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Four Steps Of Motor Selection

The basic contents required for motor selection include the type of load driven, rated power, rated voltage, rated speed, and other conditions.

1. The type of load driven

This has to be said in turn from the characteristics of the motor. Motors can be simply divided into DC motors and AC motors, and AC motors are further divided into synchronous motors and asynchronous motors.

1.1 DC motor
Four Steps Of Motor Selection插图

The advantage of a DC motor is that it can easily adjust the speed by changing the voltage, and can provide a larger torque. It is suitable for loads that need to adjust the speed frequently, 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 frequency conversion motor is not much more expensive than that of the ordinary motor, the price of the frequency converter occupies the main part of the whole set of equipment, so another advantage of the DC motor is that it is cheap.

The disadvantage of DC motors is the complex structure. As long as any equipment has a complex structure, it will inevitably lead to an increase in failure rate. Compared with AC motors, DC motors have more complicated windings (excitation windings, commutation pole windings, compensation windings, and armature windings), and slip rings, brushes, and commutators are also added. Not only does it have high requirements on the manufacturer’s workmanship, but also the later maintenance cost is relatively high. Therefore, in industrial applications, DC motors are in an embarrassing situation where they are gradually declining but are still useful in the transitional stage. If the user has sufficient funds, it is recommended to choose the solution of an AC motor with a frequency converter. After all, the use of a frequency converter can also bring many benefits.

1.2. Asynchronous motor
Four Steps Of Motor Selection插图1

The advantages of asynchronous motors are simple structure, stable performance, convenient maintenance, and low price. And the manufacturing process is also the simplest. I once heard an old technician in the workshop say that the man-hours required to assemble a DC motor can complete two synchronous motors or four asynchronous motors of similar power, which is evident. Therefore asynchronous motors are the most widely used in industry.

Asynchronous motors are divided into squirrel cage motors and wound motors, the difference lies in the rotor. Squirrel-cage motor rotors are made of metal strips, either copper or aluminum. The price of aluminum is relatively low, and my country is a big country of aluminum mines, so it is widely used in occasions with low requirements. However, the mechanical properties and electrical conductivity of copper are better than that of aluminum. Most of the rotors I have come into contact with are copper rotors. After the squirrel-cage motor solves the problem of broken rows in technology, its reliability far exceeds that of a motor with a winding rotor. The disadvantage is that the metal rotor cuts the magnetic induction line in the rotating stator magnetic field to obtain a small torque, and the starting current is large, which is difficult for loads that require a large starting torque. Although more torque can be obtained by increasing the length of the motor core, the strength is very limited. When the wound motor is started, the slip ring is used to energize the rotor winding to form a rotor magnetic field, which moves relative to the rotating stator magnetic field, so the torque is greater. And the water resistance is connected in series to reduce the starting current during the starting process, and the water resistance is controlled by a mature electronic control device to change the resistance value with the starting process. It is suitable for loads such as rolling mills and hoists. Compared with the squirrel-cage motor, the wire-wound asynchronous motor has added slip rings and water resistors, so the overall equipment price has increased to a certain extent. Compared with DC motors, the speed range is narrower and the torque is relatively small, and the corresponding value is also lower.

However, since the asynchronous motor energizes the stator winding to establish a rotating magnetic field, and the winding is an inductive element that does not perform work, it needs to absorb reactive power from the grid, which has a great impact on the grid. Intuitively experience that when high-power inductive appliances are connected to the power grid, the voltage of the power grid will drop, and the brightness of the lights will decrease at once. Therefore, the power supply bureau will limit the use of asynchronous motors, which is also something that many factories must consider. Some large power consumers, such as steel mills and aluminum mills, choose to build their own power plants to form their own independent power grids to reduce restrictions on the use of asynchronous motors. Therefore, if the asynchronous motor needs to be used for high-power loads, it needs to be equipped with a reactive power compensation device, while the synchronous motor can provide reactive power to the grid through the excitation device. The greater the power, the more obvious the advantages of synchronous motors, thus creating the stage of synchronous motors.

1.3. Synchronous motor
Four Steps Of Motor Selection插图2

The advantages of synchronous motors include reactive power compensation in the over-excited state, and 1) the speed of synchronous motors strictly abides by n=60f/p, which can precisely control the speed; 2) high operating stability. When the grid voltage suddenly drops, its The excitation system generally will force excitation to ensure the stable operation of the motor, while the torque of the asynchronous motor (proportional to the square of the voltage) will drop significantly; 3) The overload capacity is larger than that of the corresponding asynchronous motor; 4) The operating efficiency is high, especially for low-speed synchronous motors.

