Electric Motor Performance

[Rob Will]

Here is an interesting article from the U.S. Dept of Energy about how variations in voltage can affect electric motor performance.
Table 2 gets right to the point.

http://www.eere.energy.gov/industry/bestpractices/energymatters/articles.cfm/article_id=7

Rob

[Dennis Miller]

Uh...you're a brave man Rob Will...and only 2 days to recover after the 110/220 motor wiring post got started. With that said...

LET THE VERBAL GAMES BEGIN! ;)

I just can't wait...I really love this site....all the free entertainment intermixed with the camaraderie and excellent woodworking information.

Dennis

[Mike Henderson]

I don't see anything controversial in Rob's post or in the paper. The paper (especially Table 2) just outlines what to expect if you run your motor at a voltage that is above or below the rated voltage.

Mike

[Rob Will]

Does anybody have a chart showing typical amperage requirements for various HP single phase motors?

I realize that this will vary depending on the particular model. Just looking for a range or average for "good" quality USA made motors.

Thanks,

Rob

[Rod Sheridan]

Your NEC book will have a table of motor currents, however they are only aproximate values. They are for normal speed motors, normal torque curves.

If you are sizing overload protection or feeders, use the motor nameplate value.



Amperes
HP /@115V/ @230V

0.5 /9.8 /4.9
0.75/ 13.8/ 6.9
1.0/ 16/ 8
1.5/ 20/ 10
2/ 24 /12
3 /34 /17


Regards, Rod.

PS. My 3 HP tablesaw motor (General 650) is 12.5 amperes VS 17 from the table above. As stated use the nameplate information from your particular motor.

[Eric Gustafson]

The chart in the article confirmed something to me I knew from experience. That is, motor current increases with a drop in voltage. I have blown a few fuses in my day discovering that! :D

Some thing to consider, at least for the one phase customers, was mentioned by Dr. Bill Wattenburg on his KGO radio show. Bill said that electric power ultilities (he cited California) tend to run their voltages towards the higher end of their specification limits and that they do so for higher profits! The spec limits he mentioned for 110v were 105v-125v. Double that for 220v.

http://www.kgoam810.com/viewentry.asp?ID=353556&PT=PERSONALITIES

[Tim Marks]

Bill said that electric power ultilities (he cited California) tend to run their voltages towards the higher end of their specification limits and that they do so for higher profits!
Not quite.

There usually is some voltage droop characteristic in the voltage ouput of a power plant; as load on the distribution system rises, voltage naturally falls.

Because of this, the power plants tend to be set at the high end of the band to accomodate unexpected peaks in loading, thus ensuring that the voltage on the distribution system stays above the minimum.

Brownouts (sharp reduction in voltage when a large load is added) usually damages equipment, since that equipment will draw above it's rated current at low voltages.

There is a limited side benefit of less I^2 R losses (ie, the higher the voltage, the lower the current, the less energy is wasted in resistive heating of the power lines). But this is a pretty minimal benefit, since the real way to prevent those resistive losses is to use 78 kV transmission lines, not just keeping line voltage 13V higher then 230V nominal.

Back OT, the best explanation of efficiency of motor operation vs input voltage is here in a letter to the editor at Popular woodworking, where the popular myth that the electric motor "runs cooler", since the higher voltages mean lower internal current flows and less internal resistive heating:

http://www.popularwoodworking.com/features/240V.html



...when you change the connections in the motor junction box to switch between the two voltages, you are doing so to keep the winding currents constant... you are connecting the windings in series for 240V and in parallel for 120V. This is what causes the current to be halved when the voltage is doubled.

The statement that a motor running at 240V draws half the current as it does when running at 120V is only true of the total current drawn by the motor through the supply wiring. After the distribution of currents that takes place in the motor junction box, the current in each winding is exactly the same either way. So a motor running at 240V does not run cooler than the same motor running at 120V Ð there is no advantage from the "reduced current flow" -- the currents in the motor are the same.

[Mike Cutler]

Some thing to consider, at least for the one phase customers, was mentioned by Dr. Bill Wattenburg on his KGO radio show. Bill said that electric power ultilities (he cited California) tend to run their voltages towards the higher end of their specification limits and that they do so for higher profits! The spec limits he mentioned for 110v were 105v-125v. Double that for 220v.

http://www.kgoam810.com/viewentry.asp?ID=353556&PT=PERSONALITIES

If Dr. Wattenberg would like to contact me,I'd be happy to explain the inaccuracy of his statement.

A power plant puts out 345,000 volts to the grid. This voltage is developed by a large generator that is hooked to a steam driven turbine.The weight of the turbine and the generator is in 100's of tons. It is spinning at 1800 rpm. it is an incredibly large rotational mass of energy.

