Kostov Motors of EV Motors
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.
On Interpoles
Is there any empirical evidence on the effect of the interpoles?
We have performed tests on an interpoled Kostov 9" 80V motor. One would think that 80V is relatively low and does not justify the use of interpoles. Indeed it may be so for low rpm, low amp forklift motors but the data shows this not to be true for EV conversion motors. For example most good EV motors have more than 4000rpm and 200A. The Kostov 9" 80V usually has collector temperature change of 120K in 60 minutes of work (up to 140K according to standard) at 280A. Removing the interpoles results in temperature change ot 157K in JUST 10 minutes. Advancing the brushes by 8/12 degrees improves the number to 147/141K but still in only 10 minutes.
Should the brushes of an interpoled motor be advanced?
The interpoles create a magnetically neutral point whose position is independent of rotation direction, rpm, amps or voltage. The brushes are positioned exactly in this point. Advancing them will result in moving away from the neutral zone and will worsen commutation/arcing drastically. This, in turn, can have very bad consequences for the motor.
How do interpoled constructions differ from ?standard? motors?
The change is in several aspects: 1) The lack of interpoles creates a disbalance above 60-70V; the commutator is very hot while the armature/stator windings are cold. 2) This calls for few bars in the commutator so that the bars can be big to dissipate heat. 3) Few commutator bars call for a much longer armature so that torque/rpm can be reasonable. 4) The result is a long and heavy motor whose commutator is on the brink of melting but armature/stator windings stay uselessly cold. On the other hand, interpoles reduce brush arcing so that commutator temperature falls down. This allows a bigger bar count and shorter armature stack making the motor much lighter than its non-interpoled counterpart. Therefore for a comparable power, an interpoled motor has less weight (20-40%), bigger diameter but much shorter body and the ability to handle high voltage. Less weight means a cheaper motor and of course more range.
Sepex Motors
How do sepex excitation motors compare to their series counterparts?
There are three major differences between SepEx and series motors. The first is that a series motor left with no load will spin itself to destruction while a SepEx will not let rpm run out. The second is that SepEx supports (or should we rather say that it makes regen much easier) regenerative braking especially in combination with interpoles The third difference is that peak torque of SepEx is somewhat worse. The last stems from the fact that a SepEx field winding cannot be overloaded to the extend a series one can. But this applies indeed for peak torque and is of interest only to racers. There are opinions that Sepex offers smoother speed control and better overall efficiency but we have not found that to be true. These are the differences as much as the motors themselves are concerned. Controllers should not be overlooked in this comparison. There are several very good EV controllers available in the market currently. By a good EV controller one means capable of high voltage (144-300V), high nominal and peak current (300/1000A) and reliable. Unfortunatelly the ones meeting the requirements are all series controllers. The best SepEx controllers that we know off are produced by Curtis (96V) and Kelly (120V).
Motor features Explained
What is series/parallel field switching?What is series/parallel field switching?
In series field mode all 4 of the main stator windings are connected in series. This reduces significantly rpm (allowing higher voltage) and increases torque but comes at the expense of slighly reduced nominal amps (in the case of the 11" 250V amps in series mode are reduced to 210A vs 250A in parallel mode; in parallel mode the limiting factor is the armature, while the stator itself can handle even more than 250A). The series mode is optimal for higher voltages. In parallel field mode the main stator windings are connected in parallel. This increases rpm and makes the motor better suited for "lower" voltage. Hence series/parallel switching makes for a very versatile motor - one can use it at 144-192V initially with a later upgrade to higher voltage. A nice side effect is that the stator in parallel mode can now sustain much higher amp overload. This is very important because at high overloads, the tight spot is the stator (due to its much bigger resistance than the armature). In parallel, the stator has 55% more copper cross-section available for current to pass through than in the Kostov 11" 192V.
Stainless brush springs
Stainless brush springs ensure uniform pressure with time as they do not corrode. This also minimises the risk of a brush sticking in its holder.
Stainless shaft
It is quite an expensive extra. The major benefit is that the shaft ends do not corrode which makes for an easy coupling/decoupling.
Kostov Motors, 2017