4 tips to know how to determine the performance of electric motor by its no-load current

Yesterday, I just arrived at the factory, the brother who repair the motor called, rushed to ask what is the normal range of no-load current of 55kw motor?

It turns out that my brother was testing a suspected faulty motor at the machine shop, and only had a clamp-on ammeter on hand.

Generally speaking motor no-load current can roughly reflect the performance of the motor. In all aspects of motor production, testing, sales and use, the no-load current of a motor can be used as a qualitative criterion to determine the state of the motor.

Today, I will analyze the no-load current of the electric motor with you.

No-load, load current and power data relationship

The rated voltage, rated current and no-load current data have some kind of inscrutable internal relationship, once the value exceeds the conventional.

You should be alert to whether the electric motor failure.

What is no-load current?

Three-phase asynchronous motor output shaft without load or zero torque for no-load operation, when the current in the stator three-phase winding is mainly excitation current, usually called no-load current.

The excitation current, which accounts for most of the no-load current, is used to generate the rotating magnetic field.

which is the reactive component of the no-load current, and a very small proportion is the active component corresponding to the no-load iron consumption, no-load mechanical consumption, wind wear, stray consumption, etc..

which can be neglected in the qualitative analysis. Therefore, the no-load current can be considered as reactive current.

No-load current magnitude and electric motor performance

Based on the above view, the smaller the no-load current is, the better, so that the power factor of the motor is improved, which is good for the power supply of the grid.

If the no-load current is large, because the wire load area of the stator winding is certain, the allowed current is certain.

Then the active current allowed to flow through the wire can only be reduced, and the load that the electric motor can drive will be reduced.

The electric motor output will be reduced, and the winding will be easily heated up when carrying too large a load.

No-load current should also not be too small, the design is too small in the case of possible starting torque, maximum torque failed.

Relationship between no-load current and rated current

Generally, the no-load current of small motors is about 30%~70% of the rated current, and the no-load current of medium and large motors is about 20%~40% of the rated current.

The no-load current of a specific motor is usually not marked on the nameplate or product manual of the motor.

Load current is the actual value of stator current detected when the electric motor is dragging the load, and this value varies with the size of the load.

No-load current estimation of the motor: generally 1/3~1/2 of its rated current.

According to the power formula P=√3IUηcosΦ*10(-3) η=0.8, cosΦ=0.8, U=380, I≈2.3P (the unit of power P is KW) that is, in general, the rated current value is about 2 times of the rated power value, and the no-load current is approximately equal to or less than the rated power value

Example of current calculation

The no-load current of a 55kW three phase motor should be less than 110 A.

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 of the rated current, that is, about 33 to 44 A.