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Author Topic: Question for engineers versed in transformer design  (Read 4379 times)
K1ZJH
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Posts: 3297




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« on: October 07, 2017, 04:16:10 PM »

I noticed the idling current on the transformer in my HP-23A Heathkit power supply was rising rapidly as I
increased AC supply voltage above 110 volts.  At 125 VAC the idling/magnetizing current is a bit over 500 mA.
I suspect the transformer primary windings are a bit short of inductance.
 
The transformer gets warm,  but after it letting run for a few days it seems to be okay.

Curiosity got the better of me, so I tried playing around by adding capacitors across the primary winding.
A 6 uF 250 VAC cap reduced the AC idling current by 50%.

At this point I was not trusting the analog meters in my variable AC power supply.

Well, the plot thickens.  I went over to a HP meter that uses a thermal converter to measure
AC or DC sources.  That should, I would think, eliminate any false readings due to reactive
components??

Measuring the AC line current at 120 VAC showed around 440 mA of idling current.

Adding a 6 uF cap across the 125 secondary winding on the HP-23A transformer reduced the
magnetizing current drawn from the AC line to around 260 mA.

The secondary voltage remained the same with or without the cap in place.  Measurements
used AC line voltage direct from a wall socket.  When testing on the variable AC supply,
the 6 uF cap would work on the primary winding, but once the isolation transformer and
Variac were out of the picture the idling current could only be reduced by using an AC rated
cap on the low voltage secondary winding.

Am I chasing my tail, or is adding the cap internally a good idea? The test setup would seem
to indicate that the capacitor does reduce losses, but I am not convinced.

Pete
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AK0B
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Posts: 267




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« Reply #1 on: October 07, 2017, 08:21:18 PM »

I have a group of old equipment.   Back in the day one was luck to get 115 vac and never 125.  Now days my power company's meter always reads 125 plus a few tenths. I feel this is bad for my old receivers and transmitters so I use a variac to cut the voltage back to 112 to 115 vac

Stan ak0b

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G8HQP
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Posts: 595




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« Reply #2 on: October 08, 2017, 06:25:36 AM »

The problem appears to be core saturation at the higher primary voltages. This causes a drop in inductance and so a rise in magnetising current. Putting a cap across the primary will mask it, by adding capacitive current in parallel with the inductive current. Putting a load across the secondary will cure it, by causing voltage drop across the primary resistance and so reducing the effectve voltage driving the primary.

It is not uncommon for cheaper equipment to be on the verge of core saturation, especially when the transfomer is unloaded.
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K0RO
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Posts: 39




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« Reply #3 on: October 14, 2017, 01:50:58 PM »

Fascinating discussion.  I learned something new about transformers.
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WA2ISE
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Posts: 1049




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« Reply #4 on: October 14, 2017, 03:44:01 PM »


I built this autotransformer line reducer power strip.  Lowers the line voltage by 6V.
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AC7CW
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Posts: 961




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« Reply #5 on: October 14, 2017, 04:30:11 PM »


I built this autotransformer line reducer power strip.  Lowers the line voltage by 6V.

Good to know that. I had a house with 130volts at the wall receptacles. A 12 volt reduction would have been perfect.
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Novice 1958, 20WPM Extra now... (and get off my lawn)
WA2ISE
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Posts: 1049




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« Reply #6 on: October 14, 2017, 05:23:08 PM »

Good to know that. I had a house with 130volts at the wall receptacles. ...

130V at every outlet?  or just some of them, others lower?   If that's the case, you may have a bad neutral connection in the circuit breaker box.  Loose connections could do that. If you have a 240V outlet, see if that reads twice your 130V.  If that is true, call the power company, and hopefully you can get past the non-technial customer service people...

about 30 years ago I had 132V on all the outlets.  Called the power company, and got the customer service rep to tell the tech guys that I had excessive line voltage.  About a month later, some linesmen with a DVM show up, and sure enough, I had 260V from hot to hot.  They eventually found that my town was loading the 3 phase distribution network unevenly.  Took them several weeks to get it straightened out. 
« Last Edit: October 14, 2017, 05:27:50 PM by WA2ISE » Logged
KB3MDT
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Posts: 255




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« Reply #7 on: October 16, 2017, 05:31:53 PM »

Hi,
    What is the advantage of using an AutoTransformer configuration as shown in the schematics?   I always thought a bucking transformer wired the 120 Volt Primary of the Transformer across the power line.  One side of the load would go to one side of the primary.  The other side of the load would be in series with the other side of the primary phased "opposite" to the primary to reduce the output voltage.   See my cheesy diagram below.  (The \ and / represent coil windings.

<-------------|-----|
                   \  ||  |
                   /  ||  |
120 AC         \  ||  \
Primary         / ||   /  6 Volt Secondary
to house       \  ||  \
power           /  ||  |--------------->
                    \  ||              To Load
<---------------|---------------------> 

I'm always ready to learn something new.

73
Ken
KB3MDT
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K1ZJH
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Posts: 3297




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« Reply #8 on: October 17, 2017, 08:52:31 AM »

The problem appears to be core saturation at the higher primary voltages. This causes a drop in inductance and so a rise in magnetising current. Putting a cap across the primary will mask it, by adding capacitive current in parallel with the inductive current. Putting a load across the secondary will cure it, by causing voltage drop across the primary resistance and so reducing the effectve voltage driving the primary.

