06-30-2021, 07:39 AM (This post was last modified: 06-30-2021, 07:58 AM by TDKPE.)
(06-30-2021, 05:53 AM)fixtureman Wrote: Why would it draw that many Amps. My 5 HP compressor draws less then 30 amps. I only have a 60 amp service in my shop.
Your compressor motor, if it's truly 5 hp continuous output, will draw something like 20A on 240V power as the pressure approaches the cutoff point. But starting is going to jump to over 100A - your clamp-on ammeter isn't fast enough to actually show it, though. If you locked the rotor, you'd see it spike. It doesn't matter how high the starting load on the motor is, either - it's only a function of the rotor's speed and supplied voltage. High inertia or driven load (like a loaded conveyor) connected to the motor while starting only extends the time it takes to start, and with the starting current remaining high for most of the speed range while starting, heavy loads 'look' like they're drawing more current than light loads, but that's only because you can actually see it on a slow ammeter. Or the breaker trips from the extended start time.*
The circuit breakers protecting the feeder and branch circuit can absorb that current for the most part. They have both a bimetal strip to trigger a trip with an extended overload, and a magnetic trigger to instantly trip once the current exceeds some threshold, which is pretty high.
The higher the overload current, the faster the bimetal strip heats up, and the shorter the time to trip. A small overload will be sustained for potentially a couple of hours while a larger load like a stalled motor could last for seconds before tripping. Very large overloads will trip the magnetic trigger, with 'large' being ground faults and short-circuits typically. They're made like that so large but brief overloads won't trip the breaker on motor startups and such, just as large but brief overloads won't heat the wiring in the walls to excess. Not an accident that they're made that way.
I have trip curves around here somewhere showing Square D QO series small breakers where the magnetic trip threshold is something like 8 times the rated current, more or less. It's actually a range between two curves, so a pretty big window, but even the lower limit is very high.
Here's a data sheet for a 5 hp, 3600 rpm, 230V, single-phase Baldor general purpose motor. This particular motor shows kVA Code letter "G" on the nameplate, which translates to 122-137A starting current, and the data sheet shows 128A starting current, which is 6.4 times its full-load running current. 4-8 times running current is the range I typically spout for common motor types.
*As an aside, when I was little and my Dad was cutting wood on his 8" Sears contractor saw in the back yard, he'd lift the motor with his knee to slacken the belt, start the saw (toggle switch on the motor), then let the motor come back down. I don't know what he was plugged into (I was 3 or 4 at the time), but it was obviously to shorten the time to start by removing the inertia and belt friction loads to avoid tripping the breaker in the house. This was a puny 3/4 hp motor from the 50's.
Tom
“This place smells like that odd combination of flop sweat, hopelessness, aaaand feet"
06-30-2021, 08:34 AM (This post was last modified: 07-01-2021, 07:48 AM by TDKPE.)
(06-30-2021, 07:57 AM)fixtureman Wrote: Wouldn't my Balder 5HP motor trip a 30 amp breaker if it drew that much or the 60 amp main breaker
Re-read the middle paragraphs above - the breakers won't trip right away on small or even large overloads, at least until the current exceeds the magnetic threshold, in which case it trips in 1 cycle or less.
Found this trip curve for Square D QO series, 20A molded case breakers. Someone was nice enough to mark it up at various overload values (not me - I found it that way).
X-axis is overload multiplier scale, and Y-axis is time to trip in seconds. Both axes are logarithmic.
It's not marked as such, but the "1" multiplier on the X-axis goes straight up and meets the vertical portion of the trip curve, meaning the breaker will support 1 times rated current indefinitely. The right side of that skinny vertical band is a little higher than 1, so there's actually a range of current, including a small overload, that it may support indefinitely, but no less than 20A in this case, being the curve for a 20A breaker.
The "2" multiplier on the X-axis hits the curve above at 9-35 seconds. That's a pretty big uncertainty, but at no less than 9 seconds at 40A, it gives you an idea what's happening here.
"4.5" times rated current, or 90A, will hold for 1-4 seconds, and "8" times rated current will hold no longer than 1 second, but may trip instantly.
Not annotated is where you're guaranteed to trip within 1 cycle, which is the horizontal portion of the upper curve that represents the magnetic trip threshold and which in this case is something like 12x rated current. But from experience, it's typically quite a bit lower.
The magnetic threshold can be lower with other brands, and there is a 'high magnetic' type breaker where it's higher, used for motor loads usually.
And not to confuse things too much, the NEC allows 'oversized' circuit breakers for motor circuits (Article 430) to reduce the chance of the breaker tripping during start-up, where the breaker can be bigger than the conductor ampacity tables show, BUT with the motor control doing the overload/overcurrent protection for the circuit, and the breaker/fuses only there to protect against short-circuits and ground-faults. 'Oversized' breakers/fuses are allowed so they don't trip during start-ups. The A/C unit outside your house may even have an 'oversized' breaker (compared to conductor gauge) for the same reason, as they have an internal overload mechanism and you don't want the breaker to trip on a hard start event, like during a brownout (max breaker size per Article 440, as limited by the MOCP value on the A/C's data plate).
Tom
“This place smells like that odd combination of flop sweat, hopelessness, aaaand feet"
Tom I have meter that reads inrush current. Just went downstairs to check it. On my 3hp Grizzly 1023 saw on 240 volt the inrush was 91 amps with just the blade and belts for a load. Almost 22 kva. Roly
(06-30-2021, 09:31 AM)Roly Wrote: Tom I have meter that reads inrush current. Just went downstairs to check it. On my 3hp Grizzly 1023 saw on 240 volt the inrush was 91 amps with just the blade and belts for a load. Almost 22 kva. Roly
Well there ya go.
The OP could probably use a generator with less than a 22kVA intermittent rating, and the voltage will just pull down (if its breaker doesn't trip), but the mag control may not operate.
Only way to know for sure is to try, and for that, I would rent a unit. If it's too small, try a bigger one. Much cheaper than buying a 'big' one that turns out to be not big enough.
I don't know if 10kVA will be enough, but it's the biggest gas unit Sunbelt rents in my area at least. Jumping up to 20kVA makes it diesel, and that's a nice, quiet unit.
(07-01-2021, 11:46 AM)Danny in Houston Wrote: It's so enjoyable seeing competence, especially in electrical matters.
Well done TDKPE!
We are fortunate to have several on this forum we can depend on when one needs the minutiae of electric systems: TDKPE, Roly, and Blackhat come to mind quickly. I would feel confident following the advice of any of them.
(07-02-2021, 12:33 PM)srv52761 Wrote: We are fortunate to have several on this forum we can depend on when one needs the minutiae of electric systems: TDKPE, Roly, and Blackhat come to mind quickly. I would feel confident following the advice of any of them.
Glad I came back to the right place! just got home from a long trip... going to check my Baldor motor now.
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