GRR for VRM CA Fan

You can generally exceed max voltage for motor windings 136-150% for 60 seconds, depending on motor spec...check your spec, monitor amperage draw...consider SPRS system...contact manufacturer rep for recommendations....

https://www.kuttikasi.com/assets/pdf/EM-CG/3%20Phase%20Motor%20CG%20Catalogue.pdf

https://library.e.abb.com/public/a871dbcb1293adc948257d080018579f/EN_ACS800_IGBT_SPRS_Brochure_A.pdf

Wound rotor control does not directly affect the speed of the motor, it affects the torque, and the torque affects the speed. Reducing the amount of steps will simply increase the torque on the motor shaft between steps, and may or may not damage the shaft and/or distort the fan as a result. There is no way to tell in advance whether or not this is OK to do.

But GENERALLY, that is a lot of steps in a wound rotor control system, so someone must have determined that this was necessary, because they paid a lot of extra money to get this scheme. So the real question is, why? If the purpose is to minimize torque stress between steps because of the issues mentioned above, then I would be reluctant to make changes, other that to probably increase the size of the contactors so that they don't fail as often. But if the purpose of all of those steps was because of a process control issue, for example air flow control, then I would remove that system, short out the rotor and install a VFD.

But given that you only go from 90-100%, you might consider just running at full speed and using damper control for the flow. In that case, leave one step of resistance in place and start it with a soft starter.

Check the current required to accelerate the fan, using the Wk² of the fan, that may be where you find the reason for the number of steps. Also, if there is overshoot or delay in the fan acceleration, does that affect the quality of the product?

http://www.aerovent.com/docs/fan-engineering-topics/application-guide-for-selecting-ac-motors-capable-of-overcoming-fan-inertia---fe-1800.pdf

If accelerating the fan quickly exceeds the ability of the process to maintain the gas temperature, then you may find you run in overload conditions, moving cool air which could, as JRaef notes, distort the fan blading. Or perhaps your vent fan can't keep up. plugging the classifier. Your instruments may not have the accuracy over a wide range, for instance using pressure to measure flow, to allow quick changes in air flow. Maybe there is no fine trim control for air volume, like a damper, so your most efficient operation depends upon having the correct speed for your production rate.

If those all check out, then perhaps someone was covering themselves to make sure the required speed for a wide range of products was available, and you can skip those unused intermediate speeds.

Only a few select readers in your multiple postings are going to understand the operation spectrum of a vertical mill classifier supply air fan. The original design specifications would be a great source to determine the intent for the design. Once your audience knows this, your question will be easier to answer.

<...what should be the minimum speed to start the motor...> The minimum speed for starting any motor is zero.

What did the motor manufacturer say in response to the telephone calls that have been made on this topic? Did it recommend a change to variable speed/voltage drive technology?

GRR controller ----Grid rotor resistance controller

CA Fan ---- Circulating Air Fan

I presume, the rotor is Y connected and at the beginning the exterior resistance is star connected and each step the resistance decreases [5%]up to step 10 after which it goes to delta , introducing at first all the resistances and reducing gradually up to short-circuit.In my opinion R1,R2,R3,R4 have to be 5%,15%,25%,and 55% -from the total inserted resistance .In 26 steps [from 100 % to 40% in Y connection and from 33% to 13.3% in delta] the torque remains about 1 p.u.]

You intend now to reduce to 3 steps only: star and delta that means from full resistance to about 1/3 and short-circuit[100%,33%,and 0].

I think a complete calculation has to be done.

The current and the torque limits when jumping from 100% to 33% will be in the limits, I suppose, but jumping directly from 33% to zero the current and the torque will be elevated –about 3 p.u.].

Attached it is an example [of another motor!]