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# Induction Motor, Inrush Current vs. Locked Rotor Current

09/17/2010 12:15 AM

Hi all,

anyone kindly explain me the difference between inrush current and lock rotor current of an 3 phase induction motor. I thought they are the same but I were told that for the direct starting motor (induction motor) the possible peak instantaneous inruch current may be 2.82 times of lock rotor current.

I would very appreciate any theory, standard, practice reference if you have.

Thank you very much in advance.

HHung.

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#1

### Re: Induction motor, inrush current vs locked rotor current

09/17/2010 1:46 AM

To the best of my knowledge, they are exactly the same.

The peak, as you know, is 1.414 X RMS. With a max first peak asymmetry of 2, you may get at worst 2X1.414=2.828. But this is the first peak of inrush/LRA.

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#2

### Re: Induction motor, inrush current vs locked rotor current

09/17/2010 4:28 AM

Long back, we have studied in our electrical engineering theory classes that an induction motor is also a transformer with a short-circuited, rotating secondary. Haven't we?

So, like a transformer, the motor would also draw a very heavy "magnetising" in-rush current. The magnitude can be as high as 12 times the rated current of the motor and the duration could be from a few ten milli-second to about 100 milli-second. This in-rush currnet can be treated as "transient" part of the motor starting current. [What a misnomer? A motor's starting current itself is transient in nature).

This inrush current can cause spurious tripping of instantneous over current relays/releases in Breakers or fuse blowing in motor starters.

I remember having read a wonderful paper on "Motor Inrush Current Problem" authored by Mr. Rahul Gosain, that was published in Electrical Review magazine's July 2004 issue.

If you can mail me your ID thro this forum's mail box, I can send you some write-ups on this.

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#3

### Re: Induction motor, inrush current vs locked rotor current

09/17/2010 5:23 AM

dear ,

very well said.

Inrush current is the transient current - may be for few cycles, till motor starts picking up its speed. Then, gradually, it goes on decreasing till the motor reaches its full speed, & normal motor current as per the load will flow.

These in rush currents are 6 - 12 times the rated motor current, depends upon KW rating / Poles - speed.

The blocked rotor current may be equal to the in rush currents , if the motor stalls, & remain in the zero speed status.

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#4

### Re: Induction motor, inrush current vs locked rotor current

09/17/2010 5:23 AM

Hi friends

Thank you very much for your great answer. I highly appreciate if you can send me the document.

Thanks

Hung.

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#5

### Re: Induction motor, inrush current vs locked rotor current

09/17/2010 5:39 AM

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#11

### Re: Induction motor, inrush current vs locked rotor current

10/04/2010 9:38 AM

Please do not post your mail ID directly thro the reply option; it doesn't get published as per the policies of the forum. Rather, you may mail it to me thro this forum's mail box. I would respond then.

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#10

### Re: Induction motor, inrush current vs locked rotor current

10/04/2010 9:37 AM

Please do not post your mail ID directly thro the repky option; it doesn't get published as per the policies of the forum. Rather, you may mail it to me thro this forum's mail box. I would respond then.

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#6

### Re: Induction Motor, Inrush Current vs. Locked Rotor Current

09/17/2010 10:52 AM

People very often misuse the two terms Inrush Current and Starting Current interchangeably but they are not. Inrush refers to the magnetizing current, Starting Current is equivalent to Locked Rotor Current.

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#7

### Re: Induction Motor, Inrush Current vs. Locked Rotor Current

09/17/2010 2:02 PM

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.

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

http://findarticles.com/p/articles/mi_m0BPR/is_9_18/ai_78573851/

http://www1.eere.energy.gov/industry/bestpractices/pdfs/avoid_nuisance_motorsys_ts6.pdf

- MS

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#8

### Re: Induction Motor, Inrush Current vs. Locked Rotor Current

09/21/2010 1:55 AM

Whereas most the experts have dealt with the question nicely, I would like to add a bit more to further consolidate the concept. A motor or transformer is nothing but a huge Inductor with very low resistance but large inductance. When a voltage is applied across it, it is, in the first few milliseconds resisted by the pure resistance alone. Since the resistance is very low it will draw huge current (Inrush current). As the magnetic field develops around the coil it develps magnetic reactance normally designated by XL. As a result of growth of magenetic filed and subsequent growtrh of inductive reactance (XL), current starts dropping and is ultimately governed by resistance of coil plus inductive reactance (r+jXL). The reactances are of three types- subtransient, transient and steady state reactances. The inrush current is governed by subtranisient reactance, starting current by transient reactance and full load curent by steady state impedance.

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#14

### Re: Induction Motor, Inrush Current vs. Locked Rotor Current

08/04/2014 2:05 PM

You state "in the first few milliseconds resisted by the pure resistance alone. Since the resistance is very low it will draw huge current (Inrush current). As the magnetic field develops around the coil it develps magnetic reactance normally designated by XL. As a result of growth of magenetic filed and subsequent growtrh of inductive reactance (XL), current starts dropping and is ultimately governed by resistance of coil plus inductive reactance (r+jXL). " Would you mind explaining why in the first milliseconds there is only pure resistance? Is this because upon switching, the motor is reaches saturation hence the inductance behaves as an air core inductor? Also could you explain on the different types of reactances you speak of? Thanks!

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#9

### Re: Induction Motor, Inrush Current vs. Locked Rotor Current

10/04/2010 8:06 AM

Not to add more fuel to the fire, since the answers are very concise, but is stall current in inductive motor applications equivalent too? Isn't lock rotor current the same terminology as when somebody states a motor's stall current?

I've been told that as a motor stalls, at least in automotive motor applications, the current can exceed inrush currents, but not always. It's the "it depends" part, I'm confused on. Or, equivalent.

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#12

### Re: Induction Motor, Inrush Current vs. Locked Rotor Current

11/23/2010 2:56 AM

the inrush current is the current registered during start up meaning depending on the load of the motor where as the lock rotor current is the current 6 times of the rated current.

thanks,

Eng. Fidel Fornolles

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#13

### Re: Induction Motor, Inrush Current vs. Locked Rotor Current

11/23/2010 3:02 AM

I made a comment beforre that the In rush current is current being generated by the Motor during start-up while the locked rotor current usually 6 to 7 times the rated current.

thanks

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#15

### Re: Induction Motor, Inrush Current vs. Locked Rotor Current

03/15/2021 3:57 AM

They are the same, though the actual value is the supply voltage divided by the motor's impedance at rest; all of these things are measurable.

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