With the compressor wheel that you have selected the speed needs to be limited to about 58,000 rpm to limit boost to 7 psi. The motor you have selected has a KV of 2400. That means with 100% throttle and no load the motor will turn at about 79,000 rpm with 33 volt supply. Under load approximately 10% less or 71,000 rpm. So you can't run at 100% throttle if you want to limit boost to 7 psi. If you raise the battery voltage then more throttling is required to limit speed to 58,000. At 58,000 the phase voltage will be the same regardless of what the battery voltage is. Running the battery voltage up will help with voltage drop from the batteries to the ESC but it will not help the motor if the ESC is throttling to control to a constant speed.
Great questions and calculations.
Castle tech service warned me to never run the motor without load, as it would surpass its 90K self destruct RPM limit and the rotor would fly apart. I think that they said the most they ran it on was something like 36 volt power supply, which calculates to 86,400 RPM.
I am still creeping around in the design space trying to figure things out. Lots of trial and error. I have had difficulty getting an extension shaft that stays centered. I have enough damage to my compressor wheel that I may replace it with a billet one like WB did once I have a stable system. Or maybe I will end up with a different volume housing and compressor. Hopefully my new solid coupler that comes in today provides enough stability for some trial runs and experiments until the stainless laser sintered shaft comes in.
True, I established a goal for the project to achieve real boost in the 5-7 PSI. However, I am not necessarily inclined to limit it there. I will take what it can give me within reason. What were your estimates? Something like 11-12 PSI at 80K RPMs is what I was reading off the compressor map at a glance. I am not sure that the motor and ESC could handle that. I think my motor and ESC can do perhaps 300 AMPs for a 10 second burst. It will be interesting to see what the limiter in the system is. It would be great to get 50% more power and torque. That might put me in the 200 horse power / foot pounds or torque realm, which would be a blast in a 2,000 lb car like my Opel GT.
My rationale that my engine can handle more is that I have a relatively low compression engine in the realm of 8.3-8.5:1 with a strong bottom (forged crank, etc). I think I can manage the low end of the curve of the compressor by controlling / programming when the motor turns on. My engine management system (Holley Terminator X) is fully programmable, so I can just dial back the timing as a function of boost to stay away from pre detonation. Also, I am building a 2.4 liter stroker engine to replace the 2.0 liter in my car now. Waiting on getting the crank back from the machine shop.
If I end up making more boost, I am guessing / hypothesizing that heat will be the problem to manage. 1) heat in the motor, 2) heat in the ESC that is why I bought one with lots of cooling), and 3) heat in the air charge, 4) heat in engine. If I can make more boost and the support infrastructure (battery, ESC, motor) can handle it, I’ll add an intercooler to remove some of the heat from the charge.
Are you sensing that I am playing around with a compressor that is took big for my engine at 82mm diameter? Perhaps so and you are seeing how it will play out before I get there. If things don’t work out, I could migrate down to a 56 mm compressor. Honestly, I stared at a lot of compressor charts and eventually finally ordered a turbo off eBay, as at $100 price point, I was keen to start hands on learning. In retrospect, perhaps the GT 2056 is a better match. Is that what you are thinking? I think you had another name for it perhaps a T3 with certain trim. When looking at compressor maps. I did not quite understand the principles to consider when translating to an e-turbo. What you are saying makes sense (selecting a compressor that is optimized code a particular max RPM). We seem to be using a different part of the compressor curve (no 100K+ RPMs) compared to most conventional turbo systems, so it is sort of hard to translate.
