# Questions tagged [autotransformer]

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### Need to make 380V and 415V 3-phase motors work with 480V 3-phase power supply. Will a 480V->400V autotransformer cut it?

I will have multiple 415V and 380V motors to power with a 480V/3-phase supply due to an international supply chain and tight budget. I would like to know if the solution to my issue is as simple as ...
• 329
1 vote
143 views

### I would like to know the origin of the following formula

I don't understand what's in the red circle. Why is the unit 's' and what does 's' stand for? Why must we divide the current at Open-circuit test by the voltage at Open-circuit test? I don't ...
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143 views

### LTspice - auto-transformer impedance ratio

Can you please clarify what formula is used by LTspice to calculate the auto-transformer impedance transfer ratio for the equivalent circuits below? Both circuits resonate on the same frequency. I ...
• 13
852 views

### Is copper loss present in the winding of an autotransformer?

Since the autotransformer transfers power conductively in addition to through its magnetic field, does its winding also cause loss of electric power in the form of copper losses? It seems to me that a ...
• 33
109 views

### Complex Input Impedance of Autotransformer with Complex Load

I am interested in the input impedance Zin of an autotransformer with coupling k and complex load Z1 as shown in the picture. Does anyone know how this can be calculated? This is what I tried so far. ...
• 742
163 views

### How does this transformer or autotransformer work?

I came across what appeared to be a transformer. Someone non-electrical said it is a autotransformer. What does this mean and how does it work? If I apply let's say 50V AC @ 10A on terminals 150 and 0,...
• 2,371
73 views

### How does a starter transformer work?

I am searching about chargers that are specifically for starting forklifts and other things that require, say, 12V-24V and 20A-80A. They have lead-acid batteries. These starters have a switch for 12V ...
1 vote
112 views

### Auto transformer currents

I am new to autotransformers and have a slight conceptual confusion in its working. I have seen many pages stating that the current has to increase in the step-down configuration in order to conserve ...
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100 views

### Why does the secondary current change direction to increase the autotransformer capacity?

Why does the secondary current reverse direction after this connection? The primary coil acts as a passive component with a voltage drop on it and the secondary coil acts as an active source with a ...
• 288
580 views

### Can I see a schematic for a 110 <-> 220 V power converter?

I would like to see a schematic for those 110 <-> 220 V power conversion transformers-in-a-box. I've read they are autotransformers (one winding with multiple taps). I'm using one to step up 110 ...
• 299
124 views

### Convert a 12 V toroidal transformer to an autotransformer

Suppose I buy a cheap 12 V, 100 W toroidal transformer. They used to be very common in the halogen light era, so they're easy to find and very cheap. Is it possible to remove the secondary winding ...
• 1,181
164 views

### Phantom voltage on output of isolation transformer

I bought a 400->230 V isolation transformer. I measure 'infinite' resistance between primary and secondary, like I expected. When I connect a variac/autotransformer to the secondary I can still ...
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### Variac is smoking when powering a DIY transformer

I'm conducting an experiment for my Physics class. I'm testing different transformer cores and comparing the transmission efficiencies. For my circuit, I have a variable autotransformer connected to a ...
222 views

### Attempting to power a 120 V, 0.4 A cartridge heater with a 120 V, 10 A variac

I have a small AC cartridge heater that has a max. input of 120 V and 0.4 A. I would like give full power to this heater (~50 W). I am trying to power the heater with a variable auto-transformer that ...
1 vote
64 views

### Will the phase shift be mutual if one phase is stepped down through isolated transformer and added to another phase?

i came across this concept of creating phase shift ( lagging and leading conditions) by adding a stepped down line voltage in series with a different phase voltage. I physically verified the concept ...
275 views

### Power supply with output of 24V, 500mA

I am currently trying to design a power supply (non-isolated) with input of 440V AC and output of 24V/500mA. Can I use an autotransformer for this? I know I can use conventional transformer, but I ...
589 views

### Using 3-tap 127/220V to 12V transformer as a poor man's 127V to 220V autotransformer

I need to actuate a few contactor switches that need 220VAC in its coils. The problem is, my application is fed by 127VAC. Therefore, I need something like a 10W, 127V to 220V autotransformer. These ...
• 1,563
51 views

### the amount magnetic induction kVA?

the question and solution is given below: How did he know that 10 kVA is transformed by magnetic induction? SOURCE: Electrical Machines, Drives, and Power Systems by Theodore Wildi. Sixth edition, ...
• 883
637 views