Synchronous motors cannot be started directly, and require asynchronous or variable frequency starting. Asynchronous starting means that the synchronous motor is equipped with a starting winding similar to the cage winding of the asynchronous motor on the rotor, and an additional resistance about 10 times the resistance value of the exciting winding is connected in series in the excitation circuit to form a closed circuit, and the stator of the synchronous motor is directly connected to the Power grid, so that it starts as an asynchronous motor, and when the speed reaches the sub-synchronous speed (95%), the starting method of cutting off the additional resistance; the frequency conversion starting will not be repeated. So one of the disadvantages of synchronous motors is the need to add additional equipment for starting.

A synchronous motor operates on the excitation current. If there is no excitation, the motor is asynchronous. Excitation is a DC system applied to the rotor. Its rotation speed and polarity are consistent with those of the stator. If there is a problem with the excitation, the motor will lose its step and cannot be adjusted. It will trigger the protection “excitation failure” motor trip. Therefore, the second disadvantage of the synchronous motor is the need to increase the excitation device, which was directly supplied by the DC machine in the past, but is now mostly supplied by the thyristor rectifier. As the old saying goes, the more complex the structure and the more equipment, the more points of failure and the higher the failure rate.

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

To sum up, the principle of selecting a motor is to choose a motor with a simple structure, low price, reliable operation, and convenient maintenance on the premise that the performance of the motor meets the requirements of the production machinery. In this respect, AC motors are superior to DC motors, AC asynchronous motors are superior to AC synchronous motors, and squirrel cage asynchronous motors are superior to wound asynchronous motors.

For production machinery with a stable load and no special requirements for starting and braking, the continuous operation of production machinery should preferably use ordinary squirrel-cage asynchronous motors, which are widely used in machinery, water pumps, fans, etc.

Wound-type asynchronous motors should be used for production machinery that starts and brakes frequently and requires relatively large starting and braking torques, such as bridge cranes, mine hoists, air compressors, and irreversible rolling mills.

Synchronous motors should be used in occasions where there is no speed regulation requirement, constant speed is required or power factor improvement is required, such as medium and large-capacity water pumps, air compressors, hoists, mills, etc.

The speed regulation range is required to be above 1:3, and the production machinery that requires continuous, stable, and smooth speed regulation should use separately excited DC motors or squirrel-cage asynchronous motors or synchronous motors with frequency conversion speed regulation, such as large precision machine tools, gantry planers, Rolling mills, elevators, etc.

For production machinery that requires large starting torque and soft mechanical characteristics, use series or compound excitation DC motors, such as trams, electric locomotives, and heavy cranes.

2. Rated power

The rated power of the motor refers to the output power, that is, the shaft power, also known as the capacity, which is the symbolic parameter of the motor. It is the most important index to quantify the motor’s ability to drag the load, and it is also a parameter requirement that must be provided when selecting the motor.

The principle of correct selection of motor capacity is to determine the power of the motor most economically and reasonably under the premise that the motor is capable of producing mechanical load requirements. If the power is selected too large, the equipment investment will increase, resulting in waste, and the motor is often under-loaded, and the efficiency and power factor of the AC motor is low; on the contrary, if the power is selected too small, the motor will be overloaded, causing premature damage to the motor.

There are three factors that determine the main power of the motor:

1) The heating and temperature rise of the motor, which is the most important factor determining the power of the motor;

2) Allow short-time overload capability;

3) For asynchronous squirrel-cage motors, the starting ability should also be considered.

First, the specific production machinery calculates and selects the load power according to its heat generation, temperature rise, and load requirements, and then pre-selects the rated power of the motor according to the load power, duty, and overload requirements. After the rated power of the motor is pre-selected, it is necessary to check the heat generation, overload capacity, and starting capacity when necessary. If one of them is unqualified, the motor must be re-selected and checked again until all items are qualified. Therefore, the working system is also one of the requirements that must be provided. If there is no requirement, it will be handled according to the most conventional S1 working system by default; motors with overload requirements also need to provide overload multiples and corresponding running time; asynchronous squirrel-cage motors drive fans and other large rotations For inertial loads, it is also necessary to provide the moment of inertia of the load and the graph of the starting resistance torque to check the starting ability.

