Range speculation

[TEG]

Yes, that sort of does make sense. The earlier PEM perhaps had a syncronous switching limit at a lower RPM, so the intentional fall of the torque curve had to start earlier.
Now I still think that the inverter must have a fairly direct relationship between frequency and RPM, so I think the drop-off is due to intentional voltage drop as they rev the eMotor above syncronous speed. The ESS might even be able to keep delivering the max voltage, but the IGBTs couldn't handle switching that voltage that fast. So they either have to hold the eMotor at syncronous RPMs, or start reducing voltage as they ask the IGBTs to switch faster.

(At least this is what I am starting to think)

I think the torque/flux control model probably just does some conversion to frequency/voltage, but there are some subtle "weirdnesses" in the way you can send power to the motor. I recall some mention of "eddy currents" in the magnetic fields that perhaps can be optimized. I am guessing that the torque/flux control model lets them control the power flow a little more carefully based on the way the motor operates. All those Feynman lectures maybe start to come into play now.

(again there are experts that could confirm or deny all of this, but they are probably too busy designing motor controllers to "chew the fat" over here)

 

[iisjsmith]

I simply want to stand by my statement that you do not need to increase voltage when you are increasing the frequency. You can increase the frequency, thus increasing the speed at which the motor turns, without drawing additional energy from the energy source. When you do this you take a hit in available torque as the motor goes into field weakening.

The numbers I have seen on the Siemens website use an example of a 60Hz AC motor. You increase frequency and voltage linearly until it reaches 60Hz, at which point it is running at maximum frequency and maximum voltage. You can increase the frequency to 90Hz without increasing the voltage further but you can only develop 44% of rated torque. If you keep pushing the frequency up to 120Hz you can only develop 25% of rated torque.

The formula for the field weakening factor is (Rated Frequency / Extended Frequency)^2

I understand that I am missing something here when it comes to phase control and maintaining rotor control. I have gone through your post, cor_van_de_water, at least twice and I probably will review it a few times more. Any links you can send my way would be appreciated.

 

[TEG]

As much as I want to think that "Field Weakening Mode" explains the torque drop off, others have suggested that it is just intentional torque reduction to avoid overheating the motor.
Some have said that the IGBTs are fast enough that they could run base speed frequency as high as they wanted. I don't know if we will ever find out for sure.

 

[iisjsmith]

Oh, I am not insinuating that field weakening is responsible for the torque drop off on the Roadster once it hits 6000rpm. I know that is something you mentioned, but I'm not so sure.

The reason I'm not so sure field weakening is responsible for the drop in torque is because once you exceed the volts-per-hertz ratio you still see constant horsepower. As you increase voltage and frequency together from zero, keeping the volts-per-hertz ratio steady, you are in the constant torque range. Once you hit the maximum voltage (the rated voltage, or line voltage) you cannot increase it anymore, but you can continue to increase frequency. At this point torque and flux will decrease but the horsepower remains constant. The Roadster motor torque and power curve definitely show horsepower decreasing as torque decreases.

What we need is a look at the motor nameplate. If we knew what the rated frequency and voltage were for the motor we could figure many things out.

I have a feeling there are multiple things going on. They could be doing things to limit motor heating, and at the same time increasing the frequency beyond rated frequency to eke a few Nm of torque out of the engine. They have said several times that their proprietary programming of the inverter and controller are their big inventions, so there are probably many things going on behind the scenes.

 

[TEG]

Yeah, thanks. I started looking into this thinking there might be an easy answer, but it doesn't appear to be so simple.

 

[TEG]

I have come to the conclusion that the PEM is probably not the limiting factor here.
The need for battery and eMotor durability probably drives the exact HP & Torque the PEM asks them to provide.

 

[AGR]

The Roadster is being compared to "supercars" with supercar performance. If its used as a supercar would be used...what is the range? If its used as a commuter car in a traffic situation what is the range?

 

[TEG]

I doubt you will get a clear answer here. That needs to wait for production cars to get to real customer hands and then people can report what range they get.

Personally I think low speed stop and go city traffic will do well range wise.
(Regen braking is a great thing)

The part that will probably hurt is high speed cruising. It would NOT likely have a lot of range as an autobahn cruiser.

 

[AGR]

TEG,

Thank You, we are on the same thought process....low speed / commuter activity is cool, high speed / sustained highway...not so cool.

 

[TEG]

The place where the Roadster will really shine is for mountain driving.
Along with the excellent handling, the eMotor regen is really in its' element going up and down hills.
You know an ICE car is being wasteful because the brake rotors are overheating and that is wasted energy.
An electric car can recapture so much of the energy when you go back down a hill that the net effect is that the hills don't hurt your range so much, compared to a gas car which has terrible mileage going up and down hills.

 

[tonybelding]

The place where the Roadster will really shine is for mountain driving.
Along with the excellent handling, the eMotor regen is really in its' element going up and down hills.

Another point, internal combustion engines lose power as altitude increases and the air thins.  The electric motor does not.

However, climbing long grades will drain the batteries rapidly, and the range estimate will be less reliable in the mountains.  It could fool you into running out of juice before you reach the top if you aren't a bit more cautious.  This caused Kris Trexler some anxiety during his "Charge Across America" in an EV1, although he never did run out of power.  Check this:  http://kingoftheroad.net/charge_across_america/daily/day5.html