Motor current

Based on the formula I=U/R, it is possible to determine the variation curve of the motor current as a function of voltage changes. Since resistance is a constant, voltage and current vary in direct proportion to each other; when the grid voltage drops, the current also decreases accordingly.

Figure it out! This is a motor, not an electric furnace.

This post was last edited by lmsa613 on 2010-7-12 21:11. Second floor – don’t mislead people. In an AC asynchronous motor, when the grid voltage drops, the motor’s speed decreases, the speed difference between the rotor and the rotating magnetic field increases, and the current in the rotor coils rises. This is why the voltage is low, causing the motor to burn out.

Reply to 4# lmsa613: Thank you for your guidance. I’m not an expert in electrical engineering, so I still don’t quite understand it. I’d like to ask my mentor again: when the voltage drops, the rotor’s speed decreases. However, the magnetic field coils that generate the magnetic field are part of the stator and do not rotate. So what causes the decrease in rotor speed when its speed drops? When current cuts through magnetic field lines, a force is generated in the conductors carrying current, and this force is what drives the motor to rotate. The magnitude of this force depends on the strength of the stator’s magnetic field as well as the magnitude of the current in the rotor. The greater the magnetic field strength and the current, the greater this force. When the voltage drops, the magnetic field strength decreases. With the rotor’s resistance remaining constant, its speed will decrease. But what determines the increase in rotor current? First, thank you to the expert for their guidance

Reply to 5# Snowy Highland Road: I think it’s the increase in current in the stator that causes the current in the rotor to increase as well. Studying*

Let’s go into more detail! The stator does not rotate, but the magnetic field does, which is called a rotating magnetic field. As the voltage decreases, the electromagnetic torque decreases; the no-load torque remains unchanged (theoretically), and the output torque is significantly reduced. Therefore, the rotational speed decreases. To get a detailed explanation of how current changes, you’d better look up a more specialized book; my abilities are limited, and I’m unable to explain it clearly!

Once you understand the starting process of the motor, it becomes easy to comprehend the changes in motor current!

Generally, as the supply voltage decreases, the slip of the motor increases, while the load torque of the motor remains unchanged. When the motor is operating stably, the electromagnetic torque is equal to the load torque. As the slip rate S increases, the motor current also increases accordingly; the current in the motor will be higher than the rated value, and prolonged operation will affect the motor’s lifespan.

In other words, the voltage decreases while the current increases, which is not conducive to the proper operation of the motor!

The capacity of the motor is fixed; when the grid voltage decreases, the current must increase. When large-capacity motors are started directly, it causes a significant impact on the grid, with the voltage drop usually exceeding 20%, and the starting current being more than 5 times the rated current