# Transformer primary winding and energy wastage at minimum load

#### p699

Joined Oct 8, 2021
30
Hello everybody

As I started doing a new project, a few fundamental questions popped up in my head. I dont think I have understood transformers properly. Please point out if I have understood something wrong. Based on what I learn here I will decide if I have to rewind my transformer or not. This is regarding energy wastage in the primary side of a transformer. How much should my primary winding be so that almost all of my power draw from the mains goes to power the load on the secondary.In my understanding at idle whatever the current passing thru the primary just goes to magnetize the core. Any current used to power the load is over and above this magnetizing current at idle. To keep this question simple, please ignore the resistive losses for now.

To make things clearer see this example :
I have a LOAD which varies its power consumption from 15 watts to 25 watts. Then I have a transformer (extracted from an old UPS, being put to use in this new project) which at no load connected, idles at 20 watts from the mains (primary coil). Suppose I now connect the LOAD to this tranny and it pulls 15 watts from the secondary for next 1 hour, is it correct if I assume that I am wasting 5 watts from this transformer under this scenario ? Will this setup be considered efficient when my LOAD is consuming between 20 to 25 watts and be considered lossy when it is consuming between 15 to 20 watts ?..... because the transformer idles at 20 watts is it correct to think that this tranny is designed for another LOAD whose minimum consumption was 20 watts ?

For my project should I rewind (increase primary winding (inductance)) this transformer to make it consume only 15 watts or less (at idle) since my LOAD at minimum consumes 15 watts ? It should also be able to deliver max power of 25 watts with minimal resistive losses I understand that.

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#### MrChips

Joined Oct 2, 2009
30,802
Welcome to AAC!

A transformer in idle mode draws zero current and hence consumes zero watts of power.

#### p699

Joined Oct 8, 2021
30
Welcome to AAC!

A transformer in idle mode draws zero current and hence consumes zero watts of power.
Thanks for the reply MrChips. I did not understand this I am sorry. I powered few transformers I have lying around with no load at secondary and measured their AC currents at primary side. They all consumed between 1.4 watts to 18 watts at 230Vac.
Edit: I guess it is called magnetizing current.

#### MrChips

Joined Oct 2, 2009
30,802
Thanks for the reply MrChips. I did not understand this I am sorry. I powered few transformers I have lying around with no load at secondary and measured their AC currents at primary side. They all consumed between 1.4 watts to 18 watts at 230Vac.
Edit: I guess it is called magnetizing current.
Were the transformers designed for operation at 230VAC @ 50Hz?

#### LowQCab

Joined Nov 6, 2012
4,067
Welcome to AAC!

A transformer in idle mode draws zero current and hence consumes zero watts of power.
That's only in theory.
The Core of the Transformer is never perfect,
and on top of that,
some transformers are purposefully designed with lossy Cores,
and with a "barely-adequate" Primary-Winding to save costs.
This was probably made infamous by Microwave-Oven manufacturers,
who try to cheap-out in any way they can devise, and the Transformer is
the most expensive part of the Oven,
and they use about half of the Copper-Wire that they should to save costs,
then make the Core less efficient so it won't burn-down your House.

A no compromise, high quality, Transformer will consume around
~5% of its maximum rated Power-Output at idle.
Transformers wound on a Toroidal-Core may be even more efficient than this.
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.
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#### MrChips

Joined Oct 2, 2009
30,802
That's only in theory.
The Core of the Transformer is never perfect,
and on top of that,
some transformers are purposefully designed with lossy Cores,
and with a "barely-adequate" Primary-Winding to save costs.
This was probably made infamous by Microwave-Oven manufacturers,
who try to cheap-out in any way they can devise, and the Transformer is
the most expensive part of the Oven,
and they use about half of the Copper-Wire that they should to save costs,
then make the Core less efficient so it won't burn-down your House.

A no compromise, high quality, Transformer will consume around
~5% of its maximum rated Power-Output at idle.
Transformers wound on a Toroidal-Core may be even more efficient than this.
.
.
.
Thanks for this. I intentionally omitted “in theory”.

#### p699

Joined Oct 8, 2021
30
Were the transformers designed for operation at 230VAC @ 50Hz?
Yes, I bought a few from the local market, couple of them I wound my self, many I plucked out of dysfunctional UPS systems. All are definitely rated to operate at 50Hz not 60Hz.

#### p699

Joined Oct 8, 2021
30
and with a "barely-adequate" Primary-Winding to save costs.
This was probably made infamous by Microwave-Oven manufacturers,
who try to cheap-out in any way they can devise, and the Transformer is
the most expensive part of the Oven,
and they use about half of the Copper-Wire that they should to save costs,
then make the Core less efficient so it won't burn-down your House.
Thank you for writing this sir, This is exactly what my question is related to. If an application circuit demands 15 watts of power, is it right to wind a transformer primary coil that idles at 20 watts of power, or should I increase that winding so that it idles at 15 watts ? Is my understanding of power being wastage in the first case correct ?

A no compromise, high quality, Transformer will consume around
~5% of its maximum rated Power-Output at idle.
Max power consumption of my application circuit is 25 watts , so my tranny should idle at 1.25 watts (5% of max) ? But minimum power consumption is 15 watts, so wont I face resistive losses even at minimum ?

