The main dimensions of an electric motor are determined by its electromagnetic load (A) and magnetic load (Bδ). Higher electromagnetic loads result in smaller, lighter, and less expensive motors. This is why, where possible, higher values for A and Bδ are generally preferred. However, the selection of electromagnetic load values is related to many factors, affecting not only the effective material consumption of the motor but also its parameters, starting and running performance, and reliability. We discuss the impact of electromagnetic load on motor performance and economy, and then briefly introduce specific selection methods.
The impact of electromagnetic load on motor performance and economy: 1. When the electrical load A is high
● The size and volume of the motor will be smaller, saving steel materials.
●When Bδ is constant, the iron loss decreases as the core mass decreases.
● The amount of copper (aluminum) used in the winding will increase because the size of the motor is smaller. Under the condition that Bδ remains unchanged, the magnetic flux per pole will be smaller. In order to generate a certain induced electromotive force, the number of winding turns must be increased.
●Increased copper (aluminum) losses per unit surface of the armature, leading to higher winding temperature rise.
● This alters the motor parameters and characteristics. As A increases, the per-unit value of the winding reactance increases, which alters the motor's operating characteristics. For example, it will reduce the maximum torque, starting torque, and starting current of an asynchronous motor, and increase the voltage change rate, short-circuit current, short-circuit ratio, and static and dynamic stability of a synchronous motor. In a DC motor, it will worsen commutation.
2. When the air gap magnetic field Bδ is high
● The size and volume of the motor will be smaller, saving steel materials.
●Increases the basic armature iron loss. This is because, while increasing Bδ reduces the motor volume and armature core mass while other conditions remain constant, the magnetic flux density within the armature core increases accordingly due to the proportional relationship between Bδ and the magnetic flux density. The specific loss (i.e., the loss per unit mass of the core) is proportional to the square of the magnetic flux density. Therefore, as Bδ increases, the rate of increase in specific loss outpaces the rate of decrease in the armature core mass. Since the basic armature iron loss is equal to the product of its core mass and specific loss, increasing Bδ leads to increased armature iron loss, decreased efficiency, and, with unchanged cooling conditions, a higher temperature rise.
● The air gap magnetic potential drop and the saturation degree of the magnetic circuit will increase. Increasing Bδ directly increases the value of the air gap magnetic potential drop; furthermore, the increased magnetic flux density within the iron leads to increased magnetic circuit saturation. Thus, for DC and synchronous motors, the increased magnetomotive force leads to increased copper usage and excitation losses in the excitation winding, resulting in decreased efficiency; under constant cooling conditions, it also increases the temperature rise of the excitation winding. Sometimes, the excessive size of the excitation winding can cause difficulties in its arrangement (inner pole motors), or lead to an increase in the dimensions of the magnetic poles and the motor's overall dimensions (outer pole motors). For asynchronous motors, the increased excitation current will worsen the power factor.
● This alters the electrical parameters and motor characteristics. As Bδ increases, the per-unit value of the winding reactance decreases, thus affecting the motor's starting and running characteristics.
Disclaimer: This article is a reprint. If it involves copyright issues, please contact us promptly for deletion (QQ: 2737591964). We apologize for any inconvenience.
