Yesterday, Ms. Can had just gotten off the plane when Brother K, who repairs motors, called her urgently, asking what the normal range of no-load current is for a 55kW motor. It turned out that Brother K was inspecting a suspected faulty motor at a machine tool factory and only had a clamp meter with him.
Generally speaking, the no-load current of a motor can roughly reflect its performance. In all stages of motor production, testing, sales, and use, the no-load current can serve as a qualitative standard for judging the motor's condition. Today, Ms. Can will discuss the no-load current of motors with everyone.
Relationship between no-load, load current and power data
There is a certain elusive relationship between the rated voltage, rated current and no-load current. Once the values exceed the norm, one should be alert to the possibility of motor malfunction.
What is no-load current?
When a three-phase asynchronous motor operates without a load or with zero torque on its output shaft, it is considered to be in no-load operation. At this time, the current in the three-phase stator windings is primarily the magnetizing current, commonly referred to as the no-load current. The magnetizing current, which constitutes the vast majority of the no-load current, is used to generate a rotating magnetic field and is the reactive component of the no-load current. A very small proportion consists of the active component, corresponding to no-load iron losses, no-load mechanical losses, wind wear losses, and stray losses, which can be ignored in qualitative analysis. Therefore, the no-load current can be considered entirely reactive current.
No-load current magnitude and motor performance
Based on the above points, the lower the no-load current, the better. This improves the power factor of the motor and is beneficial to the power grid. If the no-load current is large, since the cross-sectional area of the stator winding conductors is fixed and the allowable current is also fixed, the allowable active current flowing through the conductors can only be reduced. This means the load that the motor can drive will be reduced, the motor output will decrease, and the windings will easily overheat when carrying an excessive load.
The no-load current should not be too small; if the design current is too small, the starting torque and maximum torque may not meet the requirements.
Relationship between no-load current and rated current
The no-load current of a small electric motor is generally 30% to 70% of its rated current, while that of a medium to large electric motor is approximately 20% to 40% of its rated current. The specific no-load current of a particular electric motor is usually not indicated on its nameplate or in its instruction manual.
Load current refers to the actual stator current value detected when the motor drives a load. This value varies with the size of the load.
Example of current calculation
The rated current of a 55kW motor should be less than 110A. The no-load current of a motor is related to the quality of the motor. Generally, the no-load current of a 55kW motor is about 0.3 to 0.4 times the rated current, that is, about 33 to 44A.
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