cts_casemod wrote: So the idea is to use the stock 7.5KW at 415V WITHOUT ANY OVERVOLTAGE that should be good for what i am looking at now: a city car.
Oh. Then you have about 7.5 kW at base speed (near 1500 RPM), and slightly less out to maximum speed.
But this 2.93x the stock power is valid at any speed?
The 2.93x is the approximate ratio of continuous power with over-voltaging by a factor of 2.93 ^ (1/0.7) = 4.65 (6885 RPM). For a motor with 4000 RPM max, the over-voltage ratio can only be 4000/1480 = 2.70x, for a continuous power ratio of about 2.7 ^ 0.7 = 2.00. The 3x that is sometimes mentioned is the common ratio of stall torque divided by nominal torque, which gives a very approximate idea of the peak torque of the motor, and hence peak power.
To answer the question, no. Don't forget that at a speed of 1000 RPM, a 7.5 kW motor only has a continuous rating of 5.0 kW, since you have about the same torque, but the speed is lower than base speed. If this is rewound by a factor of 2.70x, say, then the continuous power at 1000 RPM will still be 5.0 kW, and continuous power at 1480 RPM will still be 7.5 kW. But now you have the voltage to keep going at peak torque, all the way through to 4000 RPM. However, you have to derate the torque to allow for iron losses, so at 4000 RPM you don't have 4000/1480 x 7.5 = 20.3 kW, you only have 2.00 x 7.5 kW = 15 kW. In between, at say 2800 RPM: 2800/1480 = 1.89, and 1.89 ^ (0.7) = 1.56, so you get 1.56 x 7.5 kW = 11.7 kW continuous.
Your controller probably has a "torque boost" feature, where it does an over-voltage at lower RPM, which may improve the continuous power at lower speeds, by reducing the current for the same torque. Actually, as I write this, it seems unlikely to be useful, except the motor may overheat a little less than you might otherwise expect at low speeds.
I am going to feed my motor with 15KW (double the power) so will I be able to use this 15KW at any speed from nameplate to 4000RPM?
Yes, but not continuously. You'll only get 5 kW at 1000 RPM, 7.5 kW at 1500 RPM, and 7.5 kW thereafter (all as stock). If you don't over-voltage, you don't get higher continuous power.
Also if I am on overdrive (1500 to 4000) how much efficiency will I loose?
It's hard to say, but generally iron losses are negligible at low speed, and increase rapidly at higher speed. So it is probably worth using your gearbox to keep the motor below about 3000 RPM most of the time, for maximum efficiency.
[ Edit: I didn't check the 2.93x figure; this should be 2.0 because (4000/1480) ^ 0.7 = 2.0. I've corrected the figures to reflect this now. ]
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