The basic difference between three point and four pint stater is the connection of NVC. IN three point, NVC is in series with the field winding while in four point starter NVC is connected independently across the supply through the fourth terminal called 'N' in addition to the 'L', 'F' and 'A'.
Hence any change in the field current does not affect the performance of the NVC. Thus it is ensured that NVC always produce a force which is enough to hold the handle in 'RUN' position, against force of the spring, under all the operating conditions. Such a current is adjusted through NVC with the help of fixed resistance R connected in series with the NVC using fourth point 'N' as shown in Fig. 1.
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| Fig. 1 Four point starter |
1.1 Disadvantage
The only limitation of the four point starter is, it does not provide high speed protection to the motor. If under running condition, field gets opened, the field current reduces to zero. But there is some residual flux present and N α 1/Φ the motor tries to run with dangerously high speed. This is called high speeding action of the motor. In three point starter as NVC is in series with the field, under such field failure, NVC release handle to the OFF position. But in four point starter NVC is connected directly across the supply and its current is maintained irrespective of the current through the field winding. Hence it always maintains handle in the run position, as long as supply is there. And thus it does not protect the motor from field failure conditions which result into the high speeding of the motor.
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good explanation
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