Inspection and test methods for three-phase asynchronous motors

1、 Inspection and test of three-phase asynchronous motor mainly include:

Measurement of insulation resistance between windings and casing and between windings; Measurement of DC resistance of winding under actual cold state; No load test; Dielectric strength test of insulation between windings and casing and between windings; If necessary, conduct the dielectric strength test of winding turn to turn insulation.

2、 Inspection and test methods of three-phase asynchronous motor:

a) Appearance inspection: check whether the appearance is complete, whether the signs at the outlet end are correct, whether the fastening screws, bolts and nuts are tightened, whether the rotor rotates flexibly, and how about the radial runout of the motor shaft extension and vibration. For the wound rotor motor, the assembly quality of the brush, brush holder and collector ring should also be checked, as well as whether the contact between the brush and collector ring is good. Check the exhaust system of closed self fan cooling motor;

b) Measurement of insulation resistance: for the motor after repair, only the cold (normal temperature) insulation resistance of winding phase to phase and phase to ground is generally measured. For the wound rotor motor, the insulation resistance of rotor winding shall also be measured. For motors with multi speed windings, the insulation resistance of each winding shall be measured one by one. The insulation resistance of large motors can be measured to determine whether the windings are damp. For motors with rated voltage below 500V, 500V tramegger is generally used for measurement, and 1000V tramegger is used for motors with rated voltage between 500V and 3000V; 2500V tramegger shall be used for motors above 3000V. For motors below 500V, the insulation resistance shall not be less than 0.5M Ω. All windings replaced shall not be less than 5M Ω;

c) Measurement of DC resistance: the DC resistance of motor winding is generally measured under cold state. The ratio between the measured resistance values of each phase and the three-phase average value shall not be greater than 5%. If the difference between the resistance values is too large, it indicates that there is short circuit, open circuit, poor welding or contact in the winding, or the number of winding turns is wrong. If the three-phase resistance exceeds the specified range, it indicates that the winding wire is too thin;

d) Withstand voltage test: withstand voltage refers to that the motor stator winding can withstand a certain voltage without breakdown after being insulated by insulating materials from phase to phase and from phase to ground. AC withstand voltage test and DC withstand voltage test are both withstand voltage tests, which are methods to identify the insulation strength of power equipment.

DC withstand voltage test: the voltage is high, which has a special effect on finding some local defects of insulation, and can be carried out simultaneously with the leakage current test. Compared with AC voltage withstand test, DC voltage withstand test has the advantages of portable test equipment, less damage to insulation and easy to find local defects of equipment. Compared with the AC voltage withstand test, the main disadvantage of the DC voltage withstand test is that the voltage distribution inside the insulation is different under AC and DC, and the DC voltage withstand test is not as close to the actual insulation as AC voltage withstand test.

AC withstand voltage test: AC withstand voltage test is very strict to the test of insulation, and can effectively find more dangerous concentrated defects. It is a direct method to identify the insulation strength of electrical equipment. It is of decisive significance to judge whether electrical equipment can be put into operation. It is also an important means to ensure the insulation level of equipment and avoid insulation accidents. The AC withstand voltage test may sometimes make some weaknesses in the insulation more developed. Therefore, before the test, the insulation resistance, absorption ratio, leakage current, dielectric loss and other items must be tested on the test object, and the AC withstand voltage test can be conducted only after the test results are qualified.

1) Stator winding: during the handover test, 1 kV shall be taken for the rated voltage of 0.4 kV and below, and 10 kV for the rated voltage of 6 kV; For the motor in operation, and for the motor whose stator winding is not replaced or partially replaced during overhaul, take 1.5 times of the rated voltage, but not less than 1000 volts; For motors with all stator windings replaced, take 2 times of the rated voltage plus 1000 volts, but not less than 1500 volts; The AC withstand voltage test of low voltage motors of less importance below 100 kW can be conducted with a 2500 V megger;

2) Rotor winding: during the handover test, take 1.5 times the rated voltage for the irreversible rotor and 3 times the rated voltage for the reversible rotor.

