We can recall that the original old-fashioned washing machine, idling without load, will burn the motor. This is a typical case where the no-load current of a single-phase motor is greater than the load current.
For three-phase motors, the relationship between no-load current and rated current is an important basis for most customers to qualitatively evaluate the efficiency level of the motor. For motors with different power and number of poles, there is a roughly proportional relationship between no-load current and rated current. .
Some fans asked, for single-phase motors, what is the relationship between no-load and load current? Compared with the three-phase motor, the magnetic field of the single-phase motor is elliptical, and the magnetic field of the motor under the load state is closer to the circle. There is no absolute qualitative relationship between the no-load current and the load current of the single-phase motor. Current, and some no-load current will be greater than the load current.
For a single-phase motor, two unequal-amplitude reverse rotation vectors, or two unequal-amplitude vertical pulse vectors, are combined into an elliptical rotation vector. Usually, the stator of a single-phase asynchronous motor has two sets of windings, the main and the auxiliary, and their axes are often 90° different in space. The auxiliary winding is connected in series with an appropriate capacitor and then connected to the power grid in parallel with the main winding. Therefore, the main winding circuit is inductive, while the secondary winding may be capacitive, that is, the main winding current lags behind the secondary winding current by a phase angle in time, and the main winding and the secondary winding will generate a magnetic potential respectively. The combined action of the magnetic potential dominates the operation of the motor. Inversely analyzed, the synthetic magnetic potential can be decomposed into two rotating magnetic potentials of positive sequence and negative sequence. The positive sequence rotating magnetic potential dominates the rotation of the motor. will also be affected. Obviously, when the magnitudes of the magnetic potentials generated by the main and secondary windings are equal, that is, when the combined magnetic field of the motor is close to or completely circular, the motor basically only has a positive rotating magnetic potential, and the performance of the motor is also optimal.
For single-phase asynchronous motors, it is the basic of product design to try to eliminate the influence of negative sequence magnetic potential, that is, to make the main and auxiliary windings generate a near-circular synthetic rotating magnetic potential as much as possible to obtain the best motor performance. Point of departure.
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