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Ryan , next up is the part-wind method for starting a motor .
This method is somewhat special to our industry and I have only seen it used in refrigeration applications . Electricians do not cover this in their training .
To understand part-wind starting you have to understand what a part-wind motor is . A part-wind motor is two motors in one casing . Both motors are DOL started and , as such , will behave like DOL started motors . When the first “ half ” motor is connected to the power supply it starts like a DOL started motor ; being only “ half a motor ” it will only draw half the starting current of a whole motor started DOL . The torque produced will also be only half but this is still more than the torque of a “ whole ” motor started with star-delta starting . This is necessary as we have to overcome some inertia when starting a refrigeration compressor .
This approach is widely used in many refrigeration compressors .
The time setting is crucial as the motor will rapidly overheat if the setting is too long .
A part-wind motor showing the two sets of windings .
Because the part-wind motor has six terminals and looks like
the star delta starter motor , it can be incorrectly identified and treated like a star-delta starter motor . This results in the time delay on the starter being set too long , which defeats the whole purpose of part-wind starting .
Characteristics of Part-Wind Starting Method : Initial starting current Initial starting torque
Advantages
Disadvantages Usual starting time
Typical application
2 to 4 times running current 0.3 to 0.75 times running torque Simple Higher starting torque than Star-Delta
No break in power supply to motor during starting
Special motor required 0.1 to 0.35 seconds
Air conditioning and refrigeration compressors in packaged equipment .
Thank you for your question Ryan , we are using more and more variable speed drives which I will look at in a future publication . RACA
REFERENCES : 1 . ACRA
The part-wind motor has six terminals as below . From the diagram , the voltage across each winding is 400 volts .
" To understand partwind starting you have to understand what a part-wind motor is ."
Power circuit and control circuit for a part-wind motor .
Thank you for all your questions . Send your problems ( and sometimes your creative solutions ) to acra @ netactive . co . za with ‘ Solutions Page ’ in the subject line . You may include pictures .
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RACA Journal I January 2023 www . refrigerationandaircon . co . za