Field Weakening for Efficiency

[TomEV]

A while back, I was taking power use measurements of the Citicar at various speeds.

See thread: http://ecomodder.com/forum/showthrea...ease-1307.html

In order to get to top speed – a whopping 37 MPH – I use Field Weakening. At 37 MPH, it uses about 35 more amps than at 34 MPH (non-FW top speed). Part of the graph hinted that in certain conditions, FW might be more efficient than non-FW.

A couple of days ago, I started a new experiment with the Citicar to see if Field Weakening could be used to increase efficiency.

For reference, Field Weakening basically shunts part of the current away from the field section of the electric motor, reducing back-EMF. I use a 6” piece of ½” wide Nichrome wire as the field shunt, switched in with a solenoid.


New controller mount layout

The picture above shows the Alltrax 7245 mounted to the heat sink. The Field Weakening resistor is the curled Nicrome wire mounted on two ceramic posts at the lower right. Albright SW-202 F/R contactor mounted at top right, with Kilovac main contactor (white can on left under F/R) and weakening contactor (silver can at lower right behind weakening resistor).

In previous experiments, I determined that FW increases acceleration in my vehicle at speeds above about 2,200 RPM (roughly 19 MPH in the Citicar). For the test, I turn on FW by letting off the accelerator (turns off controller) switching on the FW contactor, and then resuming speed. I do this once I am above 25 MPH, and keep the speed around 25 MPH. The speed limit is 25 MPH on most roads in Alameda. In practice, FW is on between 19 and 30 MPH for roughly half of the trip.

Before the test, my average power use was about 230 wH/mi without the use of FW. Here are some preliminary results using FW–

Day 1 – 11.9 mi – 224 wH/mi
Day 2 – 13.9 mi – 217 wH/mi
Day 3 – 11.3 mi – 232 wH/mi
Day 4 – 23.0 mi – 227 wH/mi
Day 5 – 15.0 mi – 220 wH/mi
Day 6 – 14.1 mi – 194 wH/mi

Average for 19.55 kWh / 89.2 miles is 219 wH/mi, or about a 5% gain in efficiency. I measure kWh used at the wall plug, so the total power use includes battery charger losses, etc. I plan on continuing this test for about 30 trips to get a good average power use.

[dcb]

That's interesting, sorta sounds like there should be an inverse linear relationship between rpm and field strength.

Just curious, what do you suppose the resistance of that bit of nichrome is? And it is in series w/the field when engaged?

[TomEV]

Originally Posted by dcb

Just curious, what do you suppose the resistance of that bit of nichrome is? And it is in series w/the field when engaged?

I measured the resistance of the Nichrome a while back, and if memory serves it is about .09 ohms. This gives about 10% increase in speed compared to non-FW. Some people use a short length of small diameter (e.g. a foot or two of #6) battery cable with similar results.

This is a mild amount of Field Weakening. I have considered adding another contactor in the FW system to use half of the existing resistor (.045 ohms) for more speed, but I am concerned that the extra amperage across the armature may burn out the brushes if used too much.

The resistor is in parallel with the field only - part of the current goes through the resistor, the rest goes through the field; all the current still goes through the armature.

As to the speed/FW relationship, here is a dyno run for the Citicar from a while back. The dynamometer shows that this motor puts out 10.6 hp /46 lb ft torque (417 amps @ 44.1v input) - thin green line. Field weakened motor puts out 11.3 hp and about two lb ft above stock curve - thick green line. The end of the thin green line is the RPM limit when FW is not turned on.

The strangeness in the beginning of the FW curves is a dyno data error.

[MetroMPG]

Cool experiment, Tom.

I added a link back to your FW / speed thread in your post.

Also, I saw a very low-miles original Citicar advertised for sale near here a week or 2 ago. If I didn't already have so many vehicles, I'd have one.

[TomEV]

Originally Posted by MetroMPG

Cool experiment, Tom. I added a link back to your FW / speed thread in your post.

Thanks! I have learned much about FW since that thread. One is that since FW just changes the current path, it is unlikely to damage a PWM controller. To be absolutely safe, I don't switch FW on or off unless the controller is off or not regulating (full on).

Originally Posted by MetroMPG

Also, I saw a very low-miles original Citicar advertised for sale near here a week or 2 ago. If I didn't already have so many vehicles, I'd have one.

I have the same sort of problem. Two cars is the max I'm willing to have at any one time. I'm not ready to get rid of the Citicar, I have a Prius as my second car (my wife calls it the gas guzzler).

