# The Naked Scientists Forum

### Author Topic: How do you describe a Transformer?  (Read 6944 times)

#### Chikis

• Jr. Member
• Posts: 35
##### How do you describe a Transformer?
« on: 28/04/2013 07:48:14 »
A transformer has 500 turns in the primary coil and 300 turns in the secondary coil. If the primary is connected to 220 V mains, what voltage will be obtained from the secondry coil? What type of transformer is this?

To evaluate the magnitude of the voltage in the secondary coil:
Es/Ep = Ns/Np
Es = (Ep*Ns)/Ns
= (200*300)/500
= 132 volts. This shows that the transfor is a step down transformer because the number of volts in the primary is greater than that in secondary. I.e 220v > 132v

I also know something about the turns ratio. If the turns ratio of the transformer is greater than 1, the transformer is  secondary transformer. If the turns ratio of the  transformer is less than 1, the transformer is primary transformer.
My question now is, what is the best way to evaluate the turns ratio? Is it Ns/Np or Np/ Ns?
« Last Edit: 28/04/2013 22:19:23 by evan_au »

#### lightarrow

• Neilep Level Member
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##### Re: Transformer
« Reply #1 on: 28/04/2013 11:27:53 »
...
My question now is, what is the best way to evaluate the turns ratio? Is it Ns/Np or Np/ Ns?
Sorry, are you asking if "is better" to write "3/5" or to write "5/3"?

#### Bored chemist

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##### Re: Transformer
« Reply #2 on: 28/04/2013 12:59:22 »
This " If the turns ratio of the transformer is greater than 1, the transformer is  secondary transformer. If the turns ratio of the  transformer is less than 1, the transformer is primary transformer." looks like strange nomenclature to me.

#### teragram

• Sr. Member
• Posts: 122
##### Re: Transformer
« Reply #3 on: 28/04/2013 16:21:18 »
Consider the primary as the “input” and the secondary as the “output”:-
The turns ratio is 500/300 = 1.666666667:1.
With 220Volts on the primary (500 turns), the secondary voltage is 220/ratio = 132.
In this case the transformer is a “step down” transformer.
If used as a “step up” transformer, the ratio would be 300/500 = 0.6:1, and if 132 Volts is applied to the secondary (300 turns), the voltage on the 500 turn coil will be 132/0.6 = 220Volts.
This seems a simpler way of calculating.
In this case of course the “secondary” is now being used as the “primary”, or input. BE AWARE!! that if 220Volts were applied to the secondary (300 turns) the voltage on the 500 turn winding would be 367 Volts. Also the current will exceed the rating for the coil. It is dangerous to connect mains voltage to the secondary of a step down mains transformer
Another way of calculating is:-
The voltage per turn on the primary is 500/220, or 2.273 turns per volt, therefore the secondary voltage will be 300/2.273 = 132volts, ie the “turns per Volt” is the same for both (or even additional) windings on the core.
Needless to say, current follows similar rules except in the opposite sense, so current available on the secondary of a “step down” transformer will be greater than that on the primary.
I have never heard of a “primary” or a “secondary” transformer

#### syhprum

• Neilep Level Member
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##### Re: Transformer
« Reply #4 on: 28/04/2013 19:42:35 »
As a boy at school I was fascinated by transforms and learnt a lot about them by trial and error, one important point is that you must not over saturate the core with the laminations that were available at the time a good rule of thumb was that the turns per volt at 50Hz should be one turn per volt for a core area of 9 square inches and proportionally more turns for a smaller core area i.e your 500 turn winding should have a core area of 3.96 square inches if you are to apply 220V and you should not exceed 132V on the 300 turn winding

#### Chikis

• Jr. Member
• Posts: 35
##### Re: Transformer
« Reply #5 on: 28/04/2013 21:23:34 »
Consider the primary as the “input” and the secondary as the “output”:-
The turns ratio is 500/300 = 1.666666667:1.
With 220Volts on the primary (500 turns), the secondary voltage is 220/ratio = 132.
In this case the transformer is a “step down” transformer.
If used as a “step up” transformer, the ratio would be 300/500 = 0.6:1, and if 132 Volts is applied to the secondary (300 turns), the voltage on the 500 turn coil will be 132/0.6 = 220Volts.
This seems a simpler way of calculating.
In this case of course the “secondary” is now being used as the “primary”, or input. BE AWARE!! that if 220Volts were applied to the secondary (300 turns) the voltage on the 500 turn winding would be 367 Volts. Also the current will exceed the rating for the coil. It is dangerous to connect mains voltage to the secondary of a step down mains transformer
Another way of calculating is:-
The voltage per turn on the primary is 500/220, or 2.273 turns per volt, therefore the secondary voltage will be 300/2.273 = 132volts, ie the “turns per Volt” is the same for both (or even additional) windings on the core.
Needless to say, current follows similar rules except in the opposite sense, so current available on the secondary of a “step down” transformer will be greater than that on the primary.
I have never heard of a “primary” or a “secondary” transformer
Quote
I have never heard of a “primary” or a “secondary” transformer
Sorry that was a mistake anyway. What I intended writing is, "If the turns ratio of the transformer is greater than 1, the transformer is  step up transformer. If the turns ratio of the  transformer is less than 1, the transformer is step down transformer."

#### Chikis

• Jr. Member
• Posts: 35
##### Re: Transformer
« Reply #6 on: 28/04/2013 21:39:04 »
Sorry, are you asking if "is better" to write "3/5" or to write "5/3"?
Yes, that is what am asking.
« Last Edit: 28/04/2013 22:25:41 by Chikis »

#### evan_au

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##### Re: Transformer
« Reply #7 on: 28/04/2013 22:17:20 »
You could call it a "5:3 stepdown transformer" (specifying the Primary side first).

Other details that are useful when describing the transformer are the voltage and current ratings (the output voltage droops when the secondary current is at the maximum rated value). The rated frequency of operation is also useful if its not obvious from the context (50 or 60Hz doesn't make a big difference).

It does look a bit odd to me if you call it a "5/3 step down transformer", since I am used to reading 5/3 as a fraction > 1, which sounds like a step up transformer... but anyone who understands transformers will understand what is intended.
« Last Edit: 28/04/2013 22:31:59 by evan_au »

#### syhprum

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##### Re: How do you describe a Transformer?
« Reply #8 on: 29/04/2013 20:18:29 »
The fun begins when you have three phases

#### techmind

• Hero Member
• Posts: 934
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##### Re: How do you describe a Transformer?
« Reply #9 on: 20/05/2013 23:55:28 »
And in practice (especially in transformers with small-numbrs of turns), because of leakage inductance, you might find that the voltage ratio isn't /precisely/ what you expected it to be.

I prefer to express my transformers in terms of a turns ratio m:n (where m is the driven side, and n the output side). This way a transformer used in a "step up" configuration can be written 2:20 (for example) in a logical (left-to-right flow) order without any potentially-misleading implication of fractions...

#### The Naked Scientists Forum

##### Re: How do you describe a Transformer?
« Reply #9 on: 20/05/2013 23:55:28 »