5 Steps to Select the Right Motor

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

1. Type of load driven

This has to be said in reverse from the characteristics of the motor.

Motor can be simply divided into DC motor and AC motor, and AC motor can be divided into synchronous motor and asynchronous motor.

1. DC motor

The advantage of 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 requiring frequent speed adjustment, such as mills in steel mills, hoists in mines, etc.

But now with the development of frequency conversion technology, AC motor can also adjust the speed by changing the frequency.

However, although the price of variable-frequency motor is not much higher than that of ordinary motor, the price of variable-frequency motor accounts for the main part of the whole set of equipment, so DC motor also has an advantage of being cheap.

The disadvantage of DC motor lies in its complex structure.

As long as the structure of any equipment is complex, it will inevitably lead to an increase in the failure rate.

Compared with AC motor, DC motor not only has complex windings (excitation winding, commutation pole winding, compensation winding, armature winding), but also adds slip rings, brushes and commutators.

Not only the process requirements of manufacturers are high, but also the later maintenance costs are relatively high.

Therefore, DC motor in industrial application is in an embarrassing situation that is gradually declining but still useful in the transitional stage.

If the user has sufficient funds, it is recommended to choose the scheme of AC motor with frequency converter.

After all, using frequency converter also brings many benefits, which will not be discussed in detail.

2. Asynchronous motor

The advantages of asynchronous motor lie in its simple structure, stable performance, convenient maintenance and low price.

And the manufacturing process is also the simplest.

I have heard that the old technician in the workshop said that the man hours used to assemble a DC motor can complete two synchronous motors or four asynchronous motors with similar power, which can be seen from this.

Therefore, asynchronous motor has been the most widely used in industry.

Asynchronous motor is divided into squirrel cage motor and wound motor, and the difference lies in the rotor.

The rotor of squirrel cage motor is made of metal strip, copper or aluminum.

The price of aluminum is relatively low, and China is also a large aluminum mining country, which is widely used in low demand occasions.

However, the mechanical and electrical properties of copper are better than aluminum. Most of the rotors I contact are copper rotors.

After solving the problem of broken row in technology, the reliability of squirrel cage motor is far better than that of wound rotor motor.

The disadvantage is that the torque obtained by cutting the magnetic induction line of the metal rotor in the rotating stator magnetic field is small, and the starting current is large, so the load requiring large starting torque is not competent.

Although more torque can be obtained by increasing the length of motor core, the force is very limited.

When starting, the wound motor energizes the rotor winding through the slip ring to form the rotor magnetic field, which moves relative to the rotating stator magnetic field, so the torque is greater.

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 electric control device to change the resistance value with the starting process.

Suitable for rolling mills, hoists and other loads.

Compared with the squirrel cage motor, the wound asynchronous motor has increased the slip ring, water and electricity resistance, etc., which has a certain increase in the overall equipment price.

Compared with DC motor, the speed regulation range is relatively narrow and the torque is relatively small, and the corresponding value is also low.

However, because the asynchronous motor energizes the stator winding to establish a rotating magnetic field, and the winding is an inductive element that does not do work, it needs to absorb reactive power from the power grid, which has a great impact on the power grid.

Intuitively experience that when high-power inductive appliances are connected to the grid, the grid voltage drops and the brightness of the lights decreases at once.

Therefore, the power supply bureau will restrict the use of asynchronous motors, which is also a place that many factories must consider.

Some large power users, such as steel plants and aluminum plants, choose to establish their own power plants to form their own independent power grid, so as to reduce the restrictions on the use of asynchronous motors.

Therefore, if asynchronous motors want to meet the use of high-power loads, they need to be equipped with reactive power compensation devices, while synchronous motors can provide reactive power to the power grid through excitation devices.

The greater the power, the more obvious the advantages of synchronous motors, resulting in the dance stage of synchronous motors.

3. Synchronous motor

In addition to compensating reactive power in overexcitation state, the advantages of synchronous motor also include:

1) The speed of synchronous motor strictly complies with n=60f/p, which can accurately control the speed;

2) The operation stability is high. When the grid voltage suddenly drops, its excitation system will generally 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 greater than that of the corresponding asynchronous motor;

4) High operation efficiency, especially low-speed synchronous motor.

Synchronous motors cannot be started directly and need asynchronous or variable-frequency starting.

Asynchronous starting refers to the starting winding similar to the cage winding of asynchronous motor is installed on the rotor of synchronous motor, and the additional resistance about 10 times of the resistance value of the excitation winding is connected in series in the excitation circuit to form a closed circuit.

The stator of synchronous motor is directly connected to the power grid to start it as a asynchronous motor.

When the speed reaches the subsynchronous speed (95%), the additional resistance is cut off;

Variable frequency startup is not detailed. Therefore, one of the disadvantages of synchronous motor is the need to add additional equipment for starting.

Synchronous motor operates by excitation current.

If there is no excitation, the motor is asynchronous.

Excitation is a DC system added to the rotor, and its rotation speed and polarity are consistent with that of the stator.

If there is a problem with excitation, the motor will be out of step and cannot be adjusted, triggering the protection “excitation fault” motor trip.

Therefore, the second disadvantage of synchronous motor is the need to add excitation devices. Previously, it was directly supplied by DC machine, but now it is mostly supplied by silicon controlled rectifier.

As an old saying goes, the more complex the structure and the more equipment, the more failure points and the higher the failure rate.

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

To sum up, the principle of selecting motors is to give priority to motors with simple structure, low price, reliable operation and convenient maintenance on the premise that the motor performance meets the requirements of production machinery.

