Motor Starting Current

The meaning is clear and any electrical engg man will know it. The problem is your profile does not give us a hint of your basic, nor your questions help us to decide whether you are trying to solve your homeworks.

It could be helpful. But any way

The motor is started. You know that the motor draws a much higher current at start than its rated current (typically 5-7 times) but for a very short duration - of the order of seconds.

The normal starters are set to accept this higher current.

However if the current exceeds this value

or persists for a longer time

Then the safety device trips - obviously on over current and isolates the offending motor.

do u think u are right? if unexpected is stated it means a usual one there, if you make a wrong design so u will expect not an unexpected but an expected one......

U SEE ??? (by the way i just asked to see if u agree....)

Initial comments were generally on track.

The usual cause of tripping on start-up is that it is under fairly high load on start-up and cannot get to operating speed quickly enough.

It can also happen when the motor bearings are starting to fail, causing a drag on all running conditions, but more so during start-up (again because of slowness of getting to run speed).

Either condition indicate need for more review- Bad bearings will cause full-load power use to be higher than "normal" or rated values, and will EVENTUALLY cause a heater trip but if this is found, plan to repair/replace soon.

If the LOAD is the problem (motor full-load values at or below rated values when at speed), the motor may not be adequately sized or you may need to install a "soft-start" starter control to reduce or eliminate the problem of trip-out.

Yes you are right that starting current exceeds the relay setting? but the questions are arised, such as : 1). Is it instantaneous or time overcurrent relay? 2). Is it MPR relay or electro-mechanical relay 3). What kind of the load protected by the relay either cooling water pump or forced draft fan motors (force draft fan motor require longer time to be steady state current), 4). Is the motor protected by locked-rotor jam? 5). If the MPR relay is applied, check time setting of the relay (setting default of instantaneous is one cycle sometime).

The electromechanical has no problem against starting current as long as the setting meet the requirement, but if this setting applied to the MPR relay the starting current may trip due to inrus current (high speed recorder indicated that starting current is around 7 to 11 times FLA motor for duration more than 1 cycle).

The inrush current itself was not always high but greater than 5 times FLA, especially for motor with high efficiency (less impedance). It is depend on closing angle, residual magnitism in motor core and initial motor impedance.