Methods of diagnosing malfunctions of asynchronous electric motors

The engine does not turn when starting or its speed is abnormal... The reasons for the indicated fault can be mechanical and electrical problems.

Electrical problems include: internal breaks in the stator or rotor winding, a break in the supply network, violations of normal connections in the starting equipment. If the stator winding is broken, it rotates magnetic field, and if there is an interruption in two phases of the rotor, there will be no current in the winding of the latter interacting with the rotating field of the stator, and the motor will not be able to work. If during an interruption of the operation of the winding of the motor, it can continue to operate at the rated torque, but the speed of rotation will be significantly reduced and the force current will increase so much that, in the absence of maximum protection, the stator winding or the rotor may burn.

If the windings of the motor are connected to a triangle and one of its phases is broken, the motor will start to rotate, since its windings will be connected in an open triangle, in which a rotating magnetic field is formed, the current in the phases will be uneven and the rotation speed will be lower than the nominal one. With this error, the current in one of the phases in case of nominal motor load will be 1.73 times higher than in the other two. When all six ends of its windings are removed from the motor, the phase break is determined megohmmeter… The winding is disconnected and the resistance of each phase is measured.

Motor speed at full load lower than rated can be due to low voltage, poor contacts in the rotor winding, and also due to high resistance in the rotor circuit in the phase rotor motor. With high resistance in the rotor circuit, the slip increases the motor and its rotation speed decreases.

The resistance in the rotor circuit increases due to bad contacts in the rotor brush, starting rheostat, winding connections with slip rings, soldering of the ends of the winding, as well as insufficient cross-section of cables and wires between the slip rings and the starting rheostat.

Bad contacts in the rotor winding can be detected if a voltage equal to 20-25% of the rated voltage is applied to the motor stator. The locked rotor is slowly turned by hand and the amperage in all three phases of the stator is checked.If the rotor is straight, then in all its positions the current in the stator is the same, and in case of a break or bad contact, it will change depending on the position of the rotor.

Bad contacts when soldering the ends of the phase rotor winding are determined by the voltage drop method. The method is based on increasing the voltage drop in the places of poor soldering. In this case, the magnitude of the voltage drop across all connections is measured, and then the measurement results are compared. Soldering is considered satisfactory if the voltage drop in them exceeds the voltage drop in solders with minimum values ​​by no more than 10%.

Deep groove rotors can also break bars due to mechanical stress on the material. Bar tear in the groove portion of the squirrel cage rotor is determined as follows. The rotor is pushed out of the stator and several wooden wedges are driven into the gap between them so that the rotor cannot turn. A voltage of less than 0.25 UН is applied to the stator. On each groove of the protruding part of the rotor alternates a steel plate, which should overlap the two teeth of the rotor. If the bars are intact, the plate will be attracted to the rotor and rattle. In the presence of a tear, the pull and rattle of the plate disappear.

The motor rotates with the phase rotor open circuit. The reason for the malfunction is short circuit in the rotor winding. When switched on, the motor rotates slowly and its windings become very hot because a large current is induced in the short-circuited turns by the rotating field of the stator.Short circuits occur between the clamps of the face parts, as well as between the bars during the breakdown or weakening of the insulation in the rotor winding.

This damage is determined by careful visual inspection and measurement. insulation resistance of the rotor winding. If the inspection fails to detect a fault, then it is determined by the uneven heating of the contact rotor winding, for which the rotor is stopped and a reduced voltage is applied to the stator.

Uniform heating of the entire engine above the permissible norm can be the result of prolonged overloading and deterioration of cooling conditions. Increased heating causes premature wear of the winding insulation.

Local heating of the stator winding, which is usually accompanied by a loud hum, a decrease in the speed of rotation of the motor and uneven currents in its phases, as well as the smell of overheated insulation. This malfunction can occur as a result of incorrect connection of the coils to each other in one of the phases, a short circuit of the winding to the housing in two places, a short circuit between two phases, a short circuit between turns in one of the phases of the stator winding.

In the event of a short circuit in the motor windings, a rotating magnetic field will cause e to short circuit. etc. with which will create a current of large magnitude, depending on the resistance of the closed loop. A damaged winding can be found by the value of the measured resistance, while a damaged phase will have less resistance than a good one. Resistance is measured with a bridge or by the ammeter-voltmeter method.The faulty phase can also be determined by measuring the current in the phases if a lower voltage is applied to the motor.

When the windings are star connected, the current in the faulted phase will be greater than in others. If the windings are delta connected, the line current in the two conductors to which the faulted phase is connected will be greater than in the third conductor. When determining the indicated fault in a motor with a squirrel-cage rotor, the latter may be braked or spinning, and in wound rotor motors, the rotor winding may be open. Damaged coils are determined by the voltage drop across their ends: with damaged coils, the voltage drop will be less than with good ones.

Local heating of active stator steel occurs due to burning and melting of steel during a short circuit in the stator winding, as well as when closing steel sheets due to friction of the rotor against the stator when the motor is running or due to the breakdown of the insulation between individual steel sheets. Signs of rotor friction on the stator are smoke, sparks and a burning smell; the active steel in the places of friction has the form of a polished surface; a buzz is generated accompanied by engine vibration. The cause of grazing is a violation of the normal clearance between the rotor and the stator as a result of bearing wear, improper installation, bending of a large shaft, deformation of the stator or rotor steel, one-sided attraction of the rotor to the stator due to rotation, malfunctions in the stator winding, strong vibrations of the rotor, which are determined with a probe.

Abnormal Motor Noise… A normally running motor produces a steady humming sound common to all AC machines. Increased humming and abnormal noises from the motor can be caused by the weakening of the pressing of active steel, the packages of which will periodically shrink and weaken under the influence of the magnetic flux. To eliminate the defect, it is necessary to suppress the steel packages. Loud hums and noises in the machine can also be the result of uneven rotor and stator spacing.

Damage to the winding insulation can occur from prolonged overheating of the motor, moisture and contamination of the windings, penetration of metal dust, chips, and also as a result of natural aging of the insulation. Damage to the insulation can cause short-circuiting between phases and turns of individual windings of the windings, as well as short-circuiting of the windings to the motor housing.

Wetting of the windings occurs in case of prolonged interruptions in the operation of the motor, with direct penetration of water or steam into it as a result of storing the motor in a damp, unheated room, etc.

Metal dust trapped inside the machine creates conductive bridges that can gradually cause a short circuit between the phases of the windings and onto the housing. It is necessary to strictly observe the deadlines for inspections and scheduled engine maintenance.

The insulation resistance of motor windings with a voltage of up to 1000 V is not standardized, the insulation is considered satisfactory at a resistance of 1000 ohms to 1 in the rated voltage, but not less than 0.5 MΩ at the operating temperature of the windings.

A short circuit of the winding to the motor housing is detected with a megohmmeter, and the location of the short circuit is detected by "burning" the winding or by applying direct current.

The "burn-in" method is that one end of the damaged phase of the winding is connected to the network, and the other to the housing. With the passage of current at the place of short-circuiting of the coil to the housing, «burning» is formed, smoke and the smell of burnt insulation appear.

The motor does not run as a result of blown fuses in the armature winding, breakage of the resistor winding in the starting rheostat, or contact damage in the supply wires. A break in the resistance winding in the starting rheostat is detected with a test lamp or a megohmmeter.