The above selection of rated power is carried out under the premise that the standard ambient temperature is 40°C. If the ambient temperature of the motor changes, the rated power of the motor must be corrected. According to theoretical calculation and practice, when the ambient temperature is different, the power of the motor can be roughly increased or decreased according to the table below.

Therefore, areas with harsh climates also need to provide ambient temperature. For example, in India, the ambient temperature needs to be calibrated at 50°C. In addition, high altitude will also affect the power of the motor. The higher the altitude, the greater the temperature rise of the motor and the lower the output power. And motors used at high altitudes also need to consider the influence of the corona phenomenon.

3. Rated voltage

The rated voltage of the motor refers to the line voltage under the rated working mode.

The selection of the rated voltage of the motor depends on the power supply voltage of the power system to the enterprise and the size of the motor capacity.

The selection of AC motor voltage level is mainly determined by the power supply voltage level of the place of use. Generally, the low voltage network is 380V, so the rated voltage is 380V (Y or △ connection), 220/380V (△/Y connection), and 380/660V (△/Y connection). When the power of the low-voltage motor is increased to a certain level (such as 300KW/380V), the current is limited by the bearing capacity of the wire, so it is difficult to make it larger, or the cost is too high. It is necessary to achieve high power output by increasing the voltage. The power supply voltage of the high-voltage grid is generally 6000V or 10000V, and there are also voltage levels of 3300V, 6600V, and 11000V in foreign countries. The advantages of high-voltage motors are high power and strong impact resistance; the disadvantages are large inertia and difficulty in starting and braking.

The rated voltage of the DC motor should also match the power supply voltage. Generally 110V, 220V and 440V. Among them, 220V is the common voltage level, and the high-power motor can be increased to 600-1000V. When the AC power supply is 380V and a three-phase bridge thyristor rectifier circuit is used for the power supply, the rated voltage of the DC motor should be 440V. When a three-phase half-wave thyristor rectifier power supply is used for power supply, the rated voltage of the DC motor should be 220V.

4. Rated speed

The rated speed of the motor refers to the speed in the rated working mode.

Both the motor and the working machinery driven by it have their own rated speed. When selecting the speed of the motor, it should be noted that the speed should not be selected too low, because the lower the rated speed of the motor, the more poles, the larger the volume, and the higher the price; at the same time, the speed of the motor should not be selected too high, as this would make the drive mechanism overly complex and difficult to maintain.

In addition, when the power is constant, the motor torque is inversely proportional to the speed.

Therefore, those who do not have high requirements for starting and braking can make a comprehensive comparison with several different rated speeds in terms of equipment initial investment, floor space, and maintenance costs, and finally determine the rated speed; However, if the duration of the transition process has little effect on the productivity, in addition to considering the initial investment, the speed ratio and the rated speed of the motor are mainly selected based on the condition that the loss of the transition process is minimal. For example, the hoist motor requires frequent forward and reverse rotation and large torque, but the speed is very low. The motor is bulky and expensive.

When the motor speed is high, the critical speed of the motor also needs to be considered. The rotor of the motor will vibrate during operation. The amplitude of the rotor increases with the increase of the speed. Gradually decrease and stabilize within a certain range, the speed at which the rotor amplitude is the largest is called the critical speed of the rotor. This rotational speed is equal to the natural frequency of the rotor. When the speed continues to increase, the amplitude will increase again when it is close to 2 times the natural frequency. When the speed is equal to 2 times the natural frequency, it is called the second-order critical speed, and by analogy, there are third-order and fourth-order critical speeds. If the rotor runs at a critical speed, it will vibrate violently, and the curvature of the shaft will increase significantly. Long-term operation will also cause serious bending deformation of the shaft, or even break it. The first-order critical speed of the motor is generally above 1500 rpm, so conventional low-speed motors generally do not consider the influence of the critical speed. Conversely, for a 2-pole high-speed motor with a rated speed close to 3000 rpm, this effect needs to be considered and the motor should not be used in the critical speed range for a long time.

Generally speaking, the motor can be roughly determined by providing the type of load driven, the rated power, the rated voltage, and the rated speed of the motor. But these basic parameters are not enough if the load requirements are to be met optimally. The parameters that need to be provided include frequency, working system, overload requirements, insulation level, protection level, the moment of inertia, load resistance moment curve, installation method, ambient temperature, altitude, outdoor requirements, etc., according to specific conditions.

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