How much power the SDU really makes

[StefSonic7EV]

hi all, this question was bothering me for some time. What is your experience, anybody dyno'ed your car with the SDU?

A friend of mine created this app RaceDroid Pro, coupled with 10Hz GPS (Garmin Glo) I have done some tests.

850kg car's weight (with driver), I went for 10% drivetrain loses and 0.45 Cd x 2m2 frontal area.

Result below. What I'm getting is around 190 horsepower. Close to 200 when battery with high SoC, 170 horsepower when closing 30% SoC, much less below 30%.
What I don't understand is why only 190 hp. I'm using Torque app and OBDII device that connects to the Orion BMS. It shows 500 amps current and 370 volts on the peak. That should be giving 185kW so 250 hp. I will measure current and voltage on inverter and on BMS app directly, maybe that is voltage sag...

[collin80]

This might be due to the way an AC motor works. You're taking the peak amperage and voltage from the DC side. 500A * 370V does equal 185kw and that is about 250HP. However, AC motors need sine wave drive (not strictly true but sort of) so the inverter must chop up the DC voltage and create fluctuating voltage. If the voltage were a true sine wave then the effective AC voltage given an input DC voltage is DC / sqrt(2). This yields about 260V AC. 500 * 260 is MUCH closer to the power you're seeing (175HP). However, things are not so simple since the motor is three phase. I actually don't remember the equation for power conversion of DC to three phase. But, I think the fact that the drivetrain is AC while you are measuring DC might explain why the readings look different. You need to know the actual AC amps and volts as reported by the inverter to get a true idea of the output power.

[Isaac96]

This might be due to the way an AC motor works. You're taking the peak amperage and voltage from the DC side. 500A * 370V does equal 185kw and that is about 250HP. However, AC motors need sine wave drive (not strictly true but sort of) so the inverter must chop up the DC voltage and create fluctuating voltage. If the voltage were a true sine wave then the effective AC voltage given an input DC voltage is DC / sqrt(2). This yields about 260V AC. 500 * 260 is MUCH closer to the power you're seeing (175HP). However, things are not so simple since the motor is three phase. I actually don't remember the equation for power conversion of DC to three phase. But, I think the fact that the drivetrain is AC while you are measuring DC might explain why the readings look different. You need to know the actual AC amps and volts as reported by the inverter to get a true idea of the output power.

Well... I'd say not quite. P = VI. So 500A DC at 370V DC on the input means 700A at 260V AC output. Power in must equal power out unless you've got a resistive controller.

OP, I don't know why your power levels are lower when measured by GPS. Your drag may be higher than you have calculated, or the 10Hz GPS might be too slow. Can that app use wheel speed sensors or the phone's accelerometer? Either of those might give more accurate results.

[StefSonic7EV]

Let me draw the udc/idc at the inverter to get the accurate reading. Hard to argue with P=VI. I guess minus ~5% as inverter and motor have some losses. But not 40kW.

App works with 10Hz GPS really well for ICE cars, close to dyno numbers. Guess I need to go on a dyno to calibrate for electric powertrain.

This might be due to the way an AC motor works. You're taking the peak amperage and voltage from the DC side. 500A * 370V does equal 185kw and that is about 250HP. However, AC motors need sine wave drive (not strictly true but sort of) so the inverter must chop up the DC voltage and create fluctuating voltage. If the voltage were a true sine wave then the effective AC voltage given an input DC voltage is DC / sqrt(2). This yields about 260V AC. 500 * 260 is MUCH closer to the power you're seeing (175HP). However, things are not so simple since the motor is three phase. I actually don't remember the equation for power conversion of DC to three phase. But, I think the fact that the drivetrain is AC while you are measuring DC might explain why the readings look different. You need to know the actual AC amps and volts as reported by the inverter to get a true idea of the output power.

Well... I'd say not quite. P = VI. So 500A DC at 370V DC on the input means 700A at 260V AC output. Power in must equal power out unless you've got a resistive controller.

OP, I don't know why your power levels are lower when measured by GPS. Your drag may be higher than you have calculated, or the 10Hz GPS might be too slow. Can that app use wheel speed sensors or the phone's accelerometer? Either of those might give more accurate results.

[Isaac96]

Idc at inverter is not accurate (calculated from the AC currents), udc is generally good though.

[slow67]

It probably is voltage sag. How fast does the BMS report voltage and current?

[ManuFromParis]

I don't know yet, because we haven't rolled it on a bench, but behind the steering wheel the human feeling is much higher than the OEM Porsche 911 SC which has 200hp and around 220Nm torque.

I guess it's linked to the battery pack characteristics as well (total V, discharge perfs, etc...)

[jon volk]

I guess it comes down to how confident you are in the voltage/current sensor data accuracy.
In terms of power, calculations based on weight and acceleration over a known distance (i.e. 1/4 mile trap speed) are, to me, the foolproof standard. Particularly since you will achieve pretty consistent trap speeds regardless of the launch. Sensors can have offset errors, dynos can be manipulated in the software with a slew of correction factors, but you cant change the physics that it takes x power to accelerate a mass over a set distance.

If i had a dollar for every 3500lb car that claimed to dyno 1000 hp made dismal pass at 135mph, Id be retired.

[StefSonic7EV]

I have done some extra testing to figure it out. As Isaac96 was pointing out, the readout from the idc at the inverter was not accurate (BMS data on the left):

Udc looks to be correct.

I tend to think that BMS readout is correct. For this run Race Droid Pro has shown 187 hp.

Report based on the BMS log:

shows the current at the peak power moment was 450 amps. Voltage sag is massive though, dropped from 374 to 345. So 155kW peak power, 208hp.
Since Race Droid showed 187 hp, BMS logged power 208hp minus 5% for inverter/motor loses gives 198 hp. Even if the error of the Race Droid and BMS measurement is only 5%, we are in the ballpark.

So clearly looks like voltage sag is the answer on why I get the power of 155kW instead of factory 210kW.

Anybody with stronger battery pack could post some logs? I wonder if with a better pack the motor can really handle 210kW.

[Sparky 61]

Hi after twelve months final have the the little red race car ready for this years pilgrimage to Speed Week Down Under, interested in the set up for the controller, are you using the standard set up or have you changed any of the parameters to get more out of the motor, I'm looking for the highest top speed I can achieve.