wrote:
>That interesting that the pump is only 1/4 HP, or so. That was what I
>initially estimated, but I read other things on line that suggested it
>was 2-5 HP. One reference actually suggested that removing the PS
>pump gained someone 26 HP on the dyno. Maybe at 6000 RPM that might
>be true.
One can read the silliest things on the net
Based on many hours spent at
the console of a dyno, I can describe a scenario where that would seem to be
the case but wasn't.
>
>If it was 1/4 HP then at 12 V it should draw less than 20 Amps. The
>Toyota MR2 pump requires a 75A fuse. That would be closed to 1 HP.
Remember that HP is simply a measure of the rate of doing work. My 1/4hp
motor gets the job done because my press does its work quite slowly. The pump
itself uses practically no power, only the small frictional losses. It's just
a work translator. If you need to do work rapidly (turn the steering wheel
rapidly while stationary) then there must be more work - HP - input to the
pump.
There are two considerations. One, how fast the work must be done, usually a
consideration at very low to stationary speed and during emergency maneuver.
Two, what maximum steering wheel velocity must be supported. That is a
function of fluid volume flow.
To separate the two concepts, consider a limiting condition - the car is up on
jacks and you spin the steering wheel rapidly. If there is to be any assist,
the pump must supply the necessary volume to keep up with the actuator inside
the steering box. There is little to no resistance to the movement so the
pressure will not be very high. High volume, low pressure.
Now take the opposite situation. The car is stationary. This is the highest
tire friction situation. Even if you turn the wheel slowly, the amount of
force necessary to turn the tires is high and therefore the pump must supply
high pressure, albeit at low flow.
Both of these limiting conditions are low HP situations. The first is high
flow, low pressure and the second is high pressure low flow.
Now consider the third limiting condition, turning the wheel rapidly with the
car stationary. Now the pump must supply both high flow and high pressure.
This is probably the highest HP situation.
In fact, it's not unusual for a power steering system not to be able to handle
this situation, particularly at engine idle. As you turn the wheel faster and
faster, a point is reached where the pump is supplying the maximum flow it is
capable of. Trying to turn the wheel any faster results in great resistance,
as there is no longer any assist. Revving the engine - spinning the pump
faster so that it can supply more volume - remedies the problem.
A similar situation can arise at speed during an emergency or otherwise rapid
maneuver if the pump isn't driven fast enough. As you yank the wheel,
suddenly it feels like a brake has been applied to the steering shaft. The
power assist unit's flow demand has exceeded the pump's capability and
suddenly there is no assist. This can be quite disconcerting when one is,
say, controlling a skid or swerving to miss a road obstacle.
Where I've seen this situation most often is where an amateur hotrodder
installs a "power pulley", a smaller diameter crankshaft pulley, believing the
hype that turning the accessories slower will somehow unleash untold
horsepower. It does free up a tiny bit of power but at the expense of this
steering effect, engine overheating from lack of coolant flow and an
alternator that can't keep up with electricity demands at idle and just above
- such as at cruising speeds.
To electrically drive the pump properly, my starting place would be to
duplicate the IC engine drive. Figure out a typical cruising engine RPM, the
work back through the pulley ratios and determine how fast the PS pump is
being spun. The underlying assumption is that the OEM is spinning the pump
fast enough to handle any anticipated emergency maneuver.
The power demand when supplying an emergency maneuver will be quite high,
perhaps one of the highest. Turning a nearly stationary tire will require a
bit more force (pressure) but not necessarily more speed (volume). It is
acceptable to hit a steering wheel speed limit when turning the wheel while
parking but it isn't when trying to miss that debris in the road or dodge the
guy who just pulled out in front of you.
What that means is that though the average power demand will be low, the
driving motor has to be capable of supplying the high peak power necessary
during these maneuver situations. That's why the Toyota pump is fused at such
a high amperage, even though the idle draw is a fraction of the peak.
A PM (as on the AVS bus) or a series motor can do that as long as the
electrical supply can hold up. It may be a problem if you're running a DC/DC
converter with no aux battery. AVS solved the problem by using a high voltage
motor and running it at (near) the pack voltage. I haven't gotten deeply
enough into their system yet to know whether the motor tapped the traction
battery directly or if there was a voltage regulator involved.
When you're stationary or cruising along in a straight line, barely moving the
wheel, the power demand will be low, I'd guess significantly under 200 watts.
However, yank the wheel hard and fast and the power demand may peak at several
HP. Obviously the peak won't last long, probably not even a second, so a
flywheel on the motor might help a lot.
The easy way to do this is to copy what someone else has done. It might not
be the most energy-efficient method, though. This is an area, along with the
AC compressor drive, where I'm going to be experimenting fairly intensely over
the next few months. I want to have both system designs completed BEFORE I
start the actual conversion so that the conversion will be little more than
bolting together pre-prepared assemblies.
If you want or need to get ahead of me then we can work together on the
testing protocol.
John
--
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com <-- best little blog on the net!
Tellico Plains, Occupied TN
Unable to locate Coffee -- Operator Halted!
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