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TomEV 03-07-2008 01:04 AM

Field Weakening Experiment for Speed Increase
One thing about the Citicar - it is a relatively easy platform to experiment with!

While I had experimented with field weakening in the past, I was never able to find how much parallel field resistance should be used to have a noticeable effect. The least amount of resistance I was comfortable using was a 6 ohm, 1500 watt resistor (heater element). When connected in parallel (per various references on field weakening) the heater coil had no effect on speed, so I shelved the idea.

Reading a 100 year-old electric motor book found at Google, I came across a section regarding speed control of series motors. On page 144 there is a reference that infers the weakening resistance would need to be quite close to the field resistance to have an effect.

The field coils in my 3.5hp GE motor have about .015 ohm resistance (according to the manual). Interestingly enough, there was a part sitting in my garage that came with the Citicar, but one that I don't use because I have an electronic controller - the 'low speed' resistor. This 'resistor' (actually a nichrome strip) has a resistance of about .1 ohms - much closer to the field resistance than the 6-ohm heater coil.

I dug out the low speed resistor from the box of old parts, and hooked one end to a field connection at my F/R controller. I connected a wire to the other field connection so I could 'mash' the resistor end with the wire once I was at full, stabilized speed.

So, off I went. Made it to 33 MPH (typical max these days with 50f batteries). Stabilized there for a few seconds, then connected the low speed resistor in parallel with the field. No untoward sparks (good thing) and the speed increased (albeit gently) to a whopping 36 MPH. Not bad for a junk box experiment - almost 10% gain! :D

To make sure it wasn't just a fluke, I turned around and did the same thing a second time - stable at 33 MPH, connect the low speed resistor in parallel, and... 36 MPH! (I actually passed a Ford Excursion...) :eek: Disconnecting the field weakening resistor made the Citicar slow down, but not dramatically - took a while (about 15 seconds), but it decreased to 34+ MPH and stayed there. The next acceleration cycle (without field weakening) stabilized at 33 MPH like usual.

The book infers that you can't leave the resistor there during acceleration, as the field will be too weak to produce enough starting torque. It is only for 'steady-state' full speed (i.e. accelerator hammered down on a flat road). My next step will be to get a relay can to switch 'on' when I need a speed boost for those 35 MPH roads.

Makes me wonder how it would work with those of you that have other brands of series motors?

Disclaimer... I haven't measured the amperage or temperature of the motor in this configuration, so experiment at your own peril... ;)

bennelson 03-07-2008 01:17 AM


Originally Posted by TomEV (Post 13052)
(I actually passed a Ford Excursion...)

Take that ICE SUV!

Sounds like a cool experiment!

You have to make a final version of this connected to a big red button on the dash marked "TURBO!"

Ryland 03-07-2008 11:24 AM

I'm sitting on the edge of my seat, waiting to learn how much power your are drawing while doing this.

TomEV 03-07-2008 11:06 PM

I'll have to run a log while driving around, but I think total power will be the same. Only difference is that more is being used by the armature, because less is used by the field.

It is probably harder on the brushes this way, but a modest increase (10% in this case) is probably OK unless you run the car flat out most of the time.

TomEV 03-25-2008 01:50 AM

Here's the video with some additional information. As an aside, if you do this the field weakening resistor will need to be switched - you can't just leave it 'on' because you won't have any starting torque.

It is nice to have a 'turbo' switch... :D

bennelson 03-25-2008 09:04 AM

Interesting stuff,

I liked the note about not being able to do this on a THINK or GEM car.

I noticed you are using an Alltrax controller and a 48V system, right?

Is there any reason why field weakening couldn't be done on the Forkenswift or my ElectroMetro?

The main thing would just be having 4 power posts instead of 2 on the motor to be able to easily wire it up?

TomEV 03-25-2008 10:53 AM

I have a 48v system and an Alltrax 4844 controller, and a series DC motor. You also have a series motor - two of the posts are for the armature (A1 and A2 connect to the brushes) and the other two (S1 and S2) are for the field.

This will increase the top speed of a series motor, essentially when the Alltrax (or whatever type of controller / contactor you have) is at full, stable power. In other words, your accelerator is hammered down. If you have top speed issues (such as you can't get more than 50 MPH with ForkenSwift) then adding a field weakening setup could give you another 5 MPH or so.

It may not seem like much, but the extra 3 or 4 MPH I get is a big difference.

You can do a quick and dirty test by using a jumper cable (one of the thin, cheap ones - #6 wire) to jumper S1 and S2. The cables will have enough resistance to weaken the field somewhat without a regular resistor (some people just use cable as the resistor). Probably best to have a passenger connect the cables while you drive the car. If it has an effect, it will increase speed slowly (over a few seconds) but noticeably.

diesel_john 03-25-2008 11:54 AM

is there a way to advance the timing? mechanically or electrically
or make the pulse width shorter?

TomEV 03-25-2008 10:54 PM

Some people change the 'timing' by rotating (advancing) the brush block a few degrees. There is an art to this technique, and is probably best left to your local electric motor repair shop. From what I hear, it isn't particularly expensive, and can increase the performance of a series motor. If you have a transmission with reverse (as opposed to a forward/reverse electric contactor) it looks like a good modification since your motor will always be turning in one direction anyways.

The pulse widths are engineered by the controller manufacturer, so I'm fairly certain that can't be changed (other than to the extent that they are already changed when you step on the accelerator)

MetroMPG 04-04-2008 12:06 PM

Great experiment, Tom. A 10% increase is nothing to sneeze at. Thanks for posting it.

As I mentioned in the ForkenSwift thread, it's not as simple a matter for us because our motor field connections are internal. But that might be a reason to mod the motor case and bring the connections outside for just this reason.

I liked all the "amps @ mph" notes throughout - were they battery or motor amps? (There's no motor ammeter on the FS.)

Also - you should post a garage entry for your car - even if it's just a link to your EV Album entry. As more people read your posts, they'll have easy access to the relevant vehicle stats.

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