Except for particularly small motors, most motor windings require dipping and drying processes to ensure the insulation performance of the motor windings and at the same time reduce damage to the windings when the motor is running through the curing effect of the windings.
However, once an irreparable electrical fault occurs in the windings of the motor, the windings must be reprocessed, and the original windings will be removed. In most cases, the windings will be taken out by incineration, especially in motor repair shops. , is a more popular method. During the incineration process, the iron core will be heated together, and the iron core punched sheets will be oxidized, which is equivalent to the effective length of the motor core becoming smaller and the magnetic permeability of the iron core decreasing, which directly leads to The no-load current of the motor becomes larger, and the load current will also increase significantly in severe cases.
In order to avoid this problem, on the one hand, measures are taken in the manufacturing process of the motor to ensure the quality and reliability of the motor windings. On the other hand, the windings are taken out in other ways when the motor windings are repaired. This is a measure taken by many standardized repair shops. It is also necessary for environmental protection requirements.
Generally, it depends on the power of the motor. The no-load current of small motors can reach 60% of the rated current, or even higher. The no-load current of large-sized motors is generally only about 25% of the rated current.
The relationship between starting current and normal operating current of three-phase motor. The direct start is 5-7 times, the reduced voltage start is 3-5 times, and the three-phase motor stall current is about 7 times. Single-phase motors are about 8 times.
When the asynchronous motor is running without load, the current flowing through the three-phase winding of the stator is called no-load current. Most of the no-load current is used to generate a rotating magnetic field, which is called the no-load excitation current, which is the reactive component of the no-load current. There is also a small part of the no-load current used to generate various power losses when the motor is running without load. This part is the active component of the no-load current, and it can be ignored because it accounts for a small proportion. Therefore, no-load current can be considered as reactive current.
From this point of view, the smaller it is, the better, so that the power factor of the motor is improved, which is good for power supply to the grid. If the no-load current is large, since the conductor carrying area of the stator winding is certain and the current allowed to pass through is certain, the active current allowed to flow through the conductors can only be reduced, and the load that the motor can drive will be reduced. When the motor output is reduced and the load is too large, the windings tend to heat up.
However, the no-load current cannot be too small, otherwise it will affect other properties of the motor. Generally, the no-load current of small motors is about 30% to 70% of the rated current, and the no-load current of large and medium-sized motors is about 20% to 40% of the rated current. The specific no-load current of a certain motor is generally not marked on the motor’s nameplate or product manual. But electricians often need to know what this value is, and use this value to judge the quality of the motor repair and whether it can be used.
A simple estimate of the motor’s no-load current: divide the power by the voltage value, and multiply its quotient by six divided by ten.
Post time: Sep-28-2023