Is this buck/boost xmfr wiring correct?

[Eric R Snow]

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

 

[John Larkin]

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

Right. It's an "autotransformer."

But why raise the voltage to an AC motor?

John

 

[Chris]

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

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 www.tech-chat.de)

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

 

[Eric R Snow]

Right. It's an "autotransformer."

But why raise the voltage to an AC motor?

John

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

 

[Eric R Snow]

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 www.tech-chat.de)

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

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

 

[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 www.tech-chat.de)

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

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

 

[Jasen Betts]

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

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