Re: (ET) Solid state controls

["Christopher Zach" <czach computer org>]

Actually, on the E15 you are *not* reversing heavy current. Remember: On a
shunt motor the "big" windings are in the armature. The field currents are
quite a bit smaller; that's why they aren't really on solenoid and can be
weakened with relatively small resistors. For a demo, look at the FW
resistors as opposed to the 1A/2A ones for speeds 1-2.

Thus they could get away with a simple relay. However there is a catch...

The catch is that when you throw the field into reverse while the armature
is still going in fwd, the armature is going to have a nice big magnetic
field suddenly going opposite the (reversing) field. This is going to cause
one heck of a spark when the relays close on the field, and will blow out
the relay points rather quickly. On the E20 the field is kept locked
(weakened at worst) while the armature changes polarity. The contacts are
much bigger (they have to switch in 100+ amps), the speeds you go thru are
buffered by the resistor, and to be honest the back-EMF is absorbed by the
FW circuitry. I guess at worst you would blow a fuse in the FW system.

An interesting note: Since the motors aren't true shunt motors, they have a
small field circuit in series with the armature. When you reverse either 
the
armature and the field, you wind up with the shunt field and the series
field running in opposite directions, and somewhat cancelling each other
out. This is why:

1) The tractors go "faster" in reverse. The field is already slightly
weakened in reverse due to this effect
2) There is less power in reverse. Also the more you load down the tractor
in reverse, the more powerful the series field becomes, and the less power
the motor develops (as the series field cuts into the shunt field)
3) The FW circuitry is cut out in reverse. If you did happen to try and
weaken the field, you would probably burn out the motor.The advantage (and
this is an interesting one) is

Neat stuff.
Chris


----- Original Message -----
From: "SteveS" <ssawtelle fcc net>
To: <elec-trak cosmos phy tufts edu>
Sent: Tuesday, July 23, 2002 5:23 PM
Subject: Re: (ET) Solid state controls


> I'm enjoying this thread, learning lots.
>
> Why would the E15 system be cheaper? On the E15 you are reversing heavy
> current loads, wouldn't the solenoids for that be much more expensive 
> than
> the relatively lightweight (though more complex)E12 relay?
>
> SteveS
>
>
>
> ----- Original Message -----
> From: "Christopher Zach" <czach computer org>
> To: "Pieter Litchfield" <plitch attglobal net>; <>
> Sent: Tuesday, July 23, 2002 4:38 PM
> Subject: Re: (ET) Solid state controls
>
>
> > Yeah, the E15 does things differently than the E20. On the E20, they
> reverse
> > the armature current using a pair of push-pull solenoids. On the E15,
they
> > reverse the field current using a DPDT relay.
> >
> > The problem is when you go from fwd to reverse in the E15, the back-emf
> from
> > the armature surges the contacts and burns the daylights out of them.
When
> > you go from fwd to reverse on the E20, the arc is absorbed by the (much
> > larger) solenoid breakers. Plus the field current is relatively weak
> anyway.
> >
> > But the E15 design is a lot less expensive; I guess that's why they
> migrated
> > to it.
> >
> > Chris
> >
> > ----- Original Message -----
> > From: "Pieter Litchfield" <plitch attglobal net>
> > To: <elec-trak cosmos phy tufts edu>
> > Sent: Tuesday, July 23, 2002 4:06 PM
> > Subject: RE: (ET) Solid state controls
> >
> >
> > > As the owner of the E-15 with bucket loader that loves to eat relays,
my
> > > suggestion would be that whatever controller is designed, be sure it
can
> > > withstand the abuse hard use will heap on it.  For example, my
tendency
> to
> > > rapidly shift from reverse to forward while using the bucket has had
> dire
> > > consequences for my relays.  A better design could improve on this
> > behavior,
> > > or at least prevent the idiot behind the wheel from making the quick
> shift
> > > ( a timer or charge-up delay circuit?)
> > >
> > > As a gross observation, my E-12 while not as elegant a control design
> > seems
> > > far more robust than the E-15.  At least I haven't cooked relays 
> > > there
> > yet.
> > > But this does suggest that a prime characteristic of a power control
> > system
> > > for an E-15 ought to be "robust-ness" and "fault tolerance."
> > >
> > >  -----Original Message-----
> > > From: owner-elec-trak cosmos phy tufts edu
> > > [mailto:owner-elec-trak cosmos phy tufts edu]On Behalf Of Bob Murcek
> > > Sent: Tuesday, July 23, 2002 2:11 PM
> > > To: elec-trak cosmos phy tufts edu; ssawtelle fcc net
> > > Subject: Re: (ET) Solid state controls
> > >
> > >
> > >   Steve,
> > >   No, reducing speed with a soild-state controller does not cause 
> > > loss
> of
> > > torque.  On an E12 at least the 1st and 2nd "speeds" are created by
> adding
> > > resistors in series with the armature circuit.  When the armature
> current
> > > tries to increase, say to go up a hill, the voltage drop across the
> > > resistors increases, causing the motor to slow down.  Since the power
> lost
> > > in the resistors is wasted, resistor-based speeds should only be used
to
> > get
> > > going smoothly.
> > >
> > >   Solid-state controllers turn the power in the armature circuit on
and
> > off
> > > rapidly (not sure of the rate, but it's apparently supersonic in
mine),
> > > varying the ratio of the on time to the off time to control the
average
> > > voltage seen by the motor.  There's very little waste since the
> > solid-state
> > > switch is either on or off.  When you go uphill with a solid-state
> > > controller and the armature current tries to increase, it's free to 
> > > do
> so
> > > during the times when the controller is in the on state, so a 
> > > slowdown
> > > doesn't occur.
> > >
> > >   Possibly the biggest advantage of a solid-state control in an ET is
> the
> > > extremely fine and smooth control at very low speeds, like when 
> > > taking
> up
> > a
> > > load or parking in a tight spot...Bob
> > >
> > >
> > >   >>> "SteveS" <ssawtelle fcc net> 7/23/2002 1:39:55 PM >>>
> > >   Ah, good explanation. I see now how it makes sense on an ICD mower.
> With
> > > my
> > >   E12S, with 'only' 3 speeds forward X 4 gears, I still have pretty
much
> > all
> > >   the control I need. I do find, however, that any speed less than
full
> > >   throttle has poor power. I can climb a hill in full throttle that
> stalls
> > > out
> > >   on lower settings (same gear). That seemed illogical at first, but 
> > > I
> > > assume
> > >   it's because the motor has less than full armature current. Does 
> > > the
> E20
> > >   have the same characteristic?
> > >
> > >   I presume a solid state control would have the same effect (lower
> > settings
> > >   for slower speed sacrifice power as well)?
> > >
> > >   SteveS
> > >   E12S
> > >
> > >
> > >   ----- Original Message -----
> > >   From: "Christopher Zach" <czach computer org>
> > >   To: "SteveS" <ssawtelle fcc net>; "Elec-trak" <>
> > >   Sent: Tuesday, July 23, 2002 12:49 PM
> > >   Subject: Re: (ET) Solid state controls
> > >
> > >
> > >   > Hydrostatics are nice on an ICE based mower because you usually
have
> > to
> > >   run
> > >   > the engine at full speed in order to keep the blades spinning.
> > >
> > >
> > >   -- snip snip snip
> > >
> > >
> > >
> > >
> >
> >
>
>
>