# Motor amps/hp



## John Valdes (May 17, 2007)

He will get more torque, but his speed will remain the same. That is if he buys motors the same speed (RPM) as he already has. 
Use the formula below to find the torque of an electric motor.

*T = 5252 x HP
 rpm*​


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## JRaef (Mar 23, 2009)

"Horsepower" is a shorthand way of saying "X amount of torque at Y RPMs". If a motor says 60HP, that means the torque at the rated speed comes out to 60HP in kinetic energy.

The electrical energy used by that motor is related, but not completely dependent upon the HP. It ALSO includes the energy lost in the process of converting the electrical energy into kinetic energy. This is supposed to be shown as the efficiency value on the motor nameplate. But even then, there are truths, lies and marketing information, the latter being what often ends up on the nameplate, neither the total truth nor a total lie. Still, it's the best we have to go on without doing our own measurements. Generally speaking, if two motors have the same nameplate HP rating and the same speed, then if one says it consumes more amps, that is generally an indicator of lower efficiency, which is I think where you were coming from and it's a reasonable assumption.

But...

If the motor base speed is NOT the same, then the amps will not be the same. Just saying 60HP without discussing the motor speed is leaving out an important factor. If he is saying that he is getting more RPMs out of the motors with the higher amperage ratings, that could simply be that he is using 2 pole motors (3600 RPM synchronous speed) vs 4 pole motors (1800 RPM synch speed). That would fit the model you are describing, but it is not apples to apples and if they are centrifugal pumps, there is a HUGE difference involved. So if his pumps were designed for 2 pole motors and someone changed them to 4 pole motors, he would be seeing a MAJOR loss of pumping capacity from them. But if the pumps were designed for 4 pole motors and he has replaced some with 2 pole, he would see more flow but has created a major overload problem for his motors and failure is eminent.

But then again, if they are both the same base speed and one has a SLIGHTLY different slip speed (the difference between the synchronous speed and the actual speed), ie 1750RPM vs 1725 RPM, then he is really just perceiving that the slight difference in that speed is giving him something it is not. You cannot have higher speed AND higher torque at the same time if that is the case. Lower slip will mean higher net speed, but LESS torque, and vice versa.

Bottom line, you need to know more than you know now to pass judgement, but you are right to question it.


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## micromind (Aug 11, 2007)

Higher amps for the same HP = lower power factor. 

The reality is that if the POCO is charging him for VARs, his bill will go up. Lower PF = higher VAR.


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## Duke55 (Oct 4, 2011)

_"Horsepower" is a shorthand way of saying "X amount of torque at Y RPMs". If a motor says 60HP, that means the torque at the rated speed comes out to 60HP in kinetic energy._

_The electrical energy used by that motor is related, but not completely dependent upon the HP. It ALSO includes the energy lost in the process of converting the electrical energy into kinetic energy. This is supposed to be shown as the efficiency value on the motor nameplate. But even then, there are truths, lies and marketing information, the latter being what often ends up on the nameplate, neither the total truth nor a total lie. Still, it's the best we have to go on without doing our own measurements. Generally speaking, if two motors have the same nameplate HP rating and the same speed, then if one says it consumes more amps, that is generally an indicator of lower efficiency, which is I think where you were coming from and it's a reasonable assumption.

But...
If the motor base speed is NOT the same, then the amps will not be the same. Just saying 60HP without discussing the motor speed is leaving out an important factor. If he is saying that he is getting more RPMs out of the motors with the higher amperage ratings, that could simply be that he is using 2 pole motors (3600 RPM synchronous speed) vs 4 pole motors (1800 RPM synch speed). That would fit the model you are describing, but it is not apples to apples and if they are centrifugal pumps, there is a HUGE difference involved. So if his pumps were designed for 2 pole motors and someone changed them to 4 pole motors, he would be seeing a MAJOR loss of pumping capacity from them. But if the pumps were designed for 4 pole motors and he has replaced some with 2 pole, he would see more flow but has created a major overload problem for his motors and failure is eminent.

But then again, if they are both the same base speed and one has a SLIGHTLY different slip speed (the difference between the synchronous speed and the actual speed), ie 1750RPM vs 1725 RPM, then he is really just perceiving that the slight difference in that speed is giving him something it is not. You cannot have higher speed AND higher torque at the same time if that is the case. Lower slip will mean higher net speed, but LESS torque, and vice versa.

