Transformer Ratio Help

Thread Starter

HighVoltage!

Joined Apr 28, 2014
181
I have an educational single phase step-down/step-up transformer which is fed by a 120V Variac (Variable Transformer) that allows you to add as many turns as you wish ON THE SECONDARY (Insulated wire is used). Currently I am using the transformer as a step-down transformer with TWO TURNS ON THE SECONDARY (Ns = 2). I measured values for Vs (Secondary Voltage), Np (Set amount of turns which cannot be changed), Ns = 2, Ip (Primary Current), & Is (Secondary Current) with a Primary Voltage (Vp) = 10% on variac = 11.5Vac (.10 x 115Vac)

How would I hand calculate these values for when Ns = 3 at 10% on the variac before physically adding an extra turn? I am very familiar with using the transformer ratios for calculations (Np/Ns = Is/Ip = Vp/Vs). However, my calculation is not adding up for the new Vp? Can Volts per turn be used to calculated new values?
 

AlbertHall

Joined Jun 4, 2014
12,345
How big was the difference?
What is the physical construction of this transformer?
Note that with just two turns the wires connecting to that secondary can add or subtract a partial turn depending on the layout. Try twisting the wires like this:
 

Thread Starter

HighVoltage!

Joined Apr 28, 2014
181
How big was the difference?
What is the physical construction of this transformer?
Note that with just two turns the wires connecting to that secondary can add or subtract a partial turn depending on the layout. Try twisting the wires like this:
In other words, after adding a turn, I needed a higher primary voltage to put out more voltage on the "load". Maybe I calculated it incorrectly?
 

AlbertHall

Joined Jun 4, 2014
12,345
voltage on the "load"
Is there a load on this transformer? That changes things as the output voltage of a transformer under load depends (among other things) on the resistance of the windings. By changing from 2 to 3 turns the secondary resistance has gone up by 50%.
 

Thread Starter

HighVoltage!

Joined Apr 28, 2014
181
Is there a load on this transformer? That changes things as the output voltage of a transformer under load depends (among other things) on the resistance of the windings. By changing from 2 to 3 turns the secondary resistance has gone up by 50%.
Yes, there is a resistive load across the secondary windings.
 

Thread Starter

HighVoltage!

Joined Apr 28, 2014
181
So you need to look at the open circuit voltage.
Im assuming you mean Open circuit voltage of secondary...so with a resistive load, this is the only way to accomplish this. Would I calculate the open circuit voltage and "adjust" based on load resistance? How would I calculate my values? Thank you guys so far for responding!!!!
 

crutschow

Joined Mar 14, 2008
34,280
Im assuming you mean Open circuit voltage of secondary...so with a resistive load, this is the only way to accomplish this. Would I calculate the open circuit voltage and "adjust" based on load resistance? How would I calculate my values? Thank you guys so far for responding!!!!
Yes, you measure the voltage on the open secondary.
To calculate the effect of the load resistance you need to know (or measure) the resistance of the primary and secondary windings, and calculate the voltage drop from those (adjusting the primary resistance by the turns ratio) due to the load resistance current through those winding resistances.
 

Thread Starter

HighVoltage!

Joined Apr 28, 2014
181
Yes, you measure the voltage on the open secondary.
To calculate the effect of the load resistance you need to know (or measure) the resistance of the primary and secondary windings, and calculate the voltage drop from those (adjusting the primary resistance by the turns ratio) due to the load resistance current through those winding resistances.
Is it "safe" to put a voltmeter across the open secondary as a load to measure this voltage? Is there a formula I would use to adjust for resistance?

What if the secondary was shorted with a heavy conductor thats capable of handling high currents, how would you calculate that?
 

crutschow

Joined Mar 14, 2008
34,280
Is it "safe" to put a voltmeter across the open secondary as a load to measure this voltage? Is there a formula I would use to adjust for resistance?

What if the secondary was shorted with a heavy conductor thats capable of handling high currents, how would you calculate that?
Perfectly safe.
A voltmeter is normally such a high impedance that it will have little effect on the open circuit transformer voltage (for example typical digital multimeters have at least a 1 megohm impedance).

If the secondary is shorted then the current will be determined by the open circuit secondary voltage divided by the sum of the secondary and reflected primary winding resistances.
 

Thread Starter

HighVoltage!

Joined Apr 28, 2014
181
Perfectly safe.
A voltmeter is normally such a high impedance that it will have little effect on the open circuit transformer voltage (for example typical digital multimeters have at least a 1 megohm impedance).

If the secondary is shorted then the current will be determined by the open circuit secondary voltage divided by the sum of the secondary and reflected primary winding resistances.
What would be the best way to measure the primary and secondary resistances? Im sure those resistances will change (increase/warm up).
 

crutschow

Joined Mar 14, 2008
34,280
What would be the best way to measure the primary and secondary resistances? Im sure those resistances will change (increase/warm up).
If they are too small for reliable measurement with an ohmmeter, than you an run a known (measured) DC current through them and measure the voltage drop.

The wires may heat up some under heavy load but the resistance change is usually not that significant.
You can calculate that my measuring the transformer temperature under load after sufficient period of time for it to warm up and then calculate the resistance increase from the coefficient of resistance change with temperature for copper.
 

Thread Starter

HighVoltage!

Joined Apr 28, 2014
181
If they are too small for reliable measurement with an ohmmeter, than you an run a known (measured) DC current through them and measure the voltage drop.

The wires may heat up some under heavy load but the resistance change is usually not that significant.
You can calculate that my measuring the transformer temperature under load after sufficient period of time for it to warm up and then calculate the resistance increase from the coefficient of resistance change with temperature for copper.
Are you sure that the secondar
Perfectly safe.
A voltmeter is normally such a high impedance that it will have little effect on the open circuit transformer voltage (for example typical digital multimeters have at least a 1 megohm impedance).

If the secondary is shorted then the current will be determined by the open circuit secondary voltage divided by the sum of the secondary and reflected primary winding resistances.
Awesome!

For shorted secondary:
  1. My initial open circuit voltage measurement - Does the secondary have to have only one turn through transformer? With this open circuit voltage, I can now calculate the effect of two turns correct?
  2. I was wondering if I could figure out the secondary Open Circuit Voltage & Primary Winding resistance (Two unknowns) with knowing secondary current and secondary's wire resistance?
 

crutschow

Joined Mar 14, 2008
34,280
  1. My initial open circuit voltage measurement - Does the secondary have to have only one turn through transformer? With this open circuit voltage, I can now calculate the effect of two turns correct?
  2. I was wondering if I could figure out the secondary Open Circuit Voltage & Primary Winding resistance (Two unknowns) with knowing secondary current and secondary's wire resistance?
1. It can have as many turns as you like.
2. No.
Too many unknowns.
 

#12

Joined Nov 30, 2010
18,224
More turns = more voltage, as long as the load resistance is the same, and zero load current is a good thing.
Trying to calculate the sum of all impedances is a PIB. (Pain In Behind)
Measuring is so much easier!
Do not short a turn for more than a few seconds or you risk overheating the transformer.
 

Thread Starter

HighVoltage!

Joined Apr 28, 2014
181
More turns = more voltage, as long as the load resistance is the same, and zero load current is a good thing.
Trying to calculate the sum of all impedances is a PIB. (Pain In Behind)
Measuring is so much easier!
Do not short a turn for more than a few seconds or you risk overheating the transformer.
Secondary turns are insulated :)
 
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