Not Everything Works the First Time: EVTV's Dual Motor Experiment and Its Surprising Outcomes

Interesting show this week. Mostly a rundown on our additions to the EVTV test bench, which is coming along quite nicely.

We've added a control panel for the generator, with a contactor, current shunt, control switch and voltage/current displays. This allows us to turn on the generator or leave it off, depending on what kind of load we want to put on it. And we can measure the current back into the battery pack.

I'm once again disappointed to report that we get less power OUT of our perpetual motion machine than we put in. But it gives us a bit of a load we can cut in. I'd kind of like to add a PWM circuit or controller to this eventually so we can dial in just how much of a load. But the 150 v battery pack doesn't present much of one actually. We have to turn the generator up to a couple of thousand RPM to really get 80 amps or so out of it.

We also added a control panel to the input side, quite a bit more extensive. It has a Speedhut tachometer with the RECHARGECAR magnetic pickup to display RPM. We used a handheld laser RPM counter to check it and it probably reads about 50 rpm low - very acceptable. We're using this at 4 pulses per turn and it is working well with the tach.

We've also added a vernier 5K potentiometer to act as our throttle control. And we use a precharge resistor/contactor/shunt box originally used on our first Speedster. The control panel has a switch and a light to turn this power on. And a 1000 Amp 50mv meter that displays the current through the matching 1000 amp 50mv shunt in the box.

This gives us a little extra safety. We can of course turn the ignition voltage off to the controllers from the panel, but we can also cut off the power through the contactor. Turns out to be a handy feature as you'll see in the video.

We wired up the two Soliton1 Controllers to the two Netgain Warp 11 motors. Therein lies a tale but also one of the reasons for the test bench during the Escalade conversion.

The EVnetics Soliton1 is a relatively new controller and has some very interesting features. Most importantly to this application an IDLE function that maintains a low level RPM control on the motor with the throttle off. We know of no other controller with this feature.

Normally, with Siamese motors, a series/parallel two speed electronic shifting system is used. This requires a lot of contactors, and at the powers we are applying, that is a bit of a problem.

The advantage is of course that you can apply the full current from the controller through both motors at low speeds. This allows you to get maximum torque from both motors since in series they both get ALL the current put out by the controllers through their series armature/field windings.

The disadvantage is that it drops the voltage applied across each motor in two. So a 192v pack putting out 1000 amps would put a little over 95 volts to each motor at 1000 amps or 95kw each for 190kw. But as the motors turned up in speed, they generate back EMF (electro-motive force). Think of them as also acting as generators in the reverse direction. These motors normally would start to drop in torque at 3600-3900 rpm at 192 volts but with half the voltage, the torque drop off from BEMF would also decrease - maybe 2500 rpm or so.

And so once your vehicle is up to speed, you would shift into parallel mode. In parallel mode, the current output of the controller is applied to each motor separately, or in parallel. The advantage here is that each motor receives full voltage and so can move the torque drop off back up to 3600 rpm. The disadvantage, of course, is that each motor only sees 500 amperes maximum. This really is NOT a terrible disadvantage, because by the time you are going down the road at 2500 rpm, your need for power is quickly diminishing in normal driving, and 500 amps is generally a lot, particularly from two motors on the same shaft. It's still 192kw.

I don't like the system. The problem is:

Complexity

Failure items

Shift points

Operational complexity

The wiring is simply more complex with several contactors required. The system is of course switched with the car underway and so the contactors have to break some current. The contactors can do that - up to a point. Generally we use contactors ( a misnomer really for a high current capacity relay) to apply power or remove it but in normal operation they are not carrying ANY current at the time. You close it when you start the car. You generally open it when you shut it off. And you are parked both times and drawing near zero current.

In a pinch, you can use the contactor relay to break current in an emergency and shut down the system. The Kilovacs we use, can break up to a couple of thousand amps - about once. And they DO fail. Ergo the mechanical disconnect switch backup. There have been real incidents of contactor relay FAILURE to break current in a high current situation.

