variable transformer vs autotransformer

Thread Starter

btebo

Joined Jul 7, 2017
89
I've been using an autotransformer to give me an adjustable voltage for a hot wire cutting machine. Input is 120VAC, output is generally set to about 30-40VAC.

I've been reading that an autotransformer has safety issues especially with an open secondary.

Is a variable transformer the same animal as an autotransformer? Would a variable transformer be safer?

Thanks,
Balad
 

crutschow

Joined Mar 14, 2008
23,377
Most variable transformers are autotransformers with no isolation between primary and secondary.
You have to look at the specs to see if it is isolated.

You can use an isolation transformer at the input to the autotransformer to make it safe.
At a minimum the autotransformer should be plugged into a GFI socket to prevent fatal shocks.
 

Thread Starter

btebo

Joined Jul 7, 2017
89
Fortunately it is plugged into a GFI. I'll look for a suitable isolation transformer.

Thanks for your advice as always!
 

Dodgydave

Joined Jun 22, 2012
8,411
A variable transformer is also called a Variac, it's just a coil across the primary with a rotating carbon brush tapping the desired voltage, downside is they also use a common Neutral supply and therefore doesn't give you total isolation from the mains.
 

Thread Starter

btebo

Joined Jul 7, 2017
89
Is there such a thing as a variable transformer that provides TOTAL isolation?

I'm using a Powerstat 116CU. The data sheets I've found for it all refer to it as a variable transformer - so I can't tell if it has an isolated secondary or not. How can I test to find out? Would it be as simple as opening the secondary and measuring the output voltage?

I can add an isolation transformer. The max voltage I ever need is about 45V. If I place a step-down transformer (120VAC down to 48VAC) in front of the Powerstat unit, would that provide the isolation I need?

Since I'm learning from you experts - could I put a fuse and a MOV (rated about 50V) in the secondary to achieve the same protection?

Thanks,
Balad
 

crutschow

Joined Mar 14, 2008
23,377
I can't tell if it has an isolated secondary or not. How can I test to find out?
Measure for any continuity between the input and output with an ohmmeter.
If it's isolated there will be a very high resistance (or the meter will read OL for a resistance beyond the max it can read).
I can add an isolation transformer. The max voltage I ever need is about 45V. If I place a step-down transformer (120VAC down to 48VAC) in front of the Powerstat unit, would that provide the isolation I need?
Yes.
could I put a fuse and a MOV (rated about 50V) in the secondary to achieve the same protection?
No.
It's the continuity between the mains and the output that's the danger.
It means that you can be electrocuted by touching just one lead if you are grounded.
 

dendad

Joined Feb 20, 2016
3,028
I would place the variance before the 120V to 40V transformer, adjusting the step down transformer input, not on the output.

Edit:
I see this morning the post written on the iPad fell fowl of the Dreaded Auto Correct Feature... "variance" indeed!
 
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Thread Starter

btebo

Joined Jul 7, 2017
89
I would place the variance before the 120V to 40V transformer, adjusting the step down transformer input, not on the output.
That's an interesting thought.... If the variac is before the stepdown transformer, there is never a chance for the secondary of the variac being "open" - it will always have the primary of the stepdown transformer across the terminals.

In that case, could I just use a 1:1 isolation transformer after the variac?
 

dendad

Joined Feb 20, 2016
3,028
That's an interesting thought.... If the variac is before the stepdown transformer, there is never a chance for the secondary of the variac being "open" - it will always have the primary of the stepdown transformer across the terminals.

In that case, could I just use a 1:1 isolation transformer after the variac?
A step down transformer will be better as you are after a fair bit of current output at lower than the mains voltage. Also you will get a larger working adjustment range on the variac.
 

ebp

Joined Feb 8, 2018
2,332
One caution in using a transformer on the output of a variable xfmr:
Many variable transformers can deliver a greater than the input voltage when cranked up to the max. 140 VAC output with 120 VAC input is typical of many.

A transformer intended to have nominally 120 VAC input may be a bit unhappy with 140 VAC input. What happens is that the iron core can begin to "saturate" - the magnetic flux density limit in the core is reached. The winding of the transformer then begins to appear resistive instead of inductive to any further increase in voltage. The resistance of the winding is normally very low, so the current can become very high. This would happen on a half-cycle by half-cycle basis. For example, everything might be OK up to 170 volts (roughly the peak of 120 VAC sine wave), but at 195 volts (peak of 138 V sine) saturation is hit. While the voltage remains above 195 volts, the primary of the transformer looks resistive and the current spikes way up. This can cause considerable heating in the winding. Nothing is permanently damaged by saturating an iron core, as long as things don't get too hot for too long.

Transformers are usually designed to tolerate at least 10% above nominal input voltage to accommodate line voltage variation, but when you start stacking on some extra voltage, it can sometimes be too much. It is hard to know just how any transformer is going to behave without actually testing. With a variable transformer, just slowly turn it up while monitoring the primary current in the step-down xfmr (no load on latter). If the current stays proportional to voltage all the way to the max you'll probably be OK (again, there is the issue of high line voltage).

Core flux is actually a function of applied voltage times time (usually expressed as volt-second product). If you are operating at 60 Hz and can get a step-down transformer rated for 50 Hz, you get a nice margin. Stay away from cheap import transformers - in my experience they are more likely to be less conservatively designed.

not anything to be concerned with for your application, just tossed in in case anyone is interested:
The iron core in a transformer can retain considerable magnetism when the applied input is turned off - just depends on timing. When power is re-applied, if the polarity of the AC is "right", the core can be driven very brutally into saturation, with very high transient current. This can be rather hard on switch or relay contacts.
 

