Number of Motor Starting

Unfortunately, the IEC 60034-12 standard is not available with me ...

Here it is.

https://www.sis.se/api/document/preview/569004/

<...standard is not available...> What nonsense! A telephone call and a credit card can change that on any <...standard...> at any time, anywhere on the planet. Rumour has it that one can even do that over the internet these days...

<...think...damage the motor...> A simple telephone call to the motor manufacturer, or, better, a review of the manufacturer's data sheet for the motor, will yield information about the maximum recommended starting rate.

<...you guys...> There is nothing in this forum preventing varying the inclusion. Posting anonymously doesn't make any headway in that direction though.

_{Anonymous Poster #1 just isn't trying hard enough.}

The critical factor here, assuming you have proper sized motor for load, is the insulation class....frequent starting and stopping can increase the operating temperature of the motor....the motor must be designed to handle this heat or it's life will be cut short...

https://www.machinedesign.com/blog/5-common-motor-myths

Always best to chose a higher insulation class than you think you need...

http://sites.ieee.org/houston/files/2016/01/3-Fundementals-of-Motors-IEEE-Houston-February-2015.pdf

In most cases, induction motor designs are rotor limited, so that needs to be checked first. You need to know the Wk2 of the load, and the voltage available at the motor terminals during start, from which you can calculate the starting & acceleration torque. From that you can calculate the time when the stator field is much faster than the rotor field, which causes large amounts of current to flow in the rotor bars, raising the temperature of the rotor.

NEMA has some standards that are incorporated in standard T Frame motors. There are also rules of thumb for motor designs that are likely to be stator limited, usually high voltage and small horsepower.

NEMA motor starts Shows an excerpt of the design compliments of L&B Electric, more detail is available in the NEMA Standard.

in cases I’ve investigated, the Actual Motor Design parameters are the safest ones to use, as our PWSlack always postulates. Usually takes about 5 phone calls, detailed information of the driven load, but the designer does know at what point the heat in the rotor starts softening components.

https://www.industry.usa.siemens.com/drives/us/en/electric-motor/nema-motors/Literature-and-technical-resources/Documents/Catalog%20D81-1_2014_English.pdf

The most onerous duty for motors from rotor heating point of view is, stopping the motor before it reaches full speed and then starting again before the motor speed comes to standstill. If this has to happen again and again, better be careful in selecting the right duty motor.

"...then starting again before the motor speed comes to standstill".

Does this only apply to motors with starting centrifugal switches or all motors? I've never really thought about how that would affect motors started with motor-controllers.

But, with across-the-line starting without any special starting sequence, I'd think that the residual kinetic energy would make the subsequent start less stressful. Am I wrong?

<...the residual kinetic energy would make the subsequent start less stressful...> Sounds good from here.

True provided, the back emf has died down completely.

or as VFDs often provide, the re-application of voltage is synchronized with the rotor speed, often called 'flying start' in VFD parameter-speak.

What do you mean by "gap control"??
The standard writes that you can have two starts from cold or one from hot with a certain inertia constant & motor torque constant at rated torque during start.
What you need is the starting time & motor voltage for your fan application. An assumption that current is at "locked rotor value" during start is reasonable and will give a margin for life. You really need to work with the actual motor voltage during start, which may be as low as 80% nominal due to supply & cable impedance.
It would be essential to know the heating and cooling time constants of the motor to find the effect of any cycle of run-time & stop time. These constants are not part of a standard.
However, if you determine the expected cycle, the motor maker is best placed to advise on the acceptability of the cycle. You should investigate if the use of a variable speed drive would not have benefits in reduction of motor size/thermal shock to process or motor and high starting currents/low power factor on supply.
So your most important task is to to extract the required duty cycle for your process from its designers.....the questions "what do you usually do?" and "what would you really like to do?" come to mind.