Definition
The rated speed refers to the speed of the engine at the rated power, and the rated power is specified by the manufacturer. The rated speed is generally lower than the maximum power speed under full load. The rated speed should be determined preliminarily before the engine is designed. After the engine is developed, it will be finalized by repeated tests. The engine can not only deliver maximum power at this speed, but also must take into account reliability and durability.
The higher the rated speed, the greater the rated power. Therefore, research on increasing the rated speed of the engine is the best way to increase the power per liter. The main factors affecting the increase in rated speed are intake efficiency and mechanical efficiency.
Selection
The rated speed of the motor is selected according to the requirements of the production machinery. When determining the rated speed of the motor, the transmission ratio of the mechanical deceleration mechanism must be considered. The two cooperate with each other and can be determined after a comprehensive comparison of technology and economy.
As far as the motor itself is concerned, the higher the rated speed of the motor with the same rated power, the smaller the size of the motor and the lower the weight and cost. Therefore, it is more economical to choose a high-speed motor. However, due to certain requirements for the rotational speed of the production machinery, the higher the motor rotational speed, the greater the transmission ratio of the transmission mechanism, resulting in a complex transmission mechanism and reduced transmission efficiency. Therefore, when choosing the rated speed of the motor, both the motor and the transmission mechanism should be considered.
For a continuously working motor, generally considering the initial investment and operation and maintenance costs, several different rated speed (that is, different transmission speed ratios) schemes are compared, and the most suitable speed is selected. ; For motors that frequently start, brake and reverse, but the duration of the transition process has little effect on production, in addition to considering the initial investment, the selection is mainly based on the minimum energy consumption of the transition process; for frequent starting and braking And reverse, but the duration of the transition process has a greater impact on production (such as the main motor of the planer), mainly based on the shortest transition process duration.
Related calculations
Rated speed refers to the rotation speed of the motor when the rated voltage and rated frequency are applied to the stator of the motor, and the rated power is output on the shaft. The unit is rpm. The rated torque of the motor is not marked on the nameplate, but it can be calculated from the rated power and rated speed. The relationship is M=P/n.
Where P is the rated power, n is the rated speed, and M is the requirement. When P uses kilowatts and n uses revolutions/minutes, the unit of M is (Nu·m).
Problems and solutions
Reasons and solutions for failing to reach the rated speed
(1) The power supply is not connected (the line is off) Wire or fuse blown), check the circuit, connect the circuit or replace the fuse.
(2) If the starting equipment or other accessory equipment fails, these equipment should be checked and dealt with accordingly.
(3) The connection point or the Y contact of the motor is in poor contact, the rust should be removed and the bolts should be tightened.
(4) The brush of the wire-wound motor is in poor contact with the slip ring (oil stains, insufficient spring pressure; the brush and the brush box are too tightly matched to prevent the brush from moving up and down), should be removed Adjust the spring pressure and size of the brush and slip ring for oil and dirt in accordance with the regulations to make the brush and slip ring have good contact.
(5) The voltage is too low. Check the lead wire of the motor. If the voltage is too low, you should find out the reason, or adjust the power supply voltage, or replace the transmission wire with a large cross-sectional area.
(6) The external wiring of the motor is wrong, the △ connection is mistakenly connected to the Y connection, and the connection method should be corrected.
(7) If the stator winding is broken and the motor circuit is blocked or the motor is running in two phases, the three-phase current should be measured. The bridge checks whether the windings are connected at the lead wires to find out where the wires are broken.
(8) When the rotor is broken, disconnected or desoldered, the stator winding of the motor can be connected to a three-phase power supply with a rated voltage of 15% to 30%, and the rotor can be slowly rotated, and the rotor can be judged according to the change of the stator current Whether it is broken, broken wire or desoldering, deal with it after confirming.
(9) After changing the pole of the motor, the stator and rotor slots are not matched properly. The winding pitch should be changed according to the specific situation, or the rotor diameter should be less than 0.5mm.
(10) After the overhaul of the motor, due to internal wiring errors, or a certain phase of the lead wire was connected wrongly, an abnormal electrical buzzer will occur when the power is turned on at this time. Firstly, check whether the beginning and end of the motor are correct. If there is no problem, check whether the internal wiring of the motor is correct.
(11) The load is too large or the transmission system fails. The load and drive system should be checked.
(12) The ambient temperature is too low and the grease becomes hard or even frozen. Hot oil should be poured into the bearing.
(13) The air gap between the stator and the rotor is seriously uneven, and the unilateral magnetic pull generated by the motor "sucks" the rotor. It should be reassembled and the air gap of the motor should be adjusted according to regulations.