April 2008

Zilla FAQ29 Apr 2008 04:20 pm

I’m confused- how does the series to parallel switch happen and how does it enhance motor power and amp consumption?

Tucson AZ

Hi Rush,
Here is a snippet from the EV list some time ago. I’ve tried to edit it a bit for clarity.

EVList Post from 2004 follows:

OK. Let’s think about this a bit. Why did Otmar chose to shift at
1/2 the peak amps (and are we talking battery or motor amps,

Let’s assume it is *motor* amps that determine the shift point. When
the motor amps drop to 600A in series mode, and the controller shifts
to parallel, the motor amps stay the *same* provided there is enough
battery voltage available to push the controller back into current
limit. RPM doesn’t change instantaneously, so there is no stair step
in power; the shift to parallel just makes full pack voltage
available to each motor so that they can continue to build speed
while amps/torque continues to fall off.

I know Otmar is trying to follow this thread, so perhaps if I haven’t
put him to sleep already he can chime in with a proper explanation of
how/why the ‘Zilla implements series-parallel shifting…

Otmars Response follows:

I should mention to the uninitiated that this shifting involves utilizing the Hairball to drive contactors switching two motors electrically from series to parallel configuration. It is sometimes used in dual motor cars when trying to get the best acceleration out of a system while avoiding the higher cost of buying two controllers. It is mostly used with Z2K’s since high voltage Z1K systems do not gain as much unless the batteries sag quite a bit under load. In series mode the two motors can see up to half of the available battery voltage (don’t forget to take into account battery voltage sag) and each gets the full motor current. In parallel mode the motors can each be supplied with up to the full battery voltage but each gets only half of the available current from the controller. Parallel mode can allow both motors to draw more power when they are spinning at higher RPM.

The operation is pretty much just as you said. Initially I had the shifting happen at a certain adjustable RPM, but that’s inefficient on the street and does not compensate for battery temperature and state of charge, so I devised the auto shift algorithm and left the option of a manual shift for those who want to experiment.

When the Hairball is set to Autoshift mode it works like this:
It starts in series mode for maximum torque and for lower controller current in regular driving. Then, when you put the pedal to the metal the Hairball starts to pay attention.
In order to initiate a shift to parallel, the first requirement is that the controller is at 100% duty cycle. (this is the right side of the mountains shown below) This means the controller is full on the battery amps equal the motor amps. If the controller were still acting as a transmission then there would be no reason to switch the motors yet since the resulting power would be less after the shift.
After reaching 100% duty cycle, the motor and battery amps are dropping as the vehicle speeds up.

A simple way to view the HP to RPM curve of a series motor in a EV is to imagine it as a mountain. The upramp of the curve is the controller in current limit, the downslope on the high RPM side is when the controller is full on and the motor BEMF (Back Electromotive Force) is limiting power.

In order to maximize the power under the curve, I believe you want to shift when the falling series curve of motor power vs rpm crosses the rising curve of the motors in parallel. This turns out to be when the series mode current is half of what would be available after the shift.

Here’s a bit of ASCII art.
View with fixed width font like Courier.

Series Parallel Shift Standard

By the way, it’s interesting to note that these curves look same as the power curves seen when shifting a mechanical transmission. The big difference being that with a mechanical transmission the loads on the motors and controller are lower and so mechanical shifting is usually preferred.

Just FYI here are the same curves when the battery current limit is lower than the motor current limit. The lower the battery current, the less important the actual shift point timing becomes.

Series Parallel Shift Battery Limited

I hope this helps

Zilla FAQ07 Apr 2008 08:59 pm

Recently there have been a larger than usual number of people asking questions well beyond the usual “My car is a A, weighs B and has C components, which Zilla is the best one to use?”.

If you have been directed to this FAQ entry by our support department, then we did not have a simple answer to your design question. We are, of course, happy to answer questions about our product for those who buy direct from us, but have to limit our time expenditures for questions beyond that so we don’t get even farther behind on our production schedule.

As much as we would like to, we can’t help everyone design their EV conversion. Unfortunately, Cafe Electric llc. does not have the engineering resources available to guide the many people who are looking for help with basic vehicle design at this time. It takes significant resources to do this well; time spent in discussion with the user, calculations, and maintaining the ever changing knowledge of which components are available. There are at least a few people that we have heard of offering help with the design of EV conversions for a fee, but not knowing the quality of the offerings they will remain nameless at this time, let the seeker beware.

Here at the Cafe Electric llc. web site, we are attempting to build information resources that will assist people to make their own choices concerning the best combination of components. At first it will all be listed in this FAQ section. It is a slow process. As it grows we hope to find time to put it in a framework that we have for comparing the merits of various component options. This of course can never be fully complete, but we’ll endeavor to improve it gradually as our resources allow. In the meantime we will try to focus new FAQ answers on those questions that we see the most and feel are most important.

Many other resources are useful for designing an EV. We have a “Links” page that lists some valuable resources among which are: The EV List, an excellent place to ask specific questions. The EV List Photo Album, also a great resource since you can see what those who have gone before you have used in similar situations. The EV calculator is decent for constant speed calculations and rough estimates. The motor curves maybe based on the flawed formulas in the “build your own EV” book, so I never use the motor part of the calculator much.

Beyond that, there are many resources on the web. If you find a particularly useful one, we would appreciate hearing of it so we can add it to our links page.

Best of luck with the research!