Inrush current effects on sizing Transformer and/or Diesel

[Electriman]

I was wondering if anybody can describe how to size a transformer or Diesel generator to not only support the nominal load current but also the load inrush current. Consider the load can have much higher inrush current than nominal current like a motor. I appreciate if you support your description by articles and numeric examples.

 

[gar]

150407-0830 EDT

Electric motors, generators, transformers, and resistance heating elements can tolerate rather large short time overloads. DC motors and generators 10 times or more, except some permanent magnetic fields might get demagnitized at 10 times. AC induction and synchronous motors will have certain peak torque limits that might be 1.5 to 3 times rating. An AC generator might be similar to a DC generator.

From the diesel generator side there is a peak torque limit of the engine close to its maximum power capability. Adding a lot of mechanical inertia to the generator could provide high short time peak power capability to the generator instead of being limited by the diesel peak power capability. This is not very practical on a commericial unit.

What is an alternative to oversizing of the diesel generator? Add sufficient impedance (resistance) in series with the transformer input lines to limit peak inrush, and add a contactor controlled by a time delay relay to short out the resistors about 20 milliseconds after power is applied to the transformer.

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[Besoeker]

150407-0830 EDT

Electric motors, generators, transformers, and resistance heating elements can tolerate rather large short time overloads. DC motors and generators 10 times or more, except some permanent magnetic fields might get demagnitized at 10 times. AC induction and synchronous motors will have certain peak torque limits that might be 1.5 to 3 times rating. An AC generator might be similar to a DC generator.

From the diesel generator side there is a peak torque limit of the engine close to its maximum power capability. Adding a lot of mechanical inertia to the generator could provide high short time peak power capability to the generator instead of being limited by the diesel peak power capability. This is not very practical on a commericial unit.

What is an alternative to oversizing of the diesel generator? Add sufficient impedance (resistance) in series with the transformer input lines to limit peak inrush, and add a contactor controlled by a time delay relay to short out the resistors about 20 milliseconds after power is applied to the transformer.

.

Depending on the size of transformer you might have a problem getting a contactor to operate in that time but I wouldn't have thought it critical anyway.

 

[Electriman]

150407-0830 EDT

Electric motors, generators, transformers, and resistance heating elements can tolerate rather large short time overloads. DC motors and generators 10 times or more, except some permanent magnetic fields might get demagnitized at 10 times. AC induction and synchronous motors will have certain peak torque limits that might be 1.5 to 3 times rating. An AC generator might be similar to a DC generator.

From the diesel generator side there is a peak torque limit of the engine close to its maximum power capability. Adding a lot of mechanical inertia to the generator could provide high short time peak power capability to the generator instead of being limited by the diesel peak power capability. This is not very practical on a commericial unit.

What is an alternative to oversizing of the diesel generator? Add sufficient impedance (resistance) in series with the transformer input lines to limit peak inrush, and add a contactor controlled by a time delay relay to short out the resistors about 20 milliseconds after power is applied to the transformer.

.

Thanks, But I never heard to add resistance to reduce the inrush current. In reality what they do is they select a larger transformer or diesel when the load requires higher inrush current to start. But I don't know if there is a rule of thumb that helps to select the rating very quickly.

 

[GoldDigger]

Thanks, But I never heard to add resistance to reduce the inrush current. In reality what they do is they select a larger transformer or diesel when the load requires higher inrush current to start. But I don't know if there is a rule of thumb that helps to select the rating very quickly.

The other way to limit the inrush is with a series inductor. That way you still have a reduced current but do not have to dissipate much power in the limiting element and so do not have to be as quick to switch it out of the circuit.
But high value inductors cost a lot more than resistors for the same current limiting effect.