Determination of Undervoltage Settings (MV Motor Protection)

[Bekza]

Dear all,

Manufactures of MV motors recommend 70% to 85% of Un (norminal voltage) for undervoltage protection.What factors were considered when determining the undervoltage protection setting and is there any calculation that can be done from first principles to determine such setting value?
This question applies to motors that have 4.5 to 7 times starting current.

The limit given in the IEC 60034-1 standards for breakdown torque is 1.6 and -15% for pull up torque. Some books recommend that the safe value a motor can operate in undervoltage is when the flux is at 36%..

 

[rbulsara]

The mfrs. appear right. Need to include time delay during starting.

Rafiq Bulsara

http://www.srengineersct.com

 

[tem1234]

I agree

An important thing to look at: if synchronous motor with automatic reclosure on the supply circuit, the undervoltage protection should be set to trip the motor-control circuit before the circuit is re-energized. Fast underfrequency protection can prevent it too.