Monday, August 7, 2017

New Go-Kart!

The falloff at the corners is artistic, right?
Firstly, credit where credit is due. This project is a collaboration between myself, Ben Katz and Jared DiCarlo, with assorted contributions from Michael DeTienne, Nick Kirkby, and Fred Moore, and moral support from Austin Brown.

For the past year or so, a bunch of us at MITERS had been working on a new go-kart, built around a commercial racing kart chassis. The goal was to finally use hybrid car parts in a project which was stable enough to be fun to drive (the motorcycle was not very pleasant to ride, and the battlebot had rather serious technical issues). 

It all started with this and this. ORNL had done some tests on the hybrid starter-generator ('HSG') from the Hyundai Sonata/Kia Optima hybrids. The Sonata uses a unique hybrid system amounting to what is essentially a Honda IMA combined with a GM BAS, the difference being Hyundai's IMA pancake motor is much larger (30KW) and is capable of powering the car for a few miles in full electric mode. After dealing with remounting the rotors and stators from various Prius-derived hybrid transaxles, the HSG seemed like a dream come true - it had a housing, a water jacket, and even mounting feet.

Locally known as an 'Altermotter'
The motors are mounted via clamps to the frame and are watercooled using standard computer watercooling parts. Power transmission to the axle is via Gates Micro-V belts, soon to be replaced with PolyChain Carbon GT synchronous belts - maintaining belt tension on the 46" outer belt has proven to be quite the challenge, as the forces involved are enough to warp the frame.


The back view shows the big pile of electronics. Instead of an enclosure, the control boards are conformal coated, and the exposed HV busbar is covered in liquid electrical tape. This has done us remarkably well, even on grimy Cambridge roads. Logic is powered by a A123 12v7, which is good for about 4 hours of operation on a single charge (the converters inside the power module draw about an amp at 12V). Also visible is the rear brake (stock go-kart caliper with a 200mmx3mm Amazon moped disc) and the traction pack. The Amazon brake disc is made of some unknown alloy that discolors and warps severely under heavy load, and doesn't seem to be fit for any application.


The traction pack (42s LiPo) is made of 14 Admiral Pro 4000mAh in a 7S2P configuration. The Admiral Pro's have excellent performance when they work, but out of the original 18 we bought three have already failed - two had physically leaky cells and one has severely low capacity. The pack seems to work OK once the bad packs have been weeded out, but a 16% DOA rate is not really OK for what claims to be a premium battery...




Last but not least, a front view showing the steering and front brakes. The front caliper mounts are custom, as the cadet cart chassis we built around had no front brakes.

Key Specs:
  • Motor: 2x Sonata HSG, 20KW@270V, 50Nm@180A. Please don't read into these numbers like you would a hobby surface PM motor, as IPM's are very nonlinear in what they can achieve due to their high saliency and phase inductance.
  • Reduction: 3.5:1 using 7-groove Gates Micro-V belt (~25mm wide) with 4.5/10.0-5 racing slicks.
  • Inverter: in-house FOC firmware running on a pair of STM32F446RE's, driving the power stage out of a Gen 2 Toyota Prius.
  • Battery: 42S 8000mAh LiPo, conversion plans are in progress for 72S/5300mAh automotive cells.

Upcoming Posts:

Whew, this is a big project! Posts will be broken into two categories, construction and science, including...

Construction
  • Motor mounting
  • Brakes, including front brake conversion
  • Water loop and inverter block
  • Wiring and connectors
  • Circuit boards and resolver decoding
  • Chasing down noise
Science
  • IPM specific motor theory
  • Analytic stall torque computation from motor parameters
  • Optimizing high-speed operation
  • Real-world stall testing
  • Real-world motor tuning, including the variety of bugs we ran into

2 comments:

  1. Excellent. How are you charging the 42s pack (and subsequent 72s)?

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    Replies
    1. I charge it as a stack of separate 6S packs with several 6S hobby chargers

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