Nick Kirkby

Since last time, I paralleled the two halves of the Prius inverter in an attempt to squeeze more peak current out of it.


This was a last-ditch effort to test:

  1. if the half bridges are capable of being paralleled without getting too sad, and
  2. if the resulting (up to 50%) gain in peak current before tripping overcurrent protection would result in satisfactory power for driving around.

Yes on 1.  The half bridges paralleled fine.

meh on 2.  Performance still kind of sucked.  Top speed was about 45 MPH and acceleration was very unimpressive.  In spite of this, it was still possible to rev up the motor, drop the clutch, and spin the tires a bit.

Bottleneck of Inadequate Performance

The graph below shows the operating voltage and current ranges of the Prius inverter and the old GE brushed motor.  Shaded area represents the power that each is capable of handling before going pop or catching fire.  The rectangles have similar area because the two parts are similarly capable of handling the amount of power it takes to move a small truck under normal driving conditions.  (~150kW peak, 40kW continuous).  Unfortunately, the available power is represented by the area in which the two rectangles overlap.  The Prius inverter is designed to supply 500 volts, but at that voltage the motor would have been flung apart due to overspeeding, if the commutator hadn’t already been destroyed by arcing.  Conversely, the motor can take 1000A without burning up, but the overcurrent protection on the beefier half of the Prius inverter prevents it from ever supplying more than 400A.

motor inverter powerr

The result is that the peak power available ever is about 40kW.  This is when the motor is spinning at its rated top speed, and when the Prius inverter is pushing close to 400A, hence the bottleneck of inadequate performance.

The nominal voltages of the motor, inverter, and battery are representative of the time of their design.  Useful voltage ranges have shifted over the history of electrical energy storage and power electronics with the gaining of stuff like lithium batteries and reliable 1200V IGBTs.  For a multitude of reasons (mostly under the blanket-reason of cost reduction)

Proposed Solution 1: Replace the motor

I heard about the rear electric motor in the 4WD variant of the Toyota Highlander Hybrid from a friend.  It’s a 50kW 3 phase IPMSM, air cooled.  It is appealing because it is standalone, not attached to a bulky transaxle like the motors are in the Prius and Volt (and the front motor in a Highlander).  It is a single motor with a geartrain attached to it, ending in an open differential.  I suspect that traction control is accomplished by selectively braking the left and right wheels, as is the case in the Tesla drive units which also have open differentials.

From this thread on diyelectriccar, I learned the specifications and estimated the motor constants from some BEMF captures provided by a forum user.  The inputs and results are reproduced here:



I bought this one on ebay for $160 and picked it up at a parts yard about 1.5 hours away.


Proposed Solution 2: sell the truck

I’d been wanting to leave CA for about 6 months and it was looking like I’d be here for another few months before the getting the truck roadworthy enough to try driving away in it.


Remarkably, it sold, and I kept the battery, the inverter, and the little ECU I made.


Embrace of Greater Delights

the Promised Land didn’t quite suit me this time around, so I loaded up the goods and took a road trip back to the east coast.