# transformers

Status
Not open for further replies.

#### popeye

##### New member
I want to know how the windings in a transformer for commercial step down voltage?

#### dereckbc

##### Moderator
Staff member
Re: transformers

By the winding turn ratio. For example if a transformer primary had 100 turns, and the secondary had 43 turns, or a ratio of 100:43. With a primary input voltage of 480 volts would give you 208 volts on the secondary windings.

#### bennie

##### Esteemed Member
Re: transformers

Dereck: What is the phase and line voltage with three of the 100:43 transformers connected wye?

#### Ed MacLaren

##### Senior Member
Re: transformers

Here is a question that I used to have fun with in my apprentice classes.

Assume you have been called in to fix this problem. The customer complains that the secondary voltage, which is supposed to be 120/240, has gradually dropped over the years, and is now at 114/228 volts, about 5% too low. He wants you to restore it back to 120/240.

The transformer is now connected as shown in the sketch below. What would you do?

Ed

#### iwire

##### Moderator
Staff member
Re: transformers

OK Ed you got me curious, shouldn't the voltages at the taps be shown the opposite way?

Wouldn't lower input voltage need more windings to make the output voltage?

[ April 13, 2003, 05:30 PM: Message edited by: iwire ]

#### dereckbc

##### Moderator
Staff member
Re: transformers

Ed just guessing, move tap to 456.

#### bennie

##### Esteemed Member
Re: transformers

With a 5% decrease in secondary there must be a 2.5% decrease in the primary.

2.5% equals 12 volts. Primary voltage is 468, use appropriate tap.

[ April 13, 2003, 08:11 PM: Message edited by: bennie ]

#### Ed MacLaren

##### Senior Member
Re: transformers

Dereck has it right.

Assuming that originally a 480 volt supply produced 120/240 volts in the secondary, a reduction of 5% in the secondary can only have been caused by a drop of 5% in the primary voltage.

That means the primary voltage is now down to 456 volts, and will require that the 456 volt primary tap be used to maintain the required "volts-per-turn".

Ed

#### bennie

##### Esteemed Member
Re: transformers

Now I see where I went wrong...I used the 6 volts from the 120, instead of 12 from 240.

A 12 volt loss will be a 24 volt decrease in the primary. 456 makes sense.

#### charlie b

##### Moderator
Staff member
Re: transformers

Originally posted by bennie: Dereck: What is the phase and line voltage with three of the 100:43 transformers connected wye?
IF the primary is also connected wye, 208V. If the primary is connected delta, 360V (i.e., 208V times the square root of 3).

#### ronaldrc

##### Senior Member
Re: transformers

I would definetly go to a lower tap to get a higher voltage on the secondary.This would cause more amperage and with more amps would be more magnetic flux which would produce a higher voltage in the secondary.

Ed you no math and me don't mix I'm to lazy to figure the turn ratio.

I also used the simple logic if I doubled the number of primary windings it would cut my secondary voltage in half or to 60 volts.

I would go to a the 468 tap first and measure it and then try the 456 and then go to to closest one to the original voltage.

Ronald

[ May 06, 2003, 06:06 PM: Message edited by: ronaldrc ]

Status
Not open for further replies.