Janis59

Joined Aug 21, 2017
767
Autotransformer is such sub-kind of variable transformer, what in the aim of economy of some % of copper wire are winded on common winding. The secondary is part of primary winding. So, the full winding is 220V, and the output is got by sliding the roll higher or lower by it. Full galvanic contact with mains there is always. And probability exist 50/50%, that one may turn the plug in the way that trafo nil stays on the mains phase.

About last may help the neon lamp on the gnd wire, however better way to secure yourself is the differential automat:
This kind of fuse-switcher measures current going off and compares with coming back by gnd wire. If both currents differ more than 30 mA, in the 5 msec the fuse is blown. Price between 3 and 4 Eur at current of 10...16...25 A.
 

Janis59

Joined Aug 21, 2017
767
About saturation - yes, those round style lab-transformers of 2,2 kW 10 Amp as well the box-type 250V 8Amp with voltmeter both are prone to beat out all fuses in moment on switching on. When russia orders was in the force, it said the fuses must be slow enough and 10 A consumption had at least a 32A melting fuse, it was small thing to neglect. But now, when at 12 Amp demand the energocompany gives a A or B fast magnetic fuse of 10 Amps, or pay much more for each kilowatthour, then it becomes a real headache.
But those LATR system trafos ar eternal bless if one have it, even shortly 25A may not kill it instantly. The thermocouple welding with battery carbon, the motor checking, the DIY transformer zero-load current curve test and many many more another very essential jobs are unimaginable without it.
Or as month ago I needed to create for short one-day experiment the adjustable 200 Amp 0,5 Volt AC feed. I took a 25 Amp bi-roller type LATR, connected to half of winding at old type of classical transformer AC welding transformer (ratio 220/65/2=7) and 200/7=30A what gave it slightly warming but persisted.
 
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Janis59

Joined Aug 21, 2017
767
RE:""The safer method is a isolation transformer with a Triac controller on the secondary, many use a simple dimmer on the primary side, I preferred to place mine on the secondary.""

If to put the dimmer in secondary, You will have a sure non-sinus output with lots of killer-spikes. Not good idea for many kind of machinery. However, putting dimmer in primary is more heavy for triacs or thyristors because of probably inductive load, especially while zero-load.
But generally, idea is fine, yet less trusty than oldy-goody LATR (Laboratory Auto Transformer), and sure cheaper and less weighty.

Ou, ou, we forgot the saturable magnetic amplifier, so belowed by russian elctron microscope, vaccuum spttering device and other heavy-science machinery builders. It gives not less performance and has weight even higher as LATR :)

But if it is so dumbly insensitive thing as hot wire, the best would be indeed the classical transformer with thyristor regulator in secondary. May buy the three or four halogen transformers in serie. 4x3Eur=12 Eur. Small, lightweight and fine.
If the adjusting is premise, then Chineese 480W SMPS (24V20A, 36V etc) for about 20 Eur. Very stable construction nor You nor me ever can laid out even for tenfold of this price (oh those workers consuming a rice handful a daily salary).

P.S. I found here one for You in America (40 USD), adjusting trimmer is in the picture at left corner:
www.ebay.com/itm/High-Quality-24V-36V-48V-DC-480W-Regulated-Switching-Power-Supply-Transformer/162167259142?epid=1719637850&hash=item25c1ec0006:m:mc1iYOLlk7vAXHOUjRbGmdg
 
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Agree with isolation xformer before Variac.

lamp dimmers and transformers won;t get along. We had to use an oversized current limited SCR unit with a built-in semiconductor fuse ($25 USD) combined with a $1.00 fuse. We had an open system in a vacuum where shorts happened. We also put analog meters on the secondary. Because of the phase angle fired waveform, you could not multiply I and V to get power.

You MUST fuse the wiper of a Variac. It's OK to fuse the primary too, but the wiper must be fused.

Later, I convinced the powers that be that a 0-5 V controlled 1U form factor DC power supply would cost us less money and would actually be better and take up a lot less rack space. The temperature controller/programmer had to have an isolated analog output which they did.

We too had 30-40 V custom tantalum heaters and used 1500 W supplies. These were about $1500.00 USD each.
 

Janis59

Joined Aug 21, 2017
767
RE:""used 1500 W supplies. These were about $1500.00 USD each""

Dont know about specific qualities of such PS you tell, but price seems just as Maffia Italiana may envy.
The real price of such is 3x480W SMPS what gives a 120 USD already cleared for US market, with forward expenses included. See the link one post upper.

P.S. Those boxes slightly dislike to cooperate in parallel. Thus be better to get the 12 or 15V boxes and connect it in serie. With two of such I had done it and no trouble at all.
 

MaxHeadRoom

Joined Jul 18, 2013
19,066
The output has to be ordered voltage as needed, they are not adjustable to any wide degree.
The TRIAC/SCR design for hot-wire I used works well on the secondary for varying length of wire.
Max.
 

Thread Starter

btebo

Joined Jul 7, 2017
89
Most variable transformers are autotransformers with no isolation between primary and secondary.
You have to look at the specs to see if it is isolated.

You can use an isolation transformer at the input to the autotransformer to make it safe.
At a minimum the autotransformer should be plugged into a GFI socket to prevent fatal shocks.
Dear Crutschow,
I ended up ordering a 1:1 isolation transformer to place before my autotransformer. But I'm still having a hard time understanding how this will help.... The output of the isolation transformer will be 120VAC going to the input of the autotransformer. How will this protect the "secondary" side of the autotransformer if for some reason the secondary becomes open (broken hot wire cutter) from getting full voltage?

I'm a NOOB and am trying to learn all I can! Thanks.
Balad
 
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