Monday, July 30, 2007

Power calculations

After looking at the specs on the ElectroAuto website, I'm thinking that a maximum input power rating of 50000 watts is really too much. Here are the ElectroAuto specs:

AC24 Aircooled Motor

Length Diameter Weight Controller Efficiency
15.55" 9.45" 83.6 lbs. DMOC445 83%

Specifications @ 156 V Input
Ft.Lbs. Amps RPM HP KW
Peak/Maximum 54.57 280 rms 12,000 57.66 @
4,500 rpm
Continuous 22.12 180 rms 12,000 18.77 14

55 Ft-Lbs peak is 75 Newton-meters. The controller is programmed for 100 Newton-Meters.
I'm guessing I should probably back off the peak wattage to 43,000 as listed in the chart unless I want to severely harm the life of the motor.

The original Porsche 914 2.0L engine had about 90 peak HP so this is definitely less. It looksl like I'll have to shift more often to balance the motor torque with my desired speed. The regenerative braking makes shifting really hard, so I'll probably have to utilize the regen-disable switch more often.

I also think I figured out what the high-pitched squeal/scraping noise was. The controller puts out a 10Khz PWM signal to modulate the three AC waveforms. I hear this squeal during acceleration and regeneration, but if I let the motor spin freely by putting the accelerator in the middle, there's no squealing. The squealing never changes frequency either. I didn't expect the motor to be that noisy... Live and learn.

The last big issue I probably have with the car is that the flywheel is probably out of balance. When the motor hits about 4500 rpm, the whole car starts vibrating rather nastily. This means I'll probably have to take the transmission off again and have the flywheel balanced, or re-turned by someone before general use. Urg.

I sure hope people don't have to put up with my mistakes on their own 914 AC conversions!

Good night.


pjorg said...


1. I am surprised the Azure and Electromotive are not helping more. There are a lot of people following your blog and your progress.

2.Have you considered useing the brake light circuit ti energize the regen?

3. I have read other who have had vibration problems from Electromotive. It could also be the alignment of the adapter plate to the transmission.

4. Looks like you are not seeing the 400 amps you thought. Have you talked with Azure about this?

Looking forward to resolution before I send in my check.


TimK said...

Hi Paul,

Thanks for the comments.

1. I don't know if either company is reading this. I haven't received e-mail or notification from either company. Azure has been quick to respond. EA has not responded for a few weeks to my e-mails.

2. Good idea about the brake system. That would definitely augment the already weak manual brakes.

3. True, the adapter plate could be an issue. I think the only way to find out is to separate the transmission and motor. Then I could spin up the flywheel by itself and see if there's any vibration.

4. I'm definitely getting assistance from Azure, but they don't seem to want to explain many of the internal details of the values in the status registers. I suspect they don't want too much proprietary information getting out. They have asked for several acceleration logs so they can provide feedback.

Sometimes you just have to take a leap of faith and jump. After all, this is just a hobby (albeit an expensive one...)