electric motor amp rating

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startup load is not usually listed on the motor spec plate....listed is the running load. starting load is much higher. like 3 times higher.
 
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5,988
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Houston, Texas
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Yeah but that 23 amp running load is still based off of it actually being loaded all the way down right? So even though it says 23 amps on the name plate it may only be pulling 18 or 19 correct? Or am I wrong?
 
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2,676
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High Tax Illinois
Yes. Most likely less. I don't think you would want the motor running at it's Max or close to it all the time. I'm sure some electric motor guy will chime in here soon that will know for sure.
 
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1,906
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Canada
Yeah rated amps is at rated hp output. A synchronous motor won't exceed the rpm implied by the number of poles and the frequency of the current delivered. Nor will an induction motor which induces an excitation current in the rotor, but still relies upon phase rotation and will have slippage.
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I don't think you would want the motor running at it's Max or close to it all the time. I'm sure some electric motor guy will chime in here soon that will know for sure.
If its rated that way, I don't see the issue. That's probably the most efficient way to operate, to match an actual load to the nameplate. Of course it rarely perfectly works out that way, so reducing the motor's duty cycle, or alternatively, using a VFD to alter the (induction) motor's output can be on the table.
 
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Originally Posted by motor_oil_madman
Yeah but that 23 amp running load is still based off of it actually being loaded all the way down right? So even though it says 23 amps on the name plate it may only be pulling 18 or 19 correct? Or am I wrong?
[Linked Image] Yes you are correct. The amp draw shown on plate is at rated HP output. An idling motor will be less than what is on name plate. There can also be a SFA amp rating. That is when the motor is loaded past it's rated output. Most but not all modern fractional horse power motors have a 1.15 service factor. The old (good uns) had a 1.25 service factor. Hence the SFA rating on the tag. There may also be a locked rotor amp spec on the tag LRA.
 
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1,248
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USA
It's at the rated hp at the stated voltage and the duty ratings. If line voltage is 120 the amps will be less than at the115 stated on the plate. 2cents
 
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WY
Starting current can be many times full load current. Many motors run above full load amps and into service factor routinely, sometimes their full if somewhat shortened lives. I've seen more than one motor with a large floor fan or water hose utilized for cooling purposes. As stated above synchronous motors run at synchronous speeds without slip.
 
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Originally Posted by Farnsworth
It's at the rated hp at the stated voltage and the duty ratings. If line voltage is 120 the amps will be less than at the115 stated on the plate. 2cents
Originally Posted by sloinker
Starting current can be many times full load current. Many motors run above full load amps and into service factor routinely, sometimes their full if somewhat shortened lives. I've seen more than one motor with a large floor fan or water hose utilized for cooling purposes. As stated above synchronous motors run at synchronous speeds without slip.
^ spot on! This is exactly why I had my power company come and jack a tap position on the transformer feeding my house and shop. Prior to the tap change I was running 113. Now I have a good strong 123/242 volts. Before they would do the tap change they installed a recording VM for 2 weeks.
 
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Malaysia
Originally Posted by motor_oil_madman
Yeah but that 23 amp running load is still based off of it actually being loaded all the way down right? So even though it says 23 amps on the name plate it may only be pulling 18 or 19 correct? Or am I wrong?
Yes, you're right . It is power on demand .
 
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Location
Cheshire, England
Depending on the inertia involved, start up current can be as high as 5 times full load current for a single phase motor. For big 3 phase motors we used to say 7 times. That's when you use a starter that starts in star and then switches to delta when the motor is nearly up to speed Reduced voltage starting is not so simple on single phase motors making them generally less suitable for high inertia loads.
 
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5,988
Location
Houston, Texas
Thread starter
Originally Posted by P10crew
Op mad man... did these fellas answer the 3 ?' You had?
Well lets use my table saw as an example. Say it says 1-3/4hp 14 amps on the motor @ 115v. (Yes I know we typically have 120v at the wall outlet, that's just what it says on the motor). How much amperage would it be pulling when it's just running and only spinning the blade without feeding any wood throught it compared to sawing wood? I know we've agreed that it's not going to be pulling a full 14 amps, but how much amperage (estimate) would it be using just running with no load compared to sawing wood? Is it possible it's only using maybe 7 out of the 14 amps just spinning nothing but the blade?
 
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2,960
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Western S.C.
Originally Posted by motor_oil_madman
P10 crew not yet. Lol
Idling current will be considerably less than rated current. The percentage of rated current will vary with motor type. Spinning a saw blade in air is not zero load, although it's close.
 
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I'm sure there is idle current reference somewhere but I can't find it. Lots of variables here bearing drag, belt friction, windage drag from the fan and blade.
 
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1,248
Location
USA
If you have a digital meter on the house you can measure the current draw pretty exactly. Since they switched mine to the the new type I look at it pretty often to see the watts things are drawing and it shows the voltage too. I am pretty sure people are wondering why I am looking at the meter. The voltage does vary more than expected probably ten volts from about 233 to 243. Most of the time it is right near 240.
 
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