Shielded pump motor insulation
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This post was last edited by 156026692 on 2018-10-18 at 16:23. The resistance value of the motor’s megohmmeter reading is 10MΩ, and the motor’s power is 80KW. The power supply says that the resistance value is too low, and there is a risk of damaging the motor during operation; could someone provide the standard resistance value for shielded pump motors? Thank you.
The insulation resistance of the motor windings in shielded pumps generally exceeds 100 MΩ. If it is below 5MΩ, it is necessary to analyze the reason to check whether the insulation is damp.
I’m not sure from which point the insulation resistance was measured. It’s possible that since this pump hasn’t been operated for a long time, the motor or the wiring box has become damp, resulting in a non-zero insulation value; this indicates that the shielding cover is not damaged. It is recommended to start the pump and run it for a short period of time, then measure the resistance again once the motor has warmed up. If the resistance value increases, continue running the pump until the insulation resistance returns to normal levels.
The windings of shielded motors are the same as those of ordinary AC motors. Below is an excerpt: According to China’s standard GB755-87, at the operating temperature, the insulation resistance of motor windings must meet the value calculated by the following formula: R ≥ Un/(1000 + Pn/100) (megohms), where R represents the insulation resistance of the motor (in megohms)； Un--Motor rated voltage (volts) ； Pn--Rated power of the motor (kilowatts). When testing the cold-state insulation resistance, it is required that this resistance be no less than 1 megohm per kilovolt, that is, R ≥ 1 megohm/kilovolt. 1. When using a megohmmeter to measure insulation resistance, a 500-volt megohmmeter is generally used for motors with voltages below 500 volts ； Motors rated at 500–1000 volts should be measured using a 1000-volt megohmmeter. Motors with voltages above 1000 volts are measured using a 2500-volt megohmmeter. 2. When the motor is in a hot state (75°C), the insulation resistance value of general small and medium-sized low-voltage motors should be no less than 0.5 megohms. For high-voltage motors, the insulation resistance value of the stator at each kilovolt of operating voltage should be no less than 1 megohm, while the insulation resistance value of the wound-rotor windings at each kilovolt of operating voltage should be no less than 0.5 megohms ； The insulation resistance of the motor’s secondary circuit should not be less than 1 megohm. 3. The steps for measuring the insulation resistance of the motor are as follows: (1) Disconnect the connecting pieces at the 6 terminals inside the motor’s junction box. (2) Place the megohmmeter flat, do not connect any wires yet, and rotate the meter. The pointer should point to “∞”. Then, short-circuit the two terminals marked “l” (line) and “e” (ground) on the meter using test clips connected by a wire, and slowly turn the handle – the pointer should then point to “0”. (3) Measure the resistance between the three-phase windings of the motor. Connect the two test clamps to either end of any two phase windings respectively, place the megohmmeter horizontally, rotate it at a constant speed of 120 revolutions per minute for one minute, and then read the stable indication shown on the meter pointer. (4) Using the same method, measure the insulation resistance value between each phase winding and the casing in sequence. It should be noted, however, that the terminals marked “e” or “ground” should be connected to the uninsulated parts of the enclosure.
4th floor, correct! Shielded motors are also squirrel-cage motors