# DC Motor Characteristics



## Bill Boehme (Feb 9, 2014)

Rather than hijacking hawkeye's thread on lathes, I decided that I would start a new topic to discuss DC motor characteristics. 
"Facts", as they exist in the wild, frequently come from marketing types (you know, the guys who wear togas, frolic with unicorns, and interpret technical motor data for your entertainment). And, their knowledge about DC motors comes straight from the unicorn's mouth. But, in an imperfect world, engineers and unicorns don't always see things the same way.

For all rotary motors, the rotating element is pulled along by a rotating magnetic field. In a DC motor, the rotating magnetic field is produced by "commutation". I won't go into the mechanics of how it works, but those are the motors with carbon brushes that rub against bronze commutation segments on the armature.

Diagrams speak to engineers the way that unicorns speak to marketeers and here is my favorite diagram for DC motors because it says so much about the way that type of motor operates.









The diagram shows four parameters on a speed vs torque plot.


The vertical axis is speed where the maximum speed, No, is the maximum No Load speed. In other words, at the maximum speed all of the produced torque is needed to overcome motor losses such as friction and aerodynamic drag with none left over to power an external load. That isn't a very useful speed, obviously.
The horizontal axis is torque. The far right end of the axis, Ts, is the maximum torque that the motor can produce and is known as stall torque. The midpoint, Ts/2, is 50% of stall torque and is the point of maximum power output.
The two axes cross at zero speed and zero torque ... the combination of no torque and no speed is another one of those not very useful operating conditions.
So, the three boundary conditions that define the operating envelope ave a lot in common with marketing ... not very useful, but still necessary. The space inside this envelope is the engineer's playground.


The line that runs from No to Ts is called the load line and it represents a constant voltage ... the maximum operating voltage for the motor. There are actually a whole family of parallel constant voltage load lines. If the applied voltage is reduced the load line will be beneath the maximum voltage load line. For a constant torque load, speed is directly proportional to applied voltage.
Motor current is a linear function of torque load. There is a non-zero speed minimum current called, Io, for no torque load. Maximum load current, Is, is the stall current when speed is zero and maximum output torque is applied.
Now, lets look at the power curve ... that's the inverted parabola. Mechanical output power is the product of speed and torque. If either speed or torque are zero then obviously, power is zero which is why the two ends of the power curve are zero. The motor reaches maximum output power, Pmax, at 50% of stall torque and we want all of the motor operation to take place to the left of Ts/2. Any operation to the right of that line is known as operating on the back side of the power curve. it results in a lot of wasted energy heating the motor and diminished output power where increasing the applied voltage can actually slow the motor down.
The final very important parameter is efficiency, the funny looking inverted curve that peaks at ηmax, at very low load torque and has dropped very low by the time that maximum power output is reached. Above peak power, efficiency falls dramatically, dropping to zero at stall torque.
So, there you have it, a qualitative overview of the relationship between DC motor operating parameters. I think that without this diagram, the significance of the behavior of these parameters will be glossed over. Two significant observations are that stating horsepower output is really gilding the lilly and efficiency is probably assumed by most to be a constant. In reality, it is really hard to pin down, but it is high only over a small part of the operating envelope. Hope that this is useful info.


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## saculnhoj (May 18, 2015)

All I know is when you turn the speed down you lose power and it drives you crazy. When roughing out bowls you typically need slow speeds on small lathes because the out of balance wood really makes the lathe dance around the room. So you turn the speed down and now you have so little power you can only take 1/8" deep cut per pass. Hopefully said lathe has multiple pulleys so you can step down to a pulley that lets the motor run at less than low speed so you have at least some power. However that starts defeating the purpose of having variable speed. Which is why on larger lathes they went to 1 or 1 1/2 hp DC motors in the earlier models. So you still had some power when you the speed down. 
My friend Joe Looper had a lathe that would spin 54". He had a 5 horse motor on it. I have seen a photo of John Jordan turning on one side of the headstock and another person turning on the other side at the same time. Those were pieces about 10" in diameter and I assume he wasn't running at the slowest speed. 
Bill I know you said one time that Variable Frequency Drives lose power also but I put a 1 1/2hp 3 phase motor on my old Nova 3000 lathe used a VFD to power it. It still had lots of power at the slowest speeds. I keep wondering why they don't put 3 phase motors with VFD on the mini lathes. Probably expense.


