What are the basic requirements for motor selection (One)?

07/08/2019

First, the type of load driven

The motor can be simply divided into a DC motor and an AC motor, and the AC is divided into a synchronous motor and an asynchronous motor.

1, DC motor

The advantage of a DC motor is that it can be easily adjusted by changing the voltage and can provide a large torque. Suitable for loads that require frequent adjustment of speed, such as rolling mills in steel mills, hoists in mines, etc. But now with the development of frequency conversion technology, AC motors can also adjust the speed by changing the frequency. However, although the price of the inverter motor is not much more expensive than the ordinary motor, the price of the inverter occupies a major part of the whole set of equipment, so DC motor has another advantage that it is cheap.
The disadvantage of the DC motor is that the structure is complicated, and any device as long as the structure is complicated will inevitably lead to an increase in the failure rate. Compared with AC motors, DC motors have added slip rings, brushes and commutators in addition to complex windings (excitation windings, commutating pole windings, compensating windings, armature windings). Not only the manufacturer's process requirements are high, but also the later maintenance costs are relatively high. Therefore, DC motors are in an awkward situation in industrial applications that are gradually declining but still in use during the transition period.

2, asynchronous motor

The advantages of the asynchronous motor are simple structure, stable performance, convenient maintenance and low price. And the manufacturing process is also the simplest. I have heard from the old technicians in the workshop that the working hours of assembling a DC motor can complete two synchronous motors or four asynchronous motors of almost the same power. Therefore, asynchronous motors are the most widely used in the industry.

Asynchronous motors are further divided into squirrel-cage motors and wound-type motors, the difference being in the rotor. The squirrel cage motor rotor is made of metal strips, made of copper or aluminum. The price of aluminum is relatively low, and China is also a big country of aluminum ore, and it is widely used in places where demand is not high. But the mechanical and electrical properties of copper are better than aluminum, and most of the contacts I have come into contact with copper rotors. After the squirrel cage motor solves the problem of discontinuity in the process, the reliability far exceeds the motor of the winding rotor. The disadvantage is that the metal rotor has a small torque obtained by cutting the magnetic induction line in the rotating stator magnetic field, and the starting current is large, and the load requiring a large starting torque is difficult. Although more torque can be obtained by increasing the length of the motor core, the force is very limited. The wound-type motor energizes the rotor windings through the slip ring at startup, forming a rotor magnetic field that moves relative to the rotating stator magnetic field, thus obtaining a greater torque. And in the startup process, the water resistance is connected in series to reduce the starting current, and the water resistance is controlled by the mature electronic control device to change the resistance value with the starting process. Suitable for loads such as rolling mills and hoists. Since the wound-type asynchronous motor adds a slip ring, a water resistance, and the like to the squirrel-cage type motor, the overall equipment price is improved. Compared with the DC motor, the speed regulation range is relatively narrow and the torque is relatively small, and the corresponding value is also low.

However, the asynchronous motor establishes a rotating magnetic field by energizing the stator winding, and the winding belongs to the inductive component without doing work. The reactive power is absorbed from the power grid, which has a great impact on the power grid. Intuitive experience When a high-power inductive electrical appliance is connected to the grid, the grid voltage drops and the brightness of the lamp decreases. Therefore, the power supply bureau has restrictions on the use of asynchronous motors, which is a place that many factories must consider. Some large consumers of electricity, such as steel mills and aluminum plants, chose to establish their own power plants to form their own independent power grids to reduce the restrictions on the use of asynchronous motors. Therefore, if the asynchronous motor is to be used for high-power load, it needs to be equipped with reactive power compensation device, and the synchronous motor can provide reactive power to the grid through the excitation device. The greater the power, the more obvious the advantage of the synchronous motor, thus the synchronization is generated. The stage of the electric motor.

3, synchronous motor

The advantage of the synchronous motor is that it can compensate the reactive power in addition to the over-excited state. It also includes 1) the synchronous motor's speed strictly follows n=60f/p, which can accurately control the speed; 2) the running stability is high, when the grid voltage suddenly drops, its Excitation system will generally force excitation to ensure stable operation of the motor, while asynchronous motor torque (proportional to the square of voltage) will drop significantly; 3) overload capacity is larger than the corresponding asynchronous motor; 4) high operating efficiency, especially low-speed synchronous motor .

The synchronous motor cannot be started directly and requires asynchronous start or variable frequency start. Asynchronous start-up means that the synchronous motor is equipped with a starting winding similar to the asynchronous motor cage winding on the rotor. In the excitation circuit, an additional resistance of about 10 times the resistance of the field winding is connected in series to form a closed circuit, and the stator of the synchronous motor is directly connected. The power grid is started by the asynchronous motor. When the speed reaches the sub-synchronous speed (95%), the starting mode of the additional resistor is removed; the frequency conversion start is not repeated. One of the disadvantages of synchronous motors is the need to add additional equipment to the start-up.

The synchronous motor is operated by the excitation current. If there is no excitation, the motor is asynchronous. Excitation is a DC system applied to the rotor. Its rotational speed and polarity are consistent with the stator. If there is a problem with the excitation, the motor will lose its step, and the adjustment will not trigger the protection of the “excitation fault” motor trip. Therefore, the second disadvantage of the synchronous motor is that it needs to increase the excitation device. It was originally supplied directly by the DC machine, and now it is mostly supplied by the thyristor. Still the old saying goes, the more complex the structure, the more equipment, the more fault points, the higher the failure rate.

According to the performance characteristics of synchronous motors, their applications are mainly on loads such as hoists, mills, fans, compressors, rolling mills, and pumps.

In summary, the principle of selecting the motor is that the motor performance meets the requirements of the production machinery, and the motor with simple structure, low price, reliable operation and convenient maintenance is preferred. In this respect, the AC motor is superior to the DC motor, and the AC asynchronous motor is superior to the AC synchronous motor. The squirrel-cage asynchronous motor is superior to the wound-type asynchronous motor.

For the continuous operation of the production machinery with no special requirements for starting and braking, the ordinary squirrel-cage asynchronous motor should be preferred, which is widely used in machinery, pumps and fans.

Starting and braking are frequent, and production machinery with large starting and braking torque is required, such as bridge cranes, mine hoists, air compressors, irreversible rolling mills, etc., and wound-wound asynchronous motors should be used.

Where there is no speed regulation requirement, if the speed is constant or the power factor is required to be improved, synchronous motors, such as medium and large capacity pumps, air compressors, hoists, mills, etc., should be used.

For production machines with a speed regulation range of 1:3 or higher and continuous stable and smooth speed regulation, it is better to use a separately excited DC motor or a squirrel cage asynchronous motor or synchronous motor with variable frequency speed regulation, such as a large precision machine tool, a planer, Rolling mills, hoists, etc.

It is required to produce a machine with large starting torque and soft mechanical characteristics, using series or compound excitation DC motors, such as electric cars, electric cars, heavy cranes, etc.