Electric Motors/Motor Amps



I have a math question about amp conversion.

A 12 volt DC motor pulls 130 amps. If that 12 volt motor were to be exchanged for 120 volt AC motor would the amps be 13?  

Thanks :-)


Jeff   you are thinking   good for you,        

Lets start with just AC  to avoid confusion for now,    

To perform a task,   be it driven by an electric motor or an internal combustion engine,   it is going to take   X amount of power to perform the task.

A good example of torque or twisting power is a TORQUE WRENCH.  Take away rust, time corrosion and all that,   if a bolt is tightened to 16 foot pounds of torque [power]  it is going to take 16 foot pounds to un-tighten that bolt.

So here is a sub question to ponder    is it better to use an air powered wrench or a break over bar with leverage?    Most think it is an air powered wrench,  but the truth is a long strong breakover bar will do the job a lot faster.

It's efficiency.  Loose the mechanical components of the air wrench,  and the losses such as friction, in the gearing, the natural losses of the air motor, being friction and windage,   a breakover bar,    is much more efficient.

Your question is filled with caveats and unknowns. But a good one.

So sticking with AC for now,    if 110 volt motor draws 10 amps to perform a task,  [produce the startup torque, overcome windage and friction,]     then the very same motor wired for 220 volts will use 5 amps.

We have to have some assumptions here,  the load or task is measured by a calibrated dyno or other instrument and the load is known and proven,   so no other outside effects are going on,  it is same on same.

This sounds great at first look,   5 amps versus 10 amps,   half the amps,  must be half the cost of energy.  Not so,     amps time volts is going to give you an amount of power,  KVA, HP whatever we are dealing with and need to use.

So the 110 x 10 amps  if going to be the same as 220 x 5 amps,      amps are a result,   not a controlled part of the equation,    

In some cases the higher voltage is a bit more efficient,   but for this question, and in the real world it is so small it is not worth talking about.

So you can use that for AC  use 12 volts of AC   when connected to the same load  it will draw whatever it needs to turn the load,  so it would be  12 times x   the working amperage.

Just replace the values and you get what you get,   now  REMEMBER THE MOTOR IS THE SAME IDENTICAL MOTOR,   only configured to accept a different voltage,     the resultant amperage is  AGAIN   a result,     

Now the monkey wrench,  you went from DC to AC,    and the answer is no one knows.  We will know when we connect them to the same exact loads,  but we have no idea of the efficiency of the AC motor, or the DC motor.

Efficiencies are what most everyone ignores and is the key to it all,    for example  rough numbers,  an internal combustion engine,   [standard circa 1980  no tricked out valve systems and fuel deliver,  just an ordinary carbureted motor hung on a dyno is going to be around or less than 50% efficient,  meaning for every gallon of fuel  you get half its power at the drive shaft of the engine,    

Now take a three phase induction motor,  standard efficiency,  probably going to run in the 80% range.   Right there is a huge difference in output power,   both rated for the same output HP but one design versus the other is more efficient about burning its fuel,  or losses of electricity in the form of heat,  and other magnetizing events that go on.

So to your question   NEMA   [National Electrical Manufacturers Association] has averaged motors over various sizes and speeds, and set standards for vanilla or off the shelf motors.

NEMA or even better table 430.247 of the National Electrical code will provide you with all kinds of information it would take days to retype here,

So let me give you this    standard off the shelf motors    

The armature current  which is going to be the major rating of power for a DC motor   is rated as follows by NEMA   

40HP  DC MOTOR  @    240 volts DC running at base speed,  would draw 140 amps.

This is all averages and based on normal motors with no other outside influences or conditions,  the ambient temperature is within tolerance,  so on......

Base speed lets say 1750 RPM.

Now a standard induction AC motor,  40HP with 240 volts three phase applied,  base speed of 1750   same conditions,   same load connected at the shaft aligned  so on, would draw right at 124 amps.

Not a lot of difference but shows the generic AC induction motor to be more efficient in design than a DC motor of the same HP,   however  we use DC motors because of the ability of DC to develop low end torque or low rpm torque,     this is all changing with the invent of variable frequency drives where the hertz applied to an AC motor is manipulated,   used in electric cars,  variable speed applications,  where prior that technology  DC or slip ring configured AC motors were used.

The key to this is figuring out the HP of the DC motor,  we know it is 12 volts applied  and the resultant amperage is 130  so we can get the approx HP    


above is a great site and page that provides charts with efficiencies and other data added in,     

Basically all these are averages but your question was really   "is the DC performance the opposite or inverse of the AC motor?"      no,  is the answer to the general question,   

But if you designed a super efficient DC motor,  and a very inefficient AC motor,  maybe you get there,   

Just remember it takes x amount of power to do a task,  how you get there involves efficiencies, basic physics,  basic type of design,    so on,          

Power is power,      period     how you convert that power, be it electrical power, or fossil fuel,  determines the amount of the initial fuel that will be needed     to overcome friction and windage,  design issues,  and a whole host of other factors,      

Good question and keep on investigating  

Electric Motors

All Answers

Answers by Expert:

Ask Experts




Three phase/ AC DC single phase motors, controls, any problems or failures, motor installation, performance issues, connections. All other electric motors/gearboxes/apparatus. Specialty repair concerns, obsolete motors and solutions. Other mechanical or specialty equipment. See my profile under Home/electrical at this site


30 plus years in the electrical motor and apparatus repair industry. VP level management of repair facilities, current owner of my own specialty repair and consulting firm.

EASA, IBEW [retired], other specialty organizations, Lubrication, Vibration EDI, Tribo-electric Councils

Currently fielding concerns at this site under "Home Electrical"

4 year technical, College level specific courses, EASA repair courses, vibration analysis electronic and electrical trade school.

©2017 About.com. All rights reserved.

[an error occurred while processing this directive]