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Today, the editor of Dalan Motor will analyze the reasons for the large no-load current of the three-phase asynchronous motor through examples, and briefly share its adjustment method for the no-load current too large.
1. Problem Overview
The user sent an imported 75KW asynchronous motor and said that after the stator winding of the motor was rewinded last month, the pump was tripped when the pressure of the pump was large. It is judged that the current has become larger and the original maintenance unit has no way to deal with it.
After receiving the motor, the motor is insulated, the three phases are higher than 200MΩ, connected to the 380V power supply, the three-phase current balance reaches 89A, because the motor has no nameplate, the rated current of the 4-pole 75KW cage motor is about 140A, and the no-load current is now reached 64% of the rated current is obviously beyond the standard, the normal should be around 45A, accounting for about 30%.
It is judged that the data of the stator winding of the three-phase asynchronous motor is incorrect, and it is decided to rewind and adjust the winding data at the same time.
Second, the stator core size of the motor: outer diameter 400mm, inner diameter 255mm, length 305mm, slot number stator 72. Original winding data: 4-pole 4-way angle connection, single-layer concentric winding, span 1-18, 2-17, 3-16, 3 sets, 18 turns, using 6 enameled wires with a diameter of 1.0.
Third, the processing method
1. Under normal circumstances, the no-load current of the motor is inversely proportional to the square of the effective turns of the winding, ie
I1 / I2 = N22 / N12 (Formula 1)
In the formula: I1, I2 are the no-load current before and after the winding adjustment, N1, N2 are the effective number of turns before and after the winding adjustment, the effective number of turns N = Kdp1Nd, where: Kdp1 is the winding short-distance coefficient, Nd is the winding of each phase in series Number of turns.
2. For this motor, it is planned to calculate according to the no-load current 50A. At the same time, the winding uses double-layer winding, the span is 1-16, the short-distance coefficient is 0.96 (look-up table), the original single-layer concentric winding comprehensive span is 1 -16, it can be considered that the short distance coefficient is 0.96. Into formula one, N2 = √I1 / I2 N1 = (√89 × 18 × 3) ÷ √50 = 72, that is, the adjusted number of series turns of each phase winding is 72 turns.
3. Calculated according to the number of series turns 72 after the adjustment of the motor winding, the double-stacked winding is 6 sets, and the number of turns of each coil is 12 turns. According to the test, the full rate of the motor slot, according to the original wire cross-sectional area, can not be put into the motor slot , Need to adjust the number of turns and current density, calculated according to the rated current 140A, the original current density is 4.3A / mm2, after adjustment using four 1.12 enameled wire, the current density is 5.13, at the same time, the number of turns is also adjusted to a total of 66 per phase Turns, that is, the number of turns in each coil is 11 turns.
4. Test verification
After the motor is rewound, the no-load test is assembled. The no-load current of the motor is 52A, which is 37.1% of the rated current. There is no heat and trip after user load.
----- Responsible Editor: Dalan Oil Pump Motor 02-Procurement Consultant
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How to adjust the no-load current of 75KW three-phase asynchronous motor to be too large? Dalan Electric will teach you!