Some Problems in the Application of Electric Motor(2)

4.What is the main magnetic flux of a three-phase asynchronous motor? Is it caused by the magnetic flux of each phase generated by each phase current, or is it caused by the joint generation of three phase currents? Which parameter corresponds in the equivalent circuit? Is it the same concept as the excitation reactance in a transformer?

Answer: The main magnetic flux of a three-phase asynchronous motor refers to the magnetic flux generated by the fundamental rotational magnetomotive force synthesized by the three-phase current and reaching the rotor through the air gap. It is generated jointly by the three-phase current. The excitation reactance Xm in the equivalent circuit corresponds to it. This parameter itself is a value of one phase, which corresponds to the armature reaction reactance Xa of a synchronous motor, and is not entirely the same as the concept of excitation reactance in a transformer. The excitation reactance of a transformer is the ratio of the electromotive force induced by the main magnetic flux generated by a phase excitation current to the phase excitation current, i.e., Xm=E1/I0 (ignoring iron loss).

5. How is the rotor magnetomotive force of an asynchronous motor generated? What is the relationship between the rotation direction and rotational speed of this magnetomotive force relative to the rotor and the rotation direction and rotational speed of the rotor itself? What about the relative rotation direction and speed of the stator? Therefore, is it feasible to perform rotor frequency conversion? Is it suitable for any state of asynchronous motors?

A: Rotor magnetomotive force is a rotating magnetomotive force generated by three-phase (or multiphase) symmetrical currents induced by the three-phase (or multiphase) symmetrical windings of the rotor. The direction of rotation of this magnetomotive force relative to the rotor is determined by the phase sequence of the induced rotor current. When the rotor speed is less than synchronous speed or the rotation direction of the rotor is opposite to the rotation direction of the stator magnetomotive force, the rotor current phase sequence and the stator current phase sequence are consistent, and the rotation direction of the rotor magnetomotive force relative to the rotor is consistent with the rotation direction of the stator magnetomotive force. As long as the rotor speed is greater than the synchronous speed and both are in the same direction, the phase sequence of the rotor current and the stator current are opposite, and the rotation of the rotor magnetomotive force relative to the rotor is opposite to the rotation of the stator magnetomotive force. The rotational speed of this magnetomotive force relative to the rotor is determined by the frequency of the rotor current. When the rotor speed is n and the slip rate is s, and the frequency of the rotor current f2=sf1, the rotational speed of this magnetomotive force relative to the rotor is sn1 The rotation direction of this magnetomotive force relative to the stator is always the same as that of the stator magnetomotive force, and the relative stator rotation speed is sn1+n=((n1 n)/n1) * n1+n=n1, that is, the rotation speed is always synchronized.

There is no electrical connection between the rotor and the stator, and the effect of the rotor on the stator is achieved through the rotor magnetomotive force. From the above analysis, it can be seen that regardless of the rotational speed of the rotor and the frequency of the rotor current, the rotational speed of the rotor magnetomotive force relative to the stator is always synchronous. In this way, a rotor that does not rotate (with a rotor current frequency of f1) can be used to replace a rotor with a rotational speed of n, a slip rate of s, and a rotor current frequency of sf1, as long as the magnitude and direction of the rotor magnetomotive force are kept unchanged. This is the conversion of the rotor frequency. The frequency conversion is applicable to any rotational speed, any slip rate, or any state of an asynchronous motor, that is, the generator, motor, and electromagnetic braking states.

SIVO Electric Motor is one of the core enterprises in the manufacturing sector of China's top 500 private enterprises under the Fangyuan Group.The company has a technical management team with strong professional skills and excellent management capabilities. The company's main products include iron-shell single three-phase asynchronous motor series, aluminum-shell single three-phase asynchronous motor series, fan special motor series, frequency conversion motor series, permanent magnet DC motor series, etc.