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Is this buck/boost xmfr wiring correct?

If I want to raise the voltage to an ac 120 volt motor by, say, 12 volts, is this a way to do it?: Use a 120 volt primary, 12 volt secondary xmfr with the secondary winding rated at or above the amp rating of the motor. Then connect the 120 volt primary leads to 120 volts. Then connect one lead of the secondary to one lead of the primary. Finally, one lead of the motor is connected to the other lead of the secondary and the second lead to the lead of the primary that is not connected to the secondary. This will result in either a voltage drop or voltage gain of 12 volts. If the voltage drops then the secondary leads must be reversed. Are these statements correct? Thank you, Eric

Reply to
Eric R Snow
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Right. It's an "autotransformer."

But why raise the voltage to an AC motor?

John

Reply to
John Larkin

Hi, Eric. You're right about boosting the voltage, if I read your description properly (view in fixed font or M$ Notepad):

| | 12VAC BOOST TRANSFORMER |120VAC .----------------. | | | | L1 | _ . . | _ 132VAC | o-o--o_/ \\o--. ,---)-o_/ \\o-o | FU1 )|( | FU2 | )|( | | o-o----------' '---' | L2 | | | | '-------------------------o | L2 (created by AACircuit v1.28.5 beta 02/06/05

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Don't forget both fuses, and make sure the secondary current rating of your 12VAC xfmr is greater than the load current.

I'm a little confused about the motor business, though. What kind of motor are you driving? In addition to the question raised by Mr. Larkin, I'd be concerned about derating the xfmr (oversizing it) for the maximum current.

Cheers Chris

Reply to
Chris

Thanks John, Because it's a "universal motor" and it's right at the edge of its max load. I need its rpm to remain more constant under load. Using a variac gets me the required speed without overheating the motor. Brush life is acceptable. But the variac gets used for other stuff. Eric

Reply to
Eric R Snow

Thanks for the reply Chris. It's a "universal motor". So higher voltage means higher speed. Unlike an induction motor. I won't forget the fuses, or that the motor is not isolated from the line, and the xmfr will be rated at least twice measured motor current draw under load. Eric

Reply to
Eric R Snow

Why not a single fuse in the L1 line?

And it'll be OK if the 12 volt winding has a current rating equal to or greater than the expected load current.

John

Reply to
John Larkin

sounds good to me.

Take care when dealing with mains voltages.

Be aware that increasing the voltage will increase tha amps that flow through the motor, (but usually by a smaller fraction than the voltage increase)

Bye. Jasen

Reply to
Jasen Betts

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