There is an electric truck (one of the smaller sized Dodge 80's vintage trucks) near me that is looking for a home. Needs a battery pack and a good cleaning, but everything else looks like it is there. Tempting all the same...

[04_Sentra]

Did you ever think of moving up to 60V?

That would get you to 40mph. Not sure if the brakes could handle more than that.

[TomEV]

I will be going to a higher voltage sometime in 2011. The battery pack is a bit over three years old, and will need to be replaced within the next year as it is showing signs of age.

Apologies in advance for the long post .

The original Citicar setup used eight 6-volt batteries for 48 volts which made the Citicar weigh about 1,300 lbs. I am presently using six 8-volt batteries to save weight as my normal use of the Citicar is well within the range of six batteries (24 miles or so - I usually drive about 10 miles a day). With six batteries, it weighs 1,160 lbs on the local truck scales. A ten battery 6v/60v setup would be difficult to implement due to space constraints, and a five battery 12v/60v setup probably wouldn't have enough range.

Keeping with six batteries, I can go to 72 volts using 12-volt batteries.
↑ Increases speed to at least 45 MPH. Increases acceleration, which increases fun quotient. (Due to differential gearing, the top speed of the Citicar is about 45 MPH / 5,000 RPM, and 72 volts would reliably deliver the max RPM the motor can handle.) (-$200; six batteries instead of eight)
↓ Problem is that it will cut my range down somewhat, perhaps to about 18 miles, less if I routinely drive around at 45 MPH. As the pack ages, the range will decrease, and I may not be able to reliably get to work and back after a year or so. Replace battery charger and defroster fan ($600 - $200 = $400).

Although a nine-battery, 72v setup would increase the range significantly (36 miles) or a 12 battery 6v setup (60 miles) it would be a pain to implement in the Citicar, and would increase the weight a bit too much, making it slow to accelerate even with the higher voltage. Besides, sitting in a Citicar for more than an hour to actually use that amount of range would be a bit masochistic.

If I go back to eight lead-acid batteries, there are four options. (lithium is still too expensive) All involve moving the controller back to the OEM position behind the passenger seat, and increasing the vehicle back to OEM weight -
48 volts (eight 6v batteries) Easiest and cheapest.
↑ Increases range to about 40 miles (6v batteries have more aH capacity).
↓ Slower acceleration and no speed increase.

64 volts (eight 8v batteries)
↑ Increases range to about 32 miles. Increases speed to 41 -43 MPH (guessing 2.5 to 3 MPH gain per 8V battery)
↔ Acceleration about the same - increased weight balances increased power.
↓ Have to replace battery charger and defroster fan (both 48v items) - about $600. All costs are the price beyond the cost of the battery pack.

96 volts (eight 12v batteries) Would need major changes to realize potential.
↑↑ Potential speed increase to about 55 - 65 MPH. Faster acceleration.
↔ Keeps range at about 24 miles (12v batteries have less aH capacity than 8V).
↓↓↓ Have to replace battery charger, controller, defroster, AC/DC converter - about $2,000. Also would have to replace the transaxle/differential and motor to realize potential for a total of $3,000 - $5,000.

192 volts (eight 24v batteries)
↑↑↑↑↑ Potential speed increase to 100 MPH+. (Like anyone would want to go 100 MPH in a Citicar!) Much faster acceleration.
↓↓↓↓ Decreases range to less than 15 miles. (24v batteries don't have much aH capacity) 24v batteries are more expensive, and would have to replace all the parts as with a 96v setup. $4,000 - $6,000.

[RobertSmalls]

What about ~60V of lithium? You'd lose weight, gain cargo space and range, and you'd have much less voltage sag than lead under high load and as SoC falls. Downside is you'd need to build a BMS and a charger, and depending on how many KWh you need, it might not be exactly cheap.

Counting the number of up versus down arrows in your post, it looks like you'd favor the 192V setup!

[TomEV]

Lithium and all the accessories needed for it would add just too much to the cost of the project. Any upgrade will be expensive as is, and putting the money into a Citicar may not be the best idea...

I like the idea of 192 volts - it would surprise anyone that thinks a Citicar is slow!

[jakobnev]

Have you considered building a full field shaping device? Perhaps one part that is a DC-DC converter that feeds extra current into the field coil when you start from full stop. And one part that "steals" current from the field coil at hight speeds and converts into a higher voltage that can be fed back to the batteries. (the should be significant power going into heating you shunt currently)