|
Most PWM controllers use 200 to
2000 Hz (cycles/sec) so that the period of each pulse is less than the inductive
time constant of the coil. The net effect is that as far as the coil is
concerned, changing the duty cycle gives a change in the effective DC current in
the coil.
That said, such concepts as
field weakening still can be effected in a full PWM control. The PWM
changed the effective current, which to the coil is just a change in DC
current/voltage. I have yet to see a PWM control for this class of vehicle
that also implements field-weakening. It is more effecient, but people
don't bother. There are other 'solid-state' controls as well, mostly
relay/contactor replacements.
Larry Elie
-----Original Message-----
From: Bob Murcek [mailto:RMURCEK geisinger edu] 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 |