Manufacturer - Siemens AG
Cost - $2,000 in 1998 (non-production prototype)
Production motors available from Metric Mind Engineering
As you can imagine, the motor is the heart of the car, often defining (and limiting) its performance. My choice was 3 phase AC induction motor rather than common DC motor used in probably more than 95% of all conversions. We're not going to debate advantages or disadvantages of AC vs. DC motors. It's like debating advantages of the flat TFT screen monitor vs. CRT tube monitor. Both have their uses, but in general if one thing is more expensive and preferred by a pros, then it's not for nothing. Price aside, I have heard a lot about all kind of issues with brushes and brush plate advancing, and didn't want to deal with them. DC motors are (relatively) low voltage high current devices. At high voltages (>200VDC) the brushes will arc especially at high RPM. AC motors are made either way. High voltage units reduce I2R losses and weight of wiring. All you need is to match control electronics to the motor characteristics. I'm only going to say that while my Siemens AC setup (including electronics) is only about 1.5x more expensive than comparable raw power DC setup, it offers much more than 1.5x worth of advantages. To name a few: regenerative braking (actually also available in high end DC systems), electronic reverse, integrated components (main contactors and DC-DC converter), ability to adapt exact characteristics of the motor (and also throttle and brake potentiometers parameters, battery and other hardware parameters) via software, lack of brushes, high top RPM limit (about 10,000 for this motor, more on that - later), water cooling and high reliability.
But the fundamental disadvantage I see in the common DC solutions for EVs is using controllers and motors which are not designed or intended to be used in EV and even in outdoor environment (like common Curtis PMC controllers). At best, controllers for the fork lifts and golf carts are in common use. The same with motors. Most common ones, made by "Advanced DC motors" (can anyone tell me what exactly is advanced in these motors?) can't even normally run in both directions without some modifications! People adapt this hardware for EVs and push its limits because dominating factors are the price, availability and experience of the past, when nothing better was out there. Until recently, I was not aware of any controllers and motors designed specifically for EV.
Amodern PC or laptop has more than enough computing power to read the sensors, synthesize a 3 phase sine wave and calculate the frequency and amplitude of the voltage applied to the motor. Yet no one would dare to run their EV off of a PC bolted to the fender or laptop in the glove box. It may be acceptable for design experiments stage in the stationary lab environment, but with the temperature, shocks, vibrations and moisture extremes it's totally inadequate hardware. Why not use components intended for the job? The same about the motors and especially controllers, you will put much effort to add some more hardware (also non-automotive grade) like main and reversing contactors, precharge resistors, throttle pots, heat sinks, fans, interlock safety relays to make non-EV controller work in your EV. If integration is done right and generic components are beefed up, this will work for quite a while, I had such setup done. But it's like laptop in the EV, you can't trust it not to fail when you rely on it. Eventually it will fail, just like controller not designed from ground up with EV environment in mind.
I knew about EV AC solutions and all the advantages it provides when I built my first DC CRX. AC Propulsion's CRX was my inspiration making me hunt for this particular donor vehicle. I wanted it so bad that bought perfectly normal functioning used CRX and pulled the engine out. I still have it. I also knew, that one day I will build something much better, comparable to Alan's CRX, with the components intended for the job. Well, this day have come.
The main difference I perceive as a driver is flat torque/RPM characteristic of an AC induction machine. DC motor has giant torque at stall and it quickly diminishes as RPM goes up forcing you to shift gears. So, up to some point I feel constant acceleration on the particular gear, and above that - constant power (torque reduced proportional to RPM). That's what we want. Particular motor I used is made by Siemens, type 1PV4133WS20. Mid power high torque version. The rated continuous mechanical power is 30 kW. The max power is limited to about 80 kW by the inverter, the actual max power for the motor itself is unknown but is way more than inverter limitation. It is water cooled 3 phase 4 pole induction machine weighting about 150 lb. More information on it can be found on my Metric Mind Engineering web site. It is similar to 1PV5133 motors series, just different mounting flange and shaft option. The splined shaft is only about 30 mm long - something I worried about in the beginning, but it turn out to be well thought out and desirable feature. Without it the motor probably wouldn't fit across my engine compartment.
The motor has hall effect shaft speed sensor (two 64 teeth gears with 90° offset) as well as two winding temperature sensors. This info is fed into the power inverter and whole system works in sync adjusting frequency and amplitude of the voltage applied to the motor to control electrical slip for maximum performance and efficiency and reducing drive current in case of overheating. The motor flange is centered to the adapter plate with the round 230 mm diameter ring, recessed 8 mm into the flange. The motor mounted to the plate with four M14 bolts. For supporting the back side of the motor, special bracket linked to the rubberized engine support was made. (This support was removed in previous conversion because interfered with the back of the small DC motor running air conditioner compressor. Well, this time I needed it back, so I had to visit nearest wrecking yard, cut out this engine support and weld it in place, restoring original look. Turn out looking pretty well.)
The Siemens 3 phase water cooled induction motor, type1PV4133WS20
The motor splined shaft. Outer round centering recess is also visible.
The power inverter.
Manufacturer - Siemens AG
Cost - $3,800 in 1998
Available from Metric Mind Engineering
The power AC inverter is the main part of the electrical system of the car, and its role is simple - to convert the battery energy to the form useable for the AC motor, and to deliver right amount of this energy per driver's demand. While it doesn't sound too complicated, to perform this task flawlessly the electronics gets quite complex, and it is especially true for AC drive systems. In my case inverter has to:
- Convert (invert) the DC battery voltage to the 3 phase AC
- Adjust AC voltage amplitude and frequency based on the drive current demand and instant conditions like RPM and torque
- Provide regenerative braking turning the traction motor into a generator for recuperating kinetic energy back to the battery pack; so it has to convert generated 3 phase AC voltage back to DC before feeding the batteries,
My inverter is doing all that and much more - it also provides 12V DC voltage to run accessories in the car, hosts main contactors, displays running parameters on the screen of any PC attached to it through RS232C port, protects itself from over voltage, over current, over heating, basically over-anything, protects the motor from overheating and overload, protects the traction battery pack from under voltage during acceleration, from over voltage during regen, signaling faults, storing variable parameters in its non-volatile memory, allows to make timing diagrams of relevant internal signals and bunch of other useful things. Learn more about it here. It is made by Siemens corp. specifically for use in electric vehicles and therefore has all EV stuff implemented and debugged. The maximum battery current is 280A and the battery voltage 380 VDC which makes it about 100 kW inverter. Turns out it's pretty efficienttoo - about 94%. Since it is liquid cooled, I was hoping to run cooling water through the stock heater core and get away without additional heater. Well, because it's wasting very little energy, even when I run the car at 65 mph the water gets barely warm.
Siemens Simovert 6SV 100
kW water-cooled 3 phase AC Inverter with integrated 90A max DC-DC converter and main
No way I could resist to take the cover off to take a look at this beauty. I've seen a lot of hardware. This one is a piece of art.