Re: super series wheelslip control

[James Robinson <wascana@my-deja.com>]

"Rod [comtrain]" wrote:
> 
> James Robinson wrote:
> >
> > The radar can occasionally get confused by heavy rain or water 
> > between the rail, so the locomotive has a traditional wheel slip 
> > control system as a backup for these occasions.
> 
> The paragraph above is not incorrect, from a design point of view. 
> But it is not actually fact, in practice! Radar controlled wheel 
> creep is designed to work exactly in these wicked conditions, 
> and it does.

The radar system is not as reliable in everyday operation as you might
think.  The speed information it provides drops out frequently, even in
what could be termed normal conditions.  The system is designed to
tolerate intermittent signal loss, but in weather extreme conditions,
the dropouts can be long enough that Super Series has to resort to a
stand-by system. When I said the system is occasionally confused, it
meant in extreme weather conditions such as when there is standing water
between the rails, not just that the ballast is wet, when there is
torrential rain or heavy snow, or when there is a layer of solid ice on
top of the ballast.  The system will even lose the speed reference over
high trestles where the radar signal will just pass through the sleepers
and not bounce back to the radar head.  It does not happen that often,
but understanding that it can happen on occasion can help explain the
occasional performance problems.

> I should also add that whilst the wheel creep is operating, you cannot sand
> the rails. Locomotives so equiped, sand automatically, and more precisely
> than the human foot. 

Unless the 81s are set up differenly than other EMD locomotives, manual
sanding is typically locked out above a certain speed, usually about 12
or 15 km/hr, but the system will allow manual sanding at lower speeds,
even while the wheels are creeping.  The system will automatically sand,
when necessary, above the lock-out speed, but manual sanding will be
inoperative.

> at higher speeds, the wheel creep cuts out, and wheel slip controlls 
> are more conventional, and again you regain use of the sander.

It is actually the reverse.  Super Series is cut out below about 7
km/hr, since the doppler radar will not provide a reliable speed signal
at lower speeds.  Wheel slip control is handled by a back-up system at
these lower speeds that is similar to older wheel slip systems.  The
older system is less likely to sustain wheel creep than the Super Series
system, so tractive effort may not be as high at these lower speeds.  At
higher speed, Super Series will continue to function, and will allow
wheel creep if the conditions are right. I have seen sustained wheel
creep at speeds in excess of 80 km/hr. However, since the locomotive's
horsepower limits tractive effort at higher speeds, sustained wheel
creep is less likely than when making a heavy pull up a steep gradient
at 15 or 20 km/hr, when tractive effort and adhesion demand can be the
highest.