A special form of autotransformer called a " zig-zag transformer provides a path for current that is common to all three phases ( so-called " zero sequence " current ).
32.
The circuit has advantage over starting with a regular autotransformer, which needs to be at some point completely disconnected during the start inducing high voltage impulses, which can damage the electrical insulation of the stator.
33.
For voltage ratios that don't exceed about 3 : 1, an autotransformer is cheaper, lighter, smaller, and more efficient than an isolating ( two-winding ) transformer of the same rating.
34.
At this stage switch "'2 "'is opened and momentarily the motor is supplied by even lower voltage, because the windings of the autotransformer act as inductors connected in series with motor.
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A further transient problem is that with a single coil construction and a star switch, the autotransformer acts as a step-up transformer during the < 5 microsecond period at contact separation of the Star point switch.
36.
The autotransformer system enforces the traction return current taking its intended path, while reducing the transmission losses, and therefore achieves both required objectives, of controlling return current leakage to earth and ensuring low energy loss, simultaneously.
37.
In an autotransformer part of the current flows directly from the input to the output, and only part is transferred inductively, allowing a smaller, lighter, cheaper core to be used as well as requiring only a single winding.
38.
At periodically spaced "'Paralleling Stations "'within each electrical section the tracks'catenary wires are connected together to one side of an autotransformer and the feeder wires are connected together to the other side of the autotransformer.
39.
At periodically spaced "'Paralleling Stations "'within each electrical section the tracks'catenary wires are connected together to one side of an autotransformer and the feeder wires are connected together to the other side of the autotransformer.
40.
One basic method to reduce the starting current is with a reduced voltage autotransformer with taps at 50 %, 65 % and 80 % of the applied line voltage; once the motor is started the autotransformer is switched out of circuit.
