Discussion on Overbalance of No-load Current after Rewinding Motor

Luoyang Copper Processing Group Electromechanical Equipment Repair Company (4) Most of the electric motors repaired by the electric repair shop are three-phase asynchronous motors, accounting for about 90% of the total motor maintenance of the whole year, and the motors after rewinding are generally used for the no-load test. The problem that often arises is that although the three-phase no-load current balances but is too large, the following discussion will be made on this issue.

1 Effect of motor no-load current on operation When the motor runs without load (without load), the current flowing in the three-phase stator windings is called idle current. Most of the no-load current is used to generate a rotating magnetic field, and a small part of the no-load current is used to bear the various power losses (such as friction and core loss, etc.) when the motor is in no-load operation. The no-load current is basically reactive current. Because the conductor cross-sectional area of ​​the stator winding is certain, the allowable current is certain. If the no-load current is large, the active current flowing through the conductor is allowed to be driven by the motor. The load must be reduced; if the load is too large, the windings are prone to heat and burn. However, the no-load current cannot be too small, otherwise it will affect the deterioration of other motor performance.

2 Suitable range of no-load current When the motor is used for no-load test, the motor with technical data is mainly based on technical data. There is no technical data. Table 1 can be used to estimate the no-load current of the motor.

Table I motor no-load current and rated current percentage \ capacity pole number \ 3 Three-phase no-load current balanced but generally increasing causes and solutions (1) insufficient number of coil turns when rewinding facilities added to the stator three-phase winding The phase voltage is believed to produce a stator current I, forming a three-phase rotating magnetic field. This rotating magnetic field cuts the stator winding and induces a potential £ in the stator winding.

E―The effective value of the phase potential of the three-phase winding Vf is a power frequency Hz W—the number of series windings of each phase of the stator winding 匝F—fundamental magnetic flux WbKdp1—a winding coefficient The number of turns W of each phase winding is reduced, and the generated electromotive force is required. If E is not changed, the magnetic flux F will inevitably increase inversely. An increase in the magnetic flux will cause saturation of the core magnetic circuit, and the iron loss will be proportional to the square of the magnetic flux density, resulting in a significant increase in iron loss and a sharp increase in no-load current. When other conditions remain unchanged, but the idle current is too large due to insufficient turns of the coil, in order to adjust the no-load current, the stator coil turns should be a normal no-load current phase asynchronous motor, and the stator is single-layer Y-connected. The normal no-load current I. is about 5A. Now measured I'= 6A. When rewinding, it knows that the original stator line is 25 turns. The rest is not defective. According to the formula 2 available: the re-coil of the coil is set to 26 匝e, and the measured no-load current is 5.2 A, and it returns to normal.

(2) Winding wiring error Winding wiring error - often encountered is the f column in two cases: (a) Y connected to the motor is incorrectly connected into A connection When the power line voltage is 380V, the stator three-phase winding should be connected to a Y shape The motor is misconnected into an A-shape. The motor's rated current 0E is 220V when it is designed, and it is increased to 380V after it is connected to A-shape. According to Equation I, the main flux in the core increases accordingly to the original JT. Times. The increase in no-load current will exceed "by twice, because the magnetic flux density 5 of the core will increase with the increase of the flux Φ5, when the flux density exceeds the saturation point or the inflection point of the magnetization curve of the chamber 1. After the iron core enters the saturation region, the corresponding excitation current increases drastically, and it may exceed the rated current.

The current number of wrong connections is due to the fact that it is easy to disconnect ten sets of wires if they are connected in series and connect them in parallel so that the number of winding turns is reduced (for example, one path is connected to two paths and the number of winding turns is reduced by 50). %).

The resulting no-load current greatly increases the pitch of the coil pitch, which is smaller than the standard pitch small root excavation type 1: Kdp, winding coefficient, winding distribution coefficient Kd and winding pitch coefficient! The composition, ie, Kdpl=KdXKP, the winding distribution coefficient does not change after the position of the motor winding is determined, and the pitch factor is related to the pitch of the line, and the full pitch time. The pitch coefficient of the coil is equal to 1, and the smaller the cargo distance, the smaller the pitch coefficient, which is equivalent to the reduction of the effective number of turns of each phase of the motor winding, resulting in a larger no-load current.

The stator current of the motor generates a magnetic field in the motor. The magnetic flux of the magnetic field passes through the teeth of the stator core, the air gap, the rotor core, and the rotor core yoke and returns to the stator core to form a closed loop. At the same time, it is realized by chaining the stator winding and the turn+winding: the asynchronous motor transmits electromagnetic energy. In the magnetic circuit through which the main flux passes, the length L of the air gap is a very short part and only tens of minutes of the entire length of the magnetic circuit. Or a few hundred percent.

However, the magnetic pressure drop HL of the air gap is very large, usually accounting for 60-85% of the entire magnetic potential, that is to say. 60%-85% of the magnetic potential established by the no-load current of the asynchronous motor is consumed in the air gap. This is because 0 is the magnetic resistance of the air gap much larger than that of the silicon steel sheet, and the magnetic guide of the silicon steel sheet is hundreds or even several times that of the air, so the change in the air gap size of the asynchronous motor affects the motor. The no-load current has a greater impact. When the motor air gap is uneven or increases. The no-load current of the motor will greatly increase.

Improper assembly of the motor can also cause uneven air gaps. Because the bearing is not parallel inside or outside, the motor will produce heat from the bearing of the bull and friction between the stator and the rotor core after the operation of the motor, which will increase the mechanical losses, reduce the motor efficiency and increase the no-load current.

Remove stitches. The core is overheated. When burned, the core is overheated and the core is overheated. The core burns and the core is deformed. 1 The surface of the punching sheet is charred.> Conductor is formed> All the punched sheets are short-circuited under the alternating magnetic field. When the eddy current 1 flowing through the short circuit core 1 increases, the core loss increases, and the no-load current increases.

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