The application of geometric similarity law in motors
In order to further understand the main dimensions of the motor and some regularities between power, speed and electromagnetic load, we now analyze a series of motors with increasing power and similar geometry, which have the same current density, flux density, speed and pole. number. The so-called geometric similarity means that the corresponding ratios of the motors have the same ratio. For example, if the geometry of the motor A and B are similar, DA/DB=LA/LB=hsA/hsB=bsA/bsB=..., where hs and Bs is the slot height and slot width respectively.
It shows that for a series of power-increasing, geometrically similar motors, the quality, cost, and loss of effective material required per unit of power (kW or kVA) are maintained when the values of B and J remain the same. It is inversely proportional to the power of 1/4 power, that is, as the capacity of the single machine increases, the utilization of effective materials and the efficiency of the motor will increase. This also explains why, in the case of possible electrical equipment, there is a tendency to use high-power motors instead of several small-power motors with equal total power. Secondly, this law can be used to estimate roughly the geometry, cost or loss of a motor that is similar to the geometry of the finished motor, but can also be used to analyze the corresponding relationship between motors of various sizes in a series of geometrically similar series. .
It must be pointed out that the above geometric similarity law and some other relations are very similar. In actual situations, the relationship they reflect often has to be abandoned or destroyed due to other conditions. For example, the loss of the motor is proportional to the cube of the length dimension, but the cooling surface is proportional to the square of the length dimension. In order to ensure that the motor temperature rise does not exceed the allowable value. To withstand the increase in motor power, it is necessary to try to change the cooling system or cooling method, etc., thus giving up the similarity of their geometry.
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