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Is there a size difference ?

Is there a size difference ?

Is there a size difference ?

(OP)
Hi !

I am writing this tread to kindly request your opinions.

I came across a Client technical specification for low voltage induction motors that states:
"Motors shall be able to start under applicable load conditions with 75 % of rated voltage at motor terminals for DOL."

I believe that this requirement exceeds the conservative approach and specifies a motor bigger that necessary, resulting an overall plant poorer PF and efficiency.

It also contradicts the AS/NZS 3000 clause 3.6.2, from which I can understand that the voltage droop at the motor terminals can not exceed -15 % (-10% power system variation & -5% voltage droop in cable) when supplied from a power system or 11 % when supplied from a standalone power system.

Therefore, a motor supplied by a power system which complies with AS standards shall never have to start when the voltage at its terminals is below 80% Un.  

What do you think, specifying a minimum 80%Un at motor's terminals is conservative enough? and
Is there a considerable difference regarding the motor size, its PF and efficiency, between a motor able to DOL when 80%Un and a motor that starts DOL when 75 %Un ?

Thank you in advance for your time.


 

RE: Is there a size difference ?

That 75% limit is for the voltage dip that will happen during initial starting inrush. The motor is supposed to develop the torque needed for the load accelaration during that initial dip.  

RE: Is there a size difference ?

Usually low-voltage induction motor up to 50 HP is able to start[DOL] with 25% voltage drop from motor rated voltage. But big motors will require more voltage to start that means the drop will be limited to 16-10%.There are clients requiring only 20% for all size of motors. Large motor will be more expensive than standard but it is possible to produce such a motor.

RE: Is there a size difference ?

(OP)
Thank you edison123 and 7anoter4!

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