The basic information required for motor selection includes: the type of load being driven, rated power, rated voltage, rated speed, and other conditions.
01. Type of load being driven
The principle for selecting a motor is to prioritize motors that are simple in structure, reliable in operation, and easy to maintain, provided that the motor performance meets the mechanical requirements.
For production machinery that operates continuously under stable loads and has no special requirements for starting and braking, ordinary squirrel-cage asynchronous motors should be given priority. They are widely used in machinery, water pumps, fans, etc.
For production machinery that requires frequent starting and braking and has a large starting and braking torque, such as bridge cranes, mine hoists, air compressors, and irreversible rolling mills, wound-rotor asynchronous motors should be used.
For applications where speed regulation is not required, constant speed is needed, or power factor improvement is required, synchronous motors should be used, such as medium and large capacity water pumps, air compressors, hoists, mills, etc.
For production machinery requiring a speed range of 1 : 3 or higher and continuous, stable, and smooth speed regulation, separately excited DC motors or synchronous motors are recommended, such as large precision machine tools, gantry planers, rolling mills, and hoists.
Production machinery requiring high starting torque and soft mechanical characteristics uses series-wound or compound-wound DC motors, such as trams, locomotives, and heavy-duty cranes.
02. Rated Power
Rated power of a motor: This refers to the output power, also known as shaft power or capacity, and is a key parameter of the motor. When people ask about the size of a motor, they are generally referring to its rated power, not its physical dimensions. It is the most important indicator for quantifying a motor's load-driving capacity and is a crucial parameter that must be provided when selecting a motor.
The principle for correctly selecting motor capacity is to determine the motor power in the most economical and reasonable way, provided that the motor can meet the load requirements of the production machinery. If the power is selected too large, the equipment investment will increase, resulting in waste, and the motor will often operate under load, with low efficiency and a low power factor for AC motors. Conversely, if the power is selected too small, the motor will operate under load, causing premature motor damage.
There are three main factors that determine the power of a motor:
1 ) The heat generation and temperature rise of the motor are the most important factors determining the motor power;
2 ) Allowable short-term overload capacity;
3 ) For asynchronous squirrel-cage motors, starting capability must also be considered.
03. Rated Voltage
Rated voltage of an electric motor: refers to the line voltage under rated operating conditions.
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 mainly depends on the power supply voltage level of the application environment. Generally, low-voltage grids are 380V , so rated voltages are 380V (Y or Δ connection ) , 220/380V ( Δ /Y connection ) , and 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 conductor's carrying capacity, making it difficult to increase the output or resulting in excessive cost. Therefore, it is necessary to increase the voltage to achieve higher power output. High-voltage grids typically supply 6000V or 10000V , although 3300V , 6600V , and 11000V voltage levels are also available internationally . The advantages of high-voltage motors are high power and strong impact resistance; the disadvantages are high inertia, making starting and braking difficult.
The rated voltage of a DC motor must also match the power supply voltage. Commonly used voltages are 110V , 220V , and 440V . 220V is the most common voltage level, but high-power motors can be equipped with 600-1000V . When the AC power supply is 380V and a three-phase bridge thyristor rectifier circuit is used, the rated voltage of the DC motor should be 440V . When a three-phase half-wave thyristor rectifier power supply is used, the rated voltage of the DC motor should be 220V .
04. Rated speed
Rated speed of an electric motor: refers to the speed under rated operating conditions.
Both the electric motor and the machine it drives have their own rated speeds. When selecting the motor speed, care should be taken not to choose too low a speed, because the lower the rated speed of the motor, the more poles it has, the larger its size, and the higher its price. At the same time, the motor speed should not be selected too high either, because this will make the transmission mechanism too complex and difficult to maintain.
Furthermore, when the power is constant, the motor torque is inversely proportional to the speed.
Therefore, for equipment with low starting and braking requirements, a comprehensive comparison of several different rated speeds can be made based on factors such as initial investment, floor space, and maintenance costs to determine the final rated speed. However, for equipment that frequently starts, brakes, and reverses, but whose transition time has little impact on productivity, in addition to considering initial investment, the speed ratio and motor rated speed should be selected primarily based on minimizing losses during the transition process. For example, hoist motors require frequent forward and reverse rotation and have high torque, resulting in very low speeds, large motor sizes, and high prices.
When the motor speed is high, the critical speed must also be considered. Motor rotors vibrate during operation, and the amplitude of this vibration increases with speed. At a certain speed, the amplitude reaches its maximum (what is commonly referred to as resonance). Beyond this speed, the amplitude gradually decreases with increasing speed and stabilizes within a certain range. This speed at which the rotor amplitude is maximum is called the rotor's critical speed. This speed is equal to the rotor's natural frequency. As the speed continues to increase, approaching twice the natural frequency, the amplitude increases again. When the speed equals twice the natural frequency, it is called the second-order critical speed, and so on for the third, fourth, and so on. If the rotor operates at its critical speed, it will experience severe vibration, and the shaft bending will increase significantly. Prolonged operation can cause severe bending deformation of the shaft, or even breakage. The first-order critical speed of a motor is generally above 1500 rpm , therefore, the influence of the critical speed is generally not considered for conventional low-speed motors. Conversely, for 2- pole high - speed motors with a rated speed close to 3000 rpm , this influence must be considered, and the motor should be avoided from being used within the critical speed range for extended periods.
Generally, providing the load type, rated power, rated voltage, and rated speed of the motor is sufficient to roughly determine its suitability. However, these basic parameters are far from enough to optimally meet load requirements. Additional parameters include: frequency, duty cycle, overload requirements, insulation class, protection class, moment of inertia, load resistance torque curve, installation method, ambient temperature, altitude, outdoor requirements, etc., provided according to specific circumstances.
