I have built an electric car for drag racing, and trying to create a field weakening circuit to improve performance at higher rpms. The car uses a DC series wound motor, so connections are available to put a resistive load across the field coils. The more common way to do this is to simply use a contactor to enable a resistor (shunt), or better, to make 3 or 4 'steps' using multiple contactors and shunts as RPMs continue to rise.

Adding shunt resistance in steps works well, but is a bit crude, and I believe that if a circuit could be made that provides a variable resistive load, that the effectiveness would be better than simple steps. Weakening could be started earlier and increase proportional to RPM. So a few controls would allow adjustment of min and max shunting, ramp rate, and timing of shunt activation. Timing would simply be a switch on the throttle that is only ON at full throttle. After a certain number of seconds at full throttle, the weakening circuit would begin it's work.

So I'm trying to put together a basic circuit using mosfets in their ohmic region, that would be controlled by an analog voltage input. The input would be controlled by a number of factors, but for test purposes a pot will provide the variable input.

The test circuit is low power proof of concept. The final circuit would have to shunt maybe 400-600A at up to 100V DC, but only for a 3-4 seconds.

Can mosfets be used in this way? Or is there a better device choice to create the variable resistive load?

Or can anyone point me at an existing circuit design for this purpose?

All suggestions appreciated!

Wayne Krauth

 

May we see existing motor and control circuit schematic?

 

Here are two diagrams. One of the existing motor, motor controller, power and charging. The second file is a drawing that I just started, but is a bare minimum of a circuit to test with to see if the mosfet will work to pass a controlled amount of current through the load (12v lamp).

The shunting circuit would be connected across the S2 and S1 connections, which are the field coils.

I have only just started thinking about this as a possibility and trying to determine what components would be a good choice.

My background is in electronics, but tech school was 40 years ago, and I have been managing a software company for many years, not so much in hardware/electronics for the last 20 years, other than building up a few electric vehicles as a hobby. So to say that my electronics design skills are a little rusty would be an understatement ;-)

 

So you have no throttle? No accelerator pedal? Just an ON and OFF switch?

 

So you have no throttle? No accelerator pedal? Just an ON and OFF switch?

The diagram is primarily for the traction power circuit. The Zilla motor controller has a companion device (called the hairball) with throttle input, tach input & output, power on, start-switch sense, brake sense, reverse sense, precharge controls, contactor power, etc. I didn't include that in my custom drawing as all of that is documented in the Zilla manual.

The throttle is a dual sensor hall effect device. I would probably add a separate switch somewhere that would sense full throttle for purposes of starting the timer for the field weakening circuit.

The Zilla is a PWM device so pulses the motor with a variable duty cycle and voltage.

 

Is there any reference in your Zilla manual between standard and automatic transmissions?

 

If you use a FET as a variable resistor it will have to dissipate a massive amount of power, albeit for a few seconds only. PWM switching of a fixed shunt resistor, or of the field coil, might be an alternative (providing it didn't conflict with the Zilla PWM)?

 

I know nothing about this, so I should probably keep my mouth shut.

But, if someone said to me: "I need to control a large current into a field coil," I'd reply, "Configure a buck converter as a variable constant current controller."

You supply max current at low RPM, and drop the current as the RPM increases.

Won't work?

 

I have built an electric car for drag racing, and trying to create a field weakening circuit to improve performance at higher rpms. The car uses a DC series wound motor,
Wayne Krauth

'Performance'. Increased rpm at the expense of torque?
Max..

 

Is there any reference in your Zilla manual between standard and automatic transmissions?

The Zilla doesn't care about the transmission. The only input regarding that is the reverse switch, which enables a much lower power level to the traction motor.

I am running a powerglide transmission that has been converted to manual shift. Torque converter removed, bell housing cut off, tail shaft shortened to about 5 inches. I'm in the midst of having a new transmission input shaft machined from Vasco M300 steel, because on the last trip to the track the shaft twisted. 2000 amps makes a lot of torque in a DC motor.

