Electric motors

[DFAST]

Hello Everyone. Have you ever wondered is my H.P. accurate on my electric motor. Well here is my tale I experienced first hand. A friend of mine bought a new 80 gallon air compressor 5 HP. for his shop from a well known Farm supply store. I had wired a disconnect box 240 v 8 gauge range cable previous to installation of the compressor. With in six months he burnt up the motor. The farm store gave him a new motor to solve the problem. Four months later it burnt up again. The farm store said take it to our local electric motor rebuild shop and they would pay for it. I helped him remove the motor and take it to the rebuild shop. The service counter man said this is a problem with big stores selling compressor's. He said the motor was really about 3.5 H.P. He said on capacitor start it would for a split second of capacitor start it developed 5 H.P. . But he said we will rebuild it to a true 5H.P. He has not had a problem since the rebuild 20 years ago.
I would like to add this electric motor rebuild shop also sold a full line of electric motors and industrial motors also.

:Final thought: What a rip off selling the H.P. rating as full H.P. on the split second of capacitor start.

 

[niget2002]

All types of companies do something similar. Not just electric motors.

Burst HP vs RMS

 

[fitter30]

Manufacturers label motors with hp ratings that don't follow amperage unless it's a nema rated motor. On the motor name tag there should be a SF service factor rating.
This rating tells if the the motor can run over name plate amps without hurting the motor. If it reads 1 can't over amp the best motors( industrial) up to 1.35. 10 amp motor can run 13.5

 

[PoorUB]

Easy enough to check the amp draw, or it may be as easy as reading the label on the motor.
At 120 volt a motor will draw about 10 amps per HP. At 240 volt a motor will draw about 5 amps per HP.

Now some motors are higher efficiency, some are less, but his info will get you fairly close. In the OP's situation that motor tag was probably under 20 amps and that is a bit too far out of the range. I would expect a 5 HP motor on 240 volts to draw maybe 22 to 27 amps.

Years ago when everyone was selling these 5 to 7 HP single stage compressors they all drew 15 amps on 240 volt, so 3 HP.

 

[fitter30]

Manufacturers label motors with hp ratings that don't follow amperage unless it's a nema rated motor. On the motor name tag there should be a SF service factor rating.
This rating tells if the the motor can run over name plate amps without hurting the motor. If it reads 1 can't over amp the best motors( industrial) up to 1.35. 10 amp motor can run 13.5

 

[nadogail]

Just as cars were sold with inflated Detroit horsepower ratings compressors seem to have inflated horsepower ratings to make them look better for less money.

 

[Stuart in MN]

Kind of like the horsepower ratings for home vacuum cleaners, or the output ratings in watts for stereos.

It's pretty common for retail grade compressors to have motors where the HP is labeled as 'special', so you may have to go by the amp ratings to figure out just how big they are.

 

[marinusdees]

Hello Everyone. Have you ever wondered is my H.P. accurate on my electric motor. Well here is my tale I experienced first hand. A friend of mine bought a new 80 gallon air compressor 5 HP. for his shop from a well known Farm supply store. I had wired a disconnect box 240 v 8 gauge range cable previous to installation of the compressor. With in six months he burnt up the motor. The farm store gave him a new motor to solve the problem. Four months later it burnt up again. The farm store said take it to our local electric motor rebuild shop and they would pay for it. I helped him remove the motor and take it to the rebuild shop. The service counter man said this is a problem with big stores selling compressor's. He said the motor was really about 3.5 H.P. He said on capacitor start it would for a split second of capacitor start it developed 5 H.P. . But he said we will rebuild it to a true 5H.P. He has not had a problem since the rebuild 20 years ago.
I would like to add this electric motor rebuild shop also sold a full line of electric motors and industrial motors also.

:Final thought: What a rip off selling the H.P. rating as full H.P. on the split second of capacitor start.

Seems to me the Federal government got in the middle of this practice some years ago. I don't know what came of it though. The ampere rating on modern motors is "locked rotor" draw. The rotor is locked to prevent rotation then the motor is lit up. The amp draw just before the motor bursts into flame is the "full load" draw as seen on the nameplate. This is not a real rating, just an unrealistically high number. Sells compressors, though.

 

[ycgoat]

Yeh I am looking at a used craftsman 60gl compressor with a "7HP" motor and the new 60gl Craftsman has a 3.2HP with higher cfm at 90psi.

