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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work. |
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#41
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Another sparkies question about generators
On Thu, 28 Aug 2003 14:43:06 -0700, Eric R Snow
wrote: The motor I tested that comes closest to what I want is 2.25 long and 1.5 dia.. At 10,000 rpm it was putting out 9 volts @ 1.5 amps. It was $10.00 at H.F.. It came complete with a drill chuck, batteries, charger and a drill motor case wrapped around it. For 8 bucks they have one with a keyed chuck. If two were connected in series then spun to 15,000 rpm max then with a cheap regulator the batteries could be charged and the lights lit. Now, if a similar sized motor came along with a higher voltage output so only one would be needed then that would be great. I'm curious to see what Ax-Man has. They had several different sizes, including a couple of 24-volt motors. Prices ranged between $5 and $10 for the smaller ones -- sizes like you describe and a bit larger. In general, the larger the diameter you can use the more volts per rev you'll probably get because there's more room for magnets and peripheral velocity is greater for given RPM. I didn't see any 90-volt PM motors this time. If you could find a power seat motor at a junkyard, that might work very well. I have one here that's 2.56" dia and would be a little under 4" long if one cut the wormgear drive off or made a new endcap. It turns about 113 RPM thru 44:1 worm reduction, which makes me think it would generate pretty close to 12 volts at about 5000 RPM. The armature resistance is only 0.175 ohms so IR drop would be very low. I think there's be no problem getting several amps out of it. It draws about 15 amps before it starts slowing down noticably. AxMan had some windshieldwiper motors with gearbox attached, but they wanted way too much for them -- $19.95, I think, and you don't need gear reduction. |
#42
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Another sparkies question about generators
Jim Pentagrid is correct that the power will be much less if the
speed and field are the same. But you can either increase the speed or field to make the voltage the same. Dan (Dan Caster) wrote in message m... It will be good for the same amount of current and therefore the same amount of wattage if the voltage is the same. What are you trying to do and what kind of motor are you thinking of using? Induction motors will work well especially if you use the grid to excite them. Dan |
#43
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Another sparkies question about generators
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#44
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Another sparkies question about generators
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#46
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Another sparkies question about generators
On 29 Aug 2003 10:30:24 -0700, jim rozen wrote:
In article , Gary Coffman says... On Fri, 29 Aug 2003 00:51:51 +0100, wrote: If we now run this machine as a generator AT THE SAME SPEED it will generate the SAME 80v EMF. Note that this 80v is an open circuit voltage which is the result of the rate at which armature conductors cross the flux linkages, it is independent of the value of the armature resistance. However it has to pass through the armature resistance before reaching the output terminals At the maximum permissible 1A load current, 20v will be lost leaving a net loaded output voltage of only 60v = 60w i.e 60% of the rated input power of the same machine used as a motor. Right, but not 60% of its input power as a generator. Efficiency is still 80%, you've just arbitrarily limited the amount of input power you're allowing to be applied to the generator input shaft. Efficiency is still always the ratio of output power to input power, and now the input power is at the shaft instead of at the winding terminals. Yes and for his conditions the input shaft speed is indeed specified as being the "SAME SPEED" as the motor speed on the nameplate - he made that even clear enough that it sunk in for me. So for his given boundary conditions the explaination was quite enlightnening for me. Yeah, I'd picked up on his same speed and same current (torque) parameters. What I objected to was the statement that *efficiency* as a generator was only 64%. That was using inputs and outputs incorrectly, ie input as a motor and output as a generator. You can't do that when calculating efficiency. It is always power out divided by power in *at the same instant*. To use figures from two different modes of operation doesn't give efficiency. Note also that it isn't mandatory to follow his limits of same RPM or same current either. Those were just his arbitrary choices. Increasing either speed (voltage) or torque (current) will allow greater input power, and greater output power, up to the thermal limits of the motor/generator. Gary |
#47
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Another sparkies question about generators
Gary sez:
" Note also that it isn't mandatory to follow his limits of same RPM or same current either. Those were just his arbitrary choices. Increasing either speed (voltage) or torque (current) will allow greater input power, and greater output power, up to the thermal limits of the motor/generator." I thought Jim Pentagrid covered that very well myself. His choices were made in accordance with preserving the parameters addressed in the original question. As Jim Rozen stated, boundary conditions were specified at the outset. Bob Swinney |
#48
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Another sparkies question about generators
On Sat, 30 Aug 2003 04:30:57 +0100, wrote:
On Fri, 29 Aug 2003 22:49:30 GMT, (Gary Coffman) wrote: On 29 Aug 2003 10:30:24 -0700, jim rozen wrote: In article , Gary Coffman says... On Fri, 29 Aug 2003 00:51:51 +0100, wrote: If we now run this machine as a generator AT THE SAME SPEED it will generate the SAME 80v EMF. Note that this 80v is an open circuit voltage which is the result of the rate at which armature conductors cross the flux linkages, it is independent of the value of the armature resistance. However it has to pass through the armature resistance before reaching the output terminals At the maximum permissible 1A load current, 20v will be lost leaving a net loaded output voltage of only 60v = 60w i.e 60% of the rated input power of the same machine used as a motor. Right, but not 60% of its input power as a generator. Efficiency is still 80%, you've just arbitrarily limited the amount of input power you're allowing to be applied to the generator input shaft. Efficiency is still always the ratio of output power to input power, and now the input power is at the shaft instead of at the winding terminals. Yes and for his conditions the input shaft speed is indeed specified as being the "SAME SPEED" as the motor speed on the nameplate - he made that even clear enough that it sunk in for me. So for his given boundary conditions the explaination was quite enlightnening for me. Yeah, I'd picked up on his same speed and same current (torque) parameters. What I objected to was the statement that *efficiency* as a generator was only 64%. That was using inputs and outputs incorrectly, ie input as a motor and output as a generator. You can't do that when calculating efficiency. It is always power out divided by power in *at the same instant*. To use figures from two different modes of operation doesn't give efficiency. Note also that it isn't mandatory to follow his limits of same RPM or same current either. Those were just his arbitrary choices. Increasing either speed (voltage) or torque (current) will allow greater input power, and greater output power, up to the thermal limits of the motor/generator. Gary PLEASE read the original post carefully! The post did NOT state that the efficiency as a generator was only 64%. The postscript to the post commented that increased power could be obtained by increased speed. Increased current is not permissible because the load current is chosent to be equal to the rated full load current when operating as a motor. Currents higher than this will normally result in overheating. Reading carefully is definitely helpful! Confusion arises from the "same speed" assumption. I agree to the same-current constraint but the speedlimit should be no-load speed which the motor presumably can handle without damage. If the motor operated as a generator at no-load speed then generated EMF is rated terminal voltage, terminal voltage is that minus IR drop or 80%. We now have same current hence same losses and same torque, same power in and power out hence same efficiency, terminal voltage is 80% of nameplate rating when run as a generator. Both Gary and Jim allude to thermal limit hence max current. I concur. They both also noted that spinning the motor faster as a genny makes more volts (more power in, more power out) while loss is proportional only to current. In this case it would actually be *more* efficient, the situation being analogous to operating a motor at 20% above rated voltage with same torque and current. In Jim's example, if it were spun 20% faster than no-load speed then it would produce rated terminal voltage at rated current. I doubt if 20% overspeed would present a problem other than reducing brush and bearing life a bit. |
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