors , mechanical or electronic commutation is used for this, and in single-phase AD - additional electrical elements.

For the operation of the electric motor, two sets of electromagnets are necessary. The principle of the asynchronous electric motor is that one set is formed in the stator, since an AC source is connected to its winding. In accordance with Lenz's law, this induces an electromagnetic force (EMF) in the rotor in the same way as the voltage is induced in the secondary winding of the transformer, creating another set of electromagnets. Hence another name of AD - an induction motor. The design and operation of asynchronous motors are based on the fact that the interaction between the magnetic fields of these electromagnets generates a twisting force. As a result, the rotor rotates in the direction of the resultant moment.

Stator

The stator consists of several thin plates of aluminum or cast iron. They are pressed together to form a hollow core cylinder with grooves. Insulated wires are laid in them. Each group of windings together with the core surrounding them, after applying an alternating current to it, forms an electromagnet. The number of poles of the AD depends on the internal connection of the stator windings. It is made in such a way that when a power source is connected, a rotating magnetic field is formed.

Rotor

The rotor consists of several thin steel plates with evenly spaced aluminum or copper rods. In the most popular of its type - squirrel, or "squirrel cage" - the rods at the ends are mechanically and electrically connected by means of rings. Almost 90% of BP uses this design, because it is simple and reliable. The rotor consists of a cylindrical plate-shaped core with axially arranged parallel grooves for the installation of conductors. In each groove is laid a rod of copper, aluminum or alloy. They are short-circuited on both sides by means of end rings. Such a construction resembles a squirrel cage, which is why it received the appropriate name.

The grooves of the rotor are not completely parallel to the shaft. They are made with a slight skew for two main reasons. The first is to ensure a smooth operation of the blood pressure by reducing the magnetic noise and harmonics. The second is to reduce the probability of locking the rotor: its teeth are engaged behind the stator slots due to a direct magnetic attraction between them. This happens when their number is the same. The rotor is mounted on the shaft by means of bearings at each end. One part usually protrudes more than the other to drive the load. In some engines , speed or position sensors are attached to the non-working end of the shaft.

There is an air gap between the stator and the rotor. Energy is transmitted through it. The generated torque causes the rotor and the load to rotate. Regardless of the type of rotor used, the device and operating principle of the induction motor remain unchanged. Typically, blood pressure is classified by the number of stator windings. There are single-phase and three-phase electric motors.

Device and principle of operation of single-phase asynchronous motor

Single-phase arrays make up the largest part of electric motors. It is quite logical that the least expensive and unpretentious to service engine is used most often. As its name implies, the purpose, principle of operation of an induction motor of this type is based on the presence of only one stator winding and operation with a single-phase power supply. All rotors of this type have a rotor that is short-circuited.

Single-phase motors do not start themselves. When the motor is connected to a power source, alternating current begins to flow along the main winding. It generates a pulsating magnetic field. Due to induction, the rotor is energized. As the main magnetic field pulsates, the torque required to rotate the motor is not generated. The rotor begins to vibrate, and not rotate. Therefore, for a single-phase blood pressure, a trigger mechanism is required. It can provide an initial push, forcing the shaft to move.

The starting mechanism of a single-phase AD consists mainly of an additional winding of the stator. It may be accompanied by a series capacitor or a centrifugal switch. When the supply voltage is applied, the current in the main winding lags behind the voltage because of its resistance. At the same time, the electricity in the starting winding lags or outstrips the supply voltage, depending on the impedance of the trigger. The interaction between the magnetic fields generated by the main winding and the starting circuit creates a resultant magnetic field. It rotates in one direction. The rotor begins to rotate in the direction of the resultant magnetic field.

After the motor speed reaches about 75% of the rated value, the centrifugal switch disconnects the starting winding. Further, the engine can maintain sufficient torque to operate independently. With the exception of motors with a special starting capacitor, all single-phase motors are typically used to create power not exceeding 500 watts. Depending on the different starting methods, single-phase AD is further classified as described in the following sections.

AD with a split phase

The purpose, device and principle of operation of an asynchronous motor with a split phase are based on the use of two windings in it: starting and main. The starting is made of a wire of smaller diameter and fewer turns with respect to the main one in order to create a greater resistance. This allows you to orient its magnetic field at an angle. It differs from the direction of the main magnetic field, which leads to rotation of the rotor. The working winding, which is made of a wire of a larger diameter, ensures the operation of the engine at the rest of the time.

The starting torque is low, usually from 100 to 175% of the rated torque. The motor consumes a high starting current. It is 7-10 times higher than the nominal one. The maximum torque is also 2.5-3.5 times higher. This type of motors is used in small grinders, fans and blowers, as well as in other devices requiring low torque, with a power of 40 to 250 watts. It should avoid the use of such engines where frequent on-off cycles or high torque is required.

AD with capacitor start

Condenser asynchronous type of the engine and the principle of its operation are based on the fact that to its starting winding with a split phase in series connected capacity, providing the starting "momentum". As in the previous version of motors, there is also a centrifugal switch. It disconnects the starting circuit when the motor speed reaches 75% of the rated speed. Since the capacitor is connected in series, this creates a larger starting torque, reaching 2-4 times the working size. And the starting current, as a rule, is 4.5-5.75 times higher than the rated current, which is much lower than in the case of the split phase, due to the larger wire in the starting winding.

