what is the standard of the suitable voltage drop during motor starting?

not exceeding 20% of rated voltage

"Suitable" is a function of the process using the motor or motors in question and other facility loads: how sensitive are they to reduced voltage for short periods of time and how sensitive is the process to performance variation in those other loads. Finally, depending on impedances at your PCC and your neighbors, if any, can also cause some "bad neighbor" issues, which of course are also important. All that said if you do nothing to mitigate the inrush current will be 5-7 times FLA in most cases. So, your system impedance (which is always changing depending on load) will determine how big an affect there will be on voltage. If the system wasn't designed correctly you'll need a "Band-Aid" such as SVC, VFD, soft start, line inductors, etc.

I've always used 85% of the nominal voltage as minimum where starter contractors might drop out.

It mainly depends on the frequency of motor starts. If the motor is started infrequently, then a higher voltage drop can be tolerated; and likewise, if the motor is started frequently, then a lower voltage drop limit is necessary. These voltage drops can range from around 5% to 1% and are specified as voltage flicker limits. For large industrial motors where flicker limits are not used, the maximum voltage drop during starting will typically be governed by two criteria: (1) the maximum voltage drop must be limited to the extent that sufficient motor torque will be available to break static friction and accelerate the motor and load to rated speed in a reasonably rapid time; and (2) the voltage drop should be limited to the extent that other motors and controls are not inadvertently tripped off line. This second criteria can be reached for voltage drops exceeding about 10-15%.

Dear ;

Normally, not max of " 15% ", but this value is depending on the Starting components use to start the motor. For example, if you use the standard component " Contactor + thermal Relay " it will be better to be " around " 10% " as the voltage's tolerance of the Contactor's Coil is " 0.85 ... 1.1 Uc ".

Personally, and during the study I try to be " around 10% ".

There are no known hard and fast rules on this. However, a good practice is to limit the maximum voltage drop during motor starting to 15% (i.e.) at least 85% voltage should be available at the motor terminals during starting.

10-15% voltage sag or droop...

Keep in mind that the torque of a motor is proportional to the square of the voltage. Big volt drop means even bigger loss of torque. No good if you're trying to start loads with high inertia.

The acceptable voltage drop during start-up is %10, where the induction motor can achieve the moment of inertia of the rotor and connected mechanical load. Over %10 you may have collapse of the motor mechanics, stalling and/or burn out.

Actually type of starting is a detailed study for induction motors.