It is not uncommon for cheaper equipment to be on the verge of core saturation, especially when the transfomer is unloaded.

My curiosity was aroused since the early HP-23 doesn't show show core saturation at higher AC line
voltages while the 120/240 volt transformer in the HP-23A apparently does. 

Both cores look to be the same size.

Pete
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G3RZP
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Posts: 8123




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« Reply #9 on: October 17, 2017, 09:09:29 AM »

Were the transformers made by the same company? If not, a possibility is a different manufacturer of the steel. Or if they are made by the same company, maybe there was the usual purchasing 'success' of 'cost reduction' and a different grade of steel.
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K1ZJH
Member

Posts: 3297




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« Reply #10 on: October 17, 2017, 09:59:04 AM »

I don't know. I finished the upgrades and buttoned both units up. I am not sure the transformers bore any markings to indicate a manufacturer.

Pete
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WA2ISE
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Posts: 1049




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« Reply #11 on: October 17, 2017, 12:44:59 PM »

    What is the advantage of using an AutoTransformer configuration as shown in the schematics?   I always thought a bucking transformer wired the 120 Volt Primary of the Transformer across the power line. 
KB3MDT
In the autoformer mode, the line voltage can get higher before the transformer core starts getting saturated.  The line could go to primary voltage rating + secondary voltage rating. 
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AA4HA
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Posts: 2381




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« Reply #12 on: October 17, 2017, 01:28:23 PM »

The transformer presents inductive-reactance to the source. A capacitor presents capacitive-reactance. When you provide correction via the addition of the capacitor you are pulling the power factor back to "1". An ideal load would be purely resistive with no inductive or capacitive component.

Excessive inductive-reactance can cause heating of conductors.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
Free space loss (dB) = 32.4 + 20 × log10d + 20 × log10 f
W6EM
Member

Posts: 1641




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« Reply #13 on: October 22, 2017, 08:43:11 AM »

The transformer presents inductive-reactance to the source. A capacitor presents capacitive-reactance. When you provide correction via the addition of the capacitor you are pulling the power factor back to "1". An ideal load would be purely resistive with no inductive or capacitive component.

Excessive inductive-reactance can cause heating of conductors.
The advantage to correcting power factor close to unity is a reduction in supply losses.  Seen usually as voltage drop.  Also, an improvement in regulation, by virtue of lowering supply total inductance.  Think of adding shunt capacitance as a way of reducing series inductance in everything towards the source of supply. 

But, adding capacitance across a lumped inductance, as represented by a transformer’s self-impedance, tends to raise the overall impedance.  Simply, you are tuning the transformer’s self-inductance and approaching anti-resonance, which raises the impedance and lowers the idling current.

If there is significant power dissipated when unloaded, it does sound like insufficient self-impedance.  A combination of insufficient turns or core area or core material permiability issues.

Moving the capacitor in question to the transformer secondary only will reduce its net reactance as seen by the primary by the ratio of the square of the turns ratio.  So, a given reactance connected on the primary of a 120v to 12v transformer would be 1/100th of same value if moved to the secondary side and looked at from the primary.

As for bucking and boosting using fixed step down transformers, it’s done all the time by utilities on rural feeders.  One caveat.  If you want to use old 6.3v CT or 12.6v CT filament transformers to do that. The secondary insulation on them isn’t the same as the primary insulation.  At least, not necessarily.  To be safe, you should put the metal core/case on insulated standoffs and enclose it in a box.  That way, less chance of an accident.  In fact, you could make a “poor-man’s variac” by using a couple of them with center taps.  3.15, 6.3, 9.45, 12.6. And, either boost or buck from the incoming supply.  With a maize of switches, of course.  And, be sure to check the phasing with a good DVM.



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W6EM
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Posts: 1641




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« Reply #14 on: October 22, 2017, 09:36:31 AM »

Hi,
    What is the advantage of using an AutoTransformer configuration as shown in the schematics?   I always thought a bucking transformer wired the 120 Volt Primary of the Transformer across the power line.  One side of the load would go to one side of the primary.  The other side of the load would be in series with the other side of the primary phased "opposite" to the primary to reduce the output voltage.   See my cheesy diagram below.  (The \ and / represent coil windings.

<-------------|-----|
                   \  ||  |
                   /  ||  |
120 AC         \  ||  \
Primary         / ||   /  6 Volt Secondary
to house       \  ||  \
power           /  ||  |--------------->
                    \  ||              To Load
<---------------|---------------------> 

I'm always ready to learn something new.

73
Ken
KB3MDT
OK.  I'll give it a try.  "Auto-ing" means connecting the secondary and primary in series.  Not as your sketch shows, which simply connects the secondary to one of the primary leads as a series bucking winding.

Auto-ing, as shown in the schematics above yours, series-connects the primary and secondary transformer windings.  Doing it so the windings are "additive," as he has done (even though the result is a "substractive" output voltage) will do two things to the transformer's advantage.  First, it reduces primary winding current by adding the secondary turns so that the input voltage seen by the actual primary is reduced by the secondary winding voltage.  And, a lower primary voltage means less risk of insulation failure (important to electric utility substation transformers).

A disadvantage, although small, is regulation is not as good as the separate-winding- only involvement.  The reason being that whatever the buck winding voltage drop is will be reflected in lower primary winding voltage.  So, if significant, it will aggravate voltage drop across the transformer and make actual voltage sags larger.

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