"We" the power company have no control over the 115vac at your house. We put 345,000 volts to the grid. The grid is controlled by a central power authority that exists outside of the regualted body of electrical generators. The power is "pooled". We can't apply more voltage, or less voltage. We supply 345,000volts whether at 1000 megawatts, or 100 megawatts.
When electricity is demand is low. "Load" is taken off the generator by limiting the amount of steam to the turbine. When demand is high, "Load" is picked up, and more steam is capable of supplying the turbine.
Each Generator connected to the grid has to share equal load, or the grid can become unbalanced. It is the grid demand that dictates the power in Megawatts that each turbine generator is putting to the grid.
Intentionally dropping the regulated output voltage of a turbine connected to the grid has the very realistic potential to cause catostrophic destruction of the turbine and the generator. It simply is not done.

All generators are "synched" to the grid to ensure that the whole system is controlled at 60 hz. A deviation of grid frequency in fractions of a HZ for less that one complete cycle will trip the output breaker of a power plant to protect the turbine generator. Everything is very tightly controlled.

This is a 100,000 ft bird eye view of electrical generation. I've been doing this for almost 25 years. There is a lot to it.
Dr Wattenberg lacks an elementary understanding of power generation/distribution.

[Rick Christopherson]

Does anybody have a chart showing typical amperage requirements for various HP single phase motors?What's interesting (or possibly confusing) about the 3-phase table is that when a motor is run under-voltage, its startup current will be lower. I think this might be because a 3-phase motor gets rotating sooner than the single phase equivalent.

Either that, or the table is simply referring to the maximum startup current without regard to the length of the startup curve.

[Eric Gustafson]

Dr Wattenberg lacks an elementary understanding of power generation/distribution.

Certainly, I am no one to criticise you, Dr. Wattenburg, or anyone else for that matter. But, based on his creditials, it is safe to assume he has more than an elementary understanding of electricity, distribution, and generation. He wrote a report entitled, " ADVANCED SWITCHES FOR SOFT BLACKOUTS CRITICAL INFRASTRUCTURE PROTECTION UNANTICIPATED DISCOVERY OF EMERGENCY VOLTAGE EDUCTION FOR GRID PROTECTION."

The report was prepared for the California Energy Commission Public Interest Energy Research Program by Dr Wattenburg and the Chico Research Foundation. The following is a excerpt from that report:

"In meetings with the ENERGY COMMISSION and the governor’s office in late June 2001, the three major investor owned California utilities agreed that they could reduce the load on the grid by 250 to 300 megawatts by lowering distribution line voltages by 2 ½% in portions of their systems (but no lower than 117 volts) during power emergencies. "

Anyhow, I only mentioned it because it is something to think about.

[Mike Cutler]

Eric

I have the CEC-500-2006-058 report in front of me now. It's 114 pages and had to be printed. I didn't see an HTML option.
On cursory view of the abstract. The implementation of this plan may have had plants operating outside of their design basis.
It also looks like the deck was skewed against the residential rate payer.
let me look at it a bit.

[Eric Gustafson]

MIke,

The discussion on EVR (Emergency Voltage reduction) and its discovery starts on page 19 of the report. It seems both you are Bill Wattenburg can be right. You are discussing how power is applied to the grid. Dr Wattenburg's approach is applied to major distribution lines. According to the report, Southern Caifornia Edison has automated voltage equipment at the substation level they can use to acheive the voltage reduction.

The last sentence of the report quoted below caught my attention as different from what Wattenburg had said on his show. He said the reason they keep substation voltage high is that the consumers will pay more. He also had said that the voltage rating plates we see on motors vary not because of design differences, but largely because nominal voltages vary across the planet. 110v is more like 120-125 in the US, but may be as low as 105v in Japan. Again, according to Wattenburg, motors for all markets come off the same production line. I found all of this interesting considering the article that led to the original post.



"ENERGY COMMISSION Commissioner Dr. Arthur Rosenfeld asked the
Southern California Edison (SCE) to do an EVR test on a major distribution line. SCE has automated voltage adjustment equipment in its substations. They could do the test very easily and quickly at any time. They were rightfully worried that lowering the voltage would result in customer complaints or damage to customer equipment."

[Mike Cutler]

Eric

I don't really like the wording in that last paragraph. "Voltage Adjustment", is "voltage regulation". Yes we can "adjust" the voltage, but the purpose of the equipment is to regulate the voltage out, and maintain stability under varying load conditions. While SCE, and PG&E stated their concern for consumer equipment damage, I suspect they were equally as concerned with their own equipment damages.
The substations aren't controlling 115 and 220, they are controlling much higher voltages which are stepped down by transformers on the poles. The only place 115/120 exists is after the service panel.
If SCE is going to do this, they would have to have a feed forward strategy in place with no feedback.
I don't see how the consumer is paying more? Your billed in kilowatt hours, T&D costs, and Federal and state taxes. I could theoretically increase the voltage but the net effect would be a drop in current, and the power would be close to the same. I'm just not seeing the big benefit for SCE and PG&E. Their contracts are structured to supply X amount of power to the grid, at X amount of dollars. It's actually in their best interest for the consumer to use less power. This way they don't have to purchase power contracts for additional power to meet their contractural obligation. They also, most likely, have a mechanism that kicks in that allows them to sell their "excess power", above a certain percentage, at the going market rate, which can be much higher than retail rates, by multiples.

