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AC motors generally use three-phase system
Jul 24, 2018

AC motors generally use three-phase system, because three-phase AC motors have obvious advantages in terms of performance indicators, raw material utilization and price compared with single-phase motors. Three-phase motors of the same power are smaller, lighter, and less expensive than single-phase motors. The three-phase motor has a self-starting capability. Single-phase motors do not have starting torque. To solve the starting problem, special measures must be taken. The torque of a single-phase motor is pulsating and the noise is relatively large, but the required power supply is relatively simple, especially in the home. Therefore, small household motors and instrument motors use single-phase motors.

"AC motor": It is a machine for converting mechanical energy and AC power. Due to the tremendous development of AC power systems, AC motors have become the most commonly used motors. Compared with DC motors, AC motors have no commutator (see commutation of DC motors), so they are simple in structure, easy to manufacture, and relatively robust. They are easy to make motors with high speed, high voltage, high current and large capacity. AC motor power covers a wide range, from a few watts to hundreds of thousands of kilowatts, or even millions of kilowatts. In the early 1980s, the largest steam turbine generator had reached 1.5 million kilowatts. The AC motor was invented by the American Serbian scientist Nikola Tesla.

AC motors are divided into two categories: synchronous motors and asynchronous motors. The rotational speed ns of the synchronous motor rotor is the same as the rotational speed of the rotating magnetic field, and is called synchronous rotational speed. 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=60f/P

In China, the power frequency is 50 Hz, so the synchronous speed of the one-pole motor in the three-phase AC motor is 3000 rpm, and the synchronous speed of the two-pole motor in the three-phase AC motor is 1500 rpm, and so on. The speed of the asynchronous motor rotor is always lower or higher than the rotational speed of its rotating magnetic field, and the name of the asynchronous is thus derived. The difference between the rotor speed of an asynchronous motor and the rotational magnetic field speed (called the slip) is usually within 10%.


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