If you have the need for powerchair speed, how much are you willing to pay? Now, I should clarify,
the cost isn't in dollars – though, an upcharge cost is usually a factor, too. Rather, the precise cost
of speed to which I'm referring is in the performance efficiencies of “high-speed packages,” where an
increase in speed usually equates to decreased torque and range. |
Of course, to answer such a
question – how much torque and range are you willing to give up for the sake of speed? – we must first
discussed how powerchair technology specifically accomplishes high-speed packages.
packages” within the mobility industry typically designate alterations to a particular powerchair model's
motors and gearboxes (or, pulleys, on belt-drive systems), that allow one to order a faster version of
a powerchair's drive train than is standard. It is true that, historically, some manufacturer's “high-speed
packages” have included entirely different motors and gearboxes, visibly larger than the standard motors,
as in supplying a large 4-pole motor instead of a smaller 2-pole motor. However, most high-end powerchairs
today feature such large motors as standard that there isn't enough room on powerchair bases to add even
larger, faster motors, so the standard motors and gearboxes are “souped-up” for increased speed. Toward
this method, there are two common ways within the mobility industry to increase a powerchair's speed
without increasing the size of the drive train: Gearing can be changed to a lower numeric value, or the
motor's speed can be increased through internal changes. In either case, there are ramifications that
may – or, may not – be of ultimate consequence to a user's overall powerchair performance needs when
using a high-speed package.
Within a given powerchair drive train, the most direct way to increase
maximum speed is by changing the gearing within the gearbox. Like changing gears on a bicycle or automobile,
changing the gearing of a powerchair to a lower numerical value increases the drive shaft speed, but
simultaneously decreases torque. For example, if you change the gear ratio on a 7mph powerchair from
19:1 to 15:1, speed increases to 8.5mph, but torque and efficiency decrease – that is, the top speed
is faster, but there's less power to climb obstacles, and the overall system is working harder throughout
the entire performance spectrum, including running at top speed, so battery range decreases. Therefore,
changing a gear ratio will increase speed, but reduces torque and range in trade.
The second way
to increase the speed of a given powerchair drive train is by spinning the motors, themselves, faster,
without changing the gearing. To increase a motor's speed, it's engineered with less internal windings,
where the physical construction of the motor changes, allowing it to spin faster. In simple terms, if
the motor spins faster, it turns the gears faster, and the drive shaft speed increases. For example,
if a motor powers a drive shaft at 188rpms, equating to 7mph on a particular powerchair, but then the
motor is sped up so that it can power the drive shaft a 228rpms, the powerchair can then travel at 8.5mph.
A faster motor simply equals more speed. Yet, there are still drawbacks: Battery range is adversely
affected in that the motor then has less internal resistance, using more current, demanding more from
the batteries, decreasing overall range, and some torque is loss through the slimmer winding. In this
way, spinning a motor faster by altering its winding increases speed, but decreases range and torque.
At this point, you may be wondering, if both of these means toward speed have such obvious drawbacks,
then why would so many people choose, say, 8mph powerchairs over 6mph powerchairs?
is the answer. In the powerchair market, when a particular powerchair model offers a “high-speed package,”
it most often dictates some reduction in torque and range – but not as much as one might assume. Most
high-speed packages that are accomplished through gearing or motor alterations equate to a 10% to 15%
decrease toward torque and range. And, the ultimate consequence of such a decrease of torque and range
is truly relative to each user's needs.
If you use your standard speed powerchair in very rough
terrain, or find that even with Group-24 batteries, you're out of charge at the end of each day, 10%
to 15% less torque and range in a high-speed package would likely prove adversely noticeable in your
use. However, if you're very active around town or on campus, where extreme torque isn't often needed,
but speed is wished, and you usually don't deplete your batteries by the end of each day, the reductions
in torque and range of a high-speed package likely wouldn't be of perceivable consequence. The answer,
then, of whether a high-speed package right for you, must come from an understanding of your lifestyle,
and a consideration of what's most important in your powerchair's performance.
is, there's no magic equalizer to powerchair “high-speed packages” when it comes to speed versus torque
and range – choosing a high-speed package typically decreases overall torque and range by some extent.
However, whether or not the trade-offs of a high-speed package are of consequence is for you to decide.
Published 2/07, Copyright 2007, WheelchairJunkie.com