startring current of IM

This is mainly depends on back emf generated by the motor.The initial staring current also depends on the motor impedance.

If the three phases motor is of winded rotor type and is connected to resistances, you control the maximum value of the inrush current and also the speed

In french we call the graphic representation of the laws that describe the rotating machine "le diagramme du cercle" . This graphic has different name depending of the culture.It is not enough to translate it, but it is a diagram of the circle.

It is a polar inversion that contains alls the caracteristics of the machine, it is not logarithmic, it is linear.The reason why the starting current is six ,seven time the nominal current comes from the fact that the impedance is composed of the element jLw and omega (w) is 2pi f . if f is zero , because the speed is zero at start ,that element is zero and the total impedance is only approximately R (the resistance of the magnetic circuits)

go to wikipedia , somebody has probably written something there, I just can assure you that you will need to have a true experience with curves and have a deep understanding and ability for the abstractions of the electrotechnics and have practical training because a rotating machine is defined by its imaginaire impedance, and that is not understood after the first lesson of electrotechnics.

This graphic representation is the only help because the equations speak very little, even if they contains the same informations.It is easier to work with graphics than to write equations.This graphic are easily drawed having the caracteristics of the machine.

Every body understands that a picture says more than a speech.

There are two sources of info about inrush/start IM current":

1. Catalog of its manufacturer

2. Locked rotor test which give you the closest to true value on IM.

For student there are several academic books the give you equations, terminology, theoretical explanation etc.

Guest,

Because it is a "rule of thumb", it is based on experience. However as technology changes, the experience must catch up. Motors with very high efficiency have a higher locked-rotor current than standard designs for induction motors. Therefore the "rule of thumb" you know is low, and the equipment selection must be verified as adequate for this higher inrush current.

--JMM

This is based on design.There are specifications where starting current restrictions apply like boiler feed pump motors. I have come across cases where Starting current has been restricted to 5 times without +tolerance (Tolerance is 20% as per standards like IEC and BS,IS)If u go through motor manufacturers catalog u will find 2 pole motors have higher starting current.However special designs using special materials can be given.Most manufacturers have software for design and it is done easily.

In USA manufacturers have types from A to E. Each of them has different Inrush/Starting current. If you have access to the motor DO Locked Rotor Test. The output values from measurements gave you the closest value to the true one.

As I see the question, you are looking for mathematical explanations as to how the rule of thumb is derived. The answer is in response #2, but the "Circle Diagram" is mostly a European engineering concept (I have heard it before from othee EU engineers). In the US, we use the term "AC motor equivalent circuit", although it describes a running motor better than it does a starting motor.

This will give you some of the principals involved. Move your mouse over the elements of the diagram for additional descriptions.

Siemens STEP course on AC motor drives

This site provides a better overview of starting characteristics.

http://www.lmphotonics.com/m_control.htm