 

'Performance'. Increased rpm at the expense of torque?
Max..

Yes. As RPMs climb, back EMF does too and begins to limit torque (i.e. amps). By field weakening, there is a loss of torque, but the motor wants to go faster and the result is quite a kick to the wheels. The guy that built my motor is using 3 steps field weakening resistors in his S10 truck, and when each step of field weakening hits, it breaks the tires loose. He is running a 10.x second 1/4 mile with it, and the first resistor cuts in at 7 seconds.

 

If you use a FET as a variable resistor it will have to dissipate a massive amount of power, albeit for a few seconds only. PWM switching of a fixed shunt resistor, or of the field coil, might be an alternative (providing it didn't conflict with the Zilla PWM)?

This is something that I am concerned about too. In order to take the Zilla's PWM output into account, some feedback will be needed to sync weakening pulses, which complicates the design considerably. A straight resistive load shouldn't interact negatively with the PWM.

I am expecting to have to dump some heat, so considering mounting the mosfets, or whichever device is used, on a liquid cooled heat sink and tie that to a radiator. The duration will be short, probably no more than 3-4 seconds, and a big heat sink might be enough. Haven't gotten to the point yet where I need to consider just how many watts of heat. I do have a set of questions out to my motor builder to help understand just how much voltage would be on the field coils, and how much current to shunt around them.

So I'm very much in exploratory mode on this project at the moment...

 

I know nothing about this, so I should probably keep my mouth shut.

But, if someone said to me: "I need to control a large current into a field coil," I'd reply, "Configure a buck converter as a variable constant current controller."

You supply max current at low RPM, and drop the current as the RPM increases.

Won't work?

Not familiar with how a buck converter works, although my charger has one in it.

Another alternative would be to split the series (field and armature) into separate circuits and drive them separately, essentially making a sepex motor out of it. But that is a much bigger design challenge to tackle than I am ready for...

 

I am not an expert, but I would play with the PWM duty cycle during the acceleration.

 

I think the term "field weakening" might be being misused here, but I'm undecided.

"Field weakening" applies to separately excited motors, which run at full torque (or close to it) up to nameplate speed, and then the field current is decreased to get more RPM out of it, at the expense of torque.

The traditional concept of field weakening doesn't apply to series motors, since any decrease of current through the field results in the exact same decrease of current through the armature. Nothing gained, only lost.

What you describe by placing resistors in parallel with the field, I do not believe it weakens the field. I believe it gives the armature current an addition path around the field, so higher current can flow through the armature than the field (resulting in higher than typical torque for a given field current), which typically isn't possible. So to be technically correct, I think it should be called "armature strengthening" instead of "field weakening." But 6 of one, half a dozen of another.

Anyway, I think you need something more efficient than a resistor. Something that helps add to net power instead of take away from it. Like maybe another motor. Could you install a smaller, lower voltage motor that would gobble up that wasted energy and put it to good use helping push the car forward? i think if you wanted variable "kick" out of it, As long as you put the field on the low side of the applied motor voltage, you could put the small motor in parallel with the field of the large motor, and PWM it with a single beefy (like really beefy, 500+A) MOSFET or IGBT.

 

I am not an expert, but I would play with the PWM duty cycle during the acceleration.

The motor controller's output duty cycle would affect the whole motor. Field weakening is done by reducing the amount of current passing through the field coil, while leaving current in the armature windings unchanged. In a series wound motor they both get the same current. The change in fields makes the motor want to speed up.

A friend has an older Electrak electric lawn tractor. It has a switch to cut in field weakening resistors. He says it feels like it's shifting to a higher gear and picks up speed.

The motor has to be at or near base speed before field weakening works well. Then there is the question of how much current do you shunt away from the field coils? i.e. shunt with contactors and resistors or the current control circuit I'd like to build. Once it's enabled, the optimal amount of current shunting to apply varies with speed. Using a control system to tune it's operation, seems like it would be easier to reconfigure rather than changing resistors, in order to find that tuning sweet spot.

