Magnetic Saturation

[vfx]

Changing Our World

One of these issues, says Hofmann, is magnetic saturation. "When you're trying to get a high torque density out of one of these machines, you enter what's called a saturation region. The machine has magnetic materials in it, basically iron, that magnetically saturate as the magnetic fields in the machine get larger. The magnetic fields are what generate the torque, and as a result, the model of the machine under those conditions becomes very complicated, very nonlinear. Your controller needs to be very robust so that you can operate well in that saturated region."

 

[TEG]

From where I sit, that bit of code is the best part of Tesla's "secret sauce".
Their digital motor controls took the industry up a notch.



By the way, I read that TM may not use the motor inductor as part of the charging circuit for Model S. Are they trying to move completely away from all the old ACP technology?

 

[WarpedOne]

If someone could shed some light on this "inductor function" of motor I would be grateful

 

[TEG]

http://portal.fpsct.org/fhs/courses/designed/curriculum/Readings/Aevd/Reductive_description.pdf

What exactly is a Reductive charger?
The Reductive charger is a patented technology developed by AC Propulsion Inc. that takes the
drive system and the battery charger for an electric vehicle, and combines them into a single
unit. An EV drive system requires a powerful motor and expensive transistors called IGBTs to
convert battery current into power that drives the wheels. An EV charger also requires similar
transistors, and heavy magnetics to convert alternating current (AC) from the power outlet to
direct current (DC) for the battery.
The Reductive charger uses the motor and IGBTs from the drive system to serve as the power
elements of the charger. When the EV is being driven, the drive system works normally. When
the EV is being charged, the drive system is re-configured as a charger. In this way only one set
of large, heavy, and expensive components is needed....

Thoughts on EV Charging Interfaces

Direct AC Charging

At the beginning I assumed that any electric vehicle would use a switching power supply as its charger. This is not necessarily the case. It's an interesting coincidence that the EV1's battery pack voltage, 312 volts, is not much below that produced by direct rectification of the 240 VAC power line (340 volts). Given that the EV1's inverter (the electronic circuit that converts DC into AC for the induction motor) looks very much like a DC switching power supply in reverse, it's possible to feed 240V AC line current directly back into the inverter. This would use the inverter as if it were doing regenerative braking -- with the AC power line taking the place of the motor/generator. This approach, called reductive charging by Alan Cocconi (one of the original EV1 designers) has the advantage of requiring neither an offboard charger like the Magnecharger or the nearly equivalent onboard charger. The inverter would double as the charger, and with its existing liquid cooling system it could probably handle some fairly high charging power levels -- at least as high as those encountered in highway cruising.
This approach would dictate a conductive interface.
For safety's sake, this approach would require the complete isolation of the propulsion battery and all related circuitry from the chassis of the car. Both the negative and positive lines from the propulsion battery would be "hot" with AC with respect to ground while charging. The EV1 is already designed this way. But Cocconi reports that even with this isolation, small leakage currents can exist that will trip a GFCI in the supply circuit and careful design of the drive motor is required to eliminate them.
As an alternative, a stationary AC isolation transformer could be provided on the outlet used for charging. But because this would have to be a 60 Hz transformer rated for full charging power, it would be physically large and heavy. It could easily be heavier and possibly even more expensive than the existing Magnecharger.
The existing switch-mode chargers, for all their faults, do provide AC ground isolation "for free" through their high-frequency transformers -- either the one built into the car (conductive approach) or the one formed by the paddle and coupler in the inductive approach. And they do adapt readily to changes in AC line voltages. Both problems would have to be solved in any direct AC charging scheme.
Discussion

The reductive charging scheme seems attractive because it requires no special charging hardware, either on the car or at the charging site. But it turns out that the charger is only a fraction of the cost of many charging sites. Simply bringing power to the parking spot turns out to be much more costly in most cases. Even 120V convenience outlets, sufficient only for very slow EV charging, are rare in parking lots. Higher power feeds have to be installed specifically for EV charging. This often requires trenching under asphalt in a parking lot or running hundreds of feet of conduit in a parking garage. Sometimes the runs are long enough to require a 480V feed and a step-down transformer near the charger, further increasing the cost.

http://www.apec-conf.org/2006/APEC_2006_Plenary_3.pdf

...Traction motor acts as a boost inductor...

http://www.mail-archive.com/[email protected]/msg06130.html

...if you use the motor's winding as your converter's inductor...

 

[doug]

If someone could shed some light on this "inductor function" of motor I would be grateful

Not sure how detailed an answer you want. Any wire loop has some inductance (thus makes an inductor). An inductor is one of the basic passive linear electrical components (along with the resistor and the capacitor). The motor, with its many wire loops, of course makes a large inductor. The ACP designs used the motor as a large inductor in the charging circuit, as a clever way of reusing what is available on the car. This was also done on the Roadster.

In my brief interview with JB, he mentions that "version 2" of the Roadster drivetrain would no longer use the motor in this way, yet had better charging efficiency.

By the way, I read that TM may not use the motor inductor as part of the charging circuit for Model S.

Do you have that print reference? I'd be interested to see more detail about it.

