Causes and preventive measures of single-phase ope

Reasons and preventive measures for single-phase operation of motor

I. reasons and preventive measures for the reduction of resistance value caused by single-phase operation of motor

1. Fuse fusing ⑴ fault fusing: the fuse fusing is mainly caused by single-phase grounding or phase to phase short circuit of main circuit of motor

preventive measures: select motors that adapt to the surrounding environmental conditions and correctly installed low-voltage electrical appliances and lines, and regularly check them, strengthen daily maintenance, and eliminate various hidden dangers in time

⑵ non fault fusing: it is mainly due to improper selection of melt capacity and small capacity. When starting the motor, the fuse is blown by the impact of starting current

non fault fusing of fuse can be avoided. Don't unilaterally think that under the condition of avoiding the starting current of motor, the capacity of melt should be smaller as far as possible, so as to protect motor. We should make it clear that the fuse can only protect the motor from single-phase grounding and phase to phase short circuit accidents, and it must not be used as the overload protection of the motor

2. Correctly select the capacity of the melt

the general formula for selecting the rated current of the melt is:

rated current = rated current of K motor

⑴ fuse with large heat-resistant capacity (with filler type)  K value can be selected from 1.5 to 2.5

⑵ the K value of fuse with small heat-resistant capacity can be selected from 4 to 6

for different loads carried by motors,  The value of K is also different, for example, the motor directly drives the fan,  Then the K value can be larger. If the load of the motor is small, the K value can be smaller. The specific situation depends on the load carried by the motor

in addition, the melt of the fuse must be in good contact with the fuse base, otherwise it will cause heat at the contact, so that the melt will be subject to external heat and cause non fault fusing

in the process of installing the motor, appropriate wiring methods and correct maintenance methods should be adopted

⑴ copper aluminum transition joints shall be used for copper and aluminum connections as far as possible. If there is no copper aluminum joint, tin can be hung on the copper joint for connection

the apparent consumption of domestic crude steel will also decrease at a rate of about 4% per year. (2) for plug-in fuses with large capacity,  A thin copper sheet (0.2mm) can be padded at the wiring, which will have a better effect

⑶ check and adjust the contact pressure between the melt and the melt holder

⑷ avoid damaging the fuse during wiring. The fastening should be moderate, and the wiring should be padded with spring washers

3. Faults prone to occur in the main circuit

⑴ the dynamic and static contacts of the contactor are in poor contact

the main reason is that the contactor is improperly selected, and the arc extinguishing capacity of the contact is small,  Make the dynamic and static contacts stick together, and the action of three-phase contacts is not synchronized, resulting in phase failure operation

preventive measures: select a more suitable contactor

⑵ the use environment is harsh, such as humidity  Vibration, corrosive gas and poor heat dissipation conditions cause contact damage or wiring oxidation, poor contact and phase failure operation

preventive measures: select electrical components that meet environmental requirements, take appropriate protective measures, forcibly improve the surrounding environment, and regularly replace components

⑶ irregular inspection shows that the contactor contact is seriously worn and the surface is uneven, resulting in insufficient contact pressure and phase failure operation

preventive measures: according to the actual situation, determine a reasonable inspection and maintenance cycle, and carry out strict and careful maintenance work

⑷ improper selection of thermal relay will burn out the bimetallic sheet of thermal relay and cause phase failure operation

preventive measures: select appropriate thermal relay to avoid overload as far as possible

(5) improper installation causes wire breakage or wire damage by external force and

phase failure

preventive measures: during the construction of wires and cables, the specifications should be strictly implemented, and civilized construction should be carried out carefully

⑥ the quality of electrical components is unqualified, and the capacity cannot reach the nominal capacity, resulting in abnormal phenomena such as contact damage and sticking

preventive measures: select appropriate components, and carefully check them before installation

⑺ the quality of the motor itself is poor, and the coil winding is poorly welded or desoldered; Poor contact between lead and coil

preventive measures: select motors with good quality

II. Analysis and maintenance of single-phase operation

according to the different wiring methods of the motor, the current of single-phase operation is also different under different loads, so the protection methods are also different

for example, when the motor with Y-type wiring operates in single phase, the motor phase current is equal to the line current, and its size is related to the load carried by the motor

when the internal wire of the motor with delta connection is broken, the motor becomes a type connection, and the phase current and line current increase in proportion to the motor load. Under the rated current load, the two-phase current should increase by 1.5 times, the one-phase line current should increase to 1.5 times, and the other two-phase line current should increase by 3/2 times

when the external wire of the motor with delta connection is broken, at this time, the two-phase windings of the motor are connected in series with the third group of windings in parallel between the two-phase voltages, which indicates that the quality reliable current of the experimental machine is equal to the sum of the circuit currents of the windings in parallel, and increases in proportion to the motor load. Under the rated load, the line current increases by 3/2 times, the currents of the two windings connected in series remain unchanged, and the other phase current will increase by 1/2 times

under light load, the line current increases from light current to rated current, the current of two-phase winding keeps the light load current unchanged, and the third phase current increases by about 1.2 times

therefore, when the motor with angular connection operates in single-phase, its line current and phase current not only change with the different disconnection points, but also change according to the different loads

to sum up, the reasons for the single-phase operation of the motor are nothing more than the following reasons:

give full play to the advantages of human resources and material research and development

1. Bad environment or phase failure of one-phase power supply for some reason

2. Abnormal fuse

3. The starting equipment, wires and contacts are burned, damaged, loose, poorly contacted, improperly selected, etc., resulting in the loss of one phase of the power supply

4. One phase of motor stator winding is open circuit

5. The new motor itself fails

6. The starting equipment itself fails

as long as we carefully install it during construction, and during normal operation, maintenance and overhaul,  Strictly following the standard will certainly avoid unnecessary economic losses caused by the single-phase operation of the motor