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## NCPaladin (Aug 7, 2010)

Thanks for the info Bill but it is waaaay above me. 
I just look at it in general terms. That being for AC 10 amp = 1 HP, and DC 8 amp = 1 HP. Not that it will not fit everything but is verified by these two calculators and just raise a flag if it seems way off.
For AC
http://www.veris.com/calculator.aspx In the find HP, I used 120V, 10 amp, 70% efficiency and it came out to 1.01.
John brought up three phase and the same input yields 1.76 HP.
For DC
https://www.easycalculation.com/physics/electromagnetism/dc-motor-horsepower.php
I used 120V, 8 amp, and 80% efficiency which also give 1 HP.

I realize there are many variable that maybe only the head technician at Baldor can understand.

I know noting about this product below but I would have to give it a hearty Bravo Sierra. :laughing:
http://www.sears.com/shop-vac-6.5-p...p-00913412000P?prdNo=1&blockNo=1&blockType=G1
Maybe I should buy one and upgrade my Nova 1624 to 6.5 HP.:no:


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## Jerry Maske (Dec 29, 2013)

Bill,
There you go again picking on the Unicorns. What did they ever do to you? Great article on the motors, however. Just wish you'd stop picking on my buddies.

Jerry


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## Bill Boehme (Feb 9, 2014)

NCPaladin said:


> Thanks for the info Bill but it is waaaay above me. ....


OK, so I noticed. 

So I'll have another go at it. Basically, the whole point of the original post was to say that horsepower ratings applied to DC motors are highly suspect at best and maybe more likely bravo sierra material.

There are several kinds of DC motors. The most commonly seen one is the universal motor which is configured in such a way that it can run directly from AC without the need to convert the AC power to DC first. Tools that use universal wound motors include, drills, routers, belt sanders, power carvers, shop vacs, lawn edgers, weed eaters, electric hedge trimmers, electric chain saws, portable kitchen appliances like mixers and blenders, blow dryers, vacuum cleaners, sewing machines, treadmills, etc. Some, if not most mini and midi lathes with variable speed also use universal motors. The thing that make them so attractive is they are very low cost and very good power to weight ratio, but not so good in the efficiency department. When used in variable speed operation the controllers for these motors are very simple voltage choppers and they are also very cheap to build. Universal motors are almost always operated on AC except for battery powered devices. The following chart shows that the performance of these motors is somewhat lower when operated on AC, so that is another thing to consider and a reason that some of the data found online needs to be taken with a big grain of salt (about the size of a salt lick cube).









Basically, what the chart shows is that the speed of a universal motor is low at full load and very high at no load. It also compares the speed/load characteristics of operating the motor from AC and DC power sources.

There are also true DC motors which means that they must have a power supply that converts AC to DC. When looking at the efficiency of those motors, we need to keep in mind that the efficiency of the motor needs to be multiplied by the efficiency of the power supply in order to be truly honest.

Going back to the first post, even if the motor curves data doesn't make sense, here are the two main things to take away:


Horsepower varies with speed and load torque. There is an optimum speed and load torque where horsepower peaks. For any other speed and load torque the horsepower falls off rapidly.
The published efficiency is not a constant value ... it is the peak efficiency. Peak efficiency occurs at high speed and very light torque load ... in other words, nowhere close to peak output power. At peak output power, the efficiency is very low. That is why this type of motor gets very hot when running at high speed and fully loaded.
The bottom line is that you can't just apply these parameters as if they all happen at the same operating point because they don't. Fully loaded, the efficiency of a motor might be in the vicinity of 40% where the specs state an efficiency of 80%.

Also, take a look at the characteristic curve for motor current. The main thing that we gather from that curve is that even at zero load there is a minimum value for current that is what is known as the magnetizing current. That amount of current is necessary, but does not contribute directly to output power. It comes from a time shift difference between the voltage and current due to the energy stored in magnetic fields that exist in the windings of the motor. The result is a parameter known as power factor. This number is not a constant, but varies with load and speed. At high speed and heavy load, the power factor might be about 0.8 and perhaps .6 at mid speed and 25% of full load. This is something this is highly variable so it is hard to give exact numbers.