 

[WarpedOne]

I think I got the idea. Battery is DC, motor is AC, inverter just inverts that DC into AC.
While charging you need something that converts the AC back into DC. Part of this something are motor's big stator windings which can store a lot of energy in a form of magnetic field and thus form a strong charger. Charging is not much different than doing regen - when charging AC comes from outlet, when braking with regen, AC comes from motor.

TEG, thank you!

 

[DaveD]

An inductor is one of the basic passive linear electrical components (along with the resistor and the capacitor).

Minor point, but the memristor is also a member of this list.

 

[doug]

Minor point, but the memristor is also a member of this list.

I'm aware of the memristor, but I didn't think it was linear. The diode is also a passive device, yet non-linear.

 

[JRP3]

I wonder how adding a separate charger mechanism is going to improve the charging capability, and to a large enough degree to be worth the added complexity, expense, space, and weight of a separate unit? I always felt one of the strong suits of the ACP design was the built in charging capability.

 

[TEG]

I can see two possible reasons right away why they might want to do that:

#1: Higher current capability for the QuickCharge they want. I think I read that the ACP system maxed out around 20-30kW current.
#2: Avoid the need to pay ACP more royalties.

Also, I think they mentioned that they have a new solution in mind that is more efficient which would be another benefit.

 

[Young]

An inductor that can handle high current will be quite heavy.

I can see two possible reasons right away why they might want to do that:

#1: Higher current capability for the QuickCharge they want. I think I read that the ACP system maxed out around 20-30kW current.
#2: Avoid the need to pay ACP more royalties.

Also, I think they mentioned that they have a new solution in mind that is more efficient which would be another benefit.

 

[TEG]

Perhaps the QuickCharge would have the charger external to the vehicle and have a direct DC connection to the pack.

 

[WarpedOne]

Perhaps the QuickCharge would have the charger external to the vehicle and have a direct DC connection to the pack.

This was one thing someone from Tesla mentioned a while back as a plan for Model S, but then we heard that "standard 480V". This is not DC any more.

Edit: from today's news:

The Model S can be recharged from any 120V, 208V or 240V outlet or quick-charged from an external direct current supply in only 45 minutes. You can recharge the car during rest stops or meal breaks, enabling the Model S to go from L.A. to New York in approximately the same time as a gasoline car.

Here we have that external charger again.

 

[Joseph]

"While charging you need something that converts the AC back into DC. Part of this something are motor's big stator windings which can store a lot of energy in a form of magnetic field and thus form a strong charger. Charging is not much different than doing regen - when charging AC comes from outlet, when braking with regen, AC comes from motor."

I have NO idea what the bolded sentence is talking about. Are you saying to hook up AC electricity from the grid to the motor? What do you mean a motor "can store...energy" ? How does an emotor store energy?

I understand how reductive charging works, and I presume it's related to this since TEG posted about reductive charging; can someone explain?:biggrin:

 

[TEG]

EV Attack project [Archive] - Attack Forums

This system also utilizes the motor windings/rotor for battery charging. The induced energy as an SMPS transformer requires the motor to be electrically isolated from the frame. ACP calls this "reductive" charging.
The motor is isolated from the reduction drive input shaft using splined couplers having a delrin block between them.

Phenolic resin insulator / motor isolator (e.g.: G10 or Delrin):

SMPS is basically a circuit that operates in a closed loop system to regulate the power supply output. The AC line voltage is rectified and filtered without using a line frequency isolation transformer.

Switch-Mode Power Supplies need windings as part of the circuit.
Here is a small switching power supply:

I think that the reductive charging has a giant version of that with some components in the PEM, and the windings in the motor.

When they say "store a lot of energy", I think they mean just for brief moments as the reductive charging circuit is manipulating the current flow (temporarily converting it to magnetic field energy) to turn various AC voltages into the specific DC voltage needed to charge the battery pack.

(I am basically making educated guesses at all of this though - I don't work with this technology and I am not an EE)

 

[doug]

EV Attack project [Archive] - Attack Forums
Delrin block:

Minor point, but the spline couplers shown here are not made out of Delrin. Looks to me like G10 (or one of it's variants).

 

[TEG]

Something very strong and non-conductive...

 

[DRM]

Joseph,

I think the way the charger works is very similar to a "switching buck regulator". Wikipedia has a good summary of it. They use the motor stator windings in place of the inductor in the regulator (the inductor is the most expensive and geometrically large component). To charge the batteries, you need to regulate the current feeding into them; this can be done by modulating the switching duty cycle of the half-bridge (electronically by the car's cpu based on temperature, rate of charge, etc)

... of course I could be wrong.

//dan.

 

[TEG]

Which leaves us wondering - if they do away with reductive, do they need to put a big coil somewhere else in the vehicle for the new charger circuit?
Is there another charger technology that they could use that doesn't require a big inductor?

 

[TEG]

Coincidence, but they use G10 discs in the construction of many Tesla coils too.

"Work in Progress"

SRSG (Synchronous Rotary Spark Gap)

(In the case of the Roadster they don't want the spark to cross the gap!)