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Four Steps to Motor Selection: An Easy-to-Learn Guide

Home > Four Steps to Motor Selection: An Easy-to-Learn Guide

Four Steps to Motor Selection: An Easy-to-Learn Guide

2024-01-26 15:14:50

When selecting a motor, the following basic content is required: the type of load to be driven, rated power, rated voltage, rated speed, and other conditions.

Step 1: Type of load to be driven

This section will discuss the different types of motors based on their characteristics. Motors can be simply divided into DC motors and AC motors, with AC motors further divided into synchronous and asynchronous motors.

  1. DC Motors

DC motor advantages include the ability to easily adjust speed by changing voltage and providing large starting torque. They are suitable for loads that require frequent speed adjustments, such as rolling mills in steel plants and hoisting machines in mines. However, with the development of frequency conversion technology, AC motors can also adjust speed by changing frequency. Although the price of frequency converters is a significant part of the overall cost, DC motors still have the advantage of being less expensive.

DC motor disadvantages lie in their complex structure, which increases the likelihood of failures. In comparison to AC motors, DC motors not only have more complex windings (excitation windings, commutation pole windings, compensation windings, and armature windings) but also require additional components such as slip rings, brushes, and commutators. This increases manufacturing requirements and maintenance costs. Therefore, DC motors are in a precarious position, gradually declining but still useful in transition periods. If users have sufficient funds, it is recommended to choose an AC motor with a frequency converter, as it also brings many benefits.

  1. Asynchronous Motors

Asynchronous motors have the advantages of simple structure, stable performance, easy maintenance, and low price. They are the simplest to manufacture and have the shortest assembly time. An old technician in the workshop once said that the time it takes to assemble one DC motor is enough to complete two synchronous motors or four asynchronous motors of roughly the same power. Asynchronous motors have the widest application in industry.

Asynchronous motors can be further divided into cage-type and wound-rotor motors, depending on the rotor design. Cage-type motors use metal bars to form the rotor, which can be made of copper or aluminum. Aluminum is cheaper and widely used in low-requirement scenarios, but copper has better mechanical properties and conductivity, so most of the motors encountered are copper rotor types. Cage-type motors solve the problem of broken bars, making them more reliable than wound-rotor motors. However, their disadvantage is that metal rotors cutting magnetic lines of force in the rotating magnetic field of the stator obtain less torque, and starting current is large, requiring higher starting torque for loads. Although increasing the length of the motor's iron core can obtain more torque, the effect is limited. Wound-rotor motors, on the other hand, start by energizing the rotor windings through a slip ring, forming a rotor magnetic field that moves relative to the rotating stator magnetic field, thus obtaining larger torque. They are suitable for rolling mills, hoisting machines, and other loads. However, wound-rotor motors have additional components such as slip rings and resistance, which increase the overall cost compared to cage-type motors. Their disadvantage compared to DC motors is their narrower speed regulation range and smaller torque, and their value is correspondingly lower.

However, asynchronous motors have a disadvantage in that the stator windings are inductive components that do not perform work and need to absorb reactive power from the grid, causing a significant impact on the grid. Direct experience shows that when large inductive motors are connected to the grid, the grid voltage drops, and the brightness of the lights decreases. Therefore, power supply companies have restrictions on the use of asynchronous motors. Many factories, such as steel and aluminum plants, choose to build their power plants to form their independent power grids to avoid restrictions on the use of asynchronous motors. Thus, synchronous motors have emerged.

  1. Synchronous Motors

The advantages of synchronous motors, besides being able to compensate for reactive power in an over-excited state, include precise control of speed, high stability during operation, large overload capacity, and high efficiency, especially for low-speed synchronous motors.

Synchronous motors cannot start directly and require asynchronous or frequency conversion starting methods. Asynchronous starting involves installing a start winding similar to an asynchronous motor cage winding on the rotor, connecting it in series with a resistance approximately ten times the excitation winding resistance in the excitation circuit to form a closed circuit, and directly connecting the synchronous motor stator to the grid for starting as an asynchronous motor. When the speed reaches the sub-synchronous speed (95%), the additional resistance of the starting method is removed. Frequency conversion starting will not be elaborated on here. Therefore, one disadvantage of synchronous motors is the need for additional equipment for starting.

