Inrush Current and Stating Current of Motor

From: In Rush Current And Starting Current - Motor Starting - LMPForum

The inrush current is the current transient that flows in the first cycle or so that establishes the magnetic field in the iron.

Once the field is established, the current falls to the steady state starting current until the speed of the motor approaches full speed when the current falls to the current required to drive the load.

The inrush current can be significantly higher than the locked rotor current with 2 - 2.5 times the start current being quoted by some. The inrush current is more about the stator magnetic field than the rotor, so I do not think that the rotor will have a significant influence on the inrush current, rather the level of iron flux and the material of the stator laminations will be important.

<Minor edits by me>

JRaef or redfred orFredski or TonyS or many others can expand further.

Don't know why someone voted Lyn's response as off topic, so unfortunately all I can do is neutralize that vote with my positive one. He left nothing to be desired in that response.

The only possibility is that maybe someone misinterpreted the comment on inrush current being 2 to 2-1/2 times the starting current, thinking the value is too low because inrush can be 1100 to 2000% of FLC. But he said STARTING current, which if defined as 600% of FLC, makes his statement true.

Here I pasted from one of my previous comments from http://cr4.globalspec.com/comment/628435. Hope it will also help:

There are three types of current related to motor starting: (1) Inrush current, (2) Locked rotor current and (3) Starting current.

Many people treat them as same. I saw many technical papers where they are used to indicate the same meaning, but there are distinctive differences among them.

(1) Inrush current: At the moment the contactor closes, the full voltage is supplied to the motor terminal. This voltage is basically applied to motor stator at "short circuited condition" and hence large current flows for short time (approximately 20 to 100 ms). This is due to the transient response of the inductive stator coil and is called motor inrush current. The magnitude for motor inrush current is between 10 and 20 times of motor full load current. Inrush current (also called surge current) is a generic term applies to most of the electrical equipment (motor, transformer, capacitor bank, lightings, transmission line etc.).

(2) Locked Rotor Current: It is current drawn by the motor when the motor rotor is locked (stopped) but full voltage is applied to the motor terminal.

(3) Starting Current: It is the current drawn by the motor during its starting period. At the time of motor starting, the full voltage is applied on motor terminal with the rotor stopped. For the first few seconds (generally 1 to 10 sec), the rotor is almost stopped and it is the similar to the case of locked rotor condition. So, the starting current drawn by the motor is same as the locked rotor current for DOL start. For other starting methods, the lower voltage is applied to the motor terminal and hence the starting current is less than the locked rotor current.

See these links:

http://samplecode.rockwellautomation.com/idc/groups/literature/documents/wp/2100-wp005_-en-p.pdf

http://www.danfoss.com/NR/rdonlyres/AC8F7C52-F105-43E8-AF39-078F15225784/0/HighEfficiencyMotorsandNuisanceStartingTrips.pdf (see page 3)

- MS