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RE: (ET) Solid state controls



The mention of reversing relays reminded me that I should mention something about reversing.  Sorry, this goes on a bit...
 
There are one-, two-, and four- quadrant solid-state controllers.  The one's can only supply voltage of one polarity, the two's can supply voltage of one polarity and do regenerative braking, and the four's can supply voltage of both polarities (i.e., reverse the armature current and therefore rotation) as well as do regen braking.
 
When I bought the 4QD controller for mine I was under the impression that the motor was fully compound wound, i.e., that the series winding made up a significant part of the magnetic field.  I therefore further assumed that I couldn't/shouldn't reverse the armature current to reverse the motor and that I'd have to reverse the shunt field like is normally done on an E12.  I therefore bought a two-quadrant controller.  I plan to use a "parking brake" output from the controller, which goes active when armature current goes to zero, to enable/disable switching of the reversing relay.
 
From later discussions on this board, however, I've learned that the motor is really more of a shunt-wound motor and that the series winding is just a "compensating" winding that weakens the magnetic field a little as load (current) on the motor increases to maintain speed.  This leaves me with the feeling that I should have used a four-quadrant controller and just left the shunt field always fully energized in the "forward" direction, which would have gotten me out of the messy reversing relay circuitry.
 
A nice feature of both what I intend to do messily or what a four-quadrant controller would do cleanly is that because of the built-in current limiting there'll be no harm in throwing the lever from full forward to full reverse; in either case the controller will limit currents to safe values and do the transition smoothly...Bob

>>> "Pieter Litchfield" <plitch attglobal net> 7/23/2002 4:06:44 PM >>>
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.


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