Motor disassembly and maintenance

1. Definition and classification of motors

1. Electric motors convert electrical energy into mechanical energy and output it based on the principle of electromagnetic induction. The prime mover of mechanical torque.

2. Classification:

2. Analysis and judgment of common motor faults

Common motor faults are mainly divided into two categories: mechanical faults and electrical faults.

According to current

DC

AC

Synchronous

Asynchronous

Single Phase

Three-phase

Wound type

Squirrel cage type

3. The structure of the motor: mainly divided into stator and rotor.

Mechanical failures mainly include failures of bearings, fans, end covers, rotating shafts, casings, etc. Electrical faults mainly include stator winding, rotor winding and circuit faults.

During inspections, electricians can use their own senses to understand whether the motor is running normally - looking, listening, touching, smelling, and asking.

3. Motor disassembly

Taking the disassembly of a small-power three-phase cage motor as an example to introduce the disassembly methods and steps of the motor.

1. Preparations before disassembly

(1) Prepare common electrical tools and pull codes and other disassembly tools.

(2) Check and record the model, appearance and main technical parameters of the disassembled motor.

(3) Mark the end caps, shafts, screws, terminal blocks and other parts.

2． Disassembly steps

(1) Remove the windshield at the rear of the motor.

(2) Remove the fan blade at the rear of the motor.

(3) Remove the fastening screws of the front bearing outer cover and the front and rear end covers.

(4) Use a wooden board (or copper plate or lead plate) to pad the front end of the rotating shaft, and use a hammer to knock the rotor and back cover out of the machine base. The wooden hammer can directly hit the front end of the rotating shaft.

(5) Take out the rotor from the stator.

(6) Use a wooden stick to insert into the stator core, hold it against the inside of the front end, and use a hammer to knock the front end cover off the machine base. Finally, pull off the front and rear bearings and bearing inner covers.

(7) Remove the stator winding.

(8) Clear the trough and round the corners.

3． Disassembly methods of several main components

(1) Disassembly of bearings.

(2) Take out the small rotor.

(3) Removal of the end cover.

(4) Remove the old winding.

① Record the nameplate data and winding data of the motor in detail.

②Cold disassembly method: In small motors, semi-sealed wire ducts are generally used, and it is difficult to disassemble the windings. The method is to use a sharp flat shovel with a slope and lay the slope of the shovel flat. Use a hammer to tap on the notch to cut off the wires one by one. Do not use too much force during the operation to prevent the iron core from being damaged.

④Hot removal method: For coils that are difficult to remove, the heating method can be used to heat the old coil to a certain temperature, and then pull the stator winding out of the slot wedge. Commonly used heating methods include: electric blast constant temperature drying oven heating method; electric heating method; direct firewood burning method, etc.

4. Winding and embedding of windings

1. Introduction to special tools for winding

(1) Winding machine.

(2) Winding mold.

(3) Marking board.

(4) Pressing plate.

2． Drawing of the stator winding unfolded diagram

Now we will take a three-phase cage asynchronous motor with 4 poles and 24 slots single-layer winding as an example to illustrate the drawing process of the stator winding unfolded diagram.

(2) Steps for drawing the stator winding expansion diagram.

① Draw the slot number.

② Fixed pole distance (divided polarity).

③Mark the direction of current.

④ Phase separation zone.

⑤Connect the U-phase winding.

⑥Connect the other two phase windings.

(1) Definition of stator winding expansion diagram. Imagine making a cylinder out of paper to represent the inner circle of the stator. Use parallel straight lines drawn on the inner surface of the cylinder to represent the coil sides in the stator slot. Use numbers to indicate the number of the slot. Then, along the No. 1 slot and Cut the dot-dash line between the last slots, and after unfolding, draw the coils and their connection methods on this plan view, which is the expanded view.

(2) Winding connection method.

3． Winding method

4. Basic methods of embedding wires

(1) Cutting of insulating materials.

We use 0.2mm thick insulating paper (composite paper)

Length=slot length+5×2=910=100mm,

Width= Groove depth×2×2=15×2×2=60mm.

(2) Wire embedding method

(3) Main process requirements of wire embedding

① Wire embedding.

② Press the wire.

③Seal the slot.

④The ends are insulated between phases.

⑤ End shaping.

⑥ Bandaging.

(4) Winding wiring.

5. Motor assembly

The assembly process of the motor is opposite to the disassembly process.

Bearing assembly:

a. Cold sleeve method: Put the bearing on the shaft, use a section of iron pipe, align one end with the journal, press it against the inner ring of the bearing, tap the other end with a hammer, and slowly tap it in.

b. Hot sleeve method: The bearing can be placed in transformer oil with a temperature of 80°C to 100°C and heated for 20 to 40 minutes. Quickly push the bearing all the way to the shaft shoulder while it is hot. After cooling, it will automatically shrink and tighten.

6. Dip paint and drying

The winding insulation of the motor absorbs moisture or moisture from the outside air can invade the winding, which reduces the insulation resistance of the motor, so it must be dipped in insulating paint.

Steps: pre-baking, dipping, and drying.

7. Electrical inspection and testing after motor assembly

1. Measurement of DC resistance.

2. Measurement of insulation resistance.

3. Pressure resistance test.

4. Short circuit test.

5. No-load test.

6. Reversal test.

8. Measurement of insulation resistance

1. Megger.

The insulation resistance of insulating materials can be measured with a megger. Megger, also known as megohmmeter, is used to measure the insulation resistance of electrical equipment or power distribution equipment. Its unit is megaohm (MΩ).

The megohmmeter has three terminals: ground electrode (E), line (L) and guard ring (G).

①Measure the insulation resistance of the line to the ground: the E pole is connected to the ground wire, and the L pole is connected to the line under test.

②Measure the insulation resistance of the motor or electrical equipment shell: the E pole is connected to the shell of the device under test, and the L pole is connected to one end of the wire or winding under test.

③Measure the insulation resistance of the cable core wire: E pole is connected to the outer sheath (lead sheath) of the cable, L pole is connected to the core wire, and G pole is connected to the outermost insulation coating layer.

2. The following points should be noted when using the megometer:

① The megometer should be selected according to the voltage level of the electrical equipment under test.

② Before measurement, the equipment under test should cut off the power supply. Equipment with large capacitance should be discharged to ground.

③ Before measurement, conduct an open circuit and short circuit test with the megohmmeter.

④ During shake measurement, the megohmmeter must be placed flat and the rotation speed must be uniform, about 120 revolutions per minute. Do not cause the megohmometer to vibrate.

⑤The wiring of the megometer must use two independent insulated wires, and parallel wires or twisted wires are not allowed.

⑥After measurement, the equipment under test should be fully discharged.

⑦ When measuring resistance with a megometer, it should last for 1 minute.