### What's the right replacement for vintage "glass cloth insulation" that transfers heat in Variac?

I ripped apart a broken 1950s-era Variac autotransformer to diagnose an internal short in the core, and found a layer of gooey insulating fabric mat between the coil/taps and the mounting plate: The ...
• 2,121
911 views

### Old Variac -- chassis ground? is it safe?

I've got an old (vintage!) GenRad Variac (below) that has two prong plug and two prong socket. I want to modify it to add a safety earth pin to both input and output. In doing so, I opened it up and ...
• 2,121
95 views

### HF/HV - rectification or modulation?

Lets put it simply: The aim is to rectify the high frequency/high voltage (i.e. 200 kHz, 100 kV) output to 12-24-48 V DC. or Modulate (possible amplitude modulation?) the HF/HV output with 50 Hz. ...
5k views

### How does the component "alarm boost three pin inductor" make a piezo buzzer louder?

I have a small circuit working on 2xAA batteries (3V) with an MCU and a buzzer for alarm sound. I've already successfully used an active buzzer (with build-in driver oscillator) drived by an NPN ...
• 201
57 views

### Variac and Other dropper in line

I have a beautiful old variac, with a sliding (rotating) center contact. I would love to use this on a nice LED string inside a glass bulb on top of the device. But also, use the variac as the dimmer, ...
• 140
83 views

I'm using an audio transformer SP-70 to isolate audio. (http://catalog.triadmagnetics.com/Asset/SP-70.pdf) The question arises what is the maximum AC current allowed to operate the transformer without ...
• 181
1 vote
719 views

### Connecting a 120V/30A variac to a 240V circuit

I have a variac rated for a maximum output of 30A. The problem is it takes 120V on the input and all of my 120V circuits are 15A, as expected. I unfortunately cannot just add an additional 30A circuit ...
67 views

### Reduce switching noise on autotransformer

I am trying to achieve the same thing they are talking about in this post, which is controling an autotransformer via software. I accidently even got the exact same autotransformer, but instead of a ...
711 views

### calculation of total capacity of Bank of 3 single phase transformer?

Imgur This is a nameplate of single phase auto-transformer. However in this transformer i'm not sure how do I get the capacity of this transformer. 3 of these transformers are connected in to form ...
• 364
691 views

### Autotransformer vs Inductive Divider

Which is the advantage of using an autotransformer instead of a simple inductive divider?
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1k views

### Can a Variac be modified to output higher voltages?

Since variable transformers can be had quite cheaply these days (under AUD 45 / USD 30 / EUR 27 today for a 500 watt one through eBay from China) they are cheap enough to experiment with. I bought ...
• 1,785
337 views

### Software controlled variable voltage AC supply with SSR relays

I have an autotransformer like this one : I would like to control it with an arduino solid state relay like this one: What would happens if I wire every output of the transformer together to the ...
381 views

### What will happen when taps at secondary side of auto-transformer are connected together (short-circuited)?

In case of Auto-transformer, there is only one winding, having three lead wires (1 for primary/input, 1 for secondary/output, and 1 common between both). At the secondary side (the output side), there ...
56 views

### vario trafo 4 terminals, trouble shooting

So I got a bare vario trafo and with 4 terminals, 2 terminals are constantly showing around 3 ohms (primary maybe?) the to other 2 terminals can vary in ohm as I turn the wheel (secondary?). So I ...
2k views

### Tapped vs. Coupled Inductor

I have reverse engineered a sounder circuit which appears to be using a center tapped inductor. Since then, I have tried to find center tapped / 3-pin inductors and cannot find them from a reliable ...
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1k views

### Step up SMPS with unrealistic voltage output with a 100W LED COB load

I got some 100W LEDs to make a lamp, realized with a boost converter I could use an old ATX power supply and bingo I'd be fine. It worked at first, until I didn't like the color so I decided to ...
• 235
1 vote
2k views

### Two windings transformer to autotransformer

Let's say that we have a transformer with two windings (source: circuitstoday.com) with nominal power $S$, nominal voltages $V_1$ and $V_2$ and windings $N_1$ and $N_2$ respectively. The ...
1 vote
5k views

### Winding EI core Stepup transformer for inverter

I want to learn the method or concept for winding Inverter Transformer(12V to 220V), How to select Core size Wire gauge Bobin Size Turn Ratio and/or number of turns for different VA ratings on EI ...
1k views

### Auto-Transformer: how to understand the following diagram?

I have a question about the CW Auto-Transfomer that I will be using. The wiring diagram is above. As you can see, the equitment takes 277/240/208/120V as input. But on its specification sheets, it ...
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