 

[Zeke]

Calling a compressor 3 HP when the motor is really a 1 HP has been going on for decades. You guys wired it heavy enough, so the real problem was the motor pump relationship. Sounds to me like it was too big to be direct drive, so a smaller pulley was needed at the motor from the beginning. Now that he has a real 5 HP, it should handle the pump fine.

I wonder if the original motor was designed to run as hard as your buddy ran it, meaning probably continuously.

 

[u3b3rg33k]

if it has a SF of 1.0, then it's meant to run continuously. higher SF numbers aren't generally intended for "overloading" the motor, but to allow for start/stop operation, as DOL starting motors puts a lot of heat into the windings.

Seems to me the Federal government got in the middle of this practice some years ago. I don't know what came of it though. The ampere rating on modern motors is "locked rotor" draw. The rotor is locked to prevent rotation then the motor is lit up. The amp draw just before the motor bursts into flame is the "full load" draw as seen on the nameplate. This is not a real rating, just an unrealistically high number. Sells compressors, though.

that makes a 5HP motor with an LRA of 125A into (125A*240V=30kW) a 40HP air compressor! somehow shopvac is allowed to keep doing this for their vacuums.

 

[dogdog]

But he said we will rebuild it to a true 5H.P. He has not had a problem since the rebuild 20 years ago.

what does that mean ? or more interested in what he does to make it a true 5HP ?

 

[PWC Repair]

what does that mean ? or more interested in what he does to make it a true 5HP ?

A legit motor shop..........they redid the windings.

Crazy over rated power always reminds me or car audio amplifiers. I used to be big into that scene. My rule of thumb was always a 15A fuse for every REAL 100 watts of output @ 12.5v.

 

[manwithtools]

A legit motor shop..........they redid the windings.

Crazy over rated power always reminds me or car audio amplifiers. I used to be big into that scene. My rule of thumb was always a 15A fuse for every REAL 100 watts of output @ 12.5v.

Back when I used to install (forced to by the sales guys) the cheap ones, I always referred to the their watt ratings as an ILS rating. ILS = "If Lightning Strikes", it might make that much power for about 6 milliseconds.

 

[PoorUB]

Seems to me the Federal government got in the middle of this practice some years ago. I don't know what came of it though. The ampere rating on modern motors is "locked rotor" draw. The rotor is locked to prevent rotation then the motor is lit up. The amp draw just before the motor bursts into flame is the "full load" draw as seen on the nameplate. This is not a real rating, just an unrealistically high number. Sells compressors, though.

Yes the feds just in and put an end to the extreme HP ratings, but they don't rate motor HP by locked rotor. Motor amps are operating at full load. Locked rotor amps will be 5 times running amps. Like the other post a 5HP motor pulling 25 amps at load will pull 125 amps locked rotor.

I have measured amp draw on hundreds of motors over the years and many times they would pull nameplate amps and run like that for years.

 

[Showkey]

Not new………This is more the sales and advertising guys vs the amps and math. The some sales and ad guys used the fudged math others just claimed what ever sounded good and would pushed sales.

The same occurred ( occurs) in vehicles, small engines of all kinds and carried over to almost electric tool or devices ( audio comes to mind).

Feds and class action lawyers fixed this with years back…….but creative ads and using a foot note * ( explanation ) allowed it* to creeping back in to the market place.

* exaggerated claims of all kinds. Like vehicle MPG and EV range are common every day.
EPA tired to fix MPG multiple times.

 

[CraigStu]

Same w/ 65 inch 'class' TVs that are not 65 inches.

 

[jar944]

Compressor duty motors seem to be the only induction motors with the generous HP labels.

 

[u3b3rg33k]

:Final thought: What a rip off selling the H.P. rating as full H.P. on the split second of capacitor start.

but that's a lie too. HP = (Torque × RPM)/5252
that split second amp draw is highest at 0 RPM, and anything multiplied by zero is zero.

 

[DFAST]

Just passed along what the electric motor repair shop said.

 

[DFAST]

Back when I used to install (forced to by the sales guys) the cheap ones, I always referred to the their watt ratings as an ILS rating. ILS = "If Lightning Strikes", it might make that much power for about 6 milliseconds.

