Motor inductance problem

bigbat

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According to the Ampere loop theorem of the entire circuit, the current direction of all the wires in the single-phase winding coil of the motor is the same, and the magnetic field strength H generated by the "algebraic sum" of the currents of each wire is proportional to the magnitude of the current. If it is a copper bar whose current is the same as the "algebraic sum of the currents of each wire", then the magnetic field generated should be the same. Of course, in reality, excessive current is not only unbearable for the power supply, but also has a large resistance loss for the wires. My question is: the greater the inductive reactance of the coil, the smaller the current. In addition to the factor of line loss and installation restrictions, is it better to have more coils?

lovelessing

The more coils you have, the higher the resolution of the motor. Slowly you can get a stepper motor. In fact, the stepper motor also uses a sine wave current for further subdivision and drive. The more you make, the more difficult the machining will be. You also know the corresponding cost.

maychang

For any product, whether industrial or agricultural, there is no such thing as "the better the better", but rather a compromise between different conflicting requirements.

annysky2012

maychang posted on 2021-10-15 18:37 For any product, whether it is an industrial product or an agricultural product, there is no such thing as "the more... the better", but different...

Totally agree.

maychang

For the single-phase motor you mentioned, if the number of coil turns is large, the cross-sectional area occupied by the wire will definitely become larger. The larger the area occupied by the wire, the smaller the cross-sectional area of the magnetic circuit will be (the coil is wound in the slot, the larger the slot area, the smaller the tooth area), and the smaller the magnetic circuit area, the lower the motor efficiency will be.

maychang

The number of turns of a single-phase motor winding is determined by the supply voltage and the saturation magnetic flux density of the core material. It is usually designed so that the ferromagnetic material slightly enters magnetic saturation at the rated voltage. If the number of turns increases, the cross-sectional area of the wire will inevitably decrease and the copper loss will increase if the slot area remains unchanged. If the number of turns decreases, the magnetic flux density increases at the rated voltage and the iron loss increases.

maychang

Whether the copper loss is too large or the iron loss is too large, the motor efficiency will be reduced.

Usually, the motor is designed so that the copper loss is equal to the iron loss when it is fully loaded. It can be proved that the efficiency is the highest when the copper loss is equal to the iron loss. This is not only true for motors, but also for power frequency transformers. Power frequency transformers are often designed so that the iron loss is equal to the iron loss when fully loaded, or the copper loss is slightly greater than the iron loss when fully loaded, because the power transformer is not fully loaded very often, and in most cases it cannot reach full load, especially agricultural transformers used for irrigation.

maychang

annysky2012 posted on 2021-10-15 18:38 I totally agree.

That’s right. If we pursue a single indicator unilaterally, other indicators will not meet the requirements, or the cost, volume and weight will increase, etc.

maychang

If you have disassembled a three-phase AC asynchronous motor and measured the thickness of the silicon steel sheet, you will find that the silicon steel sheet in the motor is mostly 0.5mm thick. So why not use thinner silicon steel sheets? If the silicon steel sheet is thinner, won't the eddy current loss be reduced?

The answer is: the rolling cost of thin silicon steel sheets is higher than that of thick silicon steel sheets. The use of 0.5mm thick silicon steel sheets is precisely to achieve a balance, or a compromise, between efficiency and cost.

annysky2012

maychang posted on 2021-10-15 18:52 That's right. The one-sided pursuit of a single indicator will inevitably make other indicators fail to meet the requirements, or increase costs, volume and weight, etc.

It is best to choose the one that suits your own project.

javnson

In practical applications, the number of turns, wire diameter, and slot fullness all need to be balanced. In practical applications, there are also examples of reducing the number of turns. On Taobao, you can find a "bad" BLDC motor. The original application was to drive a drone, but because the number of turns was too large, the KV value was large, but the torque was not high, so it "didn't work". Many people (including me) bought it and rewound it with thicker wire, and the effect was quite good!

If you are interested in theories related to motor design, you can refer to the book "Motor CAD Design". Although it looks very old, it explains the design of motor parameters and their mutual influence very thoroughly. I hope it helps!

Chad4133

For the single-phase motor you mentioned, if the number of coil turns is large, the cross-sectional area occupied by the wire will definitely become larger. The larger the area occupied by the wire, the smaller the cross-sectional area of the magnetic circuit will be (the coil is wound in the slot, the larger the slot area, the smaller the tooth area), and the smaller the magnetic circuit area, the lower the motor efficiency will be.