Synchronous motors operate by excitation current, and if there is no excitation, the motor becomes asynchronous. The excitation is a direct current system added to the rotor, with a rotational speed and polarity consistent with the stator. If the excitation has problems, the motor will lose synchronization, and the protection "excitation failure" will trip the motor. Therefore, synchronous motors have a disadvantage in that they require additional excitation equipment, which was previously provided by DC machines and is now mostly provided by controllable silicon rectifiers. The more complex the structure and the more equipment and devices, the more failure points and the higher the failure rate.

Based on the performance characteristics of synchronous motors, their applications are mainly in hoisting machines, grinding machines, fans, compressors, rolling mills, and pumps.

In summary, the principle of selecting motors is to choose a motor that meets the requirements of production machinery while being simple in structure, inexpensive, reliable, and easy to maintain. In this regard, AC motors are superior to DC motors, asynchronous motors are superior to synchronous motors, and cage-type asynchronous motors are superior to wound-rotor asynchronous motors.

For loads that are stable, have no special starting and braking requirements, and run continuously, ordinary cage-type asynchronous motors are suitable and are widely used in mechanical, water pump, and fan applications.

For loads that require frequent starting and braking and have large starting and braking torque, such as bridge cranes, mine hoists, air compressors, and non-reversible rolling mills, wound-rotor asynchronous motors should be used.

For loads that do not require speed regulation and need constant speed or improved power factors, synchronous motors should be used, such as medium and large capacity water pumps, air compressors, hoists, and grinding machines.

For loads that require a speed regulation range of 1:3 or more and need continuous stable and smooth speed regulation, separately excited DC motors or cage-type asynchronous motors or synchronous motors with variable frequency speed regulation should be used, such as large precision machine tools, portal milling machines, rolling mills, and hoists.

For loads that require large starting torque and have soft mechanical characteristics, series-excited or compound-excited DC motors should be used, such as electric trains, electric trams, and heavy-duty cranes.

Step 2: Rated Power

The rated power of a motor refers to the output power, also called the axial power or capacity, and is a characteristic parameter of the motor. When asking about the size of a motor, it usually refers to the rated power rather than the physical size. It is an essential indicator for quantifying the motor's ability to drive loads and is a parameter that must be provided when selecting a motor.

(P is rated power, U is rated voltage, I is rated current, cosθ is power factor, η is efficiency)

The principle of correctly selecting motor capacity is to determine the motor's power economically and reasonably based on its ability to meet the load requirements of production machinery. If the capacity is too large, it will increase equipment investment and waste resources, and the motor will often run under-loaded, resulting in low efficiency and power factor for AC motors. On the other hand, if the capacity is too small, the motor will overload and damage prematurely.

The main factors determining the motor's primary power are:

  1. The motor's heat generation and temperature rise, which is the most critical factor in determining the motor's power.
  2. Allowable short-term overload capacity.
  3. For asynchronous cage-type motors, the starting ability must also be considered.

First, the specific production machinery's heat generation, temperature rise, and load requirements are calculated to select the load power. Then, the motor's rated power is pre-selected based on the load power, working mode, and overload requirements. After that, the motor's rated power must be checked for heating, overload ability, and, if necessary, starting ability. If any of these do not meet the requirements, the motor must be reselected and rechecked until all requirements are met.

Therefore, the working mode is also a necessary requirement to provide, and if there is no requirement, it should be processed according to the most common S1 working mode by default. Motors with overload requirements also need to provide the overload multiple and corresponding running time. For asynchronous cage-type motors driving loads with large rotational inertia, such as fans, the load's rotational inertia and starting torque curve must also be provided to check the starting ability.

