Some Problems in the Application of Electric Mators(1)

Some Problems in the Application of Electric Motor

1. Is the air gap of an asynchronous motor larger or smaller than that of a synchronous motor? Why?

Answer: The air gap of an asynchronous motor is smaller than that of a synchronous motor because the excitation current of the asynchronous motor is provided by a three-phase AC power supply. If the air gap is large, the magnetic resistance is small, and the required excitation current is large. Because the excitation current is reactive current, the large excitation current reduces the power factor of the asynchronous motor. The excitation current of a synchronous motor is provided by a DC power source. From the V-shaped curve of a synchronous motor, it can be seen that when the excitation current increases from a small amount and the excitation state changes from under excitation to over excitation, the power factor can change from lagging to leading. The power factor of asynchronous motors is always lagging.

2. What is the relationship between the rotation speed of the rotating magnetic field generated by the stator winding of an asynchronous motor through electricity and the number of pole pairs of the motor? Why is the rotor speed of an asynchronous motor always synchronous when working?

Answer: The rotational speed of the rotating magnetic field generated by energizing the stator winding, that is, the synchronous speed, whose magnitude n1 is determined by the frequency f of the power grid and the number of poles p of the motor, that is, n1=60f/p.

When an asynchronous motor operates, the rotor speed is always lower than the synchronous speed, because if the rotor speed is equal to the synchronous speed, there is no relative movement between the rotor winding and the rotating magnetic field, and thus the magnetic lines of force cannot be cut. Therefore, the induced electromotive force and current in the rotor winding are zero, and the electromagnetic torque generated by the rotor winding is zero, so it is impossible to drag the rotor to rotate. Therefore, the rotor speed of an asynchronous motor during operation is always lower than the synchronous speed.

3. 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 of excitation current to the phase of excitation current, i.e., Xm=E1/I0 (ignoring iron loss).

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