Queston RE John T. et. al. DC vs. AC motor efficiency

This isn't a trick or rhetorical question that I already have an answer for.

I've been turning wrenchs and doing electrical work for over 50 years. That being said, I just got a surprise.

This past year I've been working with something that's new to me. Also seems to be relatively new to many household electrical inspectors. 12 volt DC wiring in residential households. Especially with high-amp service panels, circuit breakers and switchs rated for DC, etc.

I am building a well pump based on an old piston-pump and converted to a 12 volt DC motor drive. Not long ago these old piston pumps were considered "junk." Now, they are considered "high end" and cost a small fortune.
Often $2000 for new one.

Here is my "surprise." My old Sears pump has a 1/4 horse, 1720 RPM, 115 VAC motor on it. These motors are common and a "dime a dozen."

Specs: Will make 1/4 horse continuous and draw 5.1 amps at 115 VAC. That comes to 586 watts.

The 1/3 horse version will draw 6.6 amps at 115 VAC. That comes to 760 watts.

Now, I just replaced with a 1/3 horse DC motor. It draws 26.4 amps at 12 volts at 1800 RPM. That comes to 317 watts.

I am amazed at this. Why is the DC motor so much more efficient then any of the AC motors?

To the converse, I know that an old DC generator on a farm tractor is inefficient because it makes AC power - and wastes much of it by custom selecting only the DC via brush placement on the commutator. Thus the reason why modern alternators using diodes make much more power per size.

I'm wondering if AC motors also waste near half the current to get rotation?

I'm also wondering if the part of the reason the DC motors are so much more efficient is the permanent magnet fields that don't use electricity?

By the way, if you EVER need to install a DC service panel in a house, I discovered a little known "secret." The only major brand that is "dual rated" for AC and DC is Square D type QO. In stock at Home Depot although they are cluelss about the obsure DC rating. Now . . . finding DC rated wall switches is another story.

This could get interesting, especially the part about the permanent magnets using no electricity.

You made the erroneous assumption that volts times amps equals watts. This is not the case with AC motors. Google "power factor" and come back after you understand it.

1 hp is 746 watts, your other math is a bit misleading but a dc motor is in general more efficient but not always.

To be more specific, the 1/3 horse AC motor is 60% efficient. 1/3 horse equals 248.6 watts. So, to make that 1/3 horse it ought to use 415 watts for grid power. The DC motor is not subject to the power-factor and is rated at 317 watts. If correct, that is substantially less power usage.

AC power=EI times the cosine of the phase angle or something like that. Dave

Did you actually make an effort to understand power factor? Do you understand that you can not determine the efficiency of a motor unless you know its power factor? Do you understand that power factor equals the cosine of the phase angle between current and voltage? Do you know how to measure phase angle?

It would seem that the answer to all these questions is "no".

Let me try to explain: For an ac load, current and voltage are only in phase if the load is purely resistive. For example, a light bulb. Motors do not generally present resistive loads; due to inductance, they are are partly resistive and partly inductive loads. A purely inductive load (a coil with no resistance) will have a current-to-voltage phase angle of 90 degrees and consumes NO POWER. Energy bounces back and forth between the coil and its power source, but the net power consumed is zero. Think of an inductor as an electric spring, storing and releasing energy. An electric motor is partly inductive and partly resistive, so the current to voltage phase angle is between zero and ninety degrees. For example, it might be 45 degrees, which would give us a power factor of .707 (cosine of 90 degrees).

Going back to your example, your AC motor is rated at 759 volt-amps (NOT 759 watts). At a power factor of .7, 759 volt-amps is 531 watts. At a power factor of .6, 759 VA is 455 watts. These are typical power factors for small motors.

The second erroneous assumption you made was to assume that the current rating for the motor is at the rated power. Unless specifically stated otherwise, the current rating is going to be greater than the current at rated power. That's because the wiring and circuit breakers for the motor need to be rated to handle a brief overload, such as starting current or during an intermittent high load.

Now with a dynamometer and other appropriate test gear, we could objectively compare the two motors. But that's not really necessary; electric motors are pretty efficient, be they ac or dc. Both motors have their advantages and uses. And there are many subtypes of ac and dc motors that have specific advantages in certain applications.

Also, regarding the difference in DC and AC rating for circuit breakers, that is easily explained: alternating current is interrupted 120 times per second, which means arcing is reduced when switching an ac load versus a dc load.

Also, the power factor for an ac motor changes depending on load. Generally ac induction motors are at peak efficiency at their max rated load so over sizing motors leads to worse efficiency.

You can perform power factor correction using capacitors but you need to switch them in a out depending on motor load.

question about ac and dc, was taught that ac changes polarity hence 60 cycles means 60 times a second, whereas dc contrary to some flows + to - true or false.

60 cycle power changes polarity 120 times per second, taking 1/60 of a second to complete a "cycle".

Whether current travels from positive to negative or negative to positive is nothing more than a convention. Military schools teach negative to positive (which is the direction of electron flow) while engineering schools teach positive to negative (which simplifies the math). Since you can't see electrons, it doesn't matter much as long as you stick to one or the other.

Not trying to be "erroneous" as you claim. These small AC motors that I'm discussing do not come with posted power-factor ratings. I went to the company's specs and they rate their 1/4 and 1/3 horse AC motors as 60% efficient but give no power factor number.

I will soon do an actual amp-draw test, under load with the 1/3 horse AC and the 1/3 horse DC and see what is actually being using via a 12 volt battery. I'll have to factor in 6% loss with the AC motor since I've be using an inverter.

As to the difference in breakers when rated for AC and DC? I understand it just fine. I was making the point that it's not always easy to find switches and breakers rated for DC when wiring residential households. Again, after researching and reading the "small print" . . the only mainstream circuit breakers that are certified for DC use is Square D QO. That doesn't mean others won't work. What it does mean is you can't pass electrical inspection unless the rating exists in writing.

Also the old wall "snap switches" like most houses had in the 1950s are rated DC and AC. The newer silent switches are not.

Yeah, it was the Hole Theory versus the Electron theory and they are both . . . still theories.

As I understand it, the choice of which theory was best often led to the choice of using negative or positive ground. At least in automotive electrical systems. Marine and military had other reasons dues to certain types of corrosion travel.

In order to do an apples-to-apples comparison, you'll need a means to measure both speed and torque (i.e. a dynamometer). For the ac motor you'll also need a means to measure the phase angle, such as a phase angle voltmeter or oscilloscope. Simply hooking up the motors to some load and measuring their current draw won't tell you anything about their comparative efficiency.

I guess I don't understand the need for household circuit breakers and switches rated for DC.