Understanding Split-phase Induction Motors

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Basic understanding as to how split-phase motors start

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Understanding Split-phase Induction Motors

  1. 1. Split-Phase Motors All single-phase motors work basically the same. It is the way the are started that differs. In order for a single-phase induction motor to work the rotor has to be subjected to a rotating magnetic field. This is produced in the stator coils of the split-phase motor by having start windings positioned with their axes 90° electrical in relation to the run windings.
  2. 2. How they work The split-phase machine has two windings from a single phase arranged in the stator. One is the main winding and other is starting winding, which is used only for starting purpose. The run winding has characteristics of low resistance but high reactance. The starting winding has high resistance but low reactance. Accordingly, the start windings are first to become magnetized when the power is applied. The current flow through the start winding begins after power is applied to the motor by 20 degrees or so. The current in the run windings begins even later by about 50 degrees. This timing creates a phase differential which creates the necessary rotating magnetic field. When the run windings become strongly magnetized, the start windings begin to fade out. As a result the magnetic field shifts to a new position creating a rotating magnetic field.
  3. 3. The start current lags less than therun current.
  4. 4. The single phase induction motor begins to work like a two phase motor!
  5. 5. These are a net magnetic field
  6. 6. After the starter and main winding has developeda rotating magnetic field, the rotor starts to rotateand for further rotation of the rotor, the starterwinding is not required. So a switch is providedthat can be opened to isolate the starter windingfrom the motor circuit. Usually the switch may beof centrifugal type and thus after the motorattaining 70 to 80% of the full load speed, thecentrifugal switch opens up, isolating the starterwinding. The centrifugal switch is connected inseries with the starter winding and it is locatedinside the motor.
  7. 7. Simply constructed split phase motors are among the leastexpensive.Theyre widely used on easy starting loads of 1/3horsepower or less.Washing machines, tool grinders and small fansand blowers are among the applications that use these motors.

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