Cooling tower motor characteristics and basic knowledge(2)

The pole number and speed of the motor:

The number of poles reflects the synchronous speed of the motor. The 2-pole synchronous speed is 3000r/min, the 4-pole synchronous speed is 1500r/min, the 6-pole synchronous speed is 1000r/min, and the 8-pole synchronous speed is 750r/min.

The circuit composed of windings----the number of pole pairs, these windings will generate a magnetic field when passing current, and correspondingly there will be magnetic poles.Each set of coils of a three-phase AC motor generates N and S magnetic poles, and the number of magnetic poles per phase of each motor is the number of poles.Since the magnetic poles appear in pairs, the motor has 2, 4, 6, 8 ... poles.

If the frequency of the three-phase alternating current is 50 Hz, the synchronous rotational speed of the combined magnetic field is 50 r/s, that is, 3000 r/min.If the rotating magnetic field of the motor is more than a pair of magnetic poles, further analysis can also obtain the relationship between the synchronous rotational speed n and the magnetic field pole pair p:n=60f/p. f is the frequency, the unit of Hz.n’s unit is r/min.

There is a strict relationship between ns and the frequency of the connected alternating current (f) and the number of magnetic pole pairs (P) of the motor. ns=f/P.

In China, the power frequency is 50HZ, so the synchronous speed of the two-pole motor is 3000 rpm, the synchronous speed of the four-pole motor is 1500 rpm, and so on.The speed of the rotor of an asynchronous machine is always lower or higher than the speed of its rotating magnetic field,the name of asynchronous comes from this.The difference between the rotor speed of an asynchronous motor and the rotational magnetic field speed (called the slip) is usually within 10%.It can be seen that the speed of the AC motor (whether synchronous or asynchronous) is limited by the power frequency.Therefore, the speed regulation of the AC motor is more difficult, the best way is to change the frequency of the power supply.

Synchronous motor speed = 60 * frequency / pole logarithm (China's power frequency is 50Hz)

Asynchronous motor speed = (60 * frequency / pole logarithm) × (1 - slip)

In addition, for motors of the same power, the higher the speed, the smaller the output torque.