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3 phase 18kw motor starter


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#1 honeywell

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Posted 10 July 2003 - 03:34 PM

I have a motor starter with four contactors main, star, delta, resistor. The resistor is phases up the same as the delta connections. On start up the main and star contactors goes in and the motor starts after the timer times out the resistor contactor goes in with the star then the next timer times out and both star and resistor contactors drop out and the delta starts. what is the resistor for and should it be going in with the star connection or should it go in before delta.
is it possible this is a waucope type continupus tourque star delta starter
Thanks for any help


Paul

#2 marke

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Posted 10 July 2003 - 07:10 PM

Hello honeywell

Yes, this is a colosed transition star delta starter, (wanchop) and the purpose of the resistors is to reduce the change over transient.
Essentially, when the motor is power in star, there is a rotoating magnetic field in the stator that induces a current to flow in the rotor. This rotor current produces a rotor magnetic field that is rotating in the stator.
When the star contactor is opened, the rotor continues to spin in the stator and the rotor field induces a voltage in the stator. In other words, the motor becomes a generator. The voltage at the stator terminals is dependent on the rotor field, and the frequency is dependent on the rotor speed. The gnereated voltage will certainly not be in synch with the supply. When the delta contactor is closed, the effective voltage applied, is the vector sum of the supply voltage and the generated voltage. This results in a very high current and torque transient. The purpose of the resistors is to keep current flowing through the stator and over ride the generated voltage. To be effective, the current flow through the resistors needs to be of a similar order of magnitude to the star current. This requires large, low value resistors, similar to what would be used in a primary resistance starter.
Most closed transition starters that I have seen employ the correct circuit, but the resistors are too small and high in resistance to be effective.
Best regards,




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