Note: For the AC voltage withstand test of synchronous motor rotor coil, the test voltage shall be 7.5 times of the excitation voltage, but shall not be lower than 1200V and not higher than 75% of the factory test voltage.

e) Turn to turn insulation test: increase the power supply voltage to 130% of the rated voltage, and let the motor idle for 5min without short circuit. It is called turn to turn insulation test to assess the insulation performance between turns;

f) Measurement of rotor open circuit voltage: when measuring the rotor open circuit voltage, the rotor is stationary, the rotor winding is open circuit, the starting rheostat is disconnected, the rated voltage is applied to the stator winding, and the voltage between lines is measured between the rotor collector rings. For motors with the rated voltage above 500V, the voltage applied to the stator winding can be appropriately reduced;

g) No load test: the no-load test is to apply three-phase balanced voltage on the stator winding of the motor to make the motor operate without load. Its purpose is to determine the no-load current and no-load loss, and separate the iron loss and mechanical loss (including wind friction loss) from the no-load loss.

During the no-load test, observe the motor operation, monitor whether there is abnormal sound, whether the iron core is overheated, whether the temperature rise and operation of the bearing are normal, and check whether the electric brush of the wound rotor motor has sparks and overheating.

For the repaired asynchronous motor, during the no-load test, usually only the no-load current is measured to check the quality of the motor after repair. No load loss test shall be conducted only when necessary.

3、 General inspection before DC motor test:

a) Carry out general inspection on the assembly quality of the motor (such as whether the fasteners are tightened and whether the rotor rotates flexibly); The brush holder shall be firmly and accurately fixed on the brush holder, the lower edge of the brush holder shall be parallel to the surface of the commutator, and the distance between the brush holders shall be equal; The brush shall be able to move up and down freely in the brush holder, but it shall not be too loose; The brush surface shall be well matched with the commutator; The spring pressure at the top of the brush shall be adjusted properly; The commutator surface shall be clean and smooth, the mica sheet between commutator sheets shall not be higher than the commutator surface, and the recessed depth shall be 1-1.5 ㎜; Check whether the outgoing line of the motor is correct;

b) Use a plug gauge to check the air gap under each magnetic pole on the circumference of the armature, and measure at both ends of the motor axis each time. The large allowable deviation value of air gap shall not exceed ± 10% of its arithmetic mean value.

The DC motor after repair is generally subject to the following tests:

a) Check the polarity of motor winding and the correctness of its connection; Check the correctness of the connection between the main magnetic pole and the commutation pole winding; Check the correctness of connection between series excitation and shunt excitation windings (or between shunt excitation windings); When measuring and measuring the insulation resistance of windings to the enclosure and between windings, in addition to measuring the insulation resistance of each winding to the enclosure and between each other, the insulation resistance between the steel wire hoop of the armature winding and the commutator, between the commutator tight ring and the commutator, and between the brush holder and the enclosure (the brush shall be lifted at this time) shall also be measured. The insulation resistance value shall not be lower than the value calculated by the following formula, that is, R=UN/(1000+PN/100), where R - insulation resistance (M Ω); UN - rated voltage of motor (V); PN - rated power of motor (KW);

b) The DC resistance of winding shall be measured by double arm bridge. The measurement shall be carried out three times, and the arithmetic mean value shall be taken. At the same time, the ambient temperature shall be measured with a thermometer; Under the condition that all windings of the motor are correctly connected, the brush of the motor must be placed on the geometric neutral line to ensure good operation performance of the motor; If the windings are replaced, the commutator is repaired, or the winding insulation is suspected, the withstand voltage test shall be conducted on the casing of each winding and commutator, and the withstand voltage test shall be conducted between windings;

c) If the above tests are qualified, the motor can be energized for no-load test;

d) The DC motor after general maintenance may not be subject to load test.