Inrush Current

Generally the motors which draws 600 to 800% starting current. CRT monitors also draws peak current during start up but don't know the exact figure that how much.

Sorry for the hazy image, the figures are 6 for induction motors, 30 for capacitors and as high as 180 times the rated current when switching multiple banks of capacitors in a large plant.

i am not sure whether you were interested in these, you did say "etc..."

Dear kvsridhhar;

From where you collected that images? I am interested in reading that manual.

This is from AEG Germany, now part of GE. Here is the link :AEG

i got involved in capacitor switching contactors way back in the 70's, when i was asked to develop special contactors for this. i was aghast at the severity of the duty and of course, my pathetic contactor promptly welded on make operation !The customer was very understanding, and i fiddled around for 72 hours non-stop and solved it somehow. i then went into some intense research into the phenomenon, and collected some valuable stuff. Mind you, no internet at that time Lot of reading from libraries, mainly the Indian Institute of Science, the best research institution in India.

Now of course, technology has advanced quite a bit, and special contactors for this duty are available from all reputed manufacturers...

You would also be interested in similar contactors of ABB, Schneider, and many others. Also the US Standard NEMA ICS2 (free download) which gives a very good common-sense solution to attenuate the very high transient peak currents. If you can't find it, let me know, i can send it through mail.

Good luck.

This was what I found out. It is interesting...Incandesent lamps has the higher inrush current.

Induction motors - 500 to 800%

Computer-CPU - 600 to 1000%

Fluorescent lights - 300 to 600%

Incandescent lights - 600 to 1500%

Battery charger - 600 to 1000%

Transformer - 1000%

auto transformer - 200 to 300%

Duration is different. Long duration is for motors.

I beg to differ. The inrush current duration will be more in case of transformers rather than for motors. One shouldn't confuse the (Magnetising) Inrush current with the starting current of motors. What you have mentioned for motors (500% to 800%) is not the inrush current but the starting current or the "locked-rotor current" of the motor.The magnetising inrush current magnitude of motors is about 12 times its rated current and it lasts only a few mili-seconds whereas it would be in seconds for transformers of same rating/capacity as that of the motor.

True! The motor's starting current duration would be more but again that too is not decided by the motor but by the inertia of the connected load.

Inrush current duration for a transformer may be longer than for a motor, but it's not in seconds. Standard rule of thumb for setting instantaneous overcurrent relay to ride out the transformer inrush is it must be above 10x FLA at 0.1 second. What's your basis for duration of seconds?

Reference is drawn to the Transformer Handbook published by ABB, Section 3.7, wherein it is stated that the duration of the inrush current can range from less than 10 cycles for small transformers up to several "minutes" for larger transformers.

Hope the above is authentication enough! If not please wait! I am searching for a book wherein I remember having studied even a formula for calculating the magnitude & duration of transformer inrush currents.

The Transformer inrush currents will be for 8-10 cycles i.e 160- 200 milli seconds.

Where as in the case of motors it might go to 8-10 seconds, depending on the load on the machine .

ramvinod

A capacitor bank.

Can you please elaborate on your original question (including what inrush current duration, as capacitive loads generally have a higher inrush current, but inductive motor loads generally have a longer sustained inrush or "starting" current).

In actual application Electrical Motors that start across the line tend to have 2 to 6 times the FLA rating for inrush. The actual starting current and the rate of decay to normal current are highly dependant on the mechnical loading. (what the motor is connected to)

However, the highest in-rush I'm aware of is the tungsten / halogen lamp at approximately 14 times the operating current.

This needs a bit more qualification as from V=I.R so depend on only the V and R to make your I.

i.e. 415 volt motor with a cold d.c. resistance of 0.2 Ω is going to take more current than a 1.2 volt led or a 5 volt lamp.

some very interesting answers about efficiency of devices systems.

I still maintain that the highest inrush is when a capacitor is switched on when several other capacitors are already on and can feed a short-circuit-like current into it. This current, though highly transient and at a high frequency, is still adequate to weld the switchgear contacts. Hence special contactors have been developed which attenuate this inrush current.

We can bring all circuits under three categories. Purely resistive (lighting bulbs), R-L-C circuits and rotating electrical machines. In bulbs in inrush current is decided by its cold value resistance and then it decays due to increase in resistance due to increasing temperature of the filament. In R-L-C- circuit the inrush current would be decided by equivalent impedance (for ac supply) and would decay as per the time constant which is decided by L/RC. For rotating machine the in rush current is decided by the armature resistance (or impedance for ac machines) and the applied voltage and would decay as back emf is built due to the rotation of the rotor (which in turn depends on the torque produced and the load).