Bottom line, you need to know more than you know now to pass judgement, but you are right to question it.
__________________
"If you don't know where you're going, then any direction will do." -- Lewis Carroll
_
Thanks for the reply
I think I left out some information.
The existing Motor is 6 pole 1180 rpm 230/460v inverter duty
60hp/45KW
1.15 S.F 
137/68.3 amps 
Also i should have be more clear with regard to speed. He(customer) is referring to his "rod speed" (this is a PCP pump on an oil well) to me his rod speed will fluctuate depending on the load and torque available. The VFD is limiting the torque to a torque max parameter in order to provide protection of the rod on the well so they don't start busting rods.

Hopefully this help a bit.

Thanks


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## JRaef (Mar 23, 2009)

Duke55;951127[I said:


> ][/I]
> Thanks for the reply
> I think I left out some information.
> The existing Motor is 6 pole 1180 rpm 230/460v inverter duty
> ...


So do you know the specs on the motors he wants to put in their place?


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## Duke55 (Oct 4, 2011)

Unfortunately I haven't gone out to site to confirm any nameplate data on the "higher amp" motor that he is using on one of his other sites. I do know that they are using some westinghouse and some toshiba motors. He is telling me that the motor that he wants say 74 amps on the nameplate.. but like I say I don't have a pic of the nameplate and haven't confirmed it myself.


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## John Valdes (May 17, 2007)

I have seen some motors exactly the same as far as NEMA nomenclature, but differing in service factor. Higher service factor can allow the manufacturer to list the particular motor at a higher current rating (FLA).

I have seen service factors as high as 3%. This increases the FLA listed on the nameplate. Whereas a motor with a 1.5% service factor will have a lower FLA on the plate.
Marketing.
These motors are the same as far as NEMA is concerned.

OP. A 6 pole motor is 1200 RPM. An 8 pole motor is 900 RPM. This is where I find my increased torque. Lowering base speed.


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## Duke55 (Oct 4, 2011)

I just had something else that I'm not sure of.... just to make things a little more confusing... lol
Because all the name plate data is based on 230/460 volts... and in this application our supply voltage is in fact 480v.. do we have to do a calculation to make adjustments to the our actual FLA? Without do this calculation could we be overloading this motor and causing premature failure?


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## JRaef (Mar 23, 2009)

No. All motors are rated for 460V, not 480V. It's the difference between "Utilization Voltage" and "Distribution Voltage". It allows for voltage drop without loss of performance.

3phase Distribution Voltages: 480, 240, 208. Single phase: 240, 120
3phase Utilization Voltages: 460, 230, 200. Single phase: 230, 115


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## xlink (Mar 12, 2012)

Are all 60 hp motors identical or might one be exactly 60 hp and another manufacturer 61 hp?


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## Duke55 (Oct 4, 2011)

I guess what I was wondering originally ... All things being equal.. Voltage, RPM, S.F, HP, the only difference being one motor having a higher FLA on the name plate.. Then will that motor have the capability of better performance.. Or is it just a less efficient motor?


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## micromind (Aug 11, 2007)

xlink said:


> Are all 60 hp motors identical or might one be exactly 60 hp and another manufacturer 61 hp?


Depends on the service factor. 

A lot of NEMA frame motors have a service factor above 1.0 Most are 1.15 A 10 HP motor with a service factor of 1.15 is actually an 11.5 HP motor. 

This motor can actually produce 11.5HP continuously, provided it has enough cooling air that's not too hot (usually 104ºF), and the voltage at its terminals is within specs (including balance, if it's 3ø). 

Some motors have both full-load amps and service factor amps. 

Once in a while, you'll see a factory-built machine (usually a fan) that'll spec HP slightly higher than the motor is rated for. This is because it's being run 'in the service factor'. 

IEC (metric) motors are a different story. There is no service factor on these motors. 10HP is 10HP. Not a bit more.


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## micromind (Aug 11, 2007)

Duke55 said:


> I guess what I was wondering originally ... All things being equal.. Voltage, RPM, S.F, HP, the only difference being one motor having a higher FLA on the name plate.. Then will that motor have the capability of better performance.. Or is it just a less efficient motor?


It may or may not be more efficient. Efficiency is watts going in vs. HP going out. Amps has little to do with it. 

The one with higher amps will have a lower power factor. This is not an issue unless the POCO is billing for VARs as well as KWhrs. Around here, they bill for VARS only on larger services, usually more than 500KVA of calculated load.


