wrote in message
...
On Thu, 29 Nov 2007 14:24:54 -0500, "Eisboch"
wrote:
wrote in message
. ..
On Thu, 29 Nov 2007 12:21:27 -0600, Vic Smith
wrote:
On Thu, 29 Nov 2007 13:12:52 -0500, wrote:
On Thu, 29 Nov 2007 12:05:58 -0600, Vic Smith
wrote:
On Thu, 29 Nov 2007 12:59:14 -0500, wrote:
Rare earth magnets?
Excuse my ignorance on this, but is that like Kryptonite?
--Vic
No.
Another big breakthrough in electric motor efficiency is PWM, or
"pulse width modulation" used for controlling the speed. In the old
days, you slowed down a battery powered motor by using a resistor. PWM
slows down the motor without wasting nearly as much energy. It offers
no savings at full throttle, but anything below that, it conserves
power rather than simply converting it to heat. This was a big deal
when they started using it for electric trolling motors about 20 years
ago.
That may well be what I read about. I was going to mention something
about "resistor" or "rectifier" but since I am so unversed on electric
components didn't want to muddle it up.
I do recall that the use of the innovation went beyond trolling
motors, and that it basically applied to all motors, but the article
gave emphasis on industrial use because of power cost savings.
--Vic
It's actually a pretty simple concept. Rather than reducing voltage
with a variable resistor to slow the motor, you maintain the voltage,
but pulse it on and off very rapidly. You vary the motor speed by
varying the duty cycle of the pulses between longer or shorter bursts
of power "on" or "off". The motor is pretty dumb and can't tell the
difference.
Silicon Controled Rectifier or "SCR". Two basic types ... phase angle
fired
or zero crossing. The zero crossing type generates less EMI and is
electrically "quieter".
They replaced the old variable resistance "rheostats" for light dimmers
and
induction motor speed control (such as in ceiling fans, etc.)
Not the same thing by itself. A silicon controlled rectifier regulates
the voltage up and down. In a PWM controller the output voltage (when
present) stays constant. In a 12 volt trolling motor, as an example,
the voltage being fed to the motor will be 12 volts regardless of the
speed of the motor. 12 volts at full "throttle" and 12 volts at 1/4
"throttle" The 12 volts is being switched on and off, and the output
of the motor is controlled by the relationship of the width of the ON
cycle to the OFF cycle.
Higher speed:
_________ ____________ _____________
|____| |____|
Lower Speed:
___ ____ ___
|_____________| |_____________|
To further expound on the Pulse width Modulation. The primary advantage
other than efficiency is that you get full torque at low RPM. The motors we
use are capable of huge over currents for short periods which is why the
electric drag cars use them. Think 2000Amps for Eight seconds. These
motors are series wound brush type motors. They are about as efficient as
you are going to get for reasonable money. There are also 3 phase motors
but the controllers get very expensive and you only gain about 3%
efficiency. Not worth it in my book. Other than that motors are pretty
much just plain heavy for the size needed. I have had several conservations
with sailors wanting to replace the old one lung internal combustion motors
with electric motors. To push a sailboat at hull speed takes so little
power that one of the motors we are using will push a 36' sailor. Some guys
are using even smaller motors.
The primary device used to control the high current application are power
mos fet"s (field effect transistors). They can be paralleled to get the
control power needed and they are easily controlled by low power
electronics. We are testing a new controller that we can connect the
control head to a computer to set up limits and various control functions.
Pretty cool stuff!
Tom