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Three-phase automatic stator resistance starters with starting resistors |
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Single or multi-step resistors are connected upstream of the three-phase squirrel-cage motors to reduce the starting
current and torque. With single-step starters, the starting current is approximately three times the motor full-load current. With
multi-step starters, the resistors can be so designed that the starting current is only 1.5 to 2 times the motor
full-load current, with a very low level of starting torque. |
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Three-phase automatic stator resistance starters with starting transformers |
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This type of starting is preferable where the same starting torque is to be obtained as with the primary resistance
starters but the starting current taken from the mains is to be further reduced. A reduced voltage Ua
(approximately 70 % of the rated operational voltage) is supplied to the motor when starting via the starting
transformer. Thus, the current taken from the mains is reduced to approximately half the direct starting current. |
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Three-phase automatic rotor starters with starting resistors |
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Resistors are connected in the rotor circuit of the motor to reduce the starting current of motors with slip-ring rotors.
The current taken from the mains is thus reduced. In contrast to stator resistance starters, the torque of the
motor is practically proportional to the current taken from the mains. The number of steps of the automatic starter is
determined by the maximum permissible starting current and by the type of the motor. |
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I: line current
Md: torque
n: speed
  Reduction of the line current
 Reduction of the torque
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