Must see! Five motor selection steps to unlock the source of powerful power for you

 

1、 The type of load driven needs to be reversed from the characteristics of the motor. Electric motors can be simply divided into DC motors and AC motors, with AC motors further divided into synchronous motors and asynchronous motors. 1. DC motor

The advantage of a DC motor is that it can easily adjust the speed by changing the voltage and provide a larger torque. Suitable for loads that require frequent speed adjustment, such as rolling mills in steel mills and elevators in mines. But now with the development of frequency conversion technology, AC motors can also adjust their speed by changing the frequency. However, although the price of variable frequency motors is not much more expensive than ordinary motors, the price of variable frequency motors accounts for the main part of the entire equipment, so DC motors also have the advantage of being cheap.

The disadvantage of DC motors is their complex structure. Any device with a complex structure will inevitably lead to an increase in failure rate. Compared to AC motors, DC motors not only have complex windings (excitation winding, commutation pole winding, compensation winding, armature winding), but also add slip rings, brushes, and commutators.

Not only does it require high craftsmanship from manufacturers, but the maintenance cost in the later stage is also relatively high. Therefore, the use of DC motors in industrial applications is in an awkward situation where they are gradually declining but still have room for use in the transitional stage. If the user has sufficient funds, it is recommended to choose the solution of using an AC motor with a frequency converter, as using a frequency converter also brings many benefits, which will not be discussed in detail.

2. Asynchronous motor

The advantages of asynchronous motors are simple structure, stable performance, easy maintenance, and low price. And the manufacturing process is also the simplest. I once heard from an old technician in the workshop that the labor required to assemble a DC motor can complete two synchronous motors or four asynchronous motors of similar power, which shows this. Therefore, asynchronous motors have been widely used in industry.

Asynchronous motors are divided into squirrel cage motors and wound motors, with the difference being the rotor. The rotor of a squirrel cage motor is made of metal strips, copper or aluminum. The price of aluminum is relatively low, and China is a major aluminum mining country, which is widely used in situations with low requirements. But copper has better mechanical and electrical properties than aluminum, and the vast majority of rotors I have come into contact with are made of copper. After solving the problem of disconnection in the manufacturing process, the reliability of squirrel cage motors far exceeds that of wound rotor motors.

However, its disadvantage is that the torque obtained by cutting magnetic induction lines in the rotating stator magnetic field of the metal rotor is relatively small, and the starting current is large, which makes it difficult to handle loads with high starting torque requirements. Although increasing the length of the motor iron core can obtain more torque, the force is very limited.

When starting a wound motor, the rotor winding is energized through a slip ring, forming a rotor magnetic field that moves relative to the rotating stator magnetic field, thus obtaining greater torque. And during the start-up process, a water resistor is connected in series to reduce the start-up current, and the water resistor is controlled by a mature electronic control device to change its resistance value with the start-up process.

Suitable for loads such as rolling mills and elevators. Due to the addition of slip rings, water and electricity resistors to wound type asynchronous motors compared to squirrel cage motors, there has been a certain increase in overall equipment prices. Compared with DC motors, it has a narrower speed range and relatively smaller torque, resulting in lower value.

However, asynchronous motors, due to the establishment of a rotating magnetic field by energizing the stator winding, which is an inductive component that does not do any work, have to absorb reactive power from the power grid, which has a significant impact on the power grid. Intuitive experience: When high-power inductive appliances are connected to the power grid, the voltage of the grid drops and the brightness of the lights decreases all at once.

Therefore, the power supply bureau may impose restrictions on the use of asynchronous motors, which is also a factor that many factories must consider. Some major electricity consumers, such as steel mills and aluminum plants, choose to establish their own power plants to form their own independent power grid, in order to reduce restrictions on the use of asynchronous motors. So if asynchronous motors want to meet high-power loads, they need to be equipped with reactive power compensation devices, while synchronous motors can provide reactive power to the grid through excitation devices. The greater the power, the more obvious the advantages of synchronous motors, thus creating the stage of synchronous motors.

3. The advantages of synchronous motors include not only compensating for reactive power in over excited states, but also:

The speed of synchronous motor strictly follows n=60f/p, which can accurately control the speed;

High operational stability, when the grid voltage suddenly drops, the excitation system will generally force excitation to ensure stable motor operation, while the torque of asynchronous motors (proportional to the square of the voltage) will significantly decrease;

The overload capacity is greater than that of the corresponding asynchronous motor;

High operational efficiency, especially for low-speed synchronous motors.

