Almost all motors commonly used in the air separation plant are three-phase AC motors. Three-phase AC motor can be divided into asynchronous motor and synchronous motor. The rotating speed of the synchronous motor and the frequency of the power grid keep a strict relationship, namely:
n=60f/P
In the formula, P -- magnetic pole logarithm;
F -- grid frequency, the normal situation is 50Hz;
N -- motor speed, r/min.
For example, for two-pole (P = 1) synchronous motor, the synchronous speed is 3000r/min; for four-pole motor, P = 2, n = 1500r/min. While the rotating speed of asynchronous motor is always less than that of synchronous motor, and it generates rotating torque by its phase difference.
Asynchronous motor is most widely used. It consists of a stator and a rotor, and has a stator and a rotor winding respectively. The stator winding is made of copper (aluminum) wire with outsourcing insulation. With the difference of rotor winding, asynchronous motor can be divided into squirrel cage type and winding type. Squirrel Cage motor rotor winding is like a cage, usually cast. Winding motor rotor winding is made of insulated wire, and the end of each phase of three-phase winding leads a wire to the slip ring.
Squirrel Cage motor has simple structure, reliable operation, low cost and simple start-up, which is used most. But the power factor is low. In the air separation plant, the low-power machines such as the liquid oxygen pump, water pump and blower all use the squirrel cage motor.
The rotor structure of winding motor is more complicated than squirrel cage motor. But it has the characteristics that squirrel cage motor does not have. When starting, the rotor coil is connected with a resistance or a frequency sensitive rheostat, which can reduce the starting current and increase the starting torque. Adjusting the resistance value of the rotor string can also adjust the rotating speed of the motor. Winding motor is used in air compressor and oxygen compressor with higher power.
The stator of the synchronous motor is the same as that of the asynchronous motor, except that the rotor iron core is covered with DC excitation winding, which has two outgoing lines connected to two slide rings, and the magnetic direct current is supplied by the outside to form a constant magnetic pole. The rotating speed of synchronous motor does not change with the change of load. In addition, it has high utilization rate of electric energy, which means high power factor. Synchronous motor is mainly used in air compressor, oxygen compressor, which requires constant speed and high power equipment.