In this respect, AC motor is better than DC motor, AC asynchronous motor is better than AC synchronous motor, and squirrel cage asynchronous motor is better than wound asynchronous motor.

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

Production machinery with frequent starting and braking and requiring large starting and braking torque, such as bridge cranes, mine hoists, air compressors, irreversible rolling mills, etc., should adopt wound asynchronous motors.

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

For production machinery whose speed regulation range is required to be more than 1:3 and which requires continuous, stable and smooth speed regulation, it is advisable to 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 rolling mills, hoists, etc.

Production machines with large starting torque and soft mechanical characteristics are required to use series or compound excitation DC motors, such as trams, electric locomotives, heavy cranes, etc.

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 often asked how big the motor is.

Generally, it does not refer to the size of the motor, but to the rated power.

It is the most important index to quantify the driving load capacity of the motor, and it is also the parameter requirement that must be provided when selecting the motor.

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

The principle of correct selection of motor capacity should determine the power of the motor most economically and reasonably on the premise that the motor can meet the load requirements of production machinery.

If the power is too large, the equipment investment increases, resulting in waste, and the motor often underruns, and the efficiency and power factor of the AC motor are low;

On the contrary, if the power is too small, the motor will overload and cause premature damage.

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

1) The heating and temperature rise of the motor are the main factors that determine the power of the motor;

2) Allowable short-time overload capacity;

3) The starting capacity of asynchronous squirrel cage motors should also be considered.

First, the specific production machinery calculates and selects the load power according to its heating, temperature rise and load requirements, and then the motor pre selects the rated power according to the load power, working system and overload requirements.

After the rated power of the motor is preselected, the heating, overload capacity and starting capacity shall be verified if necessary.

If one of them is unqualified, the motor must be re selected and checked until all items are qualified.

Therefore, the working system is also one of the requirements that must be provided. If there is no requirement, the most conventional S1 working system will be adopted by default;

Motors with overload requirements also need to provide overload multiple and corresponding operation time;

When the asynchronous squirrel cage motor drives the fan and other large moment of inertia loads, it is also necessary to provide the moment of inertia of the load and the starting resistance moment curve to check the starting capacity.

The above selection of rated power is carried out under the premise that the standard ambient temperature is 40 ℃.

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 following table.

Therefore, it is also necessary to provide ambient temperature in areas with bad weather.

For example, in India, the ambient temperature needs to be checked at 50 ℃.

In addition, high altitude will also have an impact on motor power. The higher the altitude, the greater the motor temperature rise and the smaller the output power.

And the influence of corona phenomenon should also be considered for motors used at high altitude.

For the power range of motors in the current market, I would like to list several data for reference.

DC motor: ZD9350 (mill) 9350kW

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

Winding type yrkk1000-6 (raw mill) 7400kw

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

3. Rated voltage

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

The choice 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 voltage level of AC motor mainly depends on 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), 380/660V (△ /Y connection).

When the power of low-voltage motor increases to a certain extent (such as 300KW/380V), the current is limited by the bearing capacity of the conductor, so it is difficult to increase, or the cost is too high.

High power output needs to be achieved by increasing the voltage.

The power supply voltage of high-voltage power grid is generally 6000V or 10000V, and there are also 3300V, 6600V and 11000V voltage levels abroad.

High voltage motor has the advantages of high power and strong impact resistance;

The disadvantage is that the inertia is large, and it is difficult to start and brake.

The rated voltage of DC motor should also match the power supply voltage.

Generally 110V, 220V and 440V.

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 the three-phase bridge silicon controlled rectifier circuit is used for power supply, the rated voltage of the DC motor should be 440V.

When the three-phase half wave silicon controlled 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 under the rated working mode.

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 too low, because the lower the rated speed of the motor, the more its stages, the larger the volume and the higher the price;

At the same time, the speed of the motor should not be too high, because 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.

Therefore, those with low requirements for starting and braking can comprehensively compare several different rated speeds from the aspects of initial investment, floor area and maintenance cost of the equipment, and finally determine the rated speed;

For those who often 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 the rated speed of the motor are mainly selected based on the minimum loss of the transition process.

For example, the elevator motor needs frequent forward and reverse rotation and large torque, so the speed is very low, and the motor is bulky and expensive.

When the motor speed is high, the critical speed of the motor should also be considered.

The motor rotor will vibrate during operation, and the amplitude of the rotor will increase with the increase of the speed.

At a certain speed, the amplitude reaches the maximum value (commonly known as resonance). After exceeding this speed, the amplitude will gradually decrease with the increase of the speed and stabilize within a certain range.

The speed with the maximum amplitude of the rotor 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, the amplitude will increase 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 so on.

If the rotor operates at the critical speed, there will be violent vibration, and the bending of the shaft will increase significantly.

Long-term operation will also cause serious bending deformation of the shaft, or even fracture.

The first-order critical speed of the motor is generally above 1500 rpm, so the influence of the critical speed is generally not considered for conventional low-speed motors.

On the contrary, for a 2-pole high-speed motor, if the rated speed is close to 3000 rpm, the impact should be considered and the motor should not be used in the critical speed range for a long time.

Wrap it up

Generally speaking, the motor can be roughly determined by providing the load type of the drive, the rated power, rated voltage and rated speed of the motor.

But these basic parameters are far from enough if we want to optimally meet the load requirements.

The parameters that need to be provided include: frequency, working system, overload requirements, insulation grade, protection grade, moment of inertia, load resistance moment curve, installation method, ambient temperature, altitude, outdoor requirements, etc., which are provided according to the specific situation.

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