Synchronous motors cannot be started directly and require asynchronous or variable frequency starting. Asynchronous starting refers to the installation of a starting winding similar to the cage winding of an asynchronous motor on the rotor of a synchronous motor. An additional resistor, approximately 10 times the resistance value of the excitation winding, is connected in series in the excitation circuit to form a closed circuit. The stator of the synchronous motor is directly connected to the power grid to start it as a asynchronous motor. When the speed reaches the sub synchronous speed (95%), the additional resistor is removed from the starting mode; I won't go into too much detail about variable frequency starting. So one of the disadvantages of synchronous motors is the need to add additional equipment for starting.

Synchronous motors operate on excitation current, and without excitation, the motor is asynchronous. Excitation is a DC system applied to the rotor, whose rotational speed and polarity are consistent with the stator. If there is a problem with the excitation, the motor will lose step and cannot be adjusted, triggering a protection "excitation fault" and tripping the motor.

So the second disadvantage of synchronous motors is the need to add excitation devices. Previously, they were directly supplied by DC motors, but now they are mostly supplied by thyristor rectifiers. 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 elevators, mills, fans, compressors, rolling mills, and water pumps.

In summary, the principle for selecting electric motors is to prioritize those with simple structure, low price, reliable operation, and easy maintenance, while ensuring that the motor performance meets the requirements of production machinery. In this regard, 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 type asynchronous motors.

For production machinery that operates continuously with stable loads and no special requirements for starting and braking, it is recommended to prioritize the use of ordinary squirrel cage asynchronous motors, which are widely used in machinery, water pumps, fans, etc.

Production machinery that requires frequent starting and braking and high starting and braking torque, such as bridge cranes, mine hoists, air compressors, irreversible rolling mills, etc., should use wound asynchronous motors.

In situations where there is no requirement for speed regulation and constant speed or improvement of power factor is required, synchronous motors should be used, such as medium and large capacity water pumps, air compressors, elevators, mills, etc.

Production machinery that requires a speed range of 1:3 or higher and requires continuous, stable, and smooth speed regulation should use separately excited DC motors or squirrel cage asynchronous motors or synchronous motors with variable frequency speed regulation, such as large precision machine tools, gantry planers, steel mills, elevators, etc.

Require the use of series or compound excited DC motors for production machinery with large starting torque and soft mechanical characteristics, such as electric cars, electric locomotives, heavy-duty cranes, etc.

2、 Rated power

The rated power of an electric motor refers to the output power, also known as shaft power or capacity, and is a characteristic parameter of the motor. People often ask how big the motor is, usually not the size of the motor, but the rated power. It is the most important indicator for quantifying the load capacity of a motor, and it is also a parameter requirement that must be provided when selecting a motor.

(is rated power, is rated voltage, is rated current, cos θ is power factor, and η is efficiency)

The principle of correctly selecting the capacity of an electric motor should be based on the premise that the motor is capable of meeting the requirements of production machinery loads, and the power of the motor should be determined in the most economical and reasonable way. If the power is selected too high, the equipment investment will increase, resulting in waste, and the motor often runs under load, resulting in low efficiency and power factor of the AC motor; On the contrary, if the power is selected too low, the motor will overload and cause premature damage to the motor.

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

The heating and temperature rise of an electric motor are the main factors determining its power;

Allow short-term overload capacity;

The starting ability of asynchronous squirrel cage motors should also be considered.

Firstly, the specific production machinery calculates and selects the load power based on its heat generation, temperature rise, and load requirements. Then, the motor pre selects the rated power based on the load power, working mode, and overload requirements. After pre selecting the rated power of the electric motor, it is necessary to verify its heating, overload capacity, and starting ability if necessary.

If one of the items is not qualified, the motor must be re selected and checked again until all items are qualified. Therefore, the work schedule is also one of the necessary requirements. If there is no requirement, the most conventional S1 work schedule will be followed by default; Motors with overload requirements also need to provide overload multiples and corresponding operating times; When using asynchronous squirrel cage motors to drive large inertia loads such as fans, it is also necessary to provide the inertia and starting resistance torque curve of the load to verify the starting ability.

