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17' Mahogany runabout just completed
"Eisboch" wrote in message ... 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.) Going back to Vic's question .... I think it was GE that introduced a super efficient induction motor years ago for use in refrigerator compressors, etc. I don't remember what made them more efficent though. Eisboch Most of the AC was controlled by Triacs. And AC SCR. Sort of dual SCR's. PWM is how much time the voltage is applied vs. how much off time. Should have patented the circuit when I did my senior project in engineering at university. Was a PWM dive light with a 555 timer controlling it and magnetic switches for on off and intensity. In DC you will always get full power when the power is on. On AC you can only allow power during the higher voltage part of the sine wave and gets lots of torque. Was a design out years ago, in the 1960's for drill speed controller that used SCR in a Triac mode and at low speed, the drill would about rip your arm off if it caught. Most of the modern motor controllers (Texas Insturments was the leader in parts supply) basically keep a better phase angle, and temps down via PWM and when the pwoer is applied. |
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