How should the nominal battery voltage for a certain kostov motor be determined?
We actually rate the motors at the voltage that is fed to the motor itself. In reality this is much different than the nominal battery voltage due to voltage sag. For example a Kostov 11" 192V can receive 250A at 192V at its terminals which calls for a battery that has nominal voltage of 210-220V. Of course this is assuming one uses the typical for EV conversions cell capacities of 100-200Ah (where sag can be 30-40 even 50V). As a rule of thumb our 9" and 11" motors should be paired with bateries that have 12-18V and 18-24V respectively over the motor nominal.
Why is it then that the drawing of the kostov 11" 192v states 185v?
This is because our roots are in forklift traction motors where the batteries usually weight around 1000kg often with 400-500Ah capacities and voltage sag is much less. The motor is perfectly capable of doing 192V rather than 185V and the difference is insignificant.
What is the maximum voltage that the kostovs can tolerate?
Actually the reason we rate the motors up to 144/168/192V is not because of arcing at high voltages but because going above the stated will result in unacceptably high rpm. We have performed tests up to 220V which show that as long as rpm are below 6000 (11") and 7000 (9") the motors work fine. With the currently available controllers it is very difficult to ensure the above limits are not exceeded. Therefore we in no way encourage applying more than the rated voltage to the motor's terminals.
How important is rpm for ventilation?
As a side effect of a recent lab test, we discovered that when the rpm of a 250-280A motor are reduced from 5500 to 2300 (this can happen when voltage is halved or during overload of the motor), its duty rating decreased from 60min to 27min. As the motor fan is attached to the shaft, decreased rpm greatly reduce the air flow through the motor and hence its cooling. Note that this is not crucial for 2-3x amp overload as then duty is reduced to less than 5min and ventilation is less important (there is no time for it to make a difference and the rate of heat build up under such conditions is so high that ventilation has no chance).
Can the shaft mounted stock fan be replaced by some forced air solution?
In principle it can be replaced but the standard fan sets the stakes quite high. We have performed a test with a 9" motor at 5600rpm with fan and without fan. The difference is the energy consumed by the fan - this turns up to be 440W (11" fan will probably be about 600W). A forced air solution will never be as effective as the shaft mounted fan. Therefore to match it one needs a separate ventilator with at least 500/700W rating respectively for the 9/11" motors.