The above selection of rated power is carried out under the premise of a standard ambient temperature of 40 ℃. If the ambient temperature in which the motor operates changes, the rated power of the motor must be adjusted. According to theoretical calculations and practical experience, the power of the electric motor can be roughly increased or decreased according to the table below when the ambient temperature is different.

Therefore, in areas with harsh weather conditions, environmental temperature also needs to be provided. For example, in India, the environmental temperature needs to be verified at 50 ℃. In addition, high altitude also has an impact on motor power. 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 impact of corona phenomena.

For the current power range of electric motors on the market, please provide a few data for reference.

DC motor: ZD9350 (grinder) 9350kW

Asynchronous motor: Rat cage YGF1120-4 (blast furnace fan) 28000kW

Wire wound YRKK1000-6 (raw material mill) 7400kW

Synchronous motor: TWS36000-4 (blast furnace fan) 36000kW (test unit reaches 40000kW)

3、 Rated voltage

The rated voltage of an electric motor refers to the line voltage at its rated operating mode.

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

The selection of voltage level for AC motors mainly depends on the voltage level of the power supply in the place of use. The general 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 a low-voltage motor increases to a certain extent (such as 300KW/380V), the current is limited by the carrying capacity of the wires and it is difficult to increase or the cost is too high.

High power output needs to be achieved by increasing the voltage. The supply voltage of high-voltage power 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 ability to withstand impact; The disadvantage is high inertia, making it difficult to start and brake.

The rated voltage of a DC motor should also be coordinated with the power supply voltage. Usually 110V, 220V, and 440V. Among them, 220V is a commonly used voltage level, and high-power motors can be increased to 600-1000V. When the AC power supply is 380V and a three-phase bridge thyristor rectifier circuit is used for power supply, the rated voltage of the DC motor should be selected as 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 an electric motor refers to the speed at which it operates in its rated working mode.

Both the electric motor and the working machinery driven by it have their own rated speeds. When choosing the speed of an electric 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 stages it has, the larger the volume, and the higher the price; At the same time, the speed of the electric motor should not be selected too high, as this will make the transmission mechanism too complex and difficult to maintain.

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

So for those with low requirements for starting and braking, a comprehensive comparison can be made with several different rated speeds from the aspects of initial equipment investment, footprint, and maintenance costs, and the rated speed can be finally determined; For those who frequently start, brake, and reverse, but the duration of the transition process has little impact on productivity, in addition to considering the initial investment, the speed ratio and rated motor speed are mainly selected based on the condition of minimizing the loss of the transition process. For example, the hoist motor requires frequent forward and reverse rotation with high torque, resulting in low speed, large motor volume, and expensive price.

When the motor speed is high, the critical speed of the motor also needs to be considered. The motor rotor will vibrate during operation, and the amplitude of the rotor increases with the increase of speed. At a certain speed, the amplitude reaches its maximum value (commonly known as resonance). After exceeding this speed, the amplitude gradually decreases with the increase of speed and stabilizes within a certain range. The speed at which the maximum amplitude of the rotor occurs is called the critical speed of the rotor.

This speed is equal to the natural frequency of the rotor. When the speed continues to increase and approaches twice the natural frequency, the amplitude will increase again. When the speed is equal to twice the natural frequency, it is called the second-order critical speed, and so on, there are third-order, fourth-order and other critical speeds. If the rotor operates at critical speed, it will experience severe vibration, and the bending degree of the shaft will significantly increase. Long term operation can also cause severe bending deformation and even breakage of the shaft.

The first-order critical speed of a motor is generally above 1500 revolutions per minute, so conventional low-speed motors generally do not consider the influence of critical speed. On the contrary, for a 2-pole high-speed motor with a rated speed close to 3000 revolutions per minute, this effect needs to be considered and the motor should be avoided from being used for a long time in the critical speed range.

Generally speaking, by providing the type of load being driven, the rated power, rated voltage, and rated speed of the motor, the motor can be roughly determined. But if we want to optimize and meet the load requirements, these basic parameters are far from enough. The parameters that need to be provided include: frequency, operating system, overload requirements, insulation level, protection level, moment of inertia, load resistance moment curve, installation method, ambient temperature, altitude, outdoor requirements, etc., to be provided according to specific circumstances.