Published 11/01, Copyright 2001 ,

What a Trip:
Powerchair Circuit Breaker Dynamics
by Mark E. Smith

When I was twelve, I had a Fortress powerchair.  At the time, the early 1980's, it was among the most capable chairs on the market, featuring direct-drive, 10" casters, and a pivot-beam front end.  It was slow - 5mph or so - but offroad, it beat any chair hands down.  However, as is too often the case, the chair featured one fatal flaw:  Upon ascending the slightest unpaved, uphill grade, the circuit breaker tripped, leaving me with no choice but to sit and wait while she cooled down and could be reset.

Recently, on the Message Board, I've read several posts from people frustrated by tripping circuit breakers, so, unfortunately, unlike the pop band, New Kids on the Block, poorly engineered powerchair systems didn't die with the 1980's.  

To understand circuit breakers, and why they trip, you must first understand powerchair electronics.  Let's say a chair has a 50A controller, with a 35A breaker.  The controller is channeled, meaning it has a 100A peak draw under extreme load, splitting 50A to each motor.  The breaker, as well, sees 100A peaks traveling through it, but, because it is thermally-related, it won't trip until 35A+ is sustained for a duration of time, building heat - that is, the breaker will handle a full load, till it heats up, then it trips.  

Now, the chair's design and its type of use plays a role in the interaction between the controller and circuit breaker, and whether the breaker is prone to tripping.  A low-end chair intended for mostly indoor use could get by with a 50A controller and 35A circuit breaker if used in that tame environment, never tripping the breaker, as the chair would never sustain long enough peaks of current.  Yet, in a rougher outdoor setting, the chair may sustain long peaks of current, frequently tripping the breaker.  On the other hand, a chair designed for outdoor use, featuring a 70A controller and 70A circuit breaker, can sustain higher current peaks without stressing the breaker, decreasing the likelyhood of tripping.   (And, in practical use, if a breaker's rating is on par or close to the controller's, the chair will most likely fold back, reducing power within the controller, before the breaker trips - both measures protect the system from prolonged current.)

The question is, then, which protection mechanism does the designer wish the system to rely upon first for thermal protection, the circuit breaker or controller fold back?  In my opinion, controller fold back should occur before the breaker trips, namely because it keeps the chair operable.  When the breaker trips, your chair won't move, period.  However, when fold back occurs, you can still drive the chair, though at reduced power.  I'll take some mobility over no mobility any day (think about crossing an uphill street - if the breaker trips, you're stuck in the middle of the road, but if fold back occurs, you can still get out of harm's way).  

Lastly, the intention of controller fold back is internal thermal protection, and the circuit breaker primarily protects against catastrophic shorts, so from a design standpoint, allowing each to serve their roles offers you better performance than using a circuit breaker as an end-all.

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