Universal motor...

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Externet
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Universal motor...

Post by Externet » Fri Oct 19, 2007 10:13 pm

Hi all.
A long time ago, I read a very good and simple technical explanation why a plain 120VAC circular saw motor is perfectly capable to perform very well with 12V DC, and the proof from the writer; an electric bicycle he made with a 'Skil' saw and a car battery... or two.
Can someone explain the principles of similar POWER capabilities with such different voltages? Cannot find now the article on the net explaining it. :sad:
A circular saw is not among my toys, or I would have already tested with 12V DC to compare its POWER.
Thanks,
Miguel
- Abolish the deciBel ! -

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jwax
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Post by jwax » Sat Oct 20, 2007 6:33 am

Not quite. The motor will run on 12 VDC, but not at all the same power it produces at 120 VAC. Now, 120 VDC is another story! :smile:

ecerfoglio
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Post by ecerfoglio » Mon Oct 22, 2007 6:03 am

Now, 120 VDC is another story!
No, at 120VDC it may take too much current and / or overspeed.

An universal motor is just a series wound DC motor (that is, the field winding is connected in series with the armature), but it has two small differences:

>> As the field winding will have AC, the magnetic field it produces will be variable and so the iron should be laminated to reduce losses - a "real" DC motor may be built witt a solid iron stator

>> In a DC motor, the current is given by U = E - IR, where U is the applied voltage, E is the back EMF generated by the armature, R the resistance of the windings and (of course :grin: ) I is the current.

If you use the motor with AC, the resistance "R" gets replaced with the impedance "Z", (Z = R + jX), and the inductance of the windings reduce the current for a given voltage.

So, the "correct" DC voltage for the motor will be lower than its AC one, but the exact value deppends on the design of that particular motor (on the relationship betheen R and X)

To determine that "correct" DC voltage you should run the motor with an ammeter in series, both on AC at the rated voltage and at DC with the "desired" voltage. Be shure to use the same mecanical load (ie free running) in both cases.
E. Cerfoglio
Buenos Aires
Argentina

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Externet
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Post by Externet » Mon Oct 22, 2007 9:47 pm

Yes ! :grin: that is sort of what I remember reading a long ago;

ecerfoglio wrote: <snip>
So, the "correct" DC voltage for the motor will be lower than its AC one, but the exact value deppends on the design of that particular motor (on the relationship betheen R and X)

To determine that "correct" DC voltage you should run the motor with an ammeter in series, both on AC at the rated voltage and at DC with the "desired" voltage. Be shure to use the same mecanical load (ie free running) in both cases.
Then, from what I understand, such motor should be capable of developing the same power on a lower DC voltage.

Good, time to lean the details. Thanks,
Miguel
- Abolish the deciBel ! -

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Post by dyarker » Tue Oct 23, 2007 8:25 pm

Remember that with the correct AC voltage, or the correct DC voltage, that for the same load the power used will be the same.
Dale Y

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Post by jwax » Thu Oct 25, 2007 9:04 am

Anybody recall the switch box for sale that claimed, "Run any power tool off your car"? Essentially switched the regulator out of the alternator circuit, put full battery voltage on the field winding, and the armature output would then vary with engine speed. It did indeed power a Skil saw with the correct engine speed.
Let us know how it works, Miguel! :grin:

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