It certainly decreases their life expectancy when they are opened with significant current and this is spelled out quite graphically on the data sheet of the contactor. The mechanical life is a million cycles. At 200 amp current you get 12 cycles. At 2000 amps you get 1 cycle, maybe.....

So they become failure items.

Then too, you have to have some means of initiating all this shifting. And indeed, THEN you have to actually do the shifting. This of course COULD be automated. But now we have a controller and series of relays to control our contactor relays and so complexity builds again.

How about we just use two controllers.

We put one controller on one motor and the other controller on the other motor. Now each motor can have the full 192v AND the full 1000 amps all the time.

Of course, we have the cost of the controllers. And then they have to have pretty similar outputs. You would think they would have to have exactly matching outputs or the two motors would fight. I didn't think this was the case. Both are applying torque in the same direction on the same shaft. Even if one was applying half torque while the other was applying full torque, you should get the sum of the two torques. There is no war going on here unless one is commanding it backwards.

But I had never heard of anyone doing it.

Basically, it ought to work. The outputs of the controllers really can't feed each other, they are on entirely separate sets of windings. The only common point would be the batteries and the control inputs. But having never tested it, it was kind of a theory, not a knowledge. As I stress over and over, your EV does not care IN THE SLIGHTEST what you THINK about it's operation. Your theories might entertain you, but the car just does not give a shit. It will follow the actual laws of physics as it interprets them, which is generally a might differently than how YOU interpret them.

Ergo the test bench.

And we did learn quite a bit rather quickly. One is that the output of the magnetic pickup is probably two light for two Solitons's and a tachometer. Our Soliton's were giving some erratic and very erroneous RPM readings - generally 100-150 rpm high, but also not very stable. I might be able to dress this up with some resistance value across the output. We'll have to play with that.

We did cal the two controllers to fairly precisely measured voltage outputs from the 5K pot. We tied the 5v and signal grounds together, and then the 5v signal as well. And calibrated each controller separately for 1.00v min and 4.00v max. That gets us off on the right foot. The motors turned very smoothly and had no apparent problem working out the torque sharing at any rpm. Even noise potentially fed back to the input just wasn't a problem. The controllers obviously have some capacitors on the input to smooth things a bit and we might be able to augment that (future Top Secret video). But it doesn't seem to be a problem at all.

The other target of opportunity was the idle. Obviously idling we don't NEED two motors and two controllers. To turn the transmission pump, the steering/brake pump, and an air conditioning compressor should only require 2 or 3 horsepower. So EITHER motor could be used. So we set up ONE controller with idle and the other without.

The idle function in the Soliton1 is pretty cunning. It uses a PID algorithm to seek the target RPM and provides whatever current is necessary to get there - up to a limit you can actually set separately. I like the design.

This simple concept, maintaining RPM through the controller, is actually a black art and heinously problematical. It looks easy, but any correction tends to overshoot, and cause another error input, which causes another correction, which of course overshoots again. The cycling can hit all sorts of resonances and self enhancing oscillations. Generally lumped under the term hysteresis.

This from Wikipedia:

"A proportional–integral–derivative controller (PID controller) is a generic control loop feedback mechanism (controller) widely used in industrial control systems – a PID is the most commonly used feedback controller. A PID controller calculates an "error" value as the difference between a measured process variable and a desired setpoint. The controller attempts to minimize the error by adjusting the process control inputs.

The PID controller calculation (algorithm) involves three separate constant parameters, and is accordingly sometimes called three-term control: the proportional, the integral and derivative values, denoted P, I, and D. Heuristically, these values can be interpreted in terms of time: P depends on the present error, I on the accumulation of past errors, and D is a prediction of future errors, based on current rate of change. The weighted sum of these three actions is used to adjust the process via a control element such as the position of a control valve or the power supply of a heating element.