I don't like the fact that the report itself seeks to spec Dr. Wattenberg's patent as the sole device for the "soft blackout" in the event that California adopted the EVR approach. Little bit of conflict of interest there. I also observed that he had a patent he was specifying for the vehical barriers. Interesting.
This EVR was a knee jerk reaction to a "power supply" shortage that existed on paper only. Had they ever implemented the policy they may have found thereselves in an even worsening position. They may not have been able to import power from outside the state to meet their demand at any cost. Kind of a domino effect.
The actual culprit was the poorly thoughtout and implemented deregulation bill that California signed into law. I've sat through a few seminars on the mechanics of that situation. It's the "model" for states on how not to implement deregulation. It also shelved the deregulation efforts of many states, which was a good thing. It's just too bad that it had to happen at the California rate payers expense. Of course, our situation here in CT isn't that much better. We didn't learn quick enough.:eek:

I still don't see where PG&E and SCE are actually lowering the voltage to increase profit, or how he is supporting that statement. That part I'm not catching. Am I missing it somewhere?

Motors can run within a band of voltage, but there is a trade off in longeveity. Would someone with a home shop see it? I doubt it, we're just not taxing a motor enough, we don't typically run them long enough, cycle them enough, nor operate them in a hot enough enviorment to statistically be able to quantify a decrease in life.
The document that Rob points to in his post is nothing earth shattering, or new. Any electrical engineer has volumes of info available to correctly spec a motor based on application and location. If they don't the motor manufacturer has the info.
As long as the motor is being run within it's design criteria. It's longevity can be predicted.
I would also assert that, while table 2 is informative, why would someone knowingly want to operate a motor in a low/high voltage enviorment?
If anyone suspects that they are not getting the correct voltage to the motor. Stop and get it fixed.

[Rob Will]

Your NEC book will have a table of motor currents, however they are only aproximate values. They are for normal speed motors, normal torque curves.

Thanks Rod, yes I found that chart at 430.248 in the NEC.

[Rob Will]

why would someone knowingly want to operate a motor in a low/high voltage enviorment?

That's a good question Mike but I don't know much about electricity:o.

So if I have a machine tool pulling 24 amps @ 115V on a 50 foot long 12 gauge wire fed by a 20 amp breaker..........

Can somebody tell me if this is ok for my electric motor?

Rob

[Eric Gustafson]

So if I have a machine tool pulling 24 amps @ 115V on a 50 foot long 12 gauge wire fed by a 20 amp breaker..........

Can somebody tell me if this is ok for my electric motor?

Rob

hmmmm. 24 amps on a 20 amp breaker. Won't hurt the motor at all since the breaker will trip!

[Rob Will]

hmmmm. 24 amps on a 20 amp breaker. Won't hurt the motor at all since the breaker will trip!

Eric, are you saying that I should not run this tool on that voltage?

Thanks,
Rob

[Greg Funk]

I don't see how the consumer is paying more? Your billed in kilowatt hours, T&D costs, and Federal and state taxes. I could theoretically increase the voltage but the net effect would be a drop in current, and the power would be close to the same. I'm just not seeing the big benefit for SCE and PG&E. Their contracts are structured to supply X amount of power to the grid, at X amount of dollars. It's actually in their best interest for the consumer to use less power. This way they don't have to purchase power contracts for additional power to meet their contractural obligation. They also, most likely, have a mechanism that kicks in that allows them to sell their "excess power", above a certain percentage, at the going market rate, which can be much higher than retail rates, by multiples.

For lighting loads if you increase the voltage the current will remain approximately the same and the power (and the customer's bill) will go up. I'm suspect there are other loads as well that would behave this way as well.

Here in British Columbia we buy power from California in the winter when their demand and market rates are low. Then we sell it back in the summer when demand and prices are high. Even though BC is a net importer of power we are able to use our hydroelectric dams as batteries and sell power profitably.

Greg

[Mike Seals]

The chart in the article confirmed something to me I knew from experience. That is, motor current increases with a drop in voltage. I have blown a few fuses in my day discovering that! :D

Some thing to consider, at least for the one phase customers, was mentioned by Dr. Bill Wattenburg on his KGO radio show. Bill said that electric power ultilities (he cited California) tend to run their voltages towards the higher end of their specification limits and that they do so for higher profits! The spec limits he mentioned for 110v were 105v-125v. Double that for 220v.

http://www.kgoam810.com/viewentry.asp?ID=353556&PT=PERSONALITIES


Just to join in on the discussion, I think to explain the higher voltages for transmission has a lot to do with the amount of VARs in the distribution.

You might want to look up VARs as electrical terms to get a good understanding of how they effect power. Low voltage bad for grid, high voltage good for grid.

16 years doing boiler and turbine controls for the utility company at a 2000 megawatt plant.

[mike wacker]

That's a good question Mike but I don't know much about electricity:o.

So if I have a machine tool pulling 24 amps @ 115V on a 50 foot long 12 gauge wire fed by a 20 amp breaker..........

Can somebody tell me if this is ok for my electric motor?

Rob


Ok, I'll bite, do you really have a "machine tool" with a 115V motor that has a FLA rating of 24amps or are you just pulling our collective chains?