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## Duke55 (Oct 4, 2011)

I'm really not sure what you mean about amps having little do with Efficiency... If Efficiency is watts going in vs hp going out (watt = Volt x amps).. so when your voltage is consistent and your amps increase and the hp of your motor remains the same.. then your efficiency would be less.

And to be truthful, this customer really won't care about the price of electricity, just about getting the oil out of the ground...lol But I am trying to figure out what advice to give when he is talking about spending thousands of dollars to get 60hp motors that use more amps than the 60hp motors he has now.


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## micromind (Aug 11, 2007)

Duke55 said:


> I'm really not sure what you mean about amps having little do with Efficiency... If Efficiency is watts going in vs hp going out (watt = Volt x amps).. so when your voltage is consistent and your amps increase and the hp of your motor remains the same.. then your efficiency would be less.
> 
> And to be truthful, this customer really won't care about the price of electricity, just about getting the oil out of the ground...lol But I am trying to figure out what advice to give when he is talking about spending thousands of dollars to get 60hp motors that use more amps than the 60hp motors he has now.


With any AC inductive load, watts does not equal volts times amps. The actual formula is W = V X A X PF. Power factor is the difference between true power (watts) and apparent power (volts X amps). 

If a motor is running with nothing connected to its shaft, it is producing VERY little HP. Only what is needed for its cooling fan and bearing losses. But the amps are still fairly high. Watts however are very low, as is power factor. 

As the load increases, amps will rise, but watts will rise faster. And power factor will also rise.


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## backstay (Feb 3, 2011)

Duke55 said:


> I'm really not sure what you mean about amps having little do with Efficiency... If Efficiency is watts going in vs hp going out (watt = Volt x amps).. so when your voltage is consistent and your amps increase and the hp of your motor remains the same.. then your efficiency would be less.
> 
> And to be truthful, this customer really won't care about the price of electricity, just about getting the oil out of the ground...lol But I am trying to figure out what advice to give when he is talking about spending thousands of dollars to get 60hp motors that use more amps than the 60hp motors he has now.


All the theory and tech talk is good but, if he wants to change out motors and will pay you to do what's the problem?


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## Duke55 (Oct 4, 2011)

Ok I guess I should say all things being equal ... Volts.. Power factor... RPM... SF... HP..The ONLY difference being amps.... is the one with higher amps capable of increase performance or just less efficient???


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## Duke55 (Oct 4, 2011)

Ha Ha I do see where you are coming from... if he wants to pay for them then what is the problem.. But unfortunately it's my job to try and save them from themselves sometimes. At the end of the day I can't stand by and let them spend that kind of money for what to me looks like absolutely no reason at all. There are far better upgrades that they could be spending their money on that would make my life easier.


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## JRaef (Mar 23, 2009)

Duke55 said:


> Ok I guess I should say all things being equal ... Volts.. Power factor... RPM... SF... HP..The ONLY difference being amps.... is the one with higher amps capable of increase performance or just less efficient???


All things being equal, then yes, the motor with higher amps would be less efficient.

But things are never equal, it's just not that simple. Look at it this way; the math formula for calculating 3phase amps when you know HP and Voltage is this:
A = HP x 746 / ( V x 1.732 x Eff x PF) 

In that formula there are two variables left unknown, the efficiency (Eff) and Power Factor (PF). Only if you pull two identical motors of the same make at the same time from a mfrs line will those two variables be exactly the same. So the amps will end up different if EITHER of those variables is different in your customer's motors. If it is only the power factor that is different, ie the replacement motors have a higher power factor, it might be because they have lower slip, which would then mean they will turn slightly faster. But if so, they would actually have LESS shaft torque in theory. Sometimes people _perceive_ faster speed as more torque, however you can't have it both ways. But if the PF is the same, then the one with higher amps would be less efficient.

And yes to the other question, motor nameplates reflect MINIMUM shaft HP, so it is possible that one is actually something like 61 HP vs the other being exactly 60HP.

And yes it is also possible that the customer is running his replacement motors into the service factor and is incorrectly reading the nameplate in saying that the amps are higher by reading the Service Factor amps. They are required to be listed separately so that you know what you are getting into. Running a motor into the SF is a practice that OEMs do to avoid supplying a larger motor, but the definition of SF says that you should expect less motor life, the burden of which falls on the end user not the OEM because it will still likely outlive the warranty.


