Is there a size difference ?

[sberbece]

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.

 

[edison123]

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.

 

[7anoter4]

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.

 

[sberbece]

Thank you edison123 and 7anoter4!