The above selection of rated power is based on the premise of a standard ambient temperature of 40°C. If the motor's working ambient temperature changes, the rated power of the motor must be corrected accordingly. According to theoretical calculations and practical experience, the motor's power can be roughly increased or decreased according to the following table under different ambient temperatures.

Therefore, in harsh climatic regions, the ambient temperature must also be provided for verification. For example, in India, the ambient temperature should be verified at 50°C. In addition, high altitudes also affect the motor's power, and the higher the altitude, the greater the motor's temperature rise, the lower the output power, and the motor used at high altitude also needs to consider the influence of the electric flicker phenomenon.

Step 3: Rated Voltage

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

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

The selection of AC motor voltage levels is mainly determined by the voltage levels of the power supply at the usage location. In general, low-voltage networks are 380V, and the rated voltage is 380V(Y or Δ connection), 220/380V(Δ/Y connection), or 380/660V(Δ/Y connection). When the power of a low-voltage motor increases to a certain level (e.g., 300kW/380V), the current is limited by the carrying capacity of the line, making it difficult or expensive to increase. Therefore, the voltage must be increased to achieve high-power output. High-voltage networks provide power at 6000V or 10000V, and there are also 3300V, 6600V, and 11000V voltage levels abroad. High-voltage motors have the advantages of large capacity, strong impact resistance, but their disadvantage is large inertia, making starting and braking difficult.

The rated voltage of a DC motor must also be coordinated with the voltage of the power source. Commonly, it is 110V, 220V, or 440V. Among them, 220V is the commonly used voltage level, and the rated voltage of large-capacity motors can be increased to 600V~1000V. When the AC power source is 380V and a three-phase bridge-type controllable silicon 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 controllable silicon rectifier power supply is used, the rated voltage of the DC motor should be 220V.

Step 4: Rated Speed

The rated speed of a motor refers to the speed under rated working conditions.

Both the motor and the working machinery it drives have their own rated speeds. When selecting the motor's speed, attention should be paid to not choosing speeds that are too low, as lower rated speeds result in more stages, larger volumes, and higher prices. At the same time, the motor's speed should not be chosen too high, as this will make the transmission structure more complex and difficult to maintain.

In addition, when the power is certain, the motor's torque is inversely proportional to the speed.

Therefore, for loads with low starting and braking requirements, several different rated speeds can be compared comprehensively from the perspective of initial investment, floor space, and maintenance costs to determine the rated speed. For loads that frequently start, brake, and reverse but have little impact on production efficiency during the transition process, in addition to considering the initial investment, the minimum loss during the transition process should be the primary condition for selecting the speed ratio and motor rated speed. For example, hoisting machine motors require frequent forward and reverse rotation and have large torque, resulting in low speeds, large motor sizes, and high prices.

When the motor's speed is high, the motor's critical speed must also be considered. The motor's rotor will vibrate during operation, and as the speed increases, the amplitude of the vibration will also increase. At a certain speed, the amplitude reaches its maximum value (also known as resonance), and after exceeding this speed, the amplitude gradually decreases and stabilizes within a certain range. This maximum amplitude speed of the rotor is called the rotor's critical speed and is equal to the rotor's natural frequency. If the motor runs at the critical speed for a long time, it will cause severe vibration and a significant increase in the bending of the shaft. Long-term operation may cause serious deformation or even breakage of the shaft. The first-order critical speed of a general low-speed motor is usually above 1500 rpm, so the influence of the critical speed is generally not considered. However, for 2-pole high-speed motors with rated speeds close to 3000 rpm, this influence needs to be considered, and the motor should avoid long-term operation within the critical speed range.

In general, providing the type of load to be driven, the motor's rated power, rated voltage, and rated speed can roughly determine the motor. However, if the goal is to optimally meet the load requirements, these basic parameters are far from enough. Other necessary parameters include frequency, working mode, overload requirements, insulation level, protection level, rotational inertia, load torque curve, installation method, ambient temperature, altitude, outdoor requirements, etc., which should be provided according to the specific situation.

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