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## Duke55 (Oct 4, 2011)

Thanks that all help, what I'm taking from this is that there will be minimal difference if any... certainly not enough to justify spending upwards of 75 000 dollars... I do understand that there are many variables at work here.. based on what I'm hearing I just can't recommend that it's a good idea for the customer.


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## John Valdes (May 17, 2007)

Duke55 said:


> I just had something else that I'm not sure of.... just to make things a little more confusing... lol
> Because all the name plate data is based on 230/460 volts... and in this application our supply voltage is in fact 480v.. do we have to do a calculation to make adjustments to the our actual FLA? Without do this calculation could we be overloading this motor and causing premature failure?


Many times you will hear electricians, especially guys on the forum make fun of using the term, 460 volts. 
Problem with that is NEMA electric motors are rated 230/460.
Not 230/480.




Duke55 said:


> Thanks that all help, what I'm taking from this is that there will be minimal difference if any... certainly not enough to justify spending upwards of 75 000 dollars... I do understand that there are many variables at work here.. based on what I'm hearing I just can't recommend that it's a good idea for the customer.


I cannot see upgrading/increasing HP to achieve higher efficiency.
Efficiency is choosing the correct size motor for your application and the added use of the VFD gives you more ability to be as efficient as possible. 
Seems to me, this is already present.
If anything can be done, it is replacing inefficient motors with highly efficient motors.
Baldor builds a "Super E" premium efficient motor for example.


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## Duke55 (Oct 4, 2011)

that is why I have a problem with the request from the operations that I'm dealing with... at the most basic level all I can see is that they are asking me to replace the efficient 60hp motors that they already have with less efficient 60hp motors... because one the operators thinks that when he did this (by accident, just what replacement motor was available at the time) he gained 20 RPm (rod speed, which is after a 4:1 gear)


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## xlink (Mar 12, 2012)

It's more complicated than just pump speed. Pumping more water is less profitable. There are pump controllers that vary the speed, or cycle the pump, to get better efficiency in terms of oil per stroke (or per watt). While your masters want more stokes, others are working with fewer strokes.

(I'm told that when a well is over-pumped, the void is filled with the less dense water so the well produces more water and less oil.)

But you're on a VFD. Does a VFD really know the exact torque? If so, why would a different motor drive the same load, faster with less torque?


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## JRaef (Mar 23, 2009)

xlink said:


> It's more complicated than just pump speed. Pumping more water is less profitable. There are pump controllers that vary the speed, or cycle the pump, to get better efficiency in terms of oil per stroke (or per watt). While your masters want more stokes, others are working with fewer strokes.
> 
> (I'm told that when a well is over-pumped, the void is filled with the less dense water so the well produces more water and less oil.)
> 
> But you're on a VFD. Does a VFD really know the exact torque? If so, why would a different motor drive the same load, faster with less torque?


LOL, I forgot that these were on VFDs! 
Changing the motor is NOT going to affect the pump speed them, it is the VFD that controls the pump speed. And assuming it is a Sensorless Vector Drive (if not, it should be on a PCP) it is also controlling the torque as well.

Given that reminder, I'd bet that when the drives were initially installed, somebody did not know how to set up the SVC control in the drives, or they were in too much of a hurry so they left them in V/Hz control mode, or put them in SVC but never tuned them to the motors. Then when they put in the new motors, whomever did that noticed and turned on the SVC mode thus making the pumps perform better, i.e. better torque to the pump, maybe even a perceived higher speed because of lower slip. Could be that is all he really needs to do, go through and make sure all the VFDs are properly configured for SVC control. It means uncoupling the motors and doing an Auto-tune on them, that's likely why it happened when they put in the new one.


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## Duke55 (Oct 4, 2011)

Yep these are all on vfd's (ABB 600 and a few 800) they are PCP's. and you're right they should always do an auto tune on them when replacing the motors, I bet you're right over the years there are probably a lot if these wells that weren't. Most of these wells limit out in torque and not speed... They limit at 1100 ftlbs otherwise they will break rods. Very rarely do they hit the speed limit without torquing up first. Most if the time they run at 1100 with the rod speed varying depending on load.


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## Ultrafault (Dec 16, 2012)

I just want to say thanks for the informative conversation guys but that sure was the long way around to the anwser. Same thing happens to me all the time.


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