The modified version of the start is characterized by an engine with an active resistance. In this type of motor, the capacitance is replaced by a resistor. The resistance is used in those cases where a smaller starting torque is required than with a capacitor. In addition to the lower cost, this does not give an advantage over capacitive starting. These motors are used in units with belt drive: small conveyors, large fans and pumps, as well as in many devices with direct drive or using a reducer.

AD with a working phase-shift capacitor

The device and principle of operation of the asynchronous motor of this type are based on the constant connection of a capacitor connected in series with the starting winding. After the motor has reached the rated speed, the starting circuit becomes auxiliary. Since the tank must be designed for continuous use, it can not provide the initial pulse of the starting capacitor. The starting torque of this engine is low. It is 30-150% of the nominal. The starting current is small - less than 200% of the rated current, which makes this type of electric motors ideal where there is a need for frequent on and off.

This design has several advantages. The circuit is easy to change for use with speed controllers. Electric motors can be tuned for optimum efficiency and high power factor. They are considered the most reliable single-phase motors mainly because they do not use a centrifugal start switch. They are used in fans, blowers and often switched on devices. For example, in adjusting mechanisms, gate opening systems and garage doors.

AD with starting and working capacitor

The device and the principle of operation of the asynchronous motor of this type are based on the serial connection of the starting capacitor to the starting winding. This makes it possible to create a larger torque. In addition, it has a constant capacitor, connected in series with the auxiliary winding after the starter is disconnected. This circuit allows for large torque overloads.

This type of engine is designed for lower full-load currents, which ensures its greater efficiency. This design is most costly due to the presence of starting, working capacitors and a centrifugal switch. It is used in woodworking machines, air compressors, high-pressure water pumps, vacuum pumps and where high torque is required. Power - from 0,75 to 7,5 kW.

AD with shielded pole

The device and principle of operation of an asynchronous motor of this type consist in the fact that it has only one main winding and there is no starter winding. The start is made due to the fact that around each small part of each of the stator poles there is a shielding copper ring, as a result of which the magnetic field in the given region lags behind the field in the unscreened part. Interaction of two fields leads to rotation of the shaft.

Since there is no starting coil, no switch or capacitor, the motor is electrically simple and inexpensive. In addition, its speed can be regulated by changing the voltage or through a multi-tap winding. The design of the motor with shielded poles allows its mass production. It is usually considered "disposable", as it is much cheaper to replace it than to repair it. In addition to positive qualities, this design has a number of drawbacks:

• Low starting torque, equal to 25-75% of the nominal;
• High slip (7-10%);
• Low efficiency (less than 20%).

A low initial cost allows using this type of blood pressure in low-power or rarely used devices. It's about household multi-speed fans. But low torque, low efficiency and low mechanical characteristics do not allow their commercial or industrial application.

Three-phase blood pressure

These electric motors have found wide application in industry. The device and principle of operation of a three-phase asynchronous motor are determined by its design - with a squirrel-cage or with a phase rotor. To start it, no condenser, starter winding, centrifugal switch or other device is required. The starting torque is medium and high, as is the power and efficiency. Used in grinding, turning, drilling machines, pumps, compressors, conveyors, agricultural machinery, etc.

AD with closed rotor

This is a three-phase asynchronous motor, the principle of operation and the device of which have been described above. It makes up almost 90% of all three-phase electric motors. Available in power from 250 W to several hundred kW. Compared with single-phase engines from 750 W, they are cheaper and withstand greater loads.

AD with a phase rotor

The device and the principle of operation of a three-phase asynchronous motor with a phase rotor differ from the "squirrel cage" type of blood in that there is a set of windings on the rotor, the ends of which are not short-circuited. They are led out to the contact rings. This allows you to connect external resistors and contactors to them. The maximum torque is directly proportional to the rotor resistance. Therefore, at low speeds it can be increased by additional resistance. A high resistance allows to obtain a high torque with a low starting current.

As the rotor accelerates, the resistance decreases to change the motor performance to meet the load requirements. After the motor reaches the base speed, the external resistors are disconnected. And the electric motor works like a normal blood pressure. This type is ideal for high inertia loads, requiring the application of torque at near zero speed. It provides acceleration to a maximum in minimum time with minimal current consumption.

The disadvantage of such engines is that the contact rings and brushes need regular maintenance, which is not required for a motor with a squirrel cage rotor. If the rotor winding is closed and a start attempt is made (i.e., the device becomes a standard BP), a very high current will flow in it. It is 14 times higher than the nominal at a very low torque, which is 60% of the base torque. In most cases, the application does not find it.

By varying the dependence of the rotational speed on the torque by adjusting the rotor's resistance, it is possible to vary the speed at a certain load. This allows them to effectively reduce them by about 50%, if the load requires a variable torque and speed, which is often found in printing machines, compressors, conveyors, lifts and elevators. A reduction in speed below 50% results in very low efficiency due to higher dissipated power in the rotor resistances.