Finally it’s clear. The motor never reached a full speed and burnt to death.
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When a motor is started at reduced voltage, the current at the motor terminals is reduced in direct proportion to the voltage reduction while the torque is reduced by the square of the voltage reduction. If the "typical" NEMA B motor is started at 70% of line voltage, the starting current would be 70% of the full voltage value (that is, 0.70 x 600% = 420% FLA). The torque would then be (0.70)2 or 49% of the normal starting torque (that is, 0.49 x 150% = 74% Full Load Torque). Therefore, reduced voltage starting provides an effective means of reducing both inrush current and starting torque.
If the motor has a high inertia or if the motor rating is marginal for the applied load, reducing the starting torque may prevent the motor from reaching full speed before the thermal overloads trip. Applications that require high starting torque should be reviewed to determine if reduced voltage starting is suitable.
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When a motor is started at reduced voltage, the current at the motor terminals is reduced in direct proportion to the voltage reduction while the torque is reduced by the square of the voltage reduction. If the "typical" NEMA B motor is started at 70% of line voltage, the starting current would be 70% of the full voltage value (that is, 0.70 x 600% = 420% FLA). The torque would then be (0.70)2 or 49% of the normal starting torque (that is, 0.49 x 150% = 74% Full Load Torque). Therefore, reduced voltage starting provides an effective means of reducing both inrush current and starting torque.
If the motor has a high inertia or if the motor rating is marginal for the applied load, reducing the starting torque may prevent the motor from reaching full speed before the thermal overloads trip. Applications that require high starting torque should be reviewed to determine if reduced voltage starting is suitable.
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