 

Yes, separately excited motors have separate circuits to drive the armature and field coils independently so they can change field current as you describe. The series wound motor has 4 connection points on it. Normally two of them are connected with a copper bus bar, and the other two have power applied; putting the armature and field in series.

I suspect that you are right that the armature will get more current as the field declines, so perhaps both field weakening and armature strengthing are at play. Either way, the result is good if done right.

As for adding current to the armature, at the point where base speed is near, current has already been falling away, so the battery and controller have plenty reserve to provide. Any increase in current to the armature would indeed increase torque.

As for adding another motor, yikes! This one and the transmission fills the space it's in. It's a shoehorn already. Drive shaft is 13" long. The motor power peaks at around 400 kW until about 1/3 of the way down the track, then back emf starts to erode it. By the time the rpm climbs toward base speed power has dropped by almost half, leaving ample headroom to draw more power again.

I think the term "field weakening" might be being misused here, but I'm undecided.

"Field weakening" applies to separately excited motors, which run at full torque (or close to it) up to nameplate speed, and then the field current is decreased to get more RPM out of it, at the expense of torque.

The traditional concept of field weakening doesn't apply to series motors, since any decrease of current through the field results in the exact same decrease of current through the armature. Nothing gained, only lost.

What you describe by placing resistors in parallel with the field, I do not believe it weakens the field. I believe it gives the armature current an addition path around the field, so higher current can flow through the armature than the field (resulting in higher than typical torque for a given field current), which typically isn't possible. So to be technically correct, I think it should be called "armature strengthening" instead of "field weakening." But 6 of one, half a dozen of another.

Anyway, I think you need something more efficient than a resistor. Something that helps add to net power instead of take away from it. Like maybe another motor. Could you install a smaller, lower voltage motor that would gobble up that wasted energy and put it to good use helping push the car forward? i think if you wanted variable "kick" out of it, As long as you put the field on the low side of the applied motor voltage, you could put the small motor in parallel with the field of the large motor, and PWM it with a single beefy (like really beefy, 500+A) MOSFET or IGBT.

 

Does your motor have a stub shaft on the ass end? Some do, for an encoder or tach. If so, you could maybe cut a hole in your hood and let the little guy ride on top sticking up and out, like a blower in a ICE drag car.

 

I would ask the motor manufacturer for advice.

Then I would verify your assumptions. Can you measure and monitor the voltage, current and shaft position or rpm during a run?

A small processor and a few sensors will verify and plot.

I am assuming that there is no lifetime on motor like an engine has. If this is so, it’s great for experimenting.

You will easily be able to see the results of your timing adjustments.

I would suggest a good monitoring system before any action is taken.

 

The motor was originally a 48V, 200A GE forklift motor. Dennis Berube (look up Current Eliminator electric dragster) completely rebuilt the motor specifically for drag racing, and it now can handle 200V and at least 3000 amps. He has been rebuilding and tuning motors for drag racing for decades, and has held world records for as long. Even the brushes are his own chemistry. It is his advice that I am following to add shunt resistance across the field coils. I am certain that it will work and work well, I just want to modernize the resistor method with a controllable resistive load.

Yes, you start finding the speed to enable it, by measuring field voltage and amperage during a run and find the peak voltage.

The motor controller can dump data 10 times a second and has a nice set of data to use. The attached chart is a sample of what it provides. It shows a run made at about 65% power. The transmission was clearly slipping on the shift to high gear, and you can see the power trailing off as the rpm climbs.

I would ask the motor manufacturer for advice.

Then I would verify your assumptions. Can you measure and monitor the voltage, current and shaft position or rpm during a run?

A small processor and a few sensors will verify and plot.

I am assuming that there is no lifetime on motor like an engine has. If this is so, it’s great for experimenting.

You will easily be able to see the results of your timing adjustments.

I would suggest a good monitoring system before any action is taken.