I really like the Lighting Strikes rating

 

[u3b3rg33k]

Just passed along what the electric motor repair shop said.

I'm more speaking to the mislabelling BS from the manufacturer. you can get a lot of torque out of a motor if you don't care about how long it lasts.

 

[Walkers]

Back when I used to install (forced to by the sales guys) the cheap ones, I always referred to the their watt ratings as an ILS rating. ILS = "If Lightning Strikes", it might make that much power for about 6 milliseconds.

If Lightning Strikes, I'll remember that one!

 

[DFAST]

I found this site https://www.blocklayer.com/pulley-belteng.aspx
It will allow you to find your belt speed by giving three knowns such as motor speed and bottom pulley diameter and top diameter. Click calculate and it gives final speed. Or leave a pulley size out and fill in final speed and it will tell you what size pulley you need to maintain that speed. It is a fun tool to play with or research with. Fill all information in and it will give you belt size needed. no guesstamating

 

[nadogail]

We used to use the term "Detroit Horsepower" when describing the advertised rating of engines.

 

[DFAST]

Calling a compressor 3 HP when the motor is really a 1 HP has been going on for decades. You guys wired it heavy enough, so the real problem was the motor pump relationship. Sounds to me like it was too big to be direct drive, so a smaller pulley was needed at the motor from the beginning. Now that he has a real 5 HP, it should handle the pump fine.

I wonder if the original motor was designed to run as hard as your buddy ran it, meaning probably continuously.

He is a hobby wood worker. He was leaving the compressor on 24/7 . I convinced him to turn off power at the end of the day and installed a ball valve on the bottom of his air tank. So he could take his toe and bleed off air and moisture. I do this so in our Ohio winters less drag on start up . Close the valve after 1-2 of running

 

[CraigStu]

Maybe not available for 240V but I got one of these for my new 120V compressor. I was so tired of hearing that thing run because I forgot to turn it off.

Woods 20-Amp 60-Minute In-Wall Spring Wound Countdown Timer Switch, White 59717WD - The Home Depot

Visit The Home Depot to buy Woods In-Wall 60-Minute Spring Wound Countdown Timer - White 59717

www.homedepot.com

 

[redmondjp]

Another factor which nobody has yet mentioned - what is the actual user duty cycle? In other words, how many times per hour does the compressor start and stop? If you read the ratings, the motor is probably only rated for 3-5 starts per hour else it will overheat the windings which will cause premature motor failure. A silent killer, if you will.

For some heavier-duty compressors in commercial duty, you can configure the compressor controls to leave the motor running and unload the pump head when the desired tank pressure is reached - much the same as gasoline-engine-powered compressors operate. It is much easier on the electric motor heat-wise to just stay running than to start and stop over and over again.

 

[u3b3rg33k]

Another factor which nobody has yet mentioned - what is the actual user duty cycle? In other words, how many times per hour does the compressor start and stop? If you read the ratings, the motor is probably only rated for 3-5 starts per hour else it will overheat the windings which will cause premature motor failure. A silent killer, if you will.

For some heavier-duty compressors in commercial duty, you can configure the compressor controls to leave the motor running and unload the pump head when the desired tank pressure is reached - much the same as gasoline-engine-powered compressors operate. It is much easier on the electric motor heat-wise to just stay running than to start and stop over and over again.

that's part of what a SF over 1.0 is for. not so much continuously running the motor beyond its nameplate, but for the added heat of start/stop use or intermittent heavy loads.

 

[nadogail]

Maybe not available for 240V but I got one of these for my new 120V compressor. I was so tired of hearing that thing run because I forgot to turn it off.

Woods 20-Amp 60-Minute In-Wall Spring Wound Countdown Timer Switch, White 59717WD - The Home Depot

Visit The Home Depot to buy Woods In-Wall 60-Minute Spring Wound Countdown Timer - White 59717

www.homedepot.com

A Spring Wound Timer can also be adapted to control larger loads; I use one to switch the output of a 24 Volt transformer (actually a repurposed wall wart from an irrigation timer) that energizes the coil of the relay that switches the 30 Amp 220 Volt feed to my garage space heater.

 

[markrenner371]

All types of companies do something similar. Not just electric motors.

Burst HP vs RMS

which is good HP vs RMS ,

 

[finn]

If I read the original post correctly, the air compressor story supposedly happened twenty years ago.

