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Post by parfive on Jul 13, 2013 2:17:25 GMT -5
NEC requires that 9.8 amps be used for ½ hp circuit calcs regardless of nameplate rating on motor. Overkill – tough to find a motor that bad.
Branch-circuit conductors need ampacity at least 125% of 9.8 amps = 12.25 amps, which is < 15.
Maximum cord- and plug-connected load allowed on 15 amp ckt is 12 amps, which is > 9.8.
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Post by parfive on Jul 13, 2013 2:35:01 GMT -5
What’s the amperage on that Harbor Freight Smith+Jones brand 1/2hp nameplate?
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Post by parfive on Jul 13, 2013 18:29:28 GMT -5
Well, I did “a little research” at G&G Electric & Plumbing’s pages. Some of it seems to have been written by the plumber. Motors should be fused with a time-delay fuse rated 175% of the motor's full load operating amperage, 250% of the full load operating amperage if a circuit breaker is used. The wire to a motor must be rated to carry 25% more amperage than the motor draws when operating at full load. This amperage rating is listed on most motors. The last two sentences are wrong. The electrician fixes that two paragraphs later when he says the amperage is derived from NEC Table 430-248.
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quartz
Cave Dweller
breakin' rocks in the hot sun
Member since February 2010
Posts: 3,352
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Post by quartz on Jul 13, 2013 23:19:50 GMT -5
I find the sentence about using time delay fuses interesting. With some of the equipment in my little shop, it was recommended to use time delay fuses, or breakers. They don't exist here, got lots of funny looks when hunting for them. McMinnville, Oregon. I used conventional breakers of the proper amperage, and have no problems.
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rykk
spending too much on rocks
Member since September 2011
Posts: 428
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Post by rykk on Jul 26, 2013 22:20:56 GMT -5
It sounds to me like the thermal overload is tripping. Most of these motors are rated for 40deg Celsius, which is only 104 degrees Fahrenheit. Don't know where you live or how hot it is there but I live in Florida where it is just plain unbearable (humidity makes temp feel worse) 8-9 months out of the year.
I had the same problem as you with a Lortone "jiggle pan" (vibrating lap) I got a few years ago. Took me a little bit to figure out what was going on but it made sense since it is 98 in the shade in summer here, which leaves only 6 degrees margin. All motors will normally warm up some when they run. I solved the problem by taking a high speed , 120Vac, 6in fan and putting it under the motor, blowing into the vents while the lap ran. Never shut down again. Give it a try and see if it stays running.
Anther thing that can generate heat that transfers to the motor is if you have a drive pulley on the motor that is misaligned with the load pulley. The rubbing of the belt on the side of the pulley makes it really hot and that heat is right up where the thermal sensor usually is and travels right into the motor via the shaft. Having a belt too tight or the load too far out on the shaft can create a sort of twist that stresses the bearings and that makes for heat, as well. Especially down near the base of the shaft and by causing friction when the armature and commutator rub together slightly. A loose, slipping belt gets hot, too.
Anyhow, give the fan a try. C-ya, Rick
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rykk
spending too much on rocks
Member since September 2011
Posts: 428
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Post by rykk on Jul 26, 2013 22:26:25 GMT -5
Oh yeah, another thing - 4 months ago it was Winter and a lot cooler, so that might explain why it has started acting up in the summer... Rick
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Post by johnjsgems on Jul 27, 2013 10:13:17 GMT -5
For what it is worth I believe standard circuit breakers are time delay. It takes a period of over amperage condition to trip unlike a dead short.
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rykk
spending too much on rocks
Member since September 2011
Posts: 428
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Post by rykk on Jul 28, 2013 0:50:07 GMT -5
Circuit breakers and fuses won't affect how the motor runs. The "trip switch" is the thermal overload protection. Electric motors can have one of 3 types of thermal protection: (1)Automatic Reset, which is a thermistor that goes open circuit when a spec'ed temperature is exceeded. These will automatically close the circuit once the motor cools off sufficiently based on the hysteresis of the thermistor. (2) Manual Reset, which requires the operator to manually close the circuit. These will typically pop right back out if immediately pressed after a thermal overload because there is a margin between when the sensor opens and when it closes back up. (hysteresis) Usually you have to wait awhile for the motor to cool some before it stays in again. (3) True thermal Fuses. These are a real p.i.t.a. because you have to open the motor up, desolder the thermistor fuse and install a new one.
I recently modified my Lortone jiggle pan to have an oscillating action. I needed a stronger motor than the 1/15hp that comes with the older design Lortones and so installed a 1/7hp Dayton. Got lazy and decided to slap the thing together without a fan blowing into the motor and, sure enough, the thing crapped after about 45min. Never started back up because the thermal protection was fused. Bummer!! So what I did was snagged the automatic thermistor from the old 1/15hp (same temperature spec.) and soldered it in place of the fused one in the new motor so I wouldn't have to dork with it again. Worked like a champ. Dayton used the replaceable thermistor due to the tightness of the space in their 1/7hp motors. The auto switch is a good bit bigger but I was able to squeeze it in ok. Never toss an old motor because you might need to scavenge something from it, ya know?
Anyhow, one other thing that can cause a motor to run warmer is low voltage caused by a very long, light gauge power cord. The higher gauge wires are more resistive and, over long distances, drop the voltage causing a condition called, "reluctance". The motor will draw more and more current to build the voltage across the windings to 115Vac and more current equals higher temp. If your shop is detached from your house and 115Vac is run a long distance with only 12 or 10AWG wire, this could cause a low voltage situation. Ran into that one when I was younger. I had a 1/2hp motor running a large aerated bait tank that was on a dock about 80ft from my dad's house. He'd run 220V to the dock but, since the pump motor was pre-wired for 115V, I just picked off one 115V leg. Motor ran so warm, the shiners were dying because it heated the water. And I was paying over $40 a month toward my dad's power bill because the motor drew so much extra current trying to get the proper voltage built up internally. Rewired the motor for 220 and the thing ran nice and cool. Power bill went way down, too. If you have 220Vac anywhere close, it's always a good idea to run the motor off of that rather than 110/115Vac. This concludes CyborgSam's Electric Motors 101 class for 2 day. :-) C-ya, Rick
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