Like most electric motors, an AC induction motor (AM) has a fixed external part, which is called a stator, and a rotor rotating inside. Between them there is a carefully calculated air gap.
How it works?
The device and principle of operation of induction motors, like all others, are based on the fact that the rotation of the magnetic field is used to set the rotor in motion. Three-phase blood pressure is the only type of motor in which it is created naturally due to the nature of the power supply. In DC motors , mechanical or electronic switching is used for this, and in single-phase AMs, additional electrical elements are used.
For the operation of an electric motor, two sets of electromagnets are required. The principle of operation of an induction motor is that one set is formed in the stator, since an alternating current source is connected to its winding. According to Lenz’s law, this induces electromagnetic force (EMF) in the rotor in the same way that voltage is induced in the secondary winding of a transformer, creating a different set of electromagnets. Hence the other name HELL - induction motor. The device and principle of operation of asynchronous motors are based on the fact that the interaction between the magnetic fields of these electromagnets generates torque. As a result, the rotor rotates in the direction of the resulting 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 alternating current to it forms an electromagnet. The number of AM poles 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 aluminum or copper rods evenly spaced around the periphery. In its most popular type - squirrel cage, or “squirrel cage” - the rods at the ends are mechanically and electrically connected using rings. Almost 90% of blood pressure uses this design, since it is simple and reliable. The rotor consists of a cylindrical plate core with axially spaced parallel grooves for installing conductors. A rod of copper, aluminum or alloy is placed in each groove. They are short-circuited on both sides with end rings. This design resembles a squirrel cage, which is why it received the corresponding name.
The rotor slots are not completely parallel to the shaft. They are made with a slight bias for two main reasons. The first is to ensure the smooth operation of blood pressure by reducing magnetic noise and harmonics. The second is to reduce the likelihood of stagnation of the rotor: its teeth catch on the stator slots due to direct magnetic attraction between them. This happens when their number coincides. The rotor is mounted on the shaft using bearings at each end. One part usually protrudes more than the other to drive the load. In some engines , speed or position sensors are mounted at 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 load to rotate. Regardless of the type of rotor used, the device and the principle of operation of the induction motor remain unchanged. As a rule, AM are classified by the number of stator windings. Distinguish single-phase and three-phase electric motors.
The device and principle of operation of a single-phase induction motor
Single-phase HELLs make up the largest part of electric motors. It is logical that the least expensive and low maintenance engine is used most often. As the name implies, purpose, principle of operation of an induction motor of this type are based on the presence of only one stator winding and work with a single-phase power source. For all ADs of this type, the rotor is squirrel-cage.
Single-phase motors do not start on their own. When the motor is connected to a power source, alternating current begins to flow through the main winding. It generates a pulsating magnetic field. Due to induction, the rotor is energized. Since the main magnetic field is pulsating, the torque required to rotate the motor is not generated. The rotor begins to vibrate, not rotate. Therefore, for a single-phase blood pressure, a trigger is required. It can provide an initial push forcing the shaft to move.
The starting mechanism of a single-phase HELL consists mainly of an additional stator winding. It may be accompanied by a series capacitor or centrifugal switch. When the supply voltage is applied, the current in the main winding lags behind the voltage due to its resistance. At the same time, the electricity in the starting winding lags or is ahead of the supply voltage, depending on the impedance of the starting mechanism. The interaction between the magnetic fields generated by the main winding and the starting circuit creates a resulting magnetic field. It rotates in one direction. The rotor begins to rotate in the direction of the resulting magnetic field.
After the motor speed reaches about 75% of the nominal speed, the centrifugal switch disconnects the starting winding. Further, the engine can maintain sufficient torque to act independently. With the exception of motors with a special starting capacitor, all single-phase motors are usually used to create power not exceeding 500 watts. Depending on the different starting methods, single-phase BPs are further classified as described in the following sections.
Split phase blood pressure
The purpose, structure and principle of operation of the split-phase asynchronous motor are based on the use of two windings in it: the starting one and the main one. The launcher is made of wire of smaller diameter and fewer turns in relation to the main one, in order to create more 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 the rotation of the rotor. The working winding, which is made of a larger diameter wire, ensures the functioning of the motor the rest of the time.
Starting torque is low, usually from 100 to 175% of the nominal. The motor consumes a high starting current. It is 7-10 times higher than the nominal. The maximum torque is also 2.5-3.5 times greater. This type of motor is used in small grinders, fans and blowers, as well as in other devices requiring low torque, power from 40 to 250 watts. Avoid the use of such engines where frequent on-off cycles or high torque is required.
HELL with capacitor start
The capacitor asynchronous motor type and the principle of its operation are based on the fact that a capacitance providing a starting “pulse” is connected in series to its starting winding with a split phase. As in the previous version of the motors, there is also a centrifugal switch. It disables the starting circuit when the engine speed reaches 75% of the nominal. Since the capacitor is connected in series, this creates a larger starting torque, reaching 2–4 times the size of the working one. 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 a split phase, due to the larger wire in the starting winding.