Since that time, there were several lawsuits and the FTC stepped in to rectify things.

Those old 5hp or 7hp motors are now commonly rated at 3.5 hp. This thread is pretty much irrelevant to compressors being sold today. Besides, what motor rebuilder will cost effectively rebuild a consumer grade compressor motor. I haven’t found one in fifteen years? Commercial/industrial motors costing several hundred or thousand dollars, yes, but not box store motors.

Same with advertised hp in cars. All the automakers certify hp to an SAE standard test procedure, and those numbers are used in advertising. Last time I did one of those certifications was thirty years ago, and the procedure wasn’t brand new then. We had to set up three engines, run them at the standard conditions, and use that data. I recall that all three had to be within a couple percent.

 

[FrogBlastTheVentCore]

I mean, it runs off of a sine wave, so…………….

 

[spudley]

Besides, what motor rebuilder will cost effectively rebuild a consumer grade compressor motor.

Might be a long ride and not sure about consumer grade but Quality Electric in Milwaukee is top notch. Had them rebuild a pool pump motor that worked much better after they got at it.

In my former non retired life I worked in a medium sized office bldg that had a service contract with an HVAC outfit that seemed not ready for prime time. We had an old pneumatic boiler system with a failing pump motor so their HVAC guy suggested we buy a new motor for a cool $2500. I thought this was a wee bit high so I called in another HVAC place and their guy suggested a rebuild. He pulled the motor out in the morning, Quality Electric rebuilt that day and it was replaced it late afternoon for $500.

I ended the service contract with the first guy the next day, hired number two. Everyones living happily ever after.

 

[finn]

Might be a long ride and not sure about consumer grade but Quality Electric in Milwaukee is top notch. Had them rebuild a pool pump motor that worked much better after they got at it.

In my former non retired life I worked in a medium sized office bldg that had a service contract with an HVAC outfit that seemed not ready for prime time. We had an old pneumatic boiler system with a failing pump motor so their HVAC guy suggested we buy a new motor for a cool $2500. I thought this was a wee bit high so I called in another HVAC place and their guy suggested a rebuild. He pulled the motor out in the morning, Quality Electric rebuilt that day and it was replaced it late afternoon for $500.

I ended the service contract with the first guy the next day, hired number two. Everyones living happily ever after.

This thread discussion started with consumer grade compressor motors. The entire compressor probably cast $750, or maybe $1000, and your $500 rebuild ten years ago is likely $1200 now.
I brought a hydraulic power pack motor in for a possible rebuild two years ago. $35 for an estimate. Ended up no parts available, and a brand new power pack was cheaper than an unavailable motor. The motors that were available for commercial lifts cost $750. The rep said they rarely sold one, since it was more cost effective to replace the entire unit, unless some large entity was rebuilding a large quantity of lifts. Even these $750 motors weren’t being rebuilt.

 

[tester19]

With air compressors the only thing that is of interest is the SCFM rating at different pressures. Figure out what your planning to operate and then find how much air that item consumes. HP and amps are just auxiliary numbers that are used as sales techniques. It leads the customer away from understanding that what your buying is air delivery NOT amps or HP.

It takes whatever size electric motor to acheive that.
.
.
.

 

[nadogail]

With air compressors the only thing that is of interest is the SCFM rating at different pressures. Figure out what your planning to operate and then find how much air that item consumes. HP and amps are just auxiliary numbers that are used as sales techniques. It leads the customer away from understanding that what your buying is air delivery NOT amps or HP.

It takes whatever size electric motor to achive that.
.
.
.

I could not have said it better myself; SCFM is the Steak, Amps and Horsepower are just the Sizzle.

 

[LS6 Tommy]

The ampere rating on modern motors is "locked rotor" draw. The rotor is locked to prevent rotation then the motor is lit up. The amp draw just before the motor bursts into flame is the "full load" draw as seen on the nameplate. This is not a real rating, just an unrealistically high number. Sells compressors, though.

FLA (aka RLA) and LRA ratings are not the same. FLA is just as the acronym implies, the amperage draw at full load. LRA is also just that, the amperage draw with the rotor locked. YOu want to use the FLA to check HP rating. LRA can be up to 200% of FLA, avergaing about 150%.

Tommy