For the best efficiency, you can't beat a three phase motor. Actually, you can ... a so called BLDC (brushless DC motor), but the downside is cost which can run well over ten thousand dollars for a full fledged BLDC motor with samarium-cobalt rare earth super-magnets, Hall-effect commutation sensors, digital position feedback encoder, and dedicated computer/controller. These little motors can pack a lot of power in a very small space so they are great for use on robotics, factory automation, and military/aerospace vehicles (aircraft, satellites, International Space Station, Hubble Space Telescope, Mars Rover). They are really a hybrid between AC and DC motors and somewhat like an inside-out permanent magnet DC motor where the windings are in the stationary field and the rare-earth magnets are on the rotor.

Finally, those unicorns at Sears have special magic powers that enables them to make a 6.5 HP shop vac that can run on no more power than what it takes to power an LED flashlight. :laughing:

Back in the real world, you might need to divide that number by 100 to get the actual HP. I have a Baldor 3 HP three-phase motor and it weighs almost 100 pounds. I think that a 6.5 HP motor might weigh close to 200 pounds. It looks like I might need to get one of them unicorns to work for me designing motors.


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## Jerry Maske (Dec 29, 2013)

Another fine post, Bill. I managed to wade through it and actually understand some of it. I've got a pretty good background in electronics, albeit VACUUM TUBE electronics (dating myself here). In my Navy days we used some pretty impressive electric motors and generators to power our various vacuum tube systems. Anyway, nicely done.

One hint. If you want Unicorns to come work for you, feed them Pizza. They don't like Anchovies, but anything else and you won't be able to keep them off you.

Jerry


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## Andrew LB (Oct 30, 2012)

NCPaladin said:


> I know noting about this product below but I would have to give it a hearty Bravo Sierra. :laughing:
> http://www.sears.com/shop-vac-6.5-p...p-00913412000P?prdNo=1&blockNo=1&blockType=G1
> Maybe I should buy one and upgrade my Nova 1624 to 6.5 HP.:no:


I will never own another ShopVac product again after one of their 5.5hp vacuum's almost killed me a few years ago. I was sanding a tabletop with my Bosch ROS which I had connected to the ShopVac and after about 10 minutes I started smelling smoke. I quickly turned off the sander and turned toward the vacuum and at that moment the tiny amount of smoke coming out the exhaust turned to a massive plume of the thickest jet-black smoke you could imagine. I got up quickly to turn it off and ended up grabbing the cord and yanking it from the wall. At this point my entire garage was so thick with smoke I couldn't even see the 12x 4' long T12 tube lights and made my way to the opener button and my fire extinguisher by memory. Spent the next hour hacking up a lung from all the smoke inhalation. 

It turns out something had shorted out in the motor itself, which burned through and ignited the sawdust in the bucket. I know it wasn't caused by dust buildup in the motor because I always used a drywall collection bag and a CleanStream HEPA filter, and occasionally blow the motor out with my compressor.

I now own a Ridgid 14 gallon 2-Stage vacuum and my ears have thanked me every day since.


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## NCPaladin (Aug 7, 2010)

Bill Boehme said:


> OK, so I noticed.


 Ah, seems we are on the same page. :yes: Unless you tell me a motor can be _More Than 100% efficient_ then I will use my "is it possible"
analysis.


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## Bill Boehme (Feb 9, 2014)

Any effort at comparing DC and AC motors will have so many caveats that the comparison is meaningless. Every type of motor has its strong and weak points when thinking of a particular application.

Horsepower is a term that gets bandied around a lot, but when you buy a shop vac where the motor is an integral part of the machine, truth in advertising is just a figure of speech. When that shop vac was belching like a volcano was what it looks like when producing 5.5 HP. :laughing:

Maybe horse feathers would be a better term.

I think that a more useful way to describe performance on mini and midi lathes with DC motors would be to state maximum continuous output torque at different speeds. Then, if you wanted to know horsepower then just calculate it.


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## Bill Boehme (Feb 9, 2014)

NCPaladin said:


> Ah, seems we are on the same page. :yes: Unless you tell me a motor can be _More Than 100% efficient_ then I will use my "is it possible"
> analysis.


No, it isn't possible.


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