The modified starting option is characterized by an engine with active resistance. In this type of motor, the capacitance is replaced by a resistor. Resistance is used when a lower starting torque is required than when using a capacitor. In addition to lower cost, this does not give an advantage over capacitive starting. These motors are used in units with a belt drive: small conveyors, large fans and pumps, as well as in many devices with direct drive or using a gearbox.
HELL with a working phase-shifting capacitor
The device and principle of operation of this type of induction motor are based on the constant connection of a capacitor connected in series with the starting winding. After the motor reaches rated speed, the starting circuit becomes auxiliary. Since the capacitance must be designed for continuous use, it cannot provide the initial impulse of the starting capacitor. Starting torque of such an engine is low. It is 30-150% of the nominal. Starting current is small - less than 200% of the rated current, which makes electric motors of this type ideal where frequent on and off switching is required.
This design has several advantages. The circuit is easy to change for use with speed controllers. Electric motors can be tuned for optimal efficiency and high power factor. They are considered the most reliable of single-phase motors, mainly because they do not use a centrifugal start switch. They are used in fans, blowers and frequently connected devices. For example, in adjusting mechanisms, opening systems for gates and garage doors.
HELL with starting and working capacitor
The device and principle of operation of this type of induction motor are based on the series connection of the starting capacitor to the starting winding. This makes it possible to create more torque. In addition, it has a constant capacitor connected in series with the auxiliary winding after disconnecting the starting capacitance. This design allows for large torque overloads.
This type of motor is designed for lower full load currents, which ensures its greater efficiency. This design is the most expensive 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 needed. Power - from 0.75 to 7.5 kW.
HELL with a shielded pole
The device and principle of operation of an induction motor of this type are that it has only one main winding and no start. The start-up is due to the fact that around a small part of each of the poles of the stator there is a shielding copper ring, as a result of which the magnetic field in this area lags behind the field in the unshielded part. The interaction of the two fields leads to the rotation of the shaft.
Since there is no starter coil, no switch or capacitor, the motor is electrically simple and inexpensive. In addition, its speed can be controlled 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 “one-time”, since it is much cheaper to replace it than to repair it. In addition to the positive qualities, this design has a number of disadvantages:
- low starting torque equal to 25–75% of the nominal;
- high glide (7–10%);
- low efficiency (less than 20%).
Low initial cost allows the use of this type of blood pressure in low-power or rarely used devices. We are talking about household multi-speed fans. But low torque, low efficiency and low mechanical characteristics do not allow their commercial or industrial application.
Three phase BP
These electric motors are widely used in industry. The device and the principle of operation of a three-phase induction motor are determined by its design - with a squirrel-cage or with a phase rotor. It does not require a capacitor, starting winding, centrifugal switch or other device to start it. Starting torque is medium and high, as well as power and efficiency. Used in grinding, turning, drilling machines, pumps, compressors, conveyors, agricultural machinery, etc.
Closed Rotor HELL
This is a three-phase asynchronous motor, the principle of operation and device of which were described above. It makes up almost 90% of all three-phase electric motors. Available in power from 250 watts to several hundred kW. Compared to single-phase motors from 750 watts, they are cheaper and can withstand heavy loads.
Phase Rotor HELL
The device and principle of operation of a three - phase induction motor with a phase rotor differ from the squirrel-cage type of blood pressure motor in that the rotor has a set of windings whose ends are not short-circuited. They are displayed on slip rings. This allows you to connect external resistors and contactors to them. The maximum torque is directly proportional to the resistance of the rotor. Therefore, at low speeds it can be increased by additional resistance. High resistance allows high torque at low inrush current.
As the rotor accelerates, the resistance decreases to change the characteristics of the motor to meet the load requirements. After the motor reaches base speed, the external resistors are turned off. And the electric motor works like a normal blood pressure. This type is ideal for high inertial loads, requiring the application of torque at almost zero speed. It provides acceleration to maximum in minimum time with minimum current consumption.
The disadvantage of these motors is that the contact rings and brushes need regular maintenance, which is not required for a squirrel-cage motor. If the rotor winding is closed and an attempt is made to start (i.e., the device becomes a standard AM), a very high current will flow in it. It is 14 times the nominal with a very low torque of 60% of the base. In most cases, this does not find application.
By changing the dependence of the rotational speed on the torque by adjusting the rotor resistances, one can vary the speed at a certain load. This allows you to effectively reduce them by about 50% if the load requires a variable moment and speed, which is often found in printing machines, compressors, conveyors, elevators and elevators. A decrease in speed below 50% leads to very low efficiency due to the higher power dissipation in the rotor resistances.