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17' Mahogany runabout just completed
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 |
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