#### crutschow

Joined Mar 14, 2008
34,420
I have lying around with no load at secondary and measured their AC currents at primary side.
That does not measure the real power, only the apparent power, since it does not measure the phase between the voltage and the current.
The no load transformer current is mostly reactive magnetizing current, and doesn't use much real power.
The easiest way to measure real power is to use a Kill A Watt type power meter, such as one of these.

#### LowQCab

Joined Nov 6, 2012
4,067
Thank you for writing this sir, This is exactly what my question is related to. If an application circuit demands 15 watts of power, is it right to wind a transformer primary coil that idles at 20 watts of power, or should I increase that winding so that it idles at 15 watts ? Is my understanding of power being wastage in the first case correct ?

Max power consumption of my application circuit is 25 watts , so my tranny should idle at 1.25 watts (5% of max) ? But minimum power consumption is 15 watts, so wont I face resistive losses even at minimum ?
.
Hand-Winding a High-Voltage-Primary-Winding is not recommended,
there are plenty of ways that You can do it wrong.

Why are You trying to wind your own Transformer,
when there are hundreds of high-quality units available,
for less money than You are going to spend on just the Wire for the Primary ?
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.
.

#### p699

Joined Oct 8, 2021
30
.
Hand-Winding a High-Voltage-Primary-Winding is not recommended,
there are plenty of ways that You can do it wrong.

Why are You trying to wind your own Transformer,
when there are hundreds of high-quality units available,
for less money than You are going to spend on just the Wire for the Primary ?
.
.
.
Over the years I have collected few Iron cores plucked from other devices. Then I have also bought 2 spools of enamelled copper sometime back. I have already built my own and they are working without any dangerous side effects. I like to wind my own.

The main point of this thread is about designing a transformer which matches the present power requirement more closely and thus doesnt waste much power. A store bought tranny would have been wound to meet different (or generic) power requirements isnt it ?

Transformers available in my local market are expensive (3x the price of of a full spool of good quality enameled copper wire) and often they are built with lower grade copper to keep the price low, resulting in heat and resistive losses.

So is my understanding of power being wastaged correct ? Is it right to think that I have to match primary magnetizing current to minimum load at secondary to be considered efficient ? Ignore resistive loss for now.

#### p699

Joined Oct 8, 2021
30
That does not measure the real power, only the apparent power, since it does not measure the phase between the voltage and the current.
The no load transformer current is mostly reactive magnetizing current, and doesn't use much real power.
The easiest way to measure real power is to use a Kill A Watt type power meter, such as one of these.
Thanks for the advice but can a TRMS Volt and Amp meter give close enough readings ? I have both but dont have a watt meter you are suggesting.

So is my understanding of power being wastaged correct ? Is it right to think that I have to match primary magnetizing current to minimum load at secondary to be considered efficient ? Ignore resistive loss for now.

#### crutschow

Joined Mar 14, 2008
34,420
can a TRMS Volt and Amp meter give close enough readings ?
No since, as I stated, they don't measure the phase angle between the voltage and current.
You could have one ampere of inductive current, which you would measure with your meter, but the real power would be zero.
Is it right to think that I have to match primary magnetizing current to minimum load at secondary to be considered efficient ?
No.
The magnetizing current has only a slight relation to the load power or efficiency.
Normally you want the magnetizing current to be as small as possible.

#### Ian0

Joined Aug 7, 2020
9,808
Welcome to AAC!

A transformer in idle mode draws zero current and hence consumes zero watts of power.
A perfect transformer of the kind you might find is SPICE consumes zero power. A real transformer consumes rather more than that, mainly due to core loss.
A general purpose transformer would be designed so that core loss and winding loss are about equal at full power.
If you have an application where the transformer is rarely used a full power, then winding it for low flux would decrease the core loss (which happens all the time) and increase the winding (copper) loss.

#### MrChips

Joined Oct 2, 2009
30,802
I intentionally omitted the phrase "in theory". The intent was to give the TS the opportunity to learn how transformers work.

#### p699

Joined Oct 8, 2021
30
A general purpose transformer would be designed so that core loss and winding loss are about equal at full power.
If you have an application where the transformer is rarely used a full power, then winding it for low flux would decrease the core loss (which happens all the time) and increase the winding (copper) loss.
So this means I must wind my transformer such that it has as low magnetizing current as possible and at the same time keep coil dc resistance as low as possible ?

#### p699

Joined Oct 8, 2021
30
No.
The magnetizing current has only a slight relation to the load power or efficiency.
Normally you want the magnetizing current to be as small as possible.
Thanks for that. I think all the replies here are tending towards one thing....... "Keep the magnetizing current as low as possible"

Regarding power calculation, I had postponed buying a watt meter thinking that I can use V & A to get the answer. I didnt know phase angle mattered. Yes, the voltage can be at 230Vac but the current can be at a different amplitude when phase are not in tandem. I will probably get one. Thanks.

#### p699

Joined Oct 8, 2021
30
So this means I must wind my transformer such that it has as low magnetizing current as possible and at the same time keep coil dc resistance as low as possible ?
By low dc resistance what I mean is when power is pulled from the secondary, the primary must be able to deliver it with low resistive losses.

I think the answer is clear, I will add more turns to my tranny starting now

#### crutschow

Joined Mar 14, 2008
34,420
So this means I must wind my transformer such that it has as low magnetizing current as possible
To go lower than that, may require a core size larger than needed.
By low dc resistance what I mean is when power is pulled from the secondary, the primary must be able to deliver it with low resistive losses.
You must consider the resistance of both the primary and secondary, as they both contribute to the transformer loss under load.