12voltDC ~ 5amp power supply

google up fullwave bridge rectifier Radio Shack should have something in stock

Your 12-volt transformer will work very nicely with a suitable bridge rectifier, as:

See recent posts on bridge rectifier for which terminals are which: connect the ~ or AC terminals to the transformer, the + and - terminals to the motor. I suggest using the 15-amp unit for $1.75 because it'll run nice and cool at 4.3 amps.

Hmmmm. Seems to me that a simple brush type 12 volt DC motor qualifies as a "universal motor" and therefore ought to run fine on 12 volts ac. I'd try it. If it seems to work okay, fine. If not, get the rectifier.

Pete Stanaitis

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mark wrote:

The cheapest would be to use a full wave rectifier and your 12 transformer. You can switch the low voltage side. With no load the transformer will use a little power, but not too much. You can get an idea of the power by feeling the transformer and seeing how much heat it puts out. Measuring the current on the input will not tell you anything about the power as the current will not be in phase with the voltage.

If it were me I would switch the high voltage side of the transformer. Wire the switches so either switch turns on the motor. Both have to be off to stop the motor. Or wire it with two three way switches just like lights are wired. And also have another switch to select which blower will run.

You might find a full wave rectifier in an old pc power supply. Or you could use two pc power supplies. One for each motor.

Dan

transformer

(Top posting ignored...)

The 12 v. supply in a PC may not have enough current capability... Some would, many wouldn't. You'd have to look at the spec on the supply...

Jerry

About $2.50 for a 50 amp rectifier at Rat Shack.

"Deep in her heart, every moslem woman yearns to show us her t*ts" John Griffin

RoyJ sez:

"> Later model 12 volt DC motors are typically permanent magnet based. Nogo

Interesting observation, Roy. Whaddaya figure a PM motor would do on AC ? I would guess the poor thing would attempt to turn first one way and then the other at the cyclic rate of the applied AC.

Bob Swinney

If connected to a current limited or low voltage AC supply it will buzz. If you let it have a bit more juice, it may well degauss its magnets and become useless even if you dont connect it long enough to let the holy smoke out.

If you can get to the brush terminals its fairly easy to tell its a PM motor by the absence of field connections. Otherwise, a PM motor reverses when the DC supply reverses, a universal motor does not. (You have to revese the brush connections relative to the field coils.)

A good indication can be got by connecting a voltmeter to the (otherwise not connected) terminals and spinning them by hand. PM motors generate well. Universal motors at low revs generate very weakly if at all as there is only the residual magnetism to get things started. Its woth confirming this crude test by other methods though.

Even if a motor appears to be universal, unless the field core is laminated with no solid polepieces or other solid parts in the flux loop, you'll get excessive eddy currents, and field coils that run hot or even smoke themselves.

Rectifiers are cheap, its easier to use one.

My local Radio shack (The Source by Circuit city) has the following in stock, would this be suitable. "Bridge rectifiers ( full wave ) 25A, 50 PIV."

If it is permanent magnet or not laminated core it will not work or last long on AC.

Should be fine. Go for it.

Wes S

thanks