[Mike Cutler]

That's a good question Mike but I don't know much about electricity:o.

So if I have a machine tool pulling 24 amps @ 115V on a 50 foot long 12 gauge wire fed by a 20 amp breaker..........

Can somebody tell me if this is ok for my electric motor?

Rob

The variable in question in this example is not the motor. You also don't have enough info.
Depending on the motor's size. It doesn't care about the other independents.
I'm going to discount all NEC requirements. We're only concerned with the motor. Right?

Point????

[Steve Leverich]

Rob, it's perfectly fine - the breaker will limit the amount of time this can go on :D Steve

[Rob Will]

The variable in question in this example is not the motor. You also don't have enough info.
Depending on the motor's size. It doesn't care about the other independents.
I'm going to discount all NEC requirements. We're only concerned with the motor. Right?

Point????

I just want to put a 2 hp motor on my 115V table saw. The local electric motor shop has the (Baldor) motor in stock but it draws 24 amps. What should I do?

Rob

[Greg Funk]

I just want to put a 2 hp motor on my 115V table saw. The local electric motor shop has the (Baldor) motor in stock but it draws 24 amps. What should I do?

Rob
Get a 3 phase motor;)

[Rob Will]

What's interesting (or possibly confusing) about the 3-phase table is that when a motor is run under-voltage, its startup current will be lower. I think this might be because a 3-phase motor gets rotating sooner than the single phase equivalent.

Either that, or the table is simply referring to the maximum startup current without regard to the length of the startup curve.

Hi Rick,
Looking at the chart from the Dept of Energy, it appears that all induction motors follow the trait of less voltage = less starting current.

What I can't figure out is why the motors require less starting current yet they seem to run hotter (according to the chart).

Rob

[Mike Henderson]

I just want to put a 2 hp motor on my 115V table saw. The local electric motor shop has the (Baldor) motor in stock but it draws 24 amps. What should I do?

Rob
I wouldn't do it but primarily because that motor is pretty inefficient. Two HP is about 1500 watts (rounded) while 24 amps at 115 volts is about 2760 VA (or watts, depends on how picky you are), giving an efficiency of the motor of about 54%. You should be able to find a 2HP motor that would be close to 80% efficient at full load.

A 2HP 80% efficient motor at full load would pull a bit over 16 amps, which would be okay on a 115 volt 20 amp circuit, with a slow trip circuit breaker to accomodate the startup current. But to answer your original question, I wouldn't put a 24 amp motor on 115 volts.

(assuming I did all the arithmetic correctly)

Mike

[Rob Will]

I wouldn't do it but primarily because that motor is pretty inefficient. Two HP is about 1500 watts (rounded) while 24 amps at 115 volts is about 2760 VA (or watts, depends on how picky you are), giving an efficiency of the motor of about 54%. You should be able to find a 2HP motor that would be close to 80% efficient at full load.

A 2HP 80% efficient motor at full load would pull a bit over 16 amps, which would be okay on a 115 volt 20 amp circuit, with a slow trip circuit breaker to accomodate the startup current. But to answer your original question, I wouldn't put a 24 amp motor on 115 volts.

(assuming I did all the arithmetic correctly)

Mike

That is interesting Mike, any idea where I can find an 80% efficient 3450 rpm 2 hp dual voltage motor?

Rob

[Mike Henderson]

That is interesting Mike, any idea where I can find an 80% efficient 3450 rpm 2 hp dual voltage motor?

Rob
Well, it's not exactly 80% efficient but the Weg 2HP 3450 RPM TEFC motor I have on my table saw is rated for 16.6 amps full load at 115 volts. That's pretty close to 80% (between 78% and 79%). I'm sure my motor is not that unusual - you should be able to find one like it fairly easily.

Mine is dual voltage and I run it at 220 volts. The high voltage spec is actually 208 to 230 volts but the low voltage is spec'ed for 115 volts.

Mike

[Dave MacArthur]

Pool pump shop ;)

[Mike Cutler]

I just want to put a 2 hp motor on my 115V table saw. The local electric motor shop has the (Baldor) motor in stock but it draws 24 amps. What should I do?

Rob

Source a different motor.

Once again, point???

I realize that you are trying to make some form of a point.
I also realize that you have an answer that you believe can be supported, and are trying to lead the discussion to that end, using a box argument strategy.

If you would state your premise and argument, we could, for ourselves, determine the validity of the question and your summation.
No one is learning anything right now Rob, and that is the point. Learning and exchange of knowledge.

Based upon the info you have so far elected to convey. There is a fault.

[Rick Christopherson]

I realize that you are trying to make some form of a point.
I also realize that you have an answer that you believe can be supported, and are trying to lead the discussion to that end, using a box argument strategy.
Ouch......., darn it. Oops, sorry. Please excuse me while I pick myself up off the floor from laughing too hard! I think I might have cracked a rib. :D

[Rob Will]

I wouldn't do it but primarily because that motor is pretty inefficient. Two HP is about 1500 watts (rounded) while 24 amps at 115 volts is about 2760 VA (or watts, depends on how picky you are), giving an efficiency of the motor of about 54%. You should be able to find a 2HP motor that would be close to 80% efficient at full load.

