Description

Motor speed controlling is one of the main and typical applications of industrial resistors. These resistors are used to control of torque and speed characteristics of both AC and DC motors. Limitation of inrush current is the other application of starting resistors.

  1. Starting circuit of rotor wound induction motors

In the rotor wound induction motors, the rotor windings are connected to the slip rings. The slip rings through inside brushes transmit the electrical power to the starting circuit. With regard to the change of resistances value, the current of rotor is decreased. As a result, both speed and starting torque of motor are controlled. Usually, to calculate of resistances, the following parameters are utilized:

  • Application
  • Rated power
  • Rated rotor voltage (secondary voltage)
  • Rated rotor current (secondary current)
  • NEMA class of motor
  • Step numbers
  1. Starting circuit of squirrel cage induction motors

In the squirrel cage induction motors, rotor conductors are short circuited. Therefore, rotor resistance is fixed and invariable. So, the starting resistors are connected via incoming line. In the acceleration time, the inrush current in the motor conductors is created. As a result, the mentioned resistors as reducer of voltage are operated. These resistors are typically known as ballast resistors, which are voltage divider and decrease the inrush current. Finally, motor will have a soft starting. Usually, to calculate of resistances, the following parameters are utilized:

  • Application
  • Rated power
  • Rated line voltage
  • Rated full load current
  • NEMA class of motor
  1. Starting circuit of DC series motors

In DC series motors, current is flowed by the applied voltage to the armature windings (the rotational parts in DC and AC motors are known as armature and rotor, respectively). Moreover, the resistances of armature and excitation windings have small values, so in the starting time, several resistors in sequential form are used. These resistors according to the current set points in different steps are removed from the starting circuit. As a result, the applied resistors are operated as current limiter and controller of speed and torque characteristics. To calculate of resistances, the following parameters are utilized:

  • Application
  • Rated power
  • Rated line voltage
  • Rated full load current
  • NEMA class of motor
  • Step numbers
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