In the absence of knowledge of the underlying process, a PID controller is the best controller. By tuning the three parameters in the PID controller algorithm, the controller can provide control action designed for specific process requirements. The response of the controller can be described in terms of the responsiveness of the controller to an error, the degree to which the controller overshoots the setpoint and the degree of system oscillation. Note that the use of the PID algorithm for control does not guarantee optimal control of the system or system stability.

Some applications may require using only one or two actions to provide the appropriate system control. This is achieved by setting the other parameters to zero. A PID controller will be called a PI, PD, P or I controller in the absence of the respective control actions. PI controllers are fairly common, since derivative action is sensitive to measurement noise, whereas the absence of an integral term may prevent the system from reaching its target value due to the control action. "

The Soliton1 allows you to individually specify the proportional, integral, and derivative values. I wouldn't have a clue if you e-mailed them to me. I took the defaults. It works pretty well. By cutting in and out the transmission and the generator, we could vary the load. And while the Soliton can't accurately measure RPM, it did a good job of maintaining it.

All of this scratches my ongoing itch for subtle ironies, which I mostly use to entertain myself. In this one case, I'll share. A Soliton is a standing wave, first observed in a canal of water. The soliton phenomenon was first described by John Scott Russell (1808–1882) who observed a solitary wave in the Union Canal in Scotland. He reproduced the phenomenon in a wave tank and named it the "Wave of Translation".

I rather associate this with the wavy pattern in the Soliton heat sink. I would have named Soliton Jr. the Compacton instead, but there I go.

In any event, this PID idle control was why we selected the Soliton1 for the contest and the Escalade, NOT the purported 1000 amp output. And it appears to work very well. I think it's a unique feature they almost added as an afterthought, but promises to differentiate their product from most others. Simply holding a throttle input with the A/C kicking in and out, and the transmission cycling and who knows what else would not really make this work in any satisfactory fashion. It would have required an entirely additional circuit just to control the controller had we wanted to do an automatic transmission without it.

The one fly in all of this is that to START the idle, you first have to blip the throttle past your target RPM. I don't like this and I do not think it is necessary. It might be salutary to have a separate input to start it. In this way, we could use the START signal, separately from the ignition signal, to start the idle. But if the Soliton powered up on an ignition 12v input and established idle after a brief delay, there really isn't an issue here. Automatic transmission vehicles really only let you start them in Park or Idle anyway. They did not need to take this "safety issue" on themselves.

The problem is that it makes operation of our car nonstandard. In an ICE engine vehicle with automatic transmission, you turn the key and the engine starts and idles. Period. In our Escalade, we'll have to turn the ignition key and then give it some throttle to "start it." How do I explain this useless feature to my daughter. She'll turn the key. Nothing will happen. And she'll get out of the car and ask me why it is broken.

I think we should build our cars where they operate as expected. These standard operational issues were worked out over the past 100 years without any input from me, and I don't think they need to be reworked by the crew at EVnetics.

The controllers and motors worked quite well on our test bench. Unfortunately, the transmission somewhat less so. We had it completely full of very good transmission fluid. We had no external heat exchanger but no intention of operating under any serious load, for any appreciable length of time, or at anything over about 2500 rpm. But Matt noticed early on some heating of the shell, which I measured at a peak of 140F. This is hardly warm by transmission temperature standards, but we were on the other hand hardly turning it. At one point, I pulled 80 amps out of the generator - maybe 14 horsepower - through a transmission purportedly capable of handling 800 horsepower.

But it appears to have failed anyway. At about 2000 rpm while filming, it suddenly threw on its own load and started a rather noisy vibration from within. We quickly shut off the system. Restarted it at VERY low RPM's and was immediately able to isolate the problem to the TCI transmission. We'lll be contacting them to see if they have any thoughts on the topic this morning.

You'll no doubt enjoy the onscreen panic that ensues. Kind of a KeyStone Cops meets the transmission shop.

Jack Rickard