A 2HP 80% efficient motor at full load would pull a bit over 16 amps, which would be okay on a 115 volt 20 amp circuit, with a slow trip circuit breaker to accomodate the startup current. But to answer your original question, I wouldn't put a 24 amp motor on 115 volts.

(assuming I did all the arithmetic correctly)

Mike

I think Mike gets the last word on this.
It is possible to run a true 2 hp tool on 115V IF......

You have a dedicated circuit
You have a high efficiency motor
Everything in the circuit is in good condition (cord, plug, receptacle)
You have minimal voltage drop in the wire leading from the panelFrom Chart 2. in the OP, we can see how low voltage negatively affects amperage draw and motor performance. Any deviation from the optimum conditions listed above and you probably should not run a 2 hp motor on 115V.

Personally, I would not push the limit and like Mike, I would choose to run this motor on 230V.

And yes, I was pulling our chains. This was all hypothetical (My real life TS is a 7.5 hp 3-ph).:o

I just wanted to get to the bottom of this and clarify that we can't automatically put ANY 2 hp motor on ANY 20A 115V circuit.

Thanks for playing along to solve this puzzle.

Rob

[Rick Christopherson]

I have this dog named "Checkers" and if I talk about my dog enough, no one will realize that I never really answered the original question. Well somehow or another, we got from your original posting to you claiming some sort of hollow victory without any substance in between. Sounds like Checkers to me.

[Art Mann]

Gosh, I can't believe I plowed through this whole thread only to find a statement of the obvious. What am I missisng?

[Rob Will]

I think Mike presented the data that nobody can argue with.

That is some 2hp motors will work, some will not.

Rob

[Art Mann]

Grizzly sells a tablesaw they claim is 2 hp that is pre-wired for 120V and they claim the full load current is 20A. They have another model that is also rated at 2 hp that is prewired for 240V and the 120V current is 24A. I don't see anything particularly unique about either of these saws.

[Mike Henderson]

I think Mike presented the data that nobody can argue with.

That is some 2hp motors will work, some will not.

Rob
Well, I will add one additional point. The design of induction motors is VERY mature. Most motors of the same size (HP rating) should have approximately the same efficiency. Small motors are less efficient while large motors are more efficient. In industrial sizes (let's say 50HP or larger) its not uncommon to find efficiencies of better than 90% at full load.

I would be surprised to find a 2HP motor that was 55% efficient (or so) unless that motor had some unusual characteristics. I would expect to see essentially all standard 2HP induction motors to be in the 70% - 80% efficient range. I just assumed Rob was using some arbitrary numbers in his question, just for the sake of argument.

Mike

[Rob Will]

Well, I will add one additional point. The design of induction motors is VERY mature. Most motors of the same size (HP rating) should have approximately the same efficiency. Small motors are less efficient while large motors are more efficient. In industrial sizes (let's say 50HP or larger) its not uncommon to find efficiencies of better than 90% at full load.

I would be surprised to find a 2HP motor that was 55% efficient (or so) unless that motor had some unusual characteristics. I would expect to see essentially all standard 2HP induction motors to be in the 70% - 80% efficient range. I just assumed Rob was using some arbitrary numbers in his question, just for the sake of argument.

Mike

Not really, The Baldor catalog shows 2 hp motors that draw 24 amps at 115V. Other models draw 20 amps.

Rob

[Greg Funk]

I would be surprised to find a 2HP motor that was 55% efficient (or so) unless that motor had some unusual characteristics. I would expect to see essentially all standard 2HP induction motors to be in the 70% - 80% efficient range. I just assumed Rob was using some arbitrary numbers in his question, just for the sake of argument.

Mike
Don't forget to include power factor when calculating current.

I = 746 x HP / (Voltage x PF x Efficiency)

Greg

[Mike Henderson]

Don't forget to include power factor when calculating current.

I = 746 x HP / (Voltage x PF x Efficiency)

Greg
Yes, that is included. In the motor I have, for example, the overall efficiency, including power factor, is close to 80%. At full load, the power factor approaches one. The average power factor of motors is usually given as .8 but that includes running at all load levels. When running at no load, a motor will have a power factor of .2 or even less.

Note that when you measure the current to a motor you're measuring the actual current - both in phase and out of phase. That's why you see unloaded motors drawing what you might consider to be a lot of current - most of it is out of phase (low power factor) because the motor looks like a big inductor to the line - the only real (in phase) power is that necessary to overcome friction losses and resistance losses in the wires. As the motor is loaded, the current doesn't increase that much but what happens is that the power factor increase so more of the current is in phase (and thus represents real power).

So the motor converts more electrical power to mechanical power as the load increases but the current in the line doesn't increase that much. If you were to put a watt meter on the wires feeding the motor you'd see the change in power because a watt meter only measures in phase current (and voltage).

Mike

[added note] There was a scam some years ago where the huckster would hook a voltmeter and ampmeter to a small induction motor (maybe a quarter or half HP) running at no load. He'd tell you to multiply the volts times the amps to get the watts (what you would get charged for - but note that you can't compute watts this way when you have out of phase current). Then he'd take a "magic box" and plug it into the same receptacle and the current would decrease. He'd then tell you that if you bought one of his magic boxes and pluged it into the receptacle where you have motors (like a 'frig), you'd save all kind of money. What his "magic box" was, of course, was a capacitor which corrects the power factor (it compensates for the inductance of the motor) and makes the current more in phase. Since a motor only needs a small amount of in phase current when running no load, it caused the current to the motor to decrease. But since you only pay for in phase current, it didn't change your electricity bill by a penny (well maybe a penny because of less resistance losses but not much more than that).

[Rob Will]

Most of the 2 hp single phase motors that Grainger sells are between 18 and 24 full load amps........

Here's an interesting twist: Grainger sells two similar 2 hp "air compressor" motors with very different specs. Both are made by GE and are on a NEMA 56 frame.
At 115V...
3K787 draws 15.0 FLA service factor 1.0 weight 33.4 lbs
5TB72 draws 23.8 FLA service factor 1.15 weight 32.3 lbs

In larger frame sizes (NEMA 182) they sell 2 hp motors such as:
6K313 which draws 23.6 amps at 115V, service factor 1.0 weight 61 lbs.

Just three examples.
So what is the deal here folks? How do they come up with these HP ratings?

When we get in these discussions about who can run what on 115V, are we talking about "real" horsepower? Are we talking about the same thing?

Of the motors listed above, which one would perform best on a table saw?

I have a HF milling machine with a 115V only motor (unknown hp). It will not run on any 20 amp breaker. We had to install a dedicated 30A circuit. I sure wish it would go 230V. Is this a common problem with imported motors?

Rob

[Mike Cutler]

I would be surprised to find a 2HP motor that was 55% efficient (or so) unless that motor had some unusual characteristics. I would expect to see essentially all standard 2HP induction motors to be in the 70% - 80% efficient range. I just assumed Rob was using some arbitrary numbers in his question, just for the sake of argument.

Mike


Mike the " unusual characteristic" is torgue. Efficiency is traded for torgue in high torgue application. Specifically airconditioning and refrigeration where the D/P across the rotating element is expected to be high on a start up basis, and are expected to operate at or near full load in a continuous duty application due to the D/P across the rotating element.

Rob

In your final post the "most correct", but still poor choice, motor would be the 3K787 based on all variables.
The 6K313 motor is a 1740 RPM, high torgue farm duty motor.
The 5TB72 is a high torgue motor also.

I suspect that your milling machine has a high torgue, low RPM motor. It's not an import motor issue, it's most likely the design.

However, for the sake of everyone being able to learn, the most important point that you made was not to indiscriminately choose a motor based on any one single variable. The motor needs to be selected based upon the intended application, use ,and motor nameplate data. Just as you would not want to use a high torgue motor in a tablesaw application capable of drawing 20+ amps, you wouldn't want to go the opposite dircetion either and install a high efficiency lower torgue motor in a high torgue application.
While allowing yourself more "headroom" as you termed it, by automatically defaulting to a higher voltage for margin, a person can still run into the same issues, and some even larger issues based on the design of the motor. The motor has to be selected for it's intended application.

Coupling this thread with the other thread concerning the selection of 115 or 220 for a General 2HP tablesaw. The answer is still the same. It's not necessary to rewire that particular motor for 220. The motor was sized and selected based on it's intended application by the manufacturer, mine was shipped with a 115/ 15 amp plug configuration. I actually suspect that the motor is really not a "true" 2HP motor,and General is advertising it based upon some "apparent HP " that they alone can define, (much like my 6.5hp Rigid wet/dry vac with a 15 amp plug), or it's one very efficient motor, which I doubt based on the price of the machine as a whole.

While I do, now, understand the point you were driving at. ( I'm a little slow sometimes.;) ).It is still incumbent upon the individual to select the correct replacement component should they elect not to replace "like for like" with the OEM spec'd motor.
It is an unreasonable expectation to assume that a design engineer, or manufacturer would design around the incorrect substitution of a part or component by the end user. Once you're outside of the design criteria, you're on your own.

Apologies for getting a little terse in my previous post. I always tell myself not to get involved in discussions on motors and electricity, but I seem to be drawn to them like a moth to a porch lite.

Mike

[Rob Will]

Good answer Mike, you make a good point and you do it in a professional manner. I did not catch the RPM of the heavier motor but you saw through that - it was just an example of how different these motors are.

If hp is a function of torque and rpm, then the low torque / low amp draw models are indeed NOT putting out the same hp(?).

Like you, I have come to expect this in things like the 6.5 hp shop vacs.

It seems like the machine tool manufacturers all have a different idea of what constitutes a "2hp" table saw and how much electricity that particular tool will draw.

Rob

[Eric Gustafson]

I don't see how the consumer is paying more? Your billed in kilowatt hours, T&D costs, and Federal and state taxes. I could theoretically increase the voltage but the net effect would be a drop in current, and the power would be close to the same. I'm just not seeing the big benefit for SCE and PG&E. Their contracts are structured to supply X amount of power to the grid, at X amount of dollars. It's actually in their best interest for the consumer to use less power. This way they don't have to purchase power contracts for additional power to meet their contractural obligation. They also, most likely, have a mechanism that kicks in that allows them to sell their "excess power", above a certain percentage, at the going market rate, which can be much higher than retail rates, by multiples.
Putting the report in its context: It is the kilowatt hours that were lowered in the test. Less KW's=less revenue.



I don't like the fact that the report itself seeks to spec Dr. Wattenberg's patent as the sole device for the "soft blackout" in the event that California adopted the EVR approach. Little bit of conflict of interest there. I also observed that he had a patent he was specifying for the vehical barriers. Interesting.
Actually, the the report did not tout Wattenburg's patent as the sole device forcontrolling the consumer's energy use. The report cited numerous problems with Wattenburg's patented device. Instead, it talked about EVR being an easier solution where the power companies could control wattage consumption in the event of an emergency. Wattenburg wouldn't make a penny from that.



This EVR was a knee jerk reaction to a "power supply" shortage that existed on paper only. Had they ever implemented the policy they may have found thereselves in an even worsening position. They may not have been able to import power from outside the state to meet their demand at any cost. Kind of a domino effect.
The actual culprit was the poorly thoughtout and implemented deregulation bill that California signed into law. I've sat through a few seminars on the mechanics of that situation. It's the "model" for states on how not to implement deregulation. It also shelved the deregulation efforts of many states, which was a good thing. It's just too bad that it had to happen at the California rate payers expense. Of course, our situation here in CT isn't that much better. We didn't learn quick enough.

I did realize the environment that had brought about the testing of EVR.



I still don't see where PG&E and SCE are actually lowering the voltage to increase profit, or how he is supporting that statement. That part I'm not catching. Am I missing it somewhere?

I think you meant increasing voltage to increase profit. Yes. The report never made that accusation. Wattenburg made it on his radio show. I assume (and we all know what happens when we assume) that after the EVR test lowered energy use, the question remained whether higher voltages are really neccesary, or do they merely serve to increase KW consumption. The questions about motor life or other possible ramifcations linger.

[Mike Henderson]

I didn't quote Mike Cutler's posting because it's long but this note is in reference to efficiency of motors.

Mike commented that the difference could be largely due to differences in torque in the motors. Power is force times velocity. HP in a motor is torque times angular velocity (essentially RPM). So the difference between a 2HP 3450 RPM motor and a 2HP 1725 RPM motor is that the 1725 RPM motor would have to produce twice the torque, which means that it would be physically larger and require more current. The 1725 motor would be less efficient than the faster motor.

What's not clear to me is why a two different 2HP 3450 RPM induction motors would have significantly different efficiencies. I'm assuming they would be approximately the same physical size. Differences in cooling techniques could account for some of the difference in efficiency, but the difference would be fairly small. Given the same physical size and same RPM, they would both produce the same torque (in order to produce the same HP).

It would not be starting torque because motors are rated at their operating speed and not their starting torque. Also, there are different techniques for starting a motor, some which provide higher torque than the capacitor start technique. And regarding continuous duty, it seems that cooling would be the controlling factor of continuous duty and would not affect the efficiency that much.

Mike (or anyone else) can you help me understand this? I guess I could go do some research on this but if you know, that's easier than digging.

Mike

[Rob Will]

I didn't quote Mike Cutler's posting because it's long but this note is in reference to efficiency of motors.

Mike commented that the difference could be largely due to differences in torque in the motors. Power is force times velocity. HP in a motor is torque times angular velocity (essentially RPM). So the difference between a 2HP 3450 RPM motor and a 2HP 1725 RPM motor is that the 1725 RPM motor would have to produce twice the torque, which means that it would be physically larger and require more current. The 1725 motor would be less efficient than the faster motor.

What's not clear to me is why a two different 2HP 3450 RPM induction motors would have significantly different efficiencies. I'm assuming they would be approximately the same physical size. Differences in cooling techniques could account for some of the difference in efficiency, but the difference would be fairly small. Given the same physical size and same RPM, they would both produce the same torque (in order to produce the same HP).

It would not be starting torque because motors are rated at their operating speed and not their starting torque. Also, there are different techniques for starting a motor, some which provide higher torque than the capacitor start technique. And regarding continuous duty, it seems that cooling would be the controlling factor of continuous duty and would not affect the efficiency that much.

Mike (or anyone else) can you help me understand this? I guess I could go do some research on this but if you know, that's easier than digging.

Mike

Looks like Mike will have to start digging (?)
I have no idea.

Rob

[Will Blick]

Mike, good point. I was involved in motor sales for many years.... this is a good question....and IIRC, the difference comes down to how the motor is designed and its internal components. I remember Baldor used to make two identical motors, yet, one was HE rated at 93%, while the normal model would be in the low 80's. Of course, this added efficiency added disproportionaly to the cost. Of course, if the motor ran 24 hrs a day, it would still be cost effective.

However, sometimes there is other considerations, such as, needing the HE motor cause you simply do not have enough amperage available on a given circuit, which would often justify the added cost of the HE motor, vs. re wiring...

Just guessing, i would think higher end ww tools designed to run on 12amp circuits (115V ONLY), would use very high eff. motors to achieve max. usable HP. Whereas, with 115/230V motors, i am not sure this is the case, as they user has an option if the motor is undersized for the application. So the 115/230V motor can be less eff., saving a lot on cost. Just a thought....

As for the 5HP, 12amp shop vacs....their should be a law against this type of false advertising....

[Greg Funk]

What's not clear to me is why a two different 2HP 3450 RPM induction motors would have significantly different efficiencies. I'm assuming they would be approximately the same physical size. Differences in cooling techniques could account for some of the difference in efficiency, but the difference would be fairly small. Given the same physical size and same RPM, they would both produce the same torque (in order to produce the same HP).

I suspect for the same reason different motors have different service factor ratings. Using heavier copper for the windings will result in lower I2R losses and less heat. Similarly using better steel and laminating the core will reduce magnetic losses. Of course doing this costs extra but can be economical if the motor is run continuously.

Greg

[Mike Henderson]

I had a discussion with a power engineer friend of mine. He mostly deals with LARGE 3 phase motors and power systems but I asked him to speculate on why different single phase motors of the same speed and HP might have different amperage ratings.

His best guess is the same as Greg Funk's - service factor. Service factor is the ability of a motor to provide power in excess of it's rated power for short periods of time (he did comment that service factor involves more than just increased HP but I forget what the other things are that affect service factor - tolerance of heat rise is probably one of them). For example a 2 HP motor with a service factor of 1.15 would be able to produce 2.3 HP for some period of time, while a 2 HP motor with a service factor of 1 would not be able to produce the additional HP (the 2 HP SF 1 motor might stall if presented with a 2.3 HP load). HP is torque times angular velocity (times a constant) so a motor with a higher service factor must be able to produce more torque. Torque is directly related to the flux density, which is produced by current flowing in the stator coils. So a motor with a higher service factor will require more current than a motor with a lower service factor. When measured against the RATED HP, a motor with a higher service factor will be less efficient (you never get something for nothing).

We also discussed efficiency and he commented that higher efficiency is mosty due to reducing losses - specifically resistance losses in the wires (both in the stator coil and in the rotor conductors) and losses in the steel laminations (eddy currents). Tolerances can affect efficiency - the smaller the air gap between the stator coils and the rotor, the less magnetic reluctance and the more efficient the motor. Making the air gap small requires accurate balancing of the rotor and accurate machining of both the stator and the rotor.

While not related to efficiency, he also pointed out that bearing in motors vary widely, from simple sleeve bearings, to sealed ball bearings, to sealed taper roller thrust bearings.

I only had a limited time to discuss things with him so if you have specific questions, post them here and I'll bring them up to him next time he and I meet.

Mike

[Rob Will]

I had a discussion with a power engineer friend of mine. He mostly deals with LARGE 3 phase motors and power systems but I asked him to speculate on why different single phase motors of the same speed and HP might have different amperage ratings.

His best guess is the same as Greg Funk's - service factor. Service factor is the ability of a motor to provide power in excess of it's rated power for short periods of time (he did comment that service factor involves more than just increased HP but I forget what the other things are that affect service factor - tolerance of heat rise is probably one of them). For example a 2 HP motor with a service factor of 1.15 would be able to produce 2.3 HP for some period of time, while a 2 HP motor with a service factor of 1 would not be able to produce the additional HP (the 2 HP SF 1 motor might stall if presented with a 2.3 HP load). HP is torque times angular velocity (times a constant) so a motor with a higher service factor must be able to produce more torque. Torque is directly related to the flux density, which is produced by current flowing in the stator coils. So a motor with a higher service factor will require more current than a motor with a lower service factor. When measured against the RATED HP, a motor with a higher service factor will be less efficient (you never get something for nothing).

We also discussed efficiency and he commented that higher efficiency is mosty due to reducing losses - specifically resistance losses in the wires (both in the stator coil and in the rotor conductors) and losses in the steel laminations (eddy currents). Tolerances can affect efficiency - the smaller the air gap between the stator coils and the rotor, the less magnetic reluctance and the more efficient the motor. Making the air gap small requires accurate balancing of the rotor and accurate machining of both the stator and the rotor.

While not related to efficiency, he also pointed out that bearing in motors vary widely, from simple sleeve bearings, to sealed ball bearings, to sealed taper roller thrust bearings.

I only had a limited time to discuss things with him so if you have specific questions, post them here and I'll bring them up to him next time he and I meet.

Mike

Nice work Mike! I suppose that my 6.5 hp shop vac must have VERY tight tolerances.:rolleyes: Thanks for the info.

Rob

[Mike Henderson]

Nice work Mike! I suppose that my 6.5 hp shop vac must have VERY tight tolerances.:rolleyes: Thanks